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INSTYTUT ENERGII ATOMOWEJ INSTITUTE OF ATOMIC ENERGY
PL9702390
RAPORT IAE - 20/A
RECOMBINATION METHODS IN THE DOSIMETRY OF MIXED RADIATION
NATALIA GOLNIK
Institute of Atomic Energy 05-400 Otwock-Swierk, Poland
Work partly supported by the Polish Committee for Scientific Research project No. 4 S404 010 05
OTWOCK - SWIERK 1996
VOL 2 8 Ns 2 3
Natalia Gnlnik: Recombination Methods in the Dosimetry of Mixed Radiation. This work describes in monographic manner the state of the art of recombination methods developed for the dosimetry of mixed radiation fields. The existing theories of initial recombination of ions in gases are discussed and complete formulation of a new theoretical approach is given. Recombination methods developed in the Institute of Atomic Energy in Swierk are reviewed in details and illustrated with a wealth of experimental results. The methods described here can be applied in mixed radiation fields of poorly known composition and practically unlimited energy range. Main dosimetric parameters such as absorbed dose, photon component to the absorbed dose, radiation quality factor, dose equivalent, ambient dose equivalent and some other quantities can be determined with single instrument. A novel method has been developed for determination of the energy loss distribution in the nanometric region. Experimental tests showed that the method is promising not only for radiation protection but also for radiobiological investigations, since it provides information about the energy loss in volumes of about 0.1 pm diameter, where the experimental data are not easily accessible.
Natalia Golnik: Metodv rekombinaevine w dozvmetrii nromieniowania mieszanego. Praca zawiera monografiezny opis metod rekombinacyjnych, stosowanych w dozymetrii p61 promieniowania mieszanego. Przedstawiono dyskusjs istniejqcych teorii lokalnej rekombinaeji jonow w gazach oraz peiny opis nowego modelu teoretyeznego. Szczeg&owo przedstawiono metody rekombinacyjne opracowane w Inslytucie Energii Atomowej, ilustrujqc je bogatym materialem doswiadczalnym. Opisanc metody mogq byd zastosowane w dozymetrii pol promieniowania mieszanego o nieznanym skladzie i szerokim widmie energetyeznym. Umoiliwiajq one wyznaczenie podstawowych wielkoSci dozymetrycznych, takich jak dawka pochlonicta, skladowa dawki pochodzqca od promieniowania gamma, wspticzynnik jakoici promieniowania, rdwnowaznik dawki, rdwnowaznik dawki przeslrzennej i niektorych innych, za pomocq jednego przyrzqdu. Opracowano nowq metody wyznaczania rozkladu depozycji energii w obszarach tkanki o rozmiarach dziesiqtkdw nanometrdw. Wyniki doswiadczalne wskazuja, zc metoda ta jest obiecujqca nie tylko dla celow ochrony przed promieniowaniem, ale rbwnieZ dla radiobiologii, gdyZ dostareza danych o rozkladzie depozycji energii w obszarach tkanki o srednicy okoto 0,1 pm, dla ktorych trudno uzyskad dane doswiadczalne.
HaTamw ToubHUK: PeKOMSHHaipioHKbie Meronai JoaHMerpHH CMemanHoro H3Jtvuchhh PatioTa HBJiaerca MOHorpa^mecKHM oimcaHHCM pexoMGHHauHOHHbix mctojob jtooHMexpHH CMetnaHHoro HajiyucHHH. B patioxe oGcyamemi cymecTByromne reopror HaqantHofi peKOM6nHainm hohob b raae h npeflcraBJicHo ormcaHHC hobofo TeopenmecKoro nonxona. IlonpoGHo npeacraBJiCHbi peKOMtiHHaimoHHbie Meronbi paapaGoxaratbie b HHcnrryxe AtomhoH 3Hq)nm h 6oran.rR 3KcnepHMeHTajn,Hbrft Marepnaji. OrmcaHHtie b paGoxe Mexo/rbi mobcho ncnojibaoBaxb jpm onpeflaieroM ocHOBHbix fl03HMerpmecKHx bcjupmh, xaicHx kbk nornomcHHaa aoaa, (poroHHaa yoaa, KoatjxJnmHeux xasecTBa Hanyuerom, 3KBHBajieHXHaa nosa, awGneHXHaa 3KBHBajicirrHaa no a a h HCKOxopbix npyrax, b jnoGbix nonax cMemaHHoro Hajiyuerota, raioKe b nomrx c HcnaBecTHbiM cocrasoM h c umpoKHM, npaKimecKH HeorpaHMSCHHbiM aHeprerasecKHM cneicrpoM uaenm. PaapaGojan hobbih mctos onpenejiCHmi pacnpenenemtii noabi no J1II3 b oGtexax cooTBercrByiomHX oGbCMy XCHBofi TK8HH C SHaMCTpOM paBHblM HCCKOJIbKHM neCXTKaM HaHOMCTpOB. Pe3>'JIbTaTbI HaMepcHHti noKaabraaioT, hto Meron Moxcer BbnaaTb rarrepec co cropoHbi panHononmecKofi 3aimm.i h pannoGHOJiorHH.
Wydaje Intiytut Energii Atomowej - OtNTEA Nikied 43 egz. Obfaoii: irk-wyd. 8,5; eik-druk. 13,0.
Pete zlotenie meazvnopisu 18.03.1996. Pr 3144 zdnia 1993.01,04.
II
TABLE OF CONTENTS
PageGLOSSARY - Quantities and units for radiation protection dosimetry and microdosimetry...................... 3
1. INTRODUCTION....................................................................................................................................11
2. INITIAL RECOMBINATION OF IONS IN GASES.............................................................................132.1. Classic theories of initial recombination....................................................................................... 142.2. A universal theory of initial recombination................................................................................... 16
2.2.1. Ionisation density distributions.........................................................................................192.3. A modified track structure model.................................................................................................. 22
2.3.1 An application of recombination method to measure the mean neutron energyor the gamma ray dose fraction........................................................................................23
2.4. Theoretical approach used in this work......................................................................................... 24
3. DETERMINATION OF RADIATION QUALITY...................................................................................293.1. Empirical methods..........................................................................................................................293.2. Recombination index of radiation quality......................................................................................313.3. Q4 as an approximation of the quality factor for neutrons and charged particles........................353.4. Adaptation to the new recommendations of ICRU and ICRP....................................................... 373.5. Conclusions concerning determination of radiation quality factor using a recombination
chamber...........................................................................................................................................40
4. RECOMBINATION CHAMBER AS A DETECTOR OF AMBIENT DOSE EQUIVALENT..............414.1. A method based on the concept of RIQ......................................................................................... 41
4.1.1. Experimental results...........................................................................................................424.1.2. Investigations of gas mixtures for the chamber filling..................................................... 45
4.2. Method based on microdosimetric approach.................................................................................464.2.1. Experimental results...........................................................................................................47
4.3. Practical usefulness of the method................................................................................................ 49
5. A NOVEL METHOD TO DETERMINE THE ENERGY DEPOSITION DISTRIBUTIONS...............515.1. Method............................................................................................................................................515.2. Determination of low-LET dose fraction....................................................................................... 545.3. Measurements of energy deposition distributions......................................................................... 555.4. Discussion and conclusions concerning the microdosimetric approach.........................................61
6. CONCLUSIONS....................................................................................................................................... 63
APPENDIX A RECOMBINATION CHAMBERS - DESIGN AND EXPERIMENTAL SETUP............65A. 1. Recombination chambers............................................................................................................... 65A. 2. Experimental set-up and measuring procedure............................................................................. 66A. 3. Determination of saturation current.............................................................................................. 71A. 4. The effect of volume recombination.............................................................................................. 73
APPENDIX B RADIATION FIELDS IN THE VICINITY OF HIGH-ENERGY ACCELERATORS..... 75B. 1 Measurement of dose equivalent in reference high-energy stray radiation fields.........................76
APPENDIX C APPLICATIONS OF RECOMBINATION METHODS IN RADIOTHERAPY................ 81C. 1. Determination of RIQ of high energy proton and neutron beams................................................ 82C.2. Energy expended to create an ion pair as a factor dependent on radiation quality........................85
APPENDIX D TWIN-CHAMBER TECHNIQUE FOR DETERMINATION OF PHOTONCONTRIBUTION TO THE ABSORBED DOSE................................................................ 89
BIBLIOGRAPHY...........................................................................................................................................95
ACKNOWLEDGEMENTS.......................................................................................................................... 105
1 NEXT PAQE(S) left BLANK
GLOSSARYQUANTITIES AND UNITS FOR RADIATION PROTECTION DOSIMETRY
AND MICRODOSIMETRY
In most countries, radiation protection activities are based on recommendations of two international committees: the International Commission on Radiological Protection (ICRP) and the International Commission on Radiation Units and Measurements (ICRU). Usually recommendations of ICRP concern radiation protection standards and dose limits, while ICRU defines quantities for radiation protection purposes.The definitions presented here are taken mostly from ICRU Report 5lU) and ICRP Publication 6(K2). Microdosimetric quantities are described mainly on the basis of ICRU Report 36^) and ICRU Report 16(4).
Alphabetic List of Definitions and Symbols as Used in this Work
Quantity Symbol Definitionon page
Absorbed dose D 5Absorbed dose components ®R- 5Ambient absorbed dose D*(10) 8Ambient dose equivalent H*(10) 8Distribution of absorbed dose in linear energy transfer d(L), Dl 5Dose equivalent H 6Effective dose E 7Effective dose equivalent He 7Energy deposit Ei 8Energy fluence Y 4Energy imparted e 5,9Equivalent dose ht 7Fluence <D 4ICRU sphereKerma K
84
Lineal energy y 9Linear energy transfer LET 5
- unrestricted L-L* 5- restricted La 9
Mass stopping power, of a material for charged particles S/p 5Mean energy expended in a gas per ion pair formed w 5Radiation quality factor Q 6,8Radiation weighting factors WR 7Relative biological effectiveness RBE 7Specific energy z 9Tissue weighting factor wT 7
3
RADIOMETRIC QUANTITIES
Radiation fields are characterised by radiometric quantities, which are related to the radiation itself and apply in free space as well as in the matter.
One of the most useful radiometric quantities is the fluence, which is the quotient of dN by da, where dN is the number of particles incident on a sphere of cross-sectional area da, thus:
<D=dN
da0)
The energy fluence, 'P, is the quotient of dR by da, where dR is the radiant energy incident on a sphere of cross-sectional area da:
Y =dR
da(2)
More specific radiometric quantities which are distributions of fluence in time, energy and direction are defined as differential quantities in ICRU Report 33(5X
DOSIMETRIC QUANTITIES
When radiometric quantities are combined with quantities associated with the deposition of radiation energy in matter, then the resulting quantities are called dosimetric quantities.
The fundamental dosimetric quantity is the absorbed dose, D, together with its distributions, for instance in time and in linear energy transfer. Because the definition of absorbed dose involves the quantity energy imparted, the definition of this quantity is also given below.
Both photons and neutrons deposit their energy in two steps:a) transfer of energy to secondary charged particlesb) deposition of that energy in the material through Coulomb interactions.
The first step of the energy deposition is described by the quantity called kerma (an acronym for kinetic energy released per unit mass) and the second one by the absorbed dose. Kerma is equal to absorbed dose in the presence of charged-particle equilibrium in a certain volume V in the material, i.e. when each charged particle carrying a certain energy out of the volume V is replaced by another identical charged particle which carries the same energy into the volume^).
Kerma, K, is defined as the quotient of dE%. by dm, where dE%. is the sum of the initial kinetic energies of all the charged ionising particles liberated by uncharged ionising particles in a material of mass dm:
K=&dm
(3)
For uncharged ionising radiation of energy E (excluding rest energy), the relationship between energy fluence, Y, particle fluence, <t>, and kerma, K, may be written as:
K = Y-^£- = 0-E-^l (4)P P
where p^/p is the mass energy transfer coefficient (fraction of the total primary beam energy diverted to secondary charged particles and not subsequently lost by those particles in the form of bremsstrahlung X radiation) and the term
E Ftr
P(5)
is called the kerma factor.
4
Energy imparted by ionising radiation to the matter in a volume is:
^Rin-Rout+IQ <6>
where is the radiant energy incident on the volume, i.e. the sum of energies (excluding rest energies) of all those charged and uncharged particles which enter the volume; R^ the radiant energy emerging from the volume; and IQ the sum of all changes of the rest mass energy of nuclei and elementary particles in any nuclear transformations which occur in the volume.
The absorbed dose D, is the quotient of dF by dm, where di" is the mean energy imparted by ionising radiation to matter of mass dm, thus
D = — (7)dm
The special name for the unit of absorbed dose is gray (1 Gy = 1 J kg'1).In this work, devoted to the dosimetry of mixed radiation fields, we often use the concept of absorbed
dose components <DR, which are quotients of the absorbed dose due to i-th component of the radiation field to the total absorbed dose (e g. symbols <Dn and (Dy mean the neutron and photon dose components to the total absorbed dose).
Mass stopping power, of a material for charged particles, S/p, is the quotient of dE by pd/, where dE is the energy lost by a charged particle in traversing a distance d/ in the material of density p.
i-i£ (8)P pd/
The unrestricted linear energy transfer, L (Lw) or linear collision stopping power of a material, for a charged particle, is the quotient of dE by dl, where dE is the mean energy lost by the particle, due to collisions with electrons, in traversing a distance dl, thus
L = — (9)dl
Energy may be expressed in eV and hence L may be expressed in eV nr1 or some convenient submultiple or multiple, such as keV pm*1.The more general concept of linear energy transfer (LET) involves an energy cut-off (restricted LET) and is different from that of collision stopping power. LET was introduced by ICRUf?) in order to draw attention to energy loss that could be considered as "locally imparted to the medium", whereas the stopping power (and L^) refer to loss of energy regardless of where this energy is imparted. The concept of restricted LET is of special importance for this work and is described below in the section devoted to microdosimetric quantities.
The distribution of absorbed dose in linear energy transfer, d(L), is the quotient of dD by dL, where dD is the absorbed dose contributed by charged particles with linear energy transfers between L and L + dL, thus:
d(L) =dD
dL(10)
Sometimes (e g. in ICRU Report 51) the symbol DL is used instead of d(L).
Mean energy expended in a gas per ion pair formed, W, is the quotient of E by N where N is the mean number of ion pairs formed when the kinetic energy E of a charged particle is completely dissipated in thegas.
W = E/N (11)
5
DOSE-EQUIVALENT QUANTITIES
The dose equivalent, H **), is the product of the absorbed dose D and the quality factor Q, which was introduced to take account of the relative biological effectiveness of the different types of ionising radiations at the low exposure levels encountered in routine radiation protection practice.
H = DxQ (12)The special name for the unit of dose equivalent is sieved (Sv).
Although there is no scientific solution as yet to the radiation quality problem and there is no ideal reference parameter for the quality factor, there nevertheless exists a need to account for this factor in radiation protection. For this purpose the radiation quality factor, Q, was empirically specified in terms of LET in water of directly ionising particles*10). Later the definition of Q was subjected to several changes reflecting the results of radioepidemiological and radiobiological investigations.
The present work refers mainly to formulations given by ICRP in Publication 21*11) as well as to the recent recommendations of the Commission given in Publication 60*2).
The values of the quality factor as defined in ICRP Publication 21 are presented in Table 1. Interpolated values of quality factor as a function of LET can be obtained from Figure 1, and according to the statement of ICRP*11) this curve should be considered as a common basis for dose equivalent calculations.
Table 1. Relationship between quality factor and LET, recommended in ICRP Publication 21
L in water (keV/pm) 3.5 and less 7 23 53 175 (and above)
Q(L) 1 2 5 10 20
The quality factor at a point of tissue is given byQ = d ^Q(L)d<L)dL (13)
The integration is to be performed over the distribution d(L), due to all charged particles excluding their secondary electrons.
Similarly, the dose equivalent at a point is given byH = /q(L)d(L)dL (14)
LIn 1991 ICRP has recommended*2) a new relationship between the quality factor and unrestricted LET,
to take account of recent research on effects of ionising radiation. The new function Q(L) is given in Table 2. It is also shown in Figure 1, in comparison with the old one.
•u i 'l ll mu
------ ICRP-60----- ICRP-21
keVpm"
Figure 1. Relationship between quality factor and LET, recommended in ICRP Publication 21 and ICRP Publication 60
Table 2. Relationship between quality factor and LET, recommended in ICRP Publication 60
L in water (keV pm*1)
Q(L)
<10 1
10-100 0.32L - 2.2
> 100 300/VL
6
PRIMARY LIMIT QUANTITIES AND ASSOCIATED DEFINITIONS
Radiological protection legislation in Poland and in most countries in Europe is still based on recommendations of ICRP given in Publication 26^9\ In this document the ICRP recommended that the limiting quantity for whole body irradiation should be effective dose equivalent, HE, which is expressed by the relation
Hjj = ZwtdtQt with ^w-p - 1 (15)T T
where wT is the tissue weighting factor for the relevant organ or tissue T. The ICRP has specified^) and recently revised^) numerical values for tissue weighting factors.
The basic radiation protection recommendations were revised by ICRP in 199l(2\ Dose whole-body exposure were expressed in terms of a quantity called effective dose, E, defined as
E = ]Cwtht
T
where wT are the weighting factors for organ T and HT is the equivalent dose in this organ.
The equivalent dose, HT, in a tissue or organ is given by
Ht = ZwrDt,r (I7)R
where DTR is the mean absorbed dose in the tissue or organ, T, due to radiation R, and wR is the corresponding radiation weighting factor.
Radiation weighting factors, wR, are introduced in the new recommendations of ICRP<2) and play a role analogous to mean quality factor. The ICRP specifies their numerical values in terms of particle type and energy. These specifications refer to the radiation incident on the body and do not depend on location in, or orientation of, the body.
It is important to note that according to the new recommendations the Q(L) relationship should be used almost exclusively for the purposes of external radiation monitoring by means of the ICRU operational quantities (see definition below) and is not used for calculating of effective doses.
Relative biological effectiveness (RBE) The relative biological effectiveness of one radiation compared with another is the inverse ratio of the absorbed doses producing the same degree of a defined biological effect.
It is customary to use X-rays with generating tube voltages of about 250 kV as the reference radiation for radiobiological experiments, since this was the most common type of radiation for radiobiological experiments at the time of choice. For radiotherapy, 60Co gamma rays are recommended as reference radiation by ICRU(12\
It must be realised, that the concept of RBE has important restrictions. It depends not only on the microscopic distribution of energy deposition but also on the level and kind of biological effect under investigation and on the experimental test conditions, including the administration of dose in time, the oxygen concentration and the temperature.
limits for a
(16)
7
OPERATIONAL DOSE EQUIVALENT QUANTITIES FOR AREA MONITORING.
Both effective dose equivalent and effective dose are related to a human body, therefore for practical purposes it is necessary to employ a set of operational quantities, which provide a Safe estimate of these quantities. Such a set of quantities was defined by ICRU#3'8) in terms of dose equivalent. In Publication 60#) ICRP recommends the continued use of operational dose equivalent quantities, as defined by ICRU, but with a new dependence of the quality factor on LET.
The quantity linking the external irradiation to the effective dose equivalent is the ambient dose equivalent, H*(10), which was introduced to be used as the operational quantity for strongly penetrating radiation.
The ambient dose equivalent, H*(10), at a point in a radiation field, is the dose equivalent that would be produced by the corresponding expanded and aligned field, in the ICRU sphere at a depth 10 mm, on the radius opposing the direction of the aligned field.
The ICRU sphere#) is a 30 cm diameter tissue-equivalent sphere with a density of 1 g cm-3 and a mass composition of 76.2% oxygen, 11.1% carbon, 10.1% hydrogen and 2.6% nitrogen.
Expanded radiation field is a hypothetical radiation field in which the fluence and its angular and energy distributions are the same throughout the volume of interest as in the actual field at the point of reference#). Expanded and aligned radiation field is a uniform, unidirectional field with fluence and its energy distribution equal to that of the actual field at the point of reference#).
A quantity analogous to ambient dose equivalent can be defined in terms of absorbed dose. It is called ambient absorbed dose, D*(10). The definition of the quality factor given in ICRP Report 21 was completed in ICRU Report 39 by specification of the phantom (ICRU sphere) and point of interest where the quality factor should be determined.
Introduction of radiation weighting factors, wR, in ICRP Publication 60 raised a continuous discussion concerning mainly two problems. The first is the fact that the effective dose is defined using a different concept (that with radiation weighting factors) than the ambient dose equivalent, which continue to be defined using the quality factor as a function of LET. The second problem is lack of conservatism of H*(10) for frontal neutron irradiation#4). The first problem is an important question of principle#5-16) but it almost does not influence the experimental methods described in this work. The second problem is actually under consideration and may lead in future to minor changes in definitions of wR.
MICRODOSEMETRIC QUANTITIES
The elementary quantity for microdosimetry is the energy deposit, Ej, which is needed for the description of the non uniform spatial distribution of energy in charged-particle tracks.
The energy deposit, k,, is the energy deposited in a single interaction i:
ei =Tin "Tout+QAm <18)
whereT^ = the energy of the incident ionising particle (exclusive of rest mass).Tout = the sum of the energies of all ionising particles leaving the interaction (exclusive of rest mass).
= the changes of the rest mass energy of the atom and all particles involved in the interaction.The energy deposit is a stochastic quantity and may be considered as the energy deposited at the point of interaction, if quantum mechanical uncertainties and collective effects (e g., plasmons and phonons are neglected). The point of interaction is also called the transfer point.
8
(19)The energy imparted, e, to the matter in a volume is:
e = 2> ii
where the summation is performed over all energy deposits £;, in that volume.The energy imparted is a stochastic quantity. The energy deposits over which the summation is performed may be due to one or more energy deposition events, i.e., due to one or more statistically independent particle tracks.
The specific energy, z, is the quotient of e by m, where e is the energy imparted by ionising radiation to matter of mass m.
z =—— (20)m
The unit of z is J kgr1. The special name for the unit of specific energy is gray (Gy).
The lineal energy, y, is the quotient of e by / , where e is the energy imparted to the matter in a volume by a single energy-deposition event and / is the mean chord length in that volume:
(21)
The unit of lineal energy is (J m"1), but most commonly, the unit used for this quantity is keVpm'1.For this work it is also useful to consider the dose distribution of >v Let D(y) be the fraction of absorbed dose delivered with lineal energy less than or equal to y. The dose probability density, d(y), of y is the derivative of D(y) with respect toy:
d(y) =dDQQ
dy(22)
The distribution d(y) is independent of the absorbed dose or dose rate. The expectation value,
.Vd = Jyd(y)dy (23)o
is called dose-mean lineal energy. yD is a non-stochastic quantity.
Restricted linear energy transfer or restricted linear collision stopping power, LA, of charged particle in a medium is the quotient of dE by d/, where d/ is the distance traversed by the particle and dE is the mean energy-loss due to collisions with energy transfers less than some specified value A.
La (24)
The values of A are specified in eV and L1(M), for example, designates the LET when A=I00 eV.This definition excludes not only the kinetic energy of emerging "fast" 8 rays but also the energy expen
ded against their binding energy. For large values of A it is irrelevant, but it becomes essential at values below A=100 eV. In this work, however, we do not consider so small values of the energy cut-off.
For the quantification of initial recombination of ions in gases, the radially restricted LET, Lr would be probably a better quantity.
Lrd/
(25)
where AE^is the energy deposited within a cylinder of radius r from the interaction site.A comparison of energy-restricted LET LA with the Monte Carlo calculated energy deposition in regions
not further away than distance r from points of primary ionisation shows that LA agrees with radially restricted energy deposition if r is approximately equal to the mean electron range corresponding to energy A<17X
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1. INTRODUCTION
This work is intended to describe the state of the art of recombination methods developed for dosimetry of mixed radiation fields. The work is therefore of monographic nature, although, methods recently developed at the Institute of Atomic Energy in Swierk will be reviewed in more detailed manner.
It seems to be useful to define here more precisely the keywords of the title, namely "mixed radiation fields" and "recombination methods".
Mixed radiation fields considered here are mainly the fields of neutrons accompanied by gamma radiation as well as some more complex radiation fields, e g. those existing around high energy accelerators or on board of aircraft during high-altitude flights.
In order to define the recombination methods we need to distinguish between two main processes of recombination of ions in gases - initial recombination, which occurs within a track of a single charged particle and volume recombination, which occurs when also ions from tracks of different particles may recombine. Recombination methods described here are based on the phenomenon of initial recombination and the term "recombination chamber” represents a high-pressure tissue-equivalent ionisation chamber working in unsaturated mode and so designed that ion collection efficiency is governed by initial recombination of ions.
It is essentia] for recombination methods that the initial recombination does not depend on dose rate but strongly depends on the distances between ions of opposite charge that form the track, so it can be related to a track structure. From the physical point of view, the most interesting problem is what kind of physical information on processes of interaction of radiation with tissue and on track structure can be derived from the phenomenon of initial recombination. An attempt to answer this question was one of the aims of this work. In Section 2 we present a review of existing models of initial recombination and our theoretical approach. The range of validity of our main approximations has been investigated experimentally in a series of experiments described in the same section.
These physical considerations serve as a base for the development of practical methods used for the determination of certain dosimetric quantities. Experimental verification of methods presented throughout this work is considered by us to provide additional confirmation of the basic physical approximations introduced to our theoretical approach. Main attention will be paid here to dosimetry for purposes of radiation protection and microdosimetry, however, some applications of recombination methods in radiotherapy are also shortly presented in Appendix C.
Generally, dosimetry for radiation protection should deliver a quantitative measure of the interaction of radiation with tissue, so that this measure can be related to the expected biological effect. Although the basic quantity for the estimation of health effects of radiation is the absorbed dose, the need for quantifying the biological effects of different radiations has been recognised quite early. The term relative biological effectiveness (RBE) was first used by Failla and Henshaw already in 193 U18). In 1948 Parkeri19) proposed to introduce appropriate weighting factors to account for individual contributions of different types of ionising radiation. From 1962^ a clear distinction was drawn between measured values of RBE and quality factor, Q(L), designated for radiation protection and defined in terms of linear energy transfer (L or LET).
Specification of the radiation quality factor in terms of LET created the need for detectors whose signal- to-absorbed dose ratio could follow the Q(L) function. It is obvious that such detectors have to be based on LET-dependent physical phenomena. Among such phenomena, initial recombination of ions may provide the desired properties of the detector. First recombination chambers were designed in the beginning of the 1960s by Zielczyriski in Warsaw and Dubna(20-21) and independently by Sullivan and Baarli at CERN<22\
11
As described in the Section 3 of this work, the response of any recombination chamber can be linearly correlated with the radiation quality factor defined in Report 21 of International Commission on Radiological Protection (ICRP)l11). This is one of the main advantages of recombination chambers, since due to this feature the chambers can be successfully used even in very complex radiation fields with poorly known composition and broad energy spectrum.
Since the invention of recombination chambers their suitability for evaluation of the radiation quality factor was clearly shown. A number of experimental methods was developed for approximating the quality factor by directly measurable quantities. In this work we will discuss in detail the most advanced concept of recombination index of radiation quality, RIQ^23). Special attention will be paid to more precise physical description of the RIQ concept. We also present important experimental results recently obtained for radiation fields of monoenergetic fast neutrons. Subsection (3.2) is devoted to the adaptation of the concept of RIQ to the new recommendations of ICRPf2X
For radiation protection and biomedical applications, the use of tissue equivalent (TE) ionisation chambers with TE gas filling is commonly considered to be one of the most practical methods of determining the absorbed dose in mixed radiation fields as well as for monitoring beams of ionising particles. Recombination chambers are detectors which can provide information on both the total absorbed dose and on radiation quality, so they enable determination of dose-equivalent quantities. Strictly speaking, recombination methods are used for determining operational quantities, which approximate dose equivalent quantities, by definition not measurable. In Section 4 we present the recombination methods developed for determining ambient dose equivalent, H*(10), the operational quantity recommended^) by International Commission on Radiation Units and Measurements (ICRU) for use in radiation protection against external radiation.
In Appendix B we present some spectacular examples of application of recombination methods for determining dose equivalent quantities in the vicinity of high-energy accelerators. Stray radiation fields behind the shielding of such accelerators have a very complex, usually unknown, composition and a broad energy range. Dosimetry of such fields is still a challenging task and recombination chambers were proven to be reliable, accurate and relatively simple in handling instruments.
Section 5 is of special importance for this work. In this section we present our novel method for the determination of energy deposition spectra by analysis of the saturation curve of a recombination chamber. The method is still under investigation but first results are promising. In our opinion the method opens new possibilities of using the recombination chamber in microdosimetric research.
Generally, microdosimetry deals with the spatial patterns of radiation energy deposition in tissue at cellular and molecular levels. Commonly used experimental methods are based on simulation of microscopic volumes by gaseous cavities of specially designed tissue equivalent proportional counters (TEPCs). The simulation of a microscopic volume of tissue of 1 g/ctn3 density is achieved by replacing it by a much larger cavity filled with tissue-equivalent gas of much lower density^3). Due to technical reasons it is very difficult to simulate volumes much smaller than 1 pm of diameter, while the distributions of energy deposition in nanometer targets are of main interest for radiobiology. Initial recombination by its nature occurs in small volumes with diameter equivalent to about 70 nm of tissue. Therefore, the energy deposition spectra determined by recombination chamber can provide some useful experimental information for comparison with computational methods used up to now.
12
2. INITIAL RECOMBINATION OF IONS IN GASES
The first theoretical treatment of the phenomenon of the recombination of ions was given by Langevin*24) in 1903. In following years the problem of recombination of ions in gases under varying conditions of ionisation, pressure and temperature has proven to be much more complex than had been originally anticipated. Various types, such as preferential, initial and volume recombination have been defined (for a survey of the whole subject see e.g. L. B. Loeb "Basic Processes of Gaseous Electronics"*25)).
Recombination methods described in this work are based on the phenomenon of initial recombination, i.e., the recombination wliich occurs among the ions formed in the path of a single ionising particle. Such defined initial recombination includes so called preferential recombination, which occurs when electrons, free or as negative ions, will return to the parent positive ion under the influence of the Coulomb field. Preferential recombination is small at moderate pressures. Bradbury*26) estimated that at a pressure of 1 MPa about 0.3% of the ions will recombine in this way. A Gaussian distribution of negative ions density around the positive ions has been assumed in this calculation.
Volume recombination occurs between ions produced by different ionising particles and depends on dose rate. It is usually negligible over the operating range of our chambers, however, the problem of accounting for the influence of volume recombination is considered in Appendix A.
All theories of initial ion recombination have to include some assumptions on the initial distribution of ions along tracks of ionising particles. Such assumptions are needed as a starting point for the calculation of time and spatial evolution of this initial distribution due to diffusion, recombination and drift of ions in the external electric field.
In fact, when charged particles of appreciable energy traverse matter, they lose their energy mainly through collisions with atomic electrons. Two main types of ionisation may be distinguished*4); (a) localised ionisation in the track of the ionising particle, (b) a larger energy transfer leading to the ejection of an atomic electron of sufficient energy to produce further ionising events. In the latter case the energy transferred may be large enough to produce a separate track known as a delta ray.
Tracks of heavy charged particles are essentially straight and except at higher energies they are densely ionising, i.e. the mean spacing between successive primary collisions is very small. Single ions along the track constitute the track "core". The situation is different with fast electrons and other relativistic particles. In this case the distances between successive primary collisions is relatively large and ions constitute separated clusters along the track of the primary electron and along the tracks of the delta rays*4).
The local densities of positive and negative ions produced by ionisation along the track of a charged particle are highly non-uniform. Initially, the positive ions are situated close to the track of the ionising particle and are surrounded by a broad distribution of negative ions. The negative-ion density will have a distribution depending on a variety of factors. The initial ionising action produces electrons, which have an energy distribution with zero electrons of zero energy, and a maximum number at about half the ionisation potential of the parent atom; the distribution then falls rapidly as the energy increases to the ionisation potential with a slight tail at higher energies*26). These electrons are slowed down by molecular collisions and by Coulomb field of the parent ionised molecule. At each molecular collision the electron has a probability of attachment to the molecule that depends on the energy of the electron*27).
In the models of initial recombination discussed here, it is assumed that an externally applied electric field separates positive and negative ions. The average number density is given by n+ and n„ for positive and negative carriers respectively, with n+ being set equal to n., and written as n for simplicity. During the process of separation the density distributions of ions broaden by diffusion, and, wherever they overlap, there is a loss of ions by recombination. The recombination rate is proportional to the product of the positive and negative ion densities and a constant of proportionality defines the recombination coefficient a;
13
(2-1)dn— = -a n+ n_ dt
At gas pressures used in our ionisation chambers the recombination of ions is dominated by the diffusion process, i.e. many collisions with neutral molecules occur during the period of attraction, when the ions drift together with a high probability of eventual charge neutralisation. In this case the relations between the coefficients of recombination a, diffusion D and mobility u, can be deduced from the kinetic theory of gases as:
87te2D , eDa =--------- and u =----
kT kT(2-2)
where e is the electron charge, k is Boltzmann's constant and T the absolute temperature.In real gases, however, the above relations may be considerably disturbed^5).
The problem of calculating the spatial distributions of positive and negative ions and their variation in time is hard to be solved in detail. Theoretical models developed up to now include serious simplifications introduced in order to estimate the initial recombination. In the following we will describe shortly two classic theories of initial recombination of ions in gases, developed by Jaff£(28a’28b) (columnar recombination) and by Lea<29) (recombination in isolated clusters). A very informative consideration of these theories can be also found in the work of Sullivan^30).
2.1. CLASSIC THEORIES OF INITIAL RECOMBINATION
In 1913 JaffG(28*) made the first attempt to explain the recombination of ions formed along the track of an alpha particle. His theory postulated the track in the form of a long column of positive and negative ions and considered the diffusion broadening of the columns in the presence of an external electric field. If the track is formed along the z direction and electric field, X, acts along the x direction, then the variation of the positive and negative ion densities with time at a point (x,y) outside the initial track can be expressed as:
Sn±
a= D(
d2n±
Sx2 dy dx- an, n+ - (2-3)
Mobility, u, diffusion coefficient, D and initial spatial distribution of ions were assumed to be the same for positive and negative ions.
The above equation is not directly solvable. The approximation was made by Jafite that the effect of recombination on the ion density distribution will be small and therefore the recombination term an+n_ can be neglected to obtain an approximate solution to equation (2-3).
The solution for the number of ions N surviving out of the original N0 per unit of length was obtained in the form:
N0N =------------------------- 7-r (2-4)
aN0 z/2 (i)| iz1+------ e —H0 -
8tcD 2 VlJbuX
where z = (y~—)^, b is a parameter termed the initial radius of the ion column, and is a first order
Hankel function. Since the Hankel function has an imaginary argument it can be replaced by the modified Bessel function of the second kind Kq which is usually more convenient when computer libraries are used:
— H(1) 2
loiz
= K,(z'
X2V(2-5)
14
The calculation of ion collection efficiency for the ionisation occurring in long randomly oriented columns requires averaging over the angle 0 between the column and external electric field directions:
sin 9d9
1+ ===ds sin2 9
(2-6)
where2u2X2t2
4Dt + b2(2-7)
buXy = — (2-8)
V2D
As it was pointed out by Sullivan*30), there are two assumptions which require contradictory limitations to be imposed on the range of voltage and gas pressures for which the theory is valid.
The first assumption concerns the initial distribution of ion density. For t=0 the approximate solution of equation (2-3) gives the following implied initial distribution of ions in the column of:
n± =-^~-e~r /bTib
(2-9)
where the value of parameter b is assumed to be the same for positive and negative ions. This is contrary to what may be expected from physical considerations. The effect of the implied initial distribution is least apparent at low applied electric fields, in which the major part of recombination occurs after the column has considerably diffused.
The second assumption implied in neglecting the recombination term to solve equation (2-3) is that recombination affects only the number of ions remaining in the column and not the shape of the ion density distribution. This assumption becomes valid only at high electric fields and low ionisation density, when recombination is small.
It was shown in the work of Sullivan*30) that the optimum value of the ratio of electric field and gas pressure, X/p, is:
X D— «— (2-10) p bu
where b is the initial column radius at N.T.P., which for gases is about 10 pm. Since, u/D»40 when u is in units cm2s"1V"1, the best values for X/p are about 250 Vcnr'MPa*1.
As mentioned, Jaffa's theory has been developed to describe initial recombination in tracks of heavy particles. Good agreement of the theory with experimental data for alpha particles was proved many times*e-g-31-33).
In 1934 Lea*29) pointed out that the Jaffe equation would not apply to fast electrons and y radiation as ionisation in this case is not a continuous column but in the form of individual clusters.
The primary ionising particle leaves along its track little clusters of secondary ions. The clusters produced by an a-particle are so close together that their discrete origin is unimportant and the distribution may be considered cylindrical, as Jaffe assumed. In the case of fast electrons, however, this is not so, and the clusters are separated by distances much greater than the separation of ions inside a cluster. It is apparent that the initial recombination is largely a matter of recombination within clusters.
In the theory of Lea the distribution of ions in a cluster is supposed to be spherically symmetrical initially. The assumptions and arguments of Lea follow closely those of Jaffe in deriving an equation for
15
recombination occurring in clusters of ions. The solution for the fraction of ions escaping from an isolated cluster of v0 ions is given by equation (2-11), presented here in form used by Sullivan^30).
f =1 +
avn4(27t)3/2bD P(y)
(2-11)
where P(y) = — [l-ey (1-erfVly] (2-12)y
buX r 2 >/2y = — and erfV2y =------ I e 1 dq (2-13)
V2D v2n o
The parameter b is now termed the initial radius of the cluster. The positive ions in the cluster will, at the moment of their formation, be concentrated close to the centre of the cluster. The negative ions are formed at distances from their parent atoms, determined by the distance which the electron travels before attachment to a neutral molecule to form a negative ion. This distance is appreciably greater than the distance of separation of the positive ions. The initial concentration of positive ions at the centre of the cluster rapidly decreases via diffusion and the cluster radius b which enters the recombination formula is determined by the distance travelled by the electron before negative-ion formation.
2.2. A UNIV ERSAL THEORY OF INITIAL RECOMBINATION
The theories of recombination in columns and in clusters have separate derivations and different dependencies on the electric field and gas pressure. The theory of columnar recombination uses the quantity N0, the number of ion pairs per unit length of track, whereas the theory of cluster recombination uses v0, the number of ions occurring in an isolated cluster of ionisation. In 1969 Sullivan^30) showed that some degree of universality in recombination theory may be obtained if the quantity N0 in the theory of columnar recombination is replaced by
No=— (2-14)Ax
i.e. an ion column of N0 ion pairs per unit length of track contains v0 ions over a length Ax.
The argument of Sullivan was that the value of Ax, the track length averaged over all directions in space could be considered as a cluster with respect to initial recombination. The expressions for the collection efficiency of the cluster and columnar theories were then equated. The equality could not be exact as the expressions were of different forms. However, the ranges of the parameters over which the equality could hold reasonably well and the range of uncertainty of the value of Ax could be chosen hence restrictions could be applied to the universality of the theory .
The collection efficiency for ionisation occurring in an isolated cluster is given in Lea's theory^29) by:
f2=------ ------- (2-15)i+g2p(y)
buX avnwhere P(y) is given bv eq. (2-12) y -• - and g2 =------- —— (2-16)
V2D 4(271) oD
Sullivan argues that the collection efficiency of ionisation occurring in long randomly oriented columns, given by the integral (2-6), might be approximated by an expression of the form:
16
1(2-17)
i+PgiQ(y)
aN0where g{ =------ (2-18)
8nDThe values of f, were calculated from equation (2-6) and tabulated by Sullivan for different values of g,
and y. Hence we are able to calculate the value of functions pQ(y) from eq. (2-17) for several values of gv The results are displayed in Figure 2-1 in order to show to what extend is PQ(y) a unique function of y for different gv
Figure 2-1. The functions P(y), H(y) and PQ(y). The values of function pQ(y) were deduced from calculations done by SullivanC30) for different values of gj ranging from 0.01 (upper dashed curve) to 10 (lower dashed curve).
Figure 2-2.C30) The function pQ(y) versus P(y). The cluster and columnar theories are equivalent where the relation between the functions can be approximated by a straight line. The solid straight lines indicate where the relation is linear to within ±10%. The bars indicate the range of values of PQ(y) obtained with gj between 0.01 and 10.
The equality f,=f2 holds, when Pg,=g2 and Q(y)=P(y). A plot of PQ(y) versus P(y) must therefore be a straight line through the origin with a slope of p. This plot is shown in Figure 2-2, in the form originally presented in the work of Sullivan^30).
The conclusion drawn by Sullivan from this figure was, that with 10% uncertainty in (3, fj equals f2 provided y>0.6 and the cluster theory can be considered universal. Ion collection from clusters of v ions or of length Ax containing v ions in long randomly oriented columns, is therefore given by:
f =------- --------- (2-19)1 + Cj — vP(y)
bX
where y = c2 b— ; (2-20)P
and Cj and c2 are gas constants.
17
The length of the track Ax which behaves similarly to a cluster is given by pgj=g2, which results in
Ax = V27t"pb = 4.33b (2-21)
The value of Ax is uncertain to within 10% owing to the uncertainty in p. This uncertainty depends on whether the ionisation was produced in isolated clusters of ions or in long straight columns of ionisation.
To illustrate the compatibility of the two forms of recombination equations, Sullivan*30) plotted together the calculated collection efficiencies resulting from the cluster and column equations (Figure 2-3). The plot was made over three decades of y and four decades of v.
t i t
v= 100
= 1000
Figure 2-3.(30) Ionisation collection efficiencies predicted by columnar and cluster theories, showing the degree of equivalence between the two theories. The dashed curves are from the cluster theory and the solid lines are from the columnar theory with N replaced by v/Ax and Ax=4.33b. The indicated values of v are correct for b= 15.5 pm and gas pressure of 850 kPa.
In order to be able to apply this theory as a universal one it must be demonstrated that the value of parameter b is constant and not itself a function of ionisation density *).
Inserting numerical values for the ratios of the gas coefficients derived from the kinetic theory of gases, Sullivan*30) obtained the following values of c, and c2 for TE gas: c,=2.28-10"6 cm3 kg and c2=28 kg cm*2 V-1. Then, the values of parameter b were determined experimentally for 60Co gamma radiation, Pu-Be neutrons and Pu a-particles (see Table 2-1). As it was stated by Sullivan these determinations were not very precise, however, they served to show that the gas parameter does not vary markedly with ionisation density. A systematic variation of b with V/p was noted, requiring that the range of V/p used should be restricted. Such a restriction is also required by the approximations made in the theory.
*) A similar question has been discussed earlier by Kara-Michailova and Lea*34) who argued that an a-particle column can be considered as being composed of overlapping clusters of secondary ions and, the radii of these cluster s should be much the same for a-particles as for electrons, since the ions in a cluster are in each case produced by slow electrons. When the linear distribution of spherical clusters merges, and
-r2/b2in each cluster the ion density varies with the distance from the centre according to the formula e
-r2' b2then the ion density in the resulting column is proportional to e , where r is now the perpendicular distance from the axis of the column and b has the same value as before. There thus seems to be no reason why the parameter b should be markedly different for different ionising particles in a given gas, even though the ionisation densities are different.
18
Table 2-1.(3°1 Estimated values of b in pm at a pressure of 0.1 MPa as a function of V/p and type of radiation (V in volt/cm and p in MPa).
Radiation V/p = 250 V/p = 100
60Co gamma radiation 14 15.5
Pu-Be neutrons 17 22.5
Pu alpha particles 9
2.2.1. Ionisation density distributions
It was also recognised early<30>35>36/ that the shape of the saturation curve of a recombination chamber can provide some information about the microdosimetric dose distributions versus ionisation density, and that the universal theory of initial recombination can be used for the mathematical analysis of this curve. However, since the saturation curves are smooth, special unfolding procedures have to be used. Some methods to derive the microdosimetric distributions were developed in the same work of Sullivan^30) and later on by Makarewicz et al/37X The authors reported limited resolution and reproducibility of the methods, partly due to the uncertain values of some coefficients involved in the theory of the initial recombination of ions.
The methods were based on the basic equation of the theory (2-19), which was expressed as:
f =---------------- (2-22)l+vm(p,X)
where: m = c -P(y) and y = c2b- (2-23)
The values of Cj and c2 were experimentally determined in the work of Sullivan for tissue-equivalent gas of composition (by partial pressure) of 64.4% CH4, 32.5% C02, and 3.1% N2.The symbol v (called in this Section event size) represents the number of ions produced in a cluster of ions or over a length of track which is equivalent to 0.07 pm of unit density tissue.
The measured ionisation current ? of an ionisation chamber was expressed as a function of the value of m(p,X) and represented by the sum of currents from all clusters or track segments of different v:
--"5"-------- '■*-------- (2-24)
p l+v-m(p, X)where /v is the available current per unit pressure from "events" of event size v.
The problem was to estimate the distribution of v from a series of measurements of ionisation current at different applied voltages and gas pressures.
Several methods of finding the best solution of the above equation were tested.
Algebraic and numerical approximation methods
These two methods were developed by Sullivan^30/ and used for the analysis of saturation curves measured at different gas pressures. The saturation curves were determined for 60Co, 137Cs and 226Ra gamma radiation, for 3 MeV, 15 MeV, 238Pu-Be and fission neutrons and for 5.15 MeV alpha particles.
In a simple algebraic method the measured (i/p) values were normalised and plotted in the form of (1-i/p) versus (1/m) dependence. This dependence was used as a second approximation of integral event size distribution. The third approximation of the method required a numerical derivation of the obtained data
19
and further integration of a rather complicated function. It was stated by the author that only the second approximation could be reasonably applied, giving however very poor resolution.
Somewhat better results were obtained by the numerical method. The sum (2-24) was divided into a number of terms, which had to be less than the number of experimental measurements. The dividing points were chosen in a logarithmic scale such that there were 5 terms per decade of v, giving 20 terms of the sum to be determined. The output spectrum was in the form of a histogram with 20 boxes. The author tested several numerical methods in order to avoid negative values and oscillations of the results. Unfortunately none of these methods gave stable results, reproducible for different sets of experimental data.
For this work we repeated some calculations of event size distributions performed by Sullivan. Of special interest for us were the event size distributions for 60Co and 137Cs gamma radiation sources. We used the data read from the figures given by Sullivan and his numerical approach but we applied the non-negative linear least-squares fitting routine, the same as applied in our method described in Section 5. The fits of the Equation (2-24) to the experimental data are shown in Figure 2-4.
Co- y
.5.150
lMO—a
12.17 q.
Collecting voltage, V
Figure 2-4. Saturation curves of recombination chamber filled with TE gas at different pressures as determined by Sullivan^30) in fields of 60Co and l37Cs gamma radiation sources. The values of the gas pressure are indicated in the figure in units of kg cm"2. Solid lines represent our fits of the Equation (2-24) to the experimental data.
As seen in Figure 2-4 the fits are not ideal, but they may provide a reasonable solution. The difficulties of the method arise both from the limited accuracy of the experimental results and from compromises made in the recombination theory.
A similar fitting procedure was applied to the results of measurements performed in radiation field of 15 MeV monoencrgetic neutrons and to the measurements with an alpha radiation source (Pu) inserted into the chamber. The resulting event size distributions are presented in Figure 2-5.
As could be expected, the method has low resolution, because of the smooth shape of the saturation curves. Nevertheless it can distinguish between different gamma radiations and gives the results, which are in general agreement with other available data.
The investigations of Sullivan showed that there is a possibility to use a recombination chamber for determination of event size distribution. This became the starting point for our microdosimetric approach described in Section 5 of this work.
20
3*
1 10 100 1000"1
pH 4
i i
.....#.....gamma |
p~i j■ IN* III j 1 1 *111* 1 I.Mf
2
1 -
1 10 100 1000
"i "
Pu
alpha ....w,.............. _
■
* i .tin ,4 « nut'1 II Hi.f
l37Cs
n gamma
;—'--
---1
1L; 10 100
tV#J 1 ' |'''VJ VI! 15 MeV : neutrons
1000
Figure 2-5. Differential event size (upper scale) and LA (lower scale) distributions obtained in this work using the experimental data and numerical approach of Sullivan^30* but with the non-negative linear least squares fitting routine.
Unfolding of the event size distributions from the integral equation
In 1981 we made an attempt^37) to investigate microdosimetric parameters of the medical neutron beam at the Joint Institute for Nuclear Research in Dubna. Saturation curves of the recombination chamber were measured in a water phantom exposed to the neutron beam with an average energy of 350 MeV. For comparison the same measurements were carried out in the gamma radiation field of a 137Cs source. The experimental results were used by Makarewicz*37) in his search of a mathematical method for determining the linear ionisation density spectrum.
The approach of Makarewicz was based on the same equation (2-22) used by Sullivan, but expressed in the form of a Fredholm integral equation of the first kind. The saturation curve measured using the recombination chamber was considered to be the folded spectrum, deformed by the physical process of
recording the data. A numerical regularization method was used in order to minimise the well known problem of the solution instability, which arises from inherent properties of the Fredholm integral equation. The results of calculations are shown in Figure 2-6. As stated by the author the results should be considered as preliminary due to limited resolution and reproducibility. Up to now the method remains impractical since it would require very high accuracy (better than 0.2%) of the experimental results. Moreover, the function m(p,X) contains some uncertain coefficients involved in the theory of initial recombination of ions. This uncertainty is avoided in our micro- dosimetric approach, described in Section 5.
■350 MeV neutrons
Cs gammas
1 —...1;1 2 4 8 16 32 64 128 256 512
Ions per event (v)
Figure 2-6.U7) Event size distributions for 350 MeV neutrons and for 137Cs gamma radiation source obtained by unfolding of the d(v) distributions from the integral equation.
21
2.3. A MODIFIED TRACK STRUCTURE MODEL
An important modification of Lea's model for initial recombination of electrons has been worked out by G. Makrigiorgos and A. J. WakeK38- 39) to describe the electron track structure more realistically. In the modified model the initial recombination is described in terms of restricted linear energy transfer LA, with energy cut-off A equal to 500 eV.
Lea based his theory on the assumption that the energy lost by the fast electron is deposited on the spot and there is only one track created by each fast electron. Although this is a reasonable assumption for a heavy particle track, this is no longer the case for electrons, where energetic 5 rays may emerge from the main track. Makrigiorgos and Waker stated, that it is much more reasonable to regard the resulting track geometry not as one track but as a number of different tracks each with its own recombination between the positive and negative ions it contains. From such a model it follows that not the unrestricted but the restricted ionisation density is the parameter on which the electron recombination depends. The value of the cut-off energy A was derived considering the maximum accepted cluster size. An energy deposition higher than A was assumed to create a separate electron track.
For methane-based tissue-equivalent gas the initial cluster radius at unit pressure, b0, is of the order of 15 gm. The initial cluster diameter of 2b0 was taken as a measure of the maximum distance a secondary electron can travel and still be considered as creating its ionisation in a spherical volume "fitting" with the rest of the spheres of the mam track. The extrapolated range of 2b0 corresponds to energy of 500 eV, which can be related to a cluster of 16 ion pairs.
Apart from 5-rays it also has been considered^38) that an electron at the end of its track has a high ionisation density and the clusters are created quite close together, so that, it is more appropriate to regard the end of an electron track as a column rather than as a series of discrete clusters. Consequently for very low energy electrons the Jaffe theory of columnar recombination should be applied rather than the Lea theory of clusters recombination. The electron energy which corresponds to the transition from cluster to columnar creation of ions has been estimated as being about 2.5 keV in methane based tissue equivalent gas.
The choice of the 500 eV as the cut-off for energy deposition is arbitrary to some extent, however it was experimentally verified in series of precise experiments^39) and later compared with calculations of Blohmf40).
Makrigiorgos and Waker introduced also to their model a concept of so called "spectrum equivalent electron", which is a monoenergetic electron slowing down in the chamber with energy transfers less than 500 eV and creating the same recombination as a whole electron or photon spectrum. It has been shown(39) that the collection efficiency, f}, of the ions created in electron tracks in a high pressure parallel-plate ionisation chamber operated at a high field strength Xj (X, > 400 Vcnr'MPa'1) and a constant pressure of 0.8 MPa methane-based tissue- equivalent gas is given by
(v/vq)i
l + (g0/X1)(v/v0)1(N + /k+)(2-25)
where go<xp
(2-26)8>/^bousin0
a is the recombination coefficient at unit pressure, u is mobility of ions of either sign, also at unit pressure, and 6 the angle between the primary track and the electric field X. The term (v/v0)1 is the collection efficiency at field strength X, for initial recombination in ion clusters^9) The quantity N+ is the restricted
22
dose mean ionisation density of the whole track structure created by a fast electron in the gas, assuming a constant W value and k+ is a parameter used to unify the form of Lea and Jafite theories at high voltages.
In order to eliminate uncertain gas constants Makrigiorgos and Waker measured the values of ionisation current at two different voltages applied to the chamber and used the ratios of these currents to prove the accuracy of the model and for determination of restricted dose mean LET. The ionisation currents were measured in different photon fields with an accuracy of 0.1 %. It was shown, that the modified model was clearly closer to experiment than the Lea's model. The current ratios obtained at two high voltages could be predicted to an accuracy of better than 0.5%, compared with that of about 2% from Lea's model.
The main application of the model of Makrigiorgos and Waker is experimental determination of the restricted dose mean LET of unknown photon spectra^40"42). Some results of these measurements are shown in Table 2-2.
Table 2-2.(4°l Experimental restricted dose mean LET, L500 D values relative to the 60Co L50o d value (3.37 keV pm-1) in water and comparison with the calculated values of Blohm.
Radiation (photons) Lsoo d (recombination) L500 D (calculation)
55Fe (5.9 keV) 3.82 ±0.73 3.10
153Gd (~41 keV) 1.97 ±0.19 1.88
241Am (60 keV) 1.94 (calibration) 1.94
"Tc (140 keV) 1.56 ±0.17 1.62
137Cs (660 keV) 1.09 ± 0.02 —
60Co (1252 keV) 1 1
It may be interesting to mention that our measurements^43^ of the recombination index of radiation quality, Q4 (see Section 3.2 for definition) for 241Am resulted in value of Q4=1.9 ±0.1 From this result we could estimate that the effective value of L500 D, is equal to 1.97.
2.3.1. An application of recombination method to measure the mean neutron energy or the gamma ravdose fraction
Following the development of a modified initial recombination model for electrons^39) Makrigiorgos and Waker developed a method for measuring the mean incident neutron energy or the gamma ray absorbed dose fraction of an unspecified neutron-gamma radiation field, over the neutron energy range 0.1-10 MeV<44>45). The method is based on consideration of recombination in tracks of three types of charged particles (protons, alpha particles and nitrogen ions), created in a tissue-equivalent ionisation chamber. It is assumed that each subspectrum of a certain type of particle can be replaced in the general recombination formula by a "spectrum equivalent particle" that bears the recombination and the quality characteristics of the subspectrum it represents.
Total ionisation current I(X,) collected in neutron-gamma radiation field at a certain high field strength X, is the sum of the ionisation currents collected by each spectrum-equivalent particle:
I(Xj) ~ a/e(Xj) + b/p(X,) + c,.(Xi) + diN(X,) (2-27)
where ;e, i" , /a and zN are the currents collected by the spectrum-equivalent electron, proton, alpha particle and heavy ion, respectively, and a, b, c and d are the parameters that take appropriate values to ensure that
23
each spectrum-equivalent particle contributes to the total kerma created in the chamber to the same degree as the corresponding charged particle spectrum.
Determination of all the parameters included to the model by observing the value of the total ionisation current is not possible, since the problem does not have a unique solution. Therefore the authors introduced to the model some additional simplifications. They neglected the production of alpha particles and introduced a constant heavy ion kerma component equal to 11%. Under such assumptions the ratio of the total ionisation currents collected in a mixed neutron-gamma radiation field at two high electrical field strengths Xj and X2 and a constant gas pressure can be expressed as a function of only two unknown parameters: X, which determines the gamma ray absorbed dose fraction, and E*, the initial energy of the spectrum-equivalent proton. Using a number of assumptions valid with an accuracy of a few percent the authors found a correlation of the initial energy of the spectrum-equivalent proton with the mean incident neutron energy.
With the model described above one can derive the gamma ray absorbed dose fraction and the mean incident neutron energy by performing accurate ionisation current measurements in three high field strengths. The limitation of the method to high electrical field strengths causes that the measured ionisation currents have rather similar values for all the field strengths applied, which leads to relatively large uncertainties of the unfolding procedure, even if 0.1% accuracy for the ionisation current is obtained. Therefore, the authors staled that one of the two quantities - the mean incident neutron energy or the gamma ray absorbed dose fraction - must be provided independently with a reasonable accuracy in order to obtain the other quantity.
The method led to the construction of a prototype small high-pressure tissue-equivalent ionisation chamber^45). The chamber has been used for determination of mean incident neutron energy in fields of monoenergetic neutro.is with energies of 2.4 MeV and 1.6 MeV at the National Physical Laboratory in Teddington, as well as in the field of 241 Arn-Be source. The obtained values agree very well (within 10%) with reference values.
Despite its success, the method of Makrigiorgos and Waker seems to be of little practical importance. As stated by the authors the method is limited to a narrow energy range, the measurements have to be performed with high accuracy of about 0.1-0 2% and the contribution of photon radiation has to be determined independently by another instrument. Moreover the resulting quantity - mean incident neutron energy - is not satisfying for modem radiation protection.
2.4. THEORETICAL APPROACH USED IN THIS WORK
Our theoretical approach, used throughout this work, can be considered as some development of the universal theory of initial recombination given by Sullivan(30X Several elements of the approach have been already presented in our earlier works'23-46,47^, however they were always considered as an approximation with not well known range of validity. In this work w e will point out the main assumptions of our model and we made attempt to determine experimentally the range of their validity.
One of the targets of our model was to substitute the uncertain gas parameters in the equations of recombination theory by a measurable quantity. Therefore, our approach is based on comparison of saturation curves determined in investigated mixed radiation field with the saturation curve obtained by calibration of the recombination chamber in the reference gamma radiation field. It means, however, that the operational range of the chamber has to be found. It should cover at least the range where the Sullivan's universal theory of initial recombination is valid.
24
Several series of measurements have been performed in order to determine the optimum operational range for our chambers. The main results are shortly presented in this Subsection. Our theoretical model has been also proved indirectly in large number of experiments described in this work.
General description of the model
In order to point out the main idea and assumptions of our model we present first the general description of the model, with short comments only. More detailed discussion is given in the following text.
The basic equation of the universal theory of initial recombination of ions can be represented by equation (2-19), given by Sullivan^30). In our investigations, however, we usually use more general form of this equation^23);
f = — J---- —-----dp (2-28)DJl + pF(X,p)
where f is the ion collection efficiency,F(X,p) is an unspecified here function of the electrical field strength (X) and of the gas pressure (p) p - is a local density of ions within the particle tracks, scaled such that p=l for reference gamma
radiation of 137Cs.In order to express the results in terms of energy loss, the energy required to produce an ion pair was
assumed to be constant. Under this assumption the ion collection efficiency in the recombination chamber can be expressed as:
r-.y-^-dL,
D 1 ' —^-F(X,p)1+-
(2-29)
where fmjx is the ion collection efficiency for mixed radiation, LA is the restricted LET with the energy cutoff of about 500 eV, L0 is a scaling factor and dfL^) is the differential dose distribution versus restricted LET.
An attempt was made to eliminate the uncertain function F(X,p) from Equation (2-29) and to replace it by measurable quantities derived from saturation curves for reference gamma radiation.
Basing on the long years experimental observations, that the saturation curves for different gamma radiations have similar shapes over a large range of photon energies, it was assumed that the ion collection efficiency for the reference gamma radiation can be expressed as:
fy = » ---------- (2-30)l+-^F(X,p)
Lowhere Leff, the "effective" value of LA is expected to be close to the dose mean LA value for this radiation. For simplicity, in the following the scaling factor L0 was chosen arbitrarily, so that (Leff / L0) = 1 for reference gamma radiation of 137Cs source.
Combining Equations (2-29) and (2-30) we expressed the ion collection efficiency for mixed radiation as a function of fy of reference radiation:
-y d(LA)
D* l{ la l~fy
Lq fy
dL, (2-31)
where both fmix and fy have to be determined using the same recombination chamber at the same operation conditions.
25
Physical meaning of the parameter g. and representation in terms of LA.
Initial recombination of ions in an ionisation chamber is quantitatively described by Equation (2-28), which contains a voltage dependent function F(X,p) and a dimensionless parameter g related to initial proximity of opposite charged ions within the track and characterising the particle track structure. The parameter g, called in our works the local ion density^48), is not defined explicitly by ion distances up to now. Classic theories of initial recombination express g as proportional to the linear ion density in tracks of heavy particles^8), or to the number of ion pairs in clusters of low LET radiation^29). Both models of Sullivan^30) and of Makrigiorgos and Wakeri38-39) lead to the conclusion that g can be considered as proportional to LaAV (W is the mean energy expended to create an ion pair in the gas cavity of the chamber). Both the models resulted in value of energy cut-off A of about 500 eV, however, this value has been declared to be uncertain. Our experimental results described in Section 3.2. are in agreement with the assumption of proportionality of g to L500, however, we are not able to specify the value of A directly from our measurements. It is also absolutely clear, that for low LET radiation the initial recombination cannot be described in terms of unrestricted LET.
For comparison purposes g can be also considered as proportional to yd/W (yd is a lineal energy, d is of order of 0.1 gin).
Effective value of LA for photons and scaling parameter L0.
In fact, the concept of effective LET value for photons was formulated in our work from 1980 by assumption the LET of gamma rays to be 3.5 keV-gm'1. The concept was based on observation that the ion collection efficiency in recombination chambers was similar for different gamma radiations, over large range of photon energies. At that time the value of Leff = 3.5 keV-gm*1 was chosen arbitrarily, mainly because the quality factor was defined as equal to 1 at 3.5 keV-gm"1.
Later Makrigiorgos and Waker (see Section 2.3) introduced somewhat similar, however much more justified, concept of spectrum equivalent electron. We can expect, that the effective LET, as defined by Equation (2-30), should have approximately the same values as the dose mean values of restricted LET determined by Makrigiorgos and Waker. For 137Cs these authors obtained the value of L500 D in water equal to 3.67 keV gm"'.
For this work we estimated the values of Leff from the experimental results of Sullivan^30) by fitting the Equation (2-22) to the data presented in Figure 2-4. The obtained fits were of the same rather limited quality as those performed with Equation (2-24) and resulted in values of veff=7.5 ± 0.7 for 137Cs and veff=6.7 ± 0.7 for 60Co. From these values wc obtained the approximate values of effective LA in tissue as equal to Leff = 3.4 keVgm"1 for l37Cs and Leff = 3.1 keVgnr1 for 60Co.
In this work we keep the value of L0=-3.5 keV-gm-1 because this value provides good agreement of our experimental results with the available results of other methods. Nevertheless, the value of L0 is still arbitrary to some extent and there is no clear phy sical interpretation which should be strictly associated with the L0 value chosen here.
Range of validity of the model
Our important assumption, that the values of function F(X,p) in Equation (2-28) can be approximated by experimentally determined function of fr, is based mainly on the results of investigations performed by Sullivan^30) and on his universal model of initial recombination of ions. Some information on validity of this assumption can be also derived from plots of ion collection efficiencies for different radiations versus ion collection efficiency for reference gamma radiation, measured at different filling gas pressures. Within
26
the range of validity of our model the curves for different gas pressures should be in line. Here we present two such comparisons for 222Rn alphas and for 239Pu-Be neutrons.
For alpha particles we used the data from the experiment of Zielczynski et al/49\ which was performed, using the ionisation chamber similar to REM-2 chamber (see Appendix A), but with aluminium electrodes and filled with C02 with some addition of 222Rn at different gas pressures ranging from 0.19 to 1.03 MPa. Before the experiment with alpha particles the saturation curves of the chamber were determined also in gamma radiation field of 137Cs. The results of the experiment plotted in form of fa versus fy are shown in
Figure 2-7.Since alpha particles have narrow LET distribution,
so the ion collection efficiency for a-radiation, fa, can be related to the ion collection efficiency for y-radiation by the Equation (2-31) with single value of p:
'-t-
The solid line in the figure represents the fit of the Equation (2-32) with the value of p=30. If we assume that the value of Leff in C02 is close to 3.5 MeVcm2g-1 then for p=30 we have L500 a=105 MeV cm2g'1 (restricted LET).The dose versus Lro distribution for alpha particles of 222Rn in partial equilibrium with daughter isotopes was earlier calculated by Makarewicz($0X The obtained value of the dose average LET in C02 was equal to LD a= 113 MeV cm2g"1.
All the experimental points shown in Figure 2-7 fit very well to the Equation (2-32) for whole the investigated range of the filling gas pressure. However, we want to call attention of the reader to the limited range of £y values. In fact, we don't have the experimental data at fy >0.95 for the highest pressure and for fy <0.95 for the lowest pressure. This may cause some artificial "improvement" of the fit.
We can expect that for neutrons the range of gas pressures where our model is valid may be not so broad. The Figure 2-8 shows the f^ versus fy plot derived from the results of our measurements performed in radiation field of 239Pu-Be using the recombination chamber of REM-2 type filled with the mixture of methane and nitrogen (95% of CH4 and 5% of N2).
It can be seen that the curves for 0.9 and 1.0 MPa are almost identical and the filling gas pressures around these values are considered as optimum for the REM-2 chamber filled with the mixture of methane and nitrogen. We can state also the sufficient agreement of the optimum curve with those for 1.1 MPa.
0.9 MPa
1.0 MPa
1.1 MPa
1.2 MPa
rFigure 2-8. Experimentally obtained values of f^ for 239Pu-Be neutron source at different filling gas pressures plotted as function of fy.
□ 0.187 MPa
• 0.267 MPa
A 0.519 MPa
V 1.026 MPa
d/v
0.7 0.8 - 0.9 1.0r
Figure 2-7. Experimentally obtained values of fa and f^. For each experimental point the measurements were performed for both alphas and gammas at the same gas pressure and electrical field strength. Solid line represents Equation (2-32) with p=30.
27
All the investigations presented in this work are, at least partly, based on the theoretical model described above. Rather large set of consistent experimental data confirmed well the validity of the model in the operating range of our recombination chambers.
In fact, our strongly simplified model is more general than theories of Jaffe and Lea. Comparison with the universal theory of Sullivan indicates that our approach provides better agreement with the experimental data, when the mixed radiation is considered. Basing on our model we were able to develop several measuring methods applicable in radiation protection and to work out the stable procedures for determination of energy deposition distributions. From practical point of view it is also important, that we don't need to change the gas pressure in the chamber, like it was in the method of Sullivan. On the other hand, we can obtain only very poor information on differences in energy deposition spectra for gamma radiations of different energies.
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3. DETERMINATION OF RADIATION QUALITY
The serious shortcoming of the quality factor concept is the fact that this factor has not been defined in terms of physically measurable quantities. Nevertheless, the value of the quality factor can be evaluated experimentally. Such attempt requires, however, the use of a detector with a LET dependent response. This idea became a basis for construction of recombination chambers and their application for determination of radiation quality.
As already mentioned, the ion collection efficiency in recombination chambers is governed by initial recombination of ions. The initial recombination in pressurised gases depends on the ion density along the tracks of charged particles and hence can be related to LET. This effect is used in recombination chambers to obtain information on radiation quality.
Since the invention of recombination chambers in the beginning of the 1960s(20,22) they were successfully used for determining the radiation quality in different radiation fields. Extensive experimental investigations resulted in 1980 in definition of a basic quantity called recombination index of radiation quality, RIQ^23X It was shown that the dependence of RIQ on LET could simulate well the Q(L) function, as defined in the ICRP Report 2l(n). Therefore, it was possible to determine "directly" the mean value of the quality factor in an unknown radiation field. Up to now the values of RIQ were determined experimentally in rather large number of different radiation fields and theoretically predicted dependence of RIQ on LET was well confirmed.
With the recently recommended change of the Q(L) dependence^) the need was created for an appropriate modification of recombination methods. Possible adaptation of the RIQ concept to the new requirements is shortly described in Section 3.4.
More advanced recombination methods considerably differ from the concept of RIQ, as they include determination of some microdosimetric parameters describing the dose distribution versus LET. These methods are presented in Sections 4 and 5 of this work.
3.1. EMPIRICAL METHODS
Initial recombination in a gas depends not only on the ion density, but is also a function of the gas pressure and the strength of the electric field applied to collect the ions. Gas pressure and polarising voltage are easily controllable parameters in an ionisation chamber and several attempts have been made to adjust them so that the degree of recombination occurring with widely different radiations is proportional to the accepted quality factor of the radiation.
The basic method, first described by Zielczynski^20-51) is to compare the ionisation currents collected at two different collecting voltages in a recombination chamber. In early works of Zielczynski the two polarising voltages U, and U2 (see Figure 3-1), were selected such that the ratio of current values measured at these two voltages:
f'=(—) (3-1)ll
had a value proportional to the quality factor of calibrating radiations - gamma radiation (Q=l) and n+7 radiation of a 239Pu-Be source (Q=7.2). Linear relation between f' and Q is illustrated in Figure 3-2(52) for different sources of gamma radiation (236Ra, 60Co, 170Tm), beta radiation (3H), neutrons (14 MeV generator, 238Pu-Be, 210Po-Be, 210Po-B) and for alpha particles (238U and 222Rn).
29
neutrons
Collecting voltage
Figure 3-1. General shape of saturation curves for gamma radiation, neutron and alpha particle ionisation.
14 MeVPo-Be
0.6 -
II 0.4 -
0.2 -
Quality factor
Figure 3-2.<52) Linear relation between quality factor and ratio of ionisation currents i1 and i2 measured at voltages U1 and U2, respectively.
An alternative method of determining Q was proposed by Sullivan and Baarlil22). Their approach makes use of the fact that in some range of operation conditions of a recombination chamber the saturation curve can be described as:
/ = kVn (3-2)
where i is the ionisation current, V is the applied potential, K is a linear function of dose rate and n is the "recombination number", which is function of LET.
The method of Sullivan and Baarli involves plotting the ionisation current against the electric field strength. The current was measured by a tissue-equivalent parallel-plate ionisation chamber filled with TE gas to a pressure of 0.6 MPa The electric field strength ranged from 100 to 2000 V/cm. When the plot is made on a log-log scale a straight line can be reasonably fitted, and the slope of the line gives an evaluation of the index n.
The index n does not depend on dose rate^22,53) and is a monotonic function of the quality factor of different radiations from gamma radiation to alpha particles^2). Later investigations^53-54’55) confirmed, that chambers of different construction and gas pressure can be used for determination of quality factor, when properly operated and calibrated.
The early research, mentioned above, were of primary importance for application of recombination methods in dosimetry, however, the application of recombination detectors as reference instruments was not possible yet The main disadvantage of these methods was, that both, the choice of voltages U, and U2 as well as the value of index n depended on the composition and pressure of the filling gas and on the design of the chamber. Therefore, special investigations were needed for every device in order to find the proper operating conditions. This problem has been partly solved, when the concept of recombination index of radiation quality was introduced^23) (see Paragraph 3.2).
A different method for determination of quality factor in neutron-gamma radiation fields with thermal to 10 MeV neutrons was developed by Makrigiorgos and his co-authors^56-57). Their experimental procedure consists of recording the two ionisation currents / , and /2 measured at two high field strengths and constant pressure applied to the high pressure tissue-equivalent ionisation chamber. It is assumed that in the thermal to 10 MeV neutron energy region, the kerma fraction that heavy ions contribute, relative to the kerma fraction contributed by protons, is approximately constant and equals to 11%. The mean quality factor is derived as:
30
(3-3)
where <Dy is the photon absorbed dose fraction and Lp D is the dose mean LET of the proton component.
It has been shown that the value of /,//2 defines appropriate combinations of values for <Dy and Lp D,
any of which may be used in equation (3-3) to yield the value of Q, i.e. the ratio of /,to z2 defines Q in a unique way!56). The "pairs" (<Dy ,LpD) associated witli a given z,//2 value were derived from the
recombination formulas!56). The method gives the values of Q with an accuracy of about 10 to 15% and it may be considered as a valuable experimental confirmation of the theoretical approach of Makrigiorgos and Waker, described in Section 2.3, although its practical application is limited to the neutron fields of neutron energy up to 10 MeV only.
Q = <Dy+(l-<Dy){(l-0.11) [(LpD/6) + l] + (0.11x20)}
3.2. RECOMBINATION INDEX OF RADIATION QUALITY
As it was shown in the works referenced above, the phenomenon of initial recombination may serve as a physical basis for the practical definition of radiation quality. In order to fulfil this function there is a need to specify the conditions under which the recombination is measured. The conditions have to be precisely defined and easily reproduced. These requirements have been fulfilled by the new quantity introduced by Zielczyriski et al.!23) and called the recombination index of radiation quality, (RIQ).
The definition of RIQ, involves measuring the difference in collection efficiency at the two defined voltages Us and UR and normalising so that the RIQ will be unity for reference gamma radiation. The definition of RIQ (denoted now as QR/S) is therefore:
Qr/s -(fS ) radiation(fS — fR )gamma
(3-4)
where fs and fR are the measured collection efficiencies at voltages Us and UR respectively.In the work from 1980!23) we supposed to use the radiation of 60Co gamma source as a reference,
however, later the 137Cs source was mostly used. Therefore, in this work the gamma radiation of the 137Cs source is considered as the reference, unless stated differently. The choice of reference radiation is not critical, as fCs » fCo
Supposing that QR/S is measured for an absorbed dose D of mixed radiation then it was shown!23), that
Qr/s - Z ®j '(Qr/s ) j (3-5)j
where and (QR/S)j are the absorbed dose and RIQ of the j'th component of the mixed field. Hence the recombination index has the necessary property of being the dose average of the components that make up the radiation field.
Dependence of RIQ on local ion density and on LET.
For hypothetical radiation with a single-valued ion density the ion collection efficiency can be expressed as!23) (compare also Equation 2-28):
f = 1l + pF(X,p)
(3-6)
where F(X,p) depends on the ionisation chamber parameters, such as voltage, gas pressure, etc., and g is the local ion density, associated with a sensitive volume of diameter of the order of 0.07 pm of unit density tissue (see also Section 2.4). As before, p is arbitrarily assigned to 1 for the reference gamma radiation.
31
Using the equation (3-6) and expressing m in terms of R and S, the dependence of the QR/S on ion density and measuring conditions becomes:
Qr/s -1
R-S
1-S_______1 - R
1 + S(g-1) 1 + R(p-1)(3-7)
where R = (l-fR)gainma and S = (1 -fs)g8tnma
The index RZS is always expressed in per cents, i.e. Q5/2 means that R=0.05 and S=0.02. The function given by equation (3-7) lias a maximum value at
f(T-R)(l-S)(3-8)
Of particular interest is the case when S=0; this condition is obtained when the recombination chamber is fully saturated at the upper voltage used In this case the dependence of RIQ (denoted now as QR) on ionisation density reduces to;
Qr - M-1 + ROi-l)
(3-9)
Since the quality factor is defined in terms of unrestricted LET, so for the purposes of radiological protection the RIQ may be expressed as a function of L in the following approximated form:
L/LqQr * for L 5 3.5 keV/pm (3-10)
l + R(L/L0 -1)and
QR*0.85 + 0.15L/L0 forL < 3.5 keV/pm (3-11)
where L0 = 3.5 keV/gm
The family of curves QR(L), obtained with different values of R is shown in Figure 3-3. QR isdetermined between a saturation voltage (S=0) and a voltage which gives a recombination of R% for ^7Cs gamma radiation.
The same figure presents also the dependence of the quality factor on LET, recommended in ICRP Report 2l(uX As can be seen from the figure, the form of the Q3 and of the Q4 dependence on LET matches that required by the recommended Q(L) dependence. In practice we mostly used the Q* as the approximation of the quality factor (see Section 3.3).
Figure 3-3. The recombination index of radiation quality QR as a function of LET. Also the ICRP-recommendedl1 ^ Q(L) relation is shown, for comparison.
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The form of equations (3-10) and (3-11) may be rationalised in the following way:The equation (3-10) was obtained assuming that ionisation density is proportional to LET. As discussed
already in Section 2.4 the value of L0=3.5 keV/pm was chosen mainly because the quality factor was defined as equal to 1 for L=3.5 keV/pm. This value is also close to the dose-mean restricted LET, L(5ood)=3 67 keV/pm^40), for ^37qs gamma radiation, what becomes important from the physical point of view, when we remember that the parameter p is not related to L, but rather to the restricted LET with energy cut-off of about 500 eV. For high-LET radiation the difference between L and L500 is not crucial for the value of QR and practically can be neglected, when QR is determined for radiological protection purposes.
For low-LET radiation the difference between L and LJ00 should not be neglected, as the Equation (3-10) would lead for this LET region to the values of QR much lower than 1 as shown by the dotted line on Figure 3-4. This is strongly in contrary to the experimental results of QR=0.9±0.1 for 9 MeV electrons and of about 1 for high energy muons. Therefore, in this work the dependence of QR(L) for low- LET radiation was estimated taking into account the values of L(500D), reported by Blohm^58) (see Figure 3-5) and by Makrigiorgos(40) for electrons of different energies. This estimate agrees with equation (3-10) and is depicted on Figure 3-4 by the solid line.
LET (keV pm"1)
Figure 3-4. Dependence of QR on LET for low-LET region assuming p proportional either to La- (dotted line) or to LA (solid line, A=500 eV) (see text).
raq TTfVmy
_100,D
nirmrt i .... ^ * iijimt! ■ r ,* nvd
Electron energy (MeV)
Figure 3-5. Different types of mean LET calculated for electron tracks with initial energy E0 in water (compilation of data given by Blohm(58) and by Makrigiorgos^40)).
Dependence of RIQ on composition of the filling gas mixture and on gas pressure
The value of QR may depend on the composition of the gas used for filling of the chamber. Table 3-1 presents some results of our measurements of Q4, performed with two REM-2 chambers filled with different gas mixtures. One of these chambers was filled with the tissue-equivalent (TE) gas mixture and the other with the mixture of methane and nitrogen (5%). Filling of the chamber with methane-nitrogen (MN) mixture with high excess of hydrogen contents in comparison with soft tissue, causes that the chamber sensitivity to neutrons of energy up to about 5 MeV is somewhat higher than sensitivity to photons (sensitivity means here the ratio of the detector signal to the absorbed dose). By the other hand the chamber filled with TE-gas shows too low sensitivity to neutrons of these energies. From relation (3-5) the Q4 can be written as:
Q4=<Dn(Q4)n+(DY(Ql)Y (3"12>
where (Q4)n and (Q4)y are the mean values of Q4 for neutrons and photons respectively.
33
The difference in sensitivity to neutrons and photons influences contributions of the dose components <Dnand <Dy to the detector signal. This usually results in higher values of Q4, when it is determined using the chamber filled with methane-nitrogen mixture in comparison with the TE mixture. In our practice we usually use the methane-nitrogen mixture.
Table 3-1. Values of Q4 determined with two REM-2 chambers filled with different gas mixtures: TE - methane-based tissue equivalent gas, MN - mixture of methane and nitrogen (5%).
Radiation source Mean neutronenergy
c
TE-gas
U
MN mixture
241Am-Li 0.4 MeV 2.5(43) 3.5(43)252Cf 2.5 MeV 0.35 6.8<59)239Pu-Be 4.3 MeV 0.24 6.5 6.8239Pu-Be with 0.14 7.7 8.8(»)10 cm Fe filter241Am-Be 4.5 MeV 0.24 6.5(39)
T(d,n)3He_______ 13.8 MeV 0.03 6.2 6.2
The dependence of QR on filling gas pressure has been investigated experimentally, for the REM-2 chamber filled with the TE gas and with the MN mixture. From these measurements we could estimate the
range of validity of the Equation (3-9), which predicts that the value of QR does not depend of the filling gas pressure. Results obtained for a plutonium-beryllium source are shown in Figure 3-6 for gas pressure from 0.2 MPa up to 1.4 MPa. As it can be seen from the figure the operational range of the chamber, where the value of Q4 is constant, is from about 0.2 MPa to about 0.6 MPa for TE gas and at least from 0.9 MPa to 1.2 MPa for MN mixture. There is a correlation between the operational range and molecular weight of the filling gas, however the relation has not been investigated quantitatively yet.Some information on dependence of initial recombination on gas pressure for several gases at pressures
up to 8 MPa can be also found in works of Zoetelief et al.(60>61).
Accuracy of determination of Q4
In most of practical situations the ionisation current of the REM-2 chamber can be determined with accuracy of about 0.2%, when the measuring procedure described in Appendix B is applied. This leads to accuracy of determination of Q4 of about 5%, when the value of is close to unity’. For larger values of Q* the influence of inaccuracy in determination of ionisation current became less important, but the role of inaccuracy in determination of saturation current increases. Taking into account these two factors we usually estimate accuracy of Q4 as not worse than 10%.
Filling gas pressure (MPa)
Figure 3-6. Dependence of Q4 on filling gas pressure for REM 2 chamber filled with TE-gas and with MN gas mixture, determined in radiation field of 239Pu-Be source.
34
3.3. Q4 AS AN APPROXIMATION OF THE QUALITY FACTOR FOR NEUTRONS AND CHARGED PARTICLES
Determination of Q4 involves two steps, as illustrated in Figure 3-7:1) Calibration of the chamber in a reference gamma radiation field of a ^Cs source. During the
calibration the voltage UR is determined in such a way that fy(UR)=0.96.2) Determination of ion collection efficiency at voltage UR in the investigated radiation field. This step
requires the measurement of the ionisation current at voltage UR and determination of saturation current in investigated radiation field e g. by measurements at some high voltage close to saturation conditions or by other methods as discussed in Appendix B.
The value of Q4 is then calculated from equation Q4= (1 - fn(UR)) / 0.04It may be useful here to remind Figure 3-3 which shows that both Q4 and Q3 can be used as
approximations of the quality factor. From Figure 3-7 we can see that Q4 is more convenient quantity to bedetermined experimentally than Q3. When we want to find the voltage U3 we need to determine the point of saturation curve, where fy = 0.97. This point lies closer to the saturation and the value of U3 is determined less accurate than U4. This of course influence considerably the accuracy of determination of Q3.
Experimentally obtained values of Q4 for different radiation fields are presented in Table 3-2. For neutron fields, the Q4 values in this table refer to the mixed fields of neutrons and photons. The measurements were performed mostly with the REM-2
chamber filled with methane-nitrogen mixture of gases. Only in beams of phasotron in Dubna the chamber of F-l type was used. The chamber was filled with the same MN mixture as the REM-2 chamber. Measurements for monoenergetic neutrons were performed mainly at the Physikalisch Technische Bundesanstalt (PTB) in Braunschweig (Germany), except those for neutrons of energy equal to 2.1 MeV and 5.5 MeV. These two results were obtained at TNO Rijswijk (The Netherlands). Measurements for isotopic neutron sources were performed in reference neutron fields at the Institute of Atomic Energy, Swierk. The Table 3-2 presents also the results obtained for high energy neutrons and protons, determined at the Joint Institute for Nuclear Research in Dubna (Russia). Series of measurements have been also performed in calibration fields of high-energy neutrons, created in vicinity of H6 beam of SPS at CERN. Results of these experiments are described separately in Appendix B.
reference gamma radiation
investigated radiation field
Collecting voltage, VFigure 3-7. Principle of determination of Q4. Please note, that the x-axis is logarithmic up to 100 V and linear for higher voltages.
35
Table 3-2. Experimentally obtained values of Q4.
Radiation source (reaction)Filter
E„(MeV)
Photoncomponent q4
NeutronsAccelerator 7Li(p,n)7Be 0.075 60% 5.2
7Li(p,n)7Be 0.144 43.5% 7.257Li(p,n)7Be 0.5 17.5% 10.75T(p,n)3He 0.9 0.7% 11.7T(p,n)3He 1.2 0.4% 10.8T(p,n)3He 2.1 4.5% 8.9<62)T(p,n)3He 2.5 0.5% 9.1D(d,n) 4.2 0.9% 7.0D(d,n) 5 1.8% 6.45D(d,n) 5.5 7.5% 7.0(62)T(d,n)4He 13.9 4% 6.2T(d,n)4He 14.8 1.15% 6.7T(d,n)4He 18 10% 6.2T(d,n)4He 19 9% 6.75
252Cf 2.5 35% 6.8(59>10 cm iron 10% 10.1<59>10 cm paraffin 70% 4.0(59)
241Am-Be 4.5 24% 6.5(59)10 cm iron 14% 8.8(59)10 cm paraffin 48% 5.3(59)
Reactor TBR-2 JINR Dubna 6 cmPb 1.5 unknown 8.0(62)
Phasotron JINR Dubna 350 unknown 4.810 5 cmPPMA -350 unknown 3.0
ProtonsPhasotron JINR Dubna TE phantom 200 1.2
Alphas222Rn 5 13(62)
Natural uranium 4 18(62)
According to the definition given in the 1CRP Report 21 the value of the quality factor is determined not only by the characteristics of the incident radiation but also by the geometry and composition of an irradiated object, and in general, it refers only to one point in a specific object. In 1985 ICRU recommended certain new operational quantities^3* and among them the ambient dose equivalent for monitoring of external radiation fields Therefore in this work the values of Q4 are compared with the effective quality factor in so called (CRU sphere^) i.e. at a depth of 10 mm within a 30 cm diameter sphere made from ICRU tissue substitute, irradiated by a broad parallel neutron beam. For purposes of the comparison the values of Q4 for neutrons alone were calculated using the equation (3-12) for monoenergetic neutrons of energy
36
from 75 keV up to 19 MeV. Results are presented in Figure 3-8 in comparison with calculated dependence of the effective quality factor on neutron energy!63).
Q(ICRU39)
jL—j—■ ■ ■i iii
Neutron energy, MeV
Figure 3-8. Experimentally obtained values of Q4 for monoenergetic neutrons in comparison with the effective quality factor in ICRU sphere calculated!63) according to ICRP Report 21 and plotted against neutron energy.
Experimental values of Q4 for neutrons are similar to the ICRU 39 values of the quality factor. The maximum difference of about 20% between Q4 and Q was observed for 75 keV neutrons. This experimental point is, however, the most uncertain one, as the photon component of the investigated field was high (about 60%) and not well known.
3.4. ADAPTATION TO THE NEW RECOMMENDATIONS OF ICRU AND ICRP
Adaptation to Q(y) function proposed in ICRU Report 40
In 1985 the liaison committee of ICRU and ICRP!64) proposed in Report 40 of ICRU an increase in the value of the quality factor for neutrons, but this recommendation was not adopted in European Community countries. The proposal can be represented by the equation:
Q(y) = y[l-exp(-a2y2-a3y3)] (3-13)
which utilises the lineal energy y for a specified site parameter 1 pm. Coefficients in equation. (3.4.-1) equal to: 32=5510 keV pm-1; a2=5><10"5 pm2keV'2; a3=2xl0"7 pm3keV"3.
The propositions given in ICRU Report 40 were partly adopted to the Polish state recommendations for radiological protection^65). Therefore it was advisable to develop a recombination method for experimental evaluation of such defined quality factor. In our work!66) from 1990 the recombination chamber was used for determination of the effective lineal energy and then the approximated value of the quality factor was calculated from the Q(y) relation given by ICRU. The accuracy of the approximation was of about 20% for neutrons of energy up to 15 MeV.
Since the concept proposed in ICRU Report 40 was not included to the recent recommendations of ICRP (Publication 60) and a revision of Polish recommendations concerning quality factors is expected, so the problem of quality factors defined in ICRU Report 40, Qjcru40’ became of minor importance for radiation
37
protection practice and will not be presented here in details. However, relation of quality factor to lineal energy is in our opinion more meaningful than relation to unrestricted LET.
Adaptation to Q(L) function proposed in ICRR Publication 60
It was shown above, that the dependence of RIQ on LET can be changed by an appropriate change of collecting voltages Us and UR. Unfortunately it is difficult to achieve a reasonable approximation of the new Q(L) dependence. Therefore we proposed^67) a procedure for calculation of the quality factor Qjcrpso from the measured values of Q*
Comparison of the dependencies of Q1Crp60 and Q4 on LET (see Figure 3-9) gives the relationship between both these quantities Such relationship would be valid only for hypothetical radiation with a single value of LET and would result in very crude approximation when formally used for a realistic radiation with a broad LET distribution. Mathematical simulations^67) showed, that better approximation of the quality factor can be achieved by a quantity Q4new which is a function of Q4 defined by relation shown in Figure 3-10
Figure 3-9, Dependence of Q4 and of quality factor (ICRP 60) on LET.
r| i ri'Tp turv;
Figure 3-10. Relation between Q4 and Q4new, which approximates the quality factor, as defined in ICRP Publication 60.
The difference between the quality factor Q|CRP60 and our approximation Q4new depends mainly on the width of the microdosimetric spectrum of the investigated radiation. In order to estimate this difference we simulated a number of LET distributions, for which we calculated both Q1CRP6() and Q4„ew. The results of calculations performed for some neutron radiations with relatively narrow LET distributions showed that for realistic distributions ranging from 20 keV-gnr1 up to 200 keV-pnr1 the differences between the Qicapeo and Q4new do not exceed 20%.
The values of Q4new might be much too high, when the LET distribution contains a significant photon component and component of radiation with LET higher than 300 keV-pm"1. In the most critical case, 60% radiation with LET of 500 keV-pnr1 and 40% gamma radiation, the resulting Q4=13.5 and Q4new is equal to 20, while for the same LET distribution Qicrpso"**4-
This problem can be avoided by an independent determination of the photon component of the dose. In mixed radiation fields, the photon component broadens the LET distribution and this may influence the accuracy of our approximation When neutron radiation is mixed with the photon radiation, the additional error due to the photon component may vary from zero up to more than 40% in dependence on the value of this component and on the shape of LET distribution of the neutron component. The problem is illustrated by Figure 3-11, where the Q4new and Q!CRP60 are presented in dependence on photon component.
38
The Figure shows the most disadvantageous case of mixed radiation, which consists of neutrons with Q=25 and photons.
If the gamma component is known, the can be expressed as:
Q4new = ®n'(Q4new)n +®y 1 (3'H)
When such method of calculation is used, the difference between Q4new and Q1Crp6o does not depend on the photon component. For the hypothetical radiation field mentioned above (60% radiation with LET of 500 keV-gm"1 and 40% gamma radiation) the Equation (3-14) results in the
value of Q4ikw:=Qicrp60=^ •4•Special precise measurements of Q4new were
performed in reference fields of 252Cf, 241Am-Be and 239Pu-Be neutron sources (for bare sources and
sources placed in spherical paraffin and iron filters). In these fields the photon component ranges from 10% up to 70%. The values of Q4new were calculated according to Equation (3-14) and all of them fall in agreement within 15% with the appropriate values of Qjcrpso^67^
The Q4new values, derived from Q4 values measured in monoenergetic neutron fields, are presented in Figure 3-12 in comparison with calculated^8) values of Q1CRpeo *n ICRU sphere.
Photon component cf the absorbed dose
Figure 3-11. Calculated dependence of Qicrpso and Q4new values on photon component of the absorbed dose for mixed radiation field of neutrons of highest quality factor (QiCrp60=25) and photons.
30 -... ■ICRF60
4—a. L H..I.I j_«—
Neutron energy, MeV
Figure 3-12. Experimentally obtained values of Q4new for monoenergetic neutrons in comparison with the effective quality factor in ICRU sphere calculated^68) according
to ICRP Report 60 and plotted against neutron energy.
The calculation procedure described here enables the use of Q4 for determination of the quality factor recommended by ICRP 60 for neutrons in energy range up to 19 MeV. The error resulting from calculation does not exceed 15%, however, determination of the photon component is required. Also the fields with components of very high LET should be considered with special caution.
39
3.5. CONCLUSIONS CONCERNING DETERMINATION OF RADIATION QUALITY FACTOR USING A RECOMBINATION CHAMBER
The large set of experimental results, collected in different radiation fields confirmed that a recombination chamber can be used as the reliable instrument for determination of radiation quality factor. The value of the recombination index of radiation quality Q4 practically does not differ from quality factor defined in ICRP Report 21. Therefore, equality Q4 » Q can be used for practical radiation protection. It has been shown^23), that the Q4 is an additive quantity, so the equality Q4 » Q is valid for any composition and every radiation spectrum. However, small differences between Q and Q4 may appear, because of:
1. Non-identical dependence on LET (see Figure 3-3), especially for very high-LET particles.2. Possible differences in the energy spectrum of secondary ionising particles in filling gas of the
chamber in comparison with ICRU tissue substitute, mainly because of lack of oxygen and excess of carbon in the chamber material.
3. Quality factor is related to the unrestricted LET, while Q4 is correlated rather with the restricted LET.
The results presented in this Section can be also considered as confirming the validity of our generalised model of initial recombination and particularly the validity of equation (2-28). This will be also important for our microdosimetric approach, presented in Section 5.
The recombination method can be also applied for determination of redefined quality factor, QICRp60. Both mathematical simulations and experimental results indicate, that the most crucial problem in this case is separation of the low-LET and high-LET components of the absorbed dose. If such separation is performed the QiCrp60 value can be calculated from the measured value of the recombination index of radiation quality Q4, using the simple conversion function as described in Section 3.4, however we recommend this method for fast, preliminary estimation of the quality factor only. For radiological protection purposes the value of H*(10) should be determined, as described in Section 4.
Accuracy of determination of QICRP60 can be also considerably improved, when the microdosimetric analysis is applied, as described in Section 5.
40
4. RECOMBINATION CHAMBER AS A DETECTOR OF AMBIENT DOSE EQUIVALENT
Recent recommendations of ICRU^ and ICRP(2) caused an increase of interest in dose rate meters with an instrument response turned to ambient dose equivalent, H*(10). Recombination chambers are specially suitable for such purpose, as they can be used for determination both the total absorbed dose and the quality factor of mixed radiation fields. Therefore, they may directly provide the dose equivalent data needed for radiation protection.
Recombination chamber used for determination of the ambient dose equivalent, H*(10), in mixed radiation fields should obey the following characteristics:• the mass of TE material sufficiently big to simulate the scattering properties of ICRU sphere,• the effective wall thickness close to 10 mm of tissue,• the sensitivity of the chamber to the absorbed dose similar for both neutron and gamma radiations, and• the dependence of the chamber signal on LET matching the function which defines the radiation quality
factor.It was found(69) that the above requirements could be fulfilled by the REM-2 recombination chamber
(see description in Appendix A), which used to be manufactured in Poland by POLON. Relatively big mass of the chamber (6.5 kg) and the appropriate effective wall thickness (of about 1.8 cm) cause, that the dose contribution and energy spectrum of secondaiy charged particles in the chamber cavity are similar as in the ICRU sphere at the depth of 10 mm.
4.1. A METHOD BASED ON THE CONCEPT OF RIQ
As it was described in Section 3, the response of the recombination chamber may be maintained to match the LET dependence of quality factor, as it was defined in ICRP Report 21(11X Therefore, the method for determination of ambient dose equivalent involves measurements of the absorbed dose and the recombination index of radiation quality, Q4, which approximates the quality factor.
The chamber of REM-2 type contains two sets of electrodes (see Appendix A) and can be used either in summation or in differential mode. The results presented here were obtained mostly using the summation mode, however, some data measured in differential mode, were also included.
In the summation mode the high voltage Us and the recombination voltage UR are applied to the whole chamber consecutively, so both the ambient absorbed dose D*(10) and Q4 might be determined separately. The ambient dose equivalent is then calculated as:
H*(10) = D*(10)Q4 = ”--Q4 (4.-1)kd
where qs is the ionisation charge collected at voltage Us and RD is the calibration factor.
Relative response of the chamber to H*(10) is defined as:
RH =H,(10)/H^ef(10) (4-2)
where H*(10) is measured by the recombination chamber and Href (10) means the conventionaly true value, of the ambient dose equivalent.
41
4.1.1. Experimental results
In our investigations the values of RH were experimentally determined in several gamma and mixed neutron+gamma radiation fields. We used the REM-2 chamber as made by manufacturer (without any refilling or changing the gas pressure). Two isotopic radiation sources were used for calibration of the chamber, namely the 137Cs source for photons and the 241Am-Be source for neutrons. The RD value for 137Cs was equal to RD(Cs) ~ 4-10"4 C/Gy. The response to the mixed radiation of 241Am-Be source, RD(Am-B=)’ was higher by 17% (R^Am-Be) = 4.68-10*4 C/Gy).
In order to have more clear presentation the investigated radiation fields were grupped in this work in the following way: photon sources, isotopic neutron sources with broad energy spectrum, thermal and intermediate neutrons, monoenergelic beams of fast neutrons and high energy neutrons.
Photon radiation
Response of the REM-2 chamber to H*(10) cf photon radiation was determined for energy range from 59 keV to 1.25 MeV#9). A minor difference from response to 137Cs was found for 111 keV X-rays and for 60Co source, the values of the relative response were equal to 0.99 and 1.03, respectively. However, more significant difference may be expected for low energy photons. For 241 Am source it reaches about 20%.
Table 4-1. Relative response to H*(10) of photon radiation of different energies, determined for the REM-2 chamber calibrated with 137Cs gamma radiation source.
Radiation source Mean energy Relative response
137Cs 662 keV 1
60Co 1250 keV 1.05
241Am 59 keV 0.82
X-rays 111 keV 0.99
Isotopic neutron sources
Measurements with isotopic neutron sources were performed in calibration laboratories at Institute of Atomic Energy in Swierk, at CERN and at Physikalisch Technische Bundesanstalt (FIB) in Braunschweig (Germany). Reference fields of 241Am-Be, 238Pu-Be 241Am-Li and 252Cf bare and in DzO filter were used#9). Obtained values of the relative response RH are close to the value of &H(Am-Be)' within 11% for all the investigated fields exept 241Am-Li. Lower response to 241Am-Li is due to large contribution of low energy photon radiation to the absorbed dose (about 70%); the estimated value of RH for neutrons alone is of about 1.2.
Table 4-2. Relative response to H*(10) of n+y radiation of isotopic neutron sources, determined for the REM-2 chamber calibrated with 24,Am-Be neutron source.
Radiation source Mean energy Relative response
241Am-Be 4.5 MeV 1
238Pu-Be 4.3 MeV 1.01
252Cf 2.5 MeV 1.11
241Am-Li 0.4 MeV 0.86(1.2 neutrons alone)
252Cf+D,0 0.5 MeV 0.98
42
Thermal and intermediate neutrons
Response of the chamber to thermal neutrons was determined by Htifert and Rafifnsoe(70> in reference field of thermal neutrons in PTB. The results were expressed by authors in terms of MADE'). Later, we recalculated the results to H*(10) obtaining the value of RH/RH(Am-Be) equal to 1.15.
More questionable can be the problem of response to intermiediate neutrons, especially of energies below few tens of keV. The mean energy W required to produce an ion pair increases rapidly for protons of energies below 10 keV and generally, a deep in dose equivalent response of gaseous detectors can be expected. In our REM-2 chamber this effect is, at least partly, compensated by thermalisation of the intermediate neutrons in large mass of the chamber (like it is also in ICRU sphere). The response is preasumable due to 14N(n,p)12C reaction with emmition of 600 keV secondary protons, which are well detectable by the chamber.
Some qualitative information can be also derived from measurements of Zielczynski, which were made during an intercomparison experiment, performed in soft neutron reference field in Joint Institute for Nuclear Research in Dubna^71) (Russia). This field was established for calibration of neutron dosimeters at conditions, that are similar to working environment. The neutron spectrum is relatively soft, however, it includes also a contribution from fast neutrons. The experimental results indicates that the response of the chamber to the broad spectrum of thermal and intermediate neutrons does not differ from R^Am-Be) more than 20%. Similar conclusion can be derived from our measurements with 252Cf source in D20 filter.
Monoenergetic fast neutrons
The first systematic studies of the response of the REM-2 chamber to MADE*) of monoenergetic fast neutrons were performed by Htifert and Raffnsoe^70). In our world69) from 1994 we estimated the response to H*(10) by recalculation of their results. Nowadays, the response to H*(10) was directly determined^72) in reference monoenergetic neutron fields, for 12 neutron energies ranging from 75 keV to 19 MeV.
Monoenergetic neutron beams were generated by 3.75 MV Van de Graff accelerator at Physikalisch Technische Bundesanstalt (PTB) in Braunschweig^73-74). The irradiations were performed in open geometry in the measuring hall of the accelerator facility (24 mx30 mx 14 m). The chamber was placed at the distance 2.5 m from the target. The photon contribution to the absorbed dose was below 2% for all neutron beams with energy higher than 0.5 MeV. Neutrons of lower energies were produced via the 7Li(p,n)7Be reaction, so the high-energy photons from ,9F(p,<xy)160 reaction considerably influenced the measurements.
The reference values of H*(10) were calculated from precise measurements of neutrons fluence, using the fluence to H*(10) conversion factors given by Wagner et aid63).
Since the PTB reference values of H*(10) for monoenergetic neutron fields were given for neutrons alone, therefore, the photon component of the absorbed dose, <Z)y, was determined (by analysis of the saturation curve - see Section 4.2 ) and appropriately subtracted:
(4-3)
The subscripts n refer in the above equation to the neutrons alone and <2^ is the quotient of the absorbed dose due to photons to the total absorbed dose.
The results of the investigation are shown in Figure 4-1.
*)The acronym MADE means the maximum dose equivalent per unit fluence in a 30 cm thick semiinfinite slab phantom.
43
o 2.0 r—S8.1.5 —-«1.0I f&
0.5
0.0
i O------ n i *
* 1 ■ « 1 1 ■ a!0.1 1
Neutron energy, MeV10
Figure 4.-1. The relative response of REM2 recombination chamber to H*(10) with ICRP-21 quality factor, i.e. the ratio of Hn(3Q) determined by the chamber to the reference values determined from the fluence measurements. Solid line is guide for eye.
The observed energy dependence of the chamber response to H*(10) related to the response for 24*Am-Be is generally fairly flat. In the neutron energy range from 75 keV to 4.5 MeV the relative response
equals to about 1.15 and varies less than the experimental uncertainty. There is, however, a minimum on the response curve around 14 MeV where the relative response decreases down to the value of 0.65. This minimum seems to be caused rather by some deviation of composition and dimensions of the detector from those of the ICRU sphere, and not by the LET response itself. The measured quality factor for 14.8 MeV neutrons is equal to 7, comparing with the value of 7.5 calculated for ICRU sphere^63), while the absorbed dose is underestimated by about 25%. At 18 MeV the relative response rises again to about 1.
High-energy neutrons
The first calculations of fluence to ambient dose equivalent conversion factors have been reported not far ago<75'76) and the resulting conversion factors are still considered as being uncertain. Moreover, all the measurements of a detector response have to be performed in radiation fields with broad neutron energy spectrum and with considerable component of particles other than neutrons. For this two reasons no precise value of the relative chamber response to high energy neutrons can bedetermined experimentally and given here. It can be expected, however, that our REM-2 chamber overestimates the ambient dose equivalent of high energy neutrons. The overestimation is due to higher effective wall thickness of the chamber, comparing with the depth of 10 mm in ICRU sphere recommended for H*(10) measurements.
Up to now, only one result for high energy neutrons has been reported^70) in terms of chamber response related to MADE. We tried to estimate the chamber response to HfijO), using the recently calculated conversion factors and we came to the value RH/RAm.Be=1.4±0.3 for high-energy neutrons with mean energy of 280 MeV. We want to underline here that this value should be considered only as an indication of the expected value of the relative response to high energy neutrons.
The REM-2 chamber has been also used in several series of experiments performed in high-energy stray radiation fields in the vicinity of acelerators (see Appendix B). Dosimetry of such radiation fields is still a challenging task and is based mainly on intercomparison of different measuring and calculation methods. The resultes obtained by REM-2 chamber showed, that the recombination chamber is a very good detector for determination of H*(10) in such complex fields of mixed radiation.
44
4.1.2. Investigations of gas mixtures for the chamber filling
The influence of the filling gas composition on the relative chamber response was investigated using the commercial REM-2 chamber and two chambers of the same type but filled with different gas mixtures^69). One of these two chambers was filled with the tissue-equivalent (TE) gas mixture and the other with the mixture of methane, nitrogen and argon (MNA). Filling of the chamber with methane-nitrogen mixture, with high excess of hydrogen contents in comparison with soft tissue, causes that the chamber sensitivity to neutrons of energies up to 5 MeV is somewhat higher than sensitivity to photons. By the other hand the chamber filled with TE-gas shows too low sensitivity to neutrons of these energies (see Table 4-3). The MNA mixture, with lower excess of hydrogen than in commercial REM-2 chamber, seems to be promising, however surprisingly high relative sensitivity to low energy photons was observed.
Table 4-3. Relative response of REM-2 chambers filled with different gas mixtures to H*(10), for photons and n+y radiation of isotopic neutron sources. The chambers were calibrated with 137Cs gamma radiation source. Codes of gas mixtures are as follows: MN - methane + 5% of nitrogen, TE - tissue-equivalent gas, MNA - MN + 15% of argon. ARH/RH means the relative uncertainty of the chamber response.
Radiation source Meanenergy MN
Filling gas mixture
TE MNA
arh
Rr
Photons137Cs 662 keV 1 1 1 3%60Co 1250 keV 1.05 1.04 1.04 3%241Am 59 keV 0.82 0.83 1.33 10%
X-rays 111 keV 0.99 3%
Neutron sources241Am-Be 4.5 MeV 1.17 0.8 5%238Pu-Be 4.3 MeV 1.18 082 1.09 5%2«Cf 2.5 MeV 1.3 5%241Am-Li 0.4 MeV 1.01 0.81 1.21 20%252Cf + d2o 0.5 MeV 1.15 0.75 15%
4.1.3. Impact of recent ICRP Recommendations
If the new dependence of quality factor on LET, Q(L), given in ICRP Report 60(2) will be introduced to the radiation protection regulations, the values of ambient dose equivalent, measured by the recombination chamber should be appropriately corrected. We showed in Section 3.4 of this work, that in many cases the ICRP60 quality factor can be calculated from the measured value of However, such a metod may be not reliable enough for complex radiation fields with considerable components of low LET and very high LET radiation.
A practical method for determination of H*(10) with the ICRP-60 quality factor is presented in Subsection 4.2. The method is based on our microdosimetric aproacb and is suitable for routine radiation protection.
45
4.2. METHOD BASED ON MICRODOSIMETRIC APPROACH
Generally, the ICRP-60 quality factor should be calculated from the dose distribution versus LET, d(L). Our method for determination of such distributions is considered in Section 5. However, the method requires long time of measurements (usually about one hour) and some experience in mathematical analysis. At present the method is still under investigation and cannot be considered as suitable for routine radiation protection. Taking into account the needs of radiation protection we proposed a method for the evaluation of main microdosimetric averages by simplified analysis of the saturation curved77’72). The method provides separation of low-LET and high-LET dose fractions and enables estimation of the "new" quality factor for high-LET component of the radiation field. From mathematical simulations^67) performed for several realistic d(L) distributions we expect, that with such approach the H*(10) can be determined with accuracy and "simplicity" satisfying routine radiation protection needs.
In Section 2 we showed, that if the recombination chamber is used in mixed neutron-gamma radiation field, the ion collection efficiency, fmix, for each particular collecting voltage, can be expressed as:
fmix DJ d(L)
1 +1-f, -dL (4-4)
where D is the total absorbed dose, p is the local ion density, as defined in Section 2 and fy is the ion collection efficiency in reference field of 137Cs radiation source. Both fmix and fy are measured at the same collecting voltage.
In the simplified method the integral in the equation (4-4) was approximated by the sum of low-LET and high-LET components, in the following way:
^mix ^low fy + ®high l _ f
1+ ^high4
(4-5)
where <D|ow and are the low-LET and high-LET components of the absorbed dose, respectively (®|ow + = 1), and phigh is the effective value of p for high-LET component of mixed radiation field.
The ion collection efficiencies f^x and fy were measured at the same voltages for both mixed radiation and reference gamma radiation fields. The collected data were further expressed in form of a dependence fmjx versus fy, (see examples in Figure 4-2). This facilitated the fitting of the relation (4-5) to the experimental data by the standard fitting procedures. The fitted parameters were <qow and p^.
For calculation of H*(10) we related phjgh to LET as:
f'high — Ehigh " ^0 (4-6)
where L^ is used as the effective value of LET for calculation of the quality factor of high-LET component of mixed radiation field and L0 = 3.5 keV pm*1 is a scaling factor (see discussion in Section 2.4).
Fits shown in the Figure 4-2 were performed taking into account 8 experimental points. The saturation curves were determined, however, more precisely - usually measurements were performed at about 20 different values of the collecting voltage. Then, the fitting procedure was investigated in order to find the minimum number of points that still insures the stable fit. The reliable results were obtained down to the eight experimental points. The exact values of the collecting voltage are not crucial for the method, however two points at low voltages are required (5 - 15 V for our chamber), as well as one high voltage ensuring conditions close to saturation. Other voltages, depicted in the Figure, were chosen with the aim to cover more or less uniformly the whole saturation curve.
46
1.0 -neutrons:
A 14.8 MeV
• 2.5 MeV
■ 0.9 MeV
neutrons:
o 14.8 MeV Ar 2.5 MeV V 0.9 MeV
S 0.4 -
Collecting xoltage, VY
Figure 4-2. Illustration of the fitting procedure used for determination of low LET component and of effective LET. Left plot: Saturation curves for 137Cs reference gamma radiation and for three radiation fields of monoenergetic neutrons of different energies (measured at 8 collecting voltages) Dashed lines are guides for eye. Right plot: The same experimental points as on the left, but frmx plotted as the function of f,. The solid lines represents the best fit by Equation (4-5).
4.2.1. Experimental results
Isotopic neutron sources
First tests of the method were performed in neutron fields of a 252Cf source^771. The source was exposed free in air or surrounded with 10 cm spherical paraffin or iron filter. Additional measurements were performed in a radiation field composed of the 2$2Cf and 137Cs sources.
The values of <Dj0W and L^, derived as fitted parameters, were used for calculation of H*(10) according to the following relation:
H* (10) = D* (10) • [®low + (1 - ®low) • Q(Lhigh)] (4-7)
where Q(L) is the quality factor dependence on LET recommended in ICRP Report 60. The quality factor of low LET component was taken as equal to 1.
The values of the fitted parameters 0jow and p^gh are shown in Table 4-4, together with the calculated values of quality factor. The values of <D[ow are compared with the values of the gamma component obtained for the same fields using twin-chamber technique (TE and hydrogen-free high pressure ionisation chambers).
Table 4-4. Fitted parameters ®|ow and pWgh resulting from equation (4-5) and quality factor values for neutrons alone, Qn and for mixed fields, Q^x, of 252Cf source with different filters.
Radiation source + Photon component Fitted parameters Quality factors (ICRP 60)
filter (reference value) hhigh Qn Qmix
252cf 0.36 ±0.02 0.38 16 15.7 10.1
252Cf + Fe 0.08 ±0.01 0.065 176 17.5 16.4
252Cf + 137Cs + Fe 0.56 ± 0.04 0.55 17.6 17.5 8.4
252Cf + paraffin 0.72 ±0.1 0.74 19.1 19.2 5.7
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Monocnergetic fast neutrons
The relative response of REM-2 recombination chamber to H*(10) with ICRP 60 quality factor was determined using the data from the same experiment at PTB, which has been already described in Subsection 4.1.1. The values of fitted parameters <Z}ow and ghigh are given in Table 4-5.
Table 4-5. Values of parameters <Z}0W and for monocnergetic neutrons resulting from fitting Equation (4-5) to experimental data
Neutron energy
(MeV)l*high
^high hhigh"*-'0 (keV pm-1)
0.075 0.56 ± 0.02 46 ± 17 161
0.144 0.48 ±0.01 47 ±5 164
0.5 0.225 ±0.01 47.7 ±3.5 167
0.9 0.036 ±0.01 30.1 ±1 105
1.2 0.03 ± 0.02 24.9 ± 1.3 87
2.5 0.01 ±0.01 14.5 ±0.9 51
4.2 0.05 ± 0.04 11.6 ±0.9 41
5 0.03 ± 0.03 10.4 ±0.8 36
13.9 0.25 ±0.03 11.4 ±0.9 40
14.8 0.28 ±0.03 13.2 ± 1 46
18 0.36 ±0.02 16.6 ±0.8 58
19 0.34 ± 0.02 15.5 ±0.7 54
The ambient dose equivalent response of the REM-2 chamber is shown in Figure 4-3 in the similar manner, as it was done in Figure 4-1. The values of ambient dose equivalent were calculated using Equation (4-7), exept of neutrons with energies of 75 keV, 144 keV and 0.5 MeV. These neutron beams were considerably contaminated with photons and we used a different approach, in order to calculate H*(10) for neutrons alone. The fitted parameter <2}ow was attributed to the photon component and ambient dose equivalent for neutrons was calculated simply as Hn(10) = D (10)-(l-<D,ow)-Q(Lhigh). We want to underline here, that such approach was needed for comparison purposes only (the reference values were given for neutrons alone). In normal situation, when H*(10) is determined for radiation protection purposes, the value of total quality factor for mixed radiation field is needed and Equation (4-7) should be used for all radiation fields. It is not necessary to distinguish whether the low LET component is due to accompanying photons or due to secondary particles generated by neutrons.
Two sets of fluence to H*(10) conversion factors were used for calculation of the reference values. The first one was calculated by Schuhmacher and Siebert*68) in 1992. The second, more recent set of conversion factors, calculated by Siebert and Schuhmacher*78) was based on the new values of stopping powers and ranges for protons and alpha particles, given in ICRU Report 49<79). Points and the solid line in the Figure 4-3 represent the response of the REM-2 chamber relative to the reference values calculated using the older set of conversion factors. The dashed line was obtained using the second set of conversion factors.
48
The observed energy dependence of the chamber response is not \ ery different from those obtained using ICRP-21 quality factors. The minimum value is equal now to 0.85, and the maximum one equals to 1.55. As might be also seen from the Figure 4-3, the difference due to the old and new conversion factors (solid and dashed lines) is pronounced for low-energy neutrons only and is smaller than experimental uncertainty of our points.
Aaupt-j ii Am Aul ,
Neutron energy, MeV
Figure 4-3. The relative response of REM-2 recombination chamber to H*(10) with ICRP-69 quality factor, i.e. the ratio of H*(10) determined by the chamber to the reference values determined from the fluence measurements using two different sets of conversion factors (see text).
4.3. PRACTICAL USEFULNESS OF THE METHOD
The main advantages of using the recombination chamber in monitoring of the ambient dose equivalent in mixed radiation fields are:— similar sensitivity to H*(10) for photon and neutron radiation,— weak dependence of the relative response to H*(10) on photon and neutron energy,— practically, no limitation on LET and energy range of detected radiation.
We investigated the relative response of the REM-2 chamber to H*(10) with both ICRP-21 and ICRP-60 quality factors. The observed energy dependence of the chamber response to H*(10) is relatively flat as compared with most of instrumentation used in radiation protection.
For monoenergetic neutrons we observed an overestimation of the H* (10) for neutron energies lower than 5 MeV and underestimation for fast neutrons of energies between 13 and 19 MeV, with respect to calibration with 241Am-Be source. The ratio between maximum and minimum values of the response curve is within factor 1.8 in the energy range considered here.
Usually in radiation protection practice, one deals with a broad neutron energy spectrum. It can be estimated that for realistic neutron fields the response of the chamber to H*(10) is constant within about 25% and fulfils well the requirements of radiation protection. This statement is confirmed by the results obtained for isotopic neutron sources as well as for thermal and high-energy neutrons.
The measuring range of the dose rate is, for routine routine operation, from 1 fiGy h"1 to 0.25 Gy h"1, which corresponds to the values of H*(10) from 1 pSv h*1 up to about 1 Sv h'1.
Our studies have also shown that the simplified method of the saturation curve analysis can be successfully used for determination of the main microdosimetric parameters of mixed radiation fields. The fitting procedure used in our method was proved to be stable and weakly influenced by experimental uncertainties.
49
Therefore, a large recombination chamber, like the REM-2, may serve as an useful detector for determination of ambient dose equivalent in mixed radiation fields, especially in stray radiation fields in the vicinity of accelerators and nuclear facilities.
50
5. A NOVEL METHOD TO DETERMINE THE ENERGY DEPOSITION DISTRIBUTIONS
In two our earlier works*46,47) we showed, that some information about the energy loss distributions in mixed radiation fields can be obtained by comparison of saturation curve for a mixed radiation with the curve measured in a reference gamma radiation field. In this Section we will present the state of the art of our method, as well as recent experimental results. We will also try to point out the problems, which are still under investigation.
Our mathematical approach was inspired by the numerical approximation used by Sullivan*30), however, some important changes were introduced*46):1. Theoretical function m(p,X) from Equation (4-1) was substituted by the experimental values derived
from measurements in the reference gamma radiation field. This eliminated the uncertain theoretical and gas parameters involved to the function m(p,X).
2. Ion collection efficiency for all components and energies of low-LET radiation was approximated by ion collection efficiency measured in the reference gamma radiation field.
3. Number of the considered LET intervals was reduced from 20 to 6 or 8.4. A linear least squares fitting method was used, with fitted parameters constrained to be non-negative.
Micredosimetric distributions obtained by means of recombination chamber concern nanometric regionsof tissue, since the length of track, over which the energy loss is averaged is equivalent to about 0.07 microns of unit density tissue*30). Such data may be important in radiobiological studies as nanometers is the scale of the critical biological targets. The distributions will be also very useful in the determination of the quality factor, especially in view of recent recommendations of ICRP.
5.1. METHOD
In Subsection 2.4 we expressed the ion collection efficiency in recombination chamber irradiated in mixed radiation field, fmix> as a function of ion collection efficiency measured for the same chamber but in reference gamma field, f^ (Equation 2-31):
fmix = —f d(LA)la i-fY
dL<1 +
(5-1)
where both f^x and fy have to be determined at the same operation conditions.In order to determine the dose fractions associated with certain intervals of the LET, the integral in
Equation (5-1) was approximated by the sum:n
fmix = (5-2)j=l
where (Dj are the dose fractions associated with the LA interval from L^j to LAj+1 (i.e. quotients of the absorbed dose due to j-th LET interval of the radiation field to the total absorbed dose) and
Sj are the analytical functions of ^ given by Equation (5-3).
sj '■=------- 1------- T-rW^—........f'A.rt-l f"A,j L^. 1 + —A------ L f'A.j+l ^Ajl-fy jl + (l-f.
— f> )(LA,}+I /Lq
-fY)(LA,j.Lo -1)(5-3)
51
Because of the similarity of saturation curves for any low LET radiation we decided to replace the function s; for the first interval of LA by fy. In our opinion such procedure should be more accurate, than integration according to Equation 5-3, as the integration range is not well defined for low LET radiation.
Finally the following equation was fitted to the experimental data.
n
L,, (5-4)j=2
where <Z)jow is interpreted as the low LET component of the absorbed dose.The major steps of our method are illustrated in Figure 5-1. The ion collection efficiencies f^% and fy
were measured at the same voltages for both radiation fields (Figure 5-la). The values of ^rnix were plotted versus fy (Figure 5-lb) what helped us to use standard fitting routines. The dose fractions <Dj were determined by fitting of Equation (5-4) to the plotted data (Figure 5-lc) and usually displayed in the form of histograms (Figure 5-1 d).
L&, keV pm'1
Figure 5-1. Illustration of our method of determination of restricted LET distribution.a) Saturation curves for 137Cs reference gamma radiation and for three radiation fields of monoenergetic
neutrons of different energies measured using REM-2 recombination chamber. Dashed lines are guides for eye.
b) The same experimental points as in (a), but plotted as the function of fy. The solid lines represents the best fit by Equation (5-4).
c) The experimental points for 0.9 MeV neutrons and the fit as in (b). Dashed lines shows the functions S: that contribute essentially to the fitted curve. The functions S: are associated with the following LET intervals: s3 from 50 to lOOkeVgnr5, s4 from 100 to 200 key-pro'1, s6 from 500 to 1000 keV-pm-1. The function £, is shown for comparison. Photon component of the absorbed dose was below 1%.
d) Dose distribution versus restricted LET obtained for 0.9 MeV neutrons.
52
The Equation (5-4) is linear, so the determination of coefficients l)j from the experimental data was relatively easy. Linear least squares fitting method was used with fitted parameters constrained to be nonnegative. The whole range of LET was arbitrarily divided to six - eight intervals (for larger number of intervals our fitting procedure was unstable).
Moreover, the linearity of the Equation (5-4) enables the easy estimation of the saturation current. Practically the saturation curves were normalised by dividing all ionisation currents by the value of the current at the highest applied voltage, i0. The saturation current is given by the product of i0 and the sum of the fitted dose fractions.
As the first interval in Equation (5-4) was assumed to be attributed to low LET radiation, so it seemed reasonable to choose the range of the first interval to be similar to those of gamma radiation of 137Cs used for calibration of the chamber. Our choice was based on calculations of dose distribution against lineal energy, kindly performed for us by P. Olko(79\ The results obtained for the volume of 100 nm effective
diameter are presented in Figure 5-2, as the solid line over the first interval of the histogram Taking into account the shape of the distribution for I37Cs source, we decided to present the low LET range in the interval from 0.3 up to 20 keV/pm. Such a presentation does not influence the shape of liigh LET part of the distribution.
The high LET fractions are graphically displayed on the figures in the form of histograms versus logarithmic x-axis. The area of each histogram bar represents the dose fraction associat ed with the certain interval of LET.
Strictly speaking, the assumption of constant dose distribution within intervals of LA involved in the Integral (5-3) should be depicted in the logarithmic plots by the curves like the solid one in the Figure 5-2 (for LA>30 keV/pm) Such presentation would be however much less informative than the histogram form.
It is possible to make the fits with the assumption of constant distribution of LAd(I.A) within intervals of La, what strictly corresponds to the shape of the histogram in the loga/itlunic scale. The integrals Sj would then have the form:
1 t 1 + (1 ~ fy )(L ̂j+] /Lq - 1) •LajSj ln<LA>>+1)-ln(LAj)
l + (l-fY)(LAj/L0 -1)
Such calculated distributions may differ from those obtained using formula (5-4), however, differences are usually small. We are not able to state a priori which assumption approximates better the real physical conditions. Therefore, in this work we use the distributions based on Equation (5-3), since they are more convenient for calculation of dosimetric quantities averaged over the LET distribution, like the quality factor and ambient dose equivalent.
^0.4
i if..y"WYrtVj*1 10 100 1000
(keV pm'1)
Figure 5-2. Illustration of the approach used in this work for presentation of microdosimetric distributions. The histogram represents the dose distribution versus restricted LET (A*500 eV) obtained for 252Cf radiation. The solid line over the first interval of the histogram is the microdosimetric distribution calculated for 137Cs gamma radiation^79) and normalised to the same area as for the photon component of 252Cf radiation. Solid line over the intervals with LA>30 keV/pm shows the real shape of the distribution in logarithmic scale (see text).
53
5.2 DETERMINATION OF LOW LET DOSE FRACTION
Before presentation of microdosimetric results we would like to digress from the main subject in order to underline an important and up to know not well recognised feature of the recombination chamber and of our microdosimetric approach, namely the ability to separate the dose fractions due to low LET and high LET particles in mixed radiation fields In many cases of neutron-photon radiation fields of limited neutron energy (up to about 10 MeV), the only low LET particles are electrons from photon interactions. Therefore, the recombination chamber can be used both for determination of the total absorbed dose and of its photon component.
A special emphasis was laid in our investigations on testing accuracy of the separation of the low-LET dose fraction. First measurements were performed in reference fields of a 252Cf source!7*). The source was exposed as a bare one or placed in a spherical paraffin or iron filter of 10 cm in diameter. Additional measurements were performed in a radiation field composed of the 252Cf and iron filter and of 137Cs sources. The experimentally obtained results are shown in Figure 5-3, where the solid lines represent the fits according to the Equation (5-2). The most important are now the values of ion collection efficiency at low collecting voltages, where large part of ions from tracks of high LET particles recombine. Ions from tracks of electrons are still collected with efficiency of about 90% and the photon component of the absorbed dose can be determined almost "by eye" from the value of ion collection efficiency.
For all the radiation fields considered here the results obtained using the recombination method (values of the fitted parameter ©|ow) agree very well with the reference values of the photon component (<Dy) measured using a twin chamber technique!59) (see caption for Figure 5-3).
D =74%0.8 -
0.6 -
0.5 - Cf (paraffin filter)
D, =12%0.4 - Cf (bare)
'Cf (iron filter)
Ion collection efficiency for Cs, f
Figure 5-3.147) Plots of ion collection efficiency for mixed radiation against those for reference gamma radiation (points - experiment, solid lines - the fits; see text). The percentages of the photon component of the dose resulting from the fits and the reference values (in brackets) are equal to: 74% (70±9) for 252Cf with paraffin filter; 55% (56±4) for the composed field - 2$2Cf with iron filter + ,37Cs; 38% (35±2) for bare 2$2Cf source and 12% (1(»1) for 252Cf with iron filter.
Our later measurements performed in stray radiation fields!47) and in fields of monoenergetic neutrons (see next Subsection) confirmed well the practical usefulness of recombination chamber for separation of photon (or more general low LET) dose fraction in mixed radiation fields.
54
5.3. MEASUREMENTS OF ENERGY LOSS DISTRIBUTIONS
Following our theoretical approach, we present here the energy loss distributions determined by recombination method. The data from several series of our experiments performed in different mixed radiation fields were used for determination of such distributions. The results are presented here in the form of differential dose distributions versus restricted LET per logarithmic interval, LAd(LA).
The usual way to investigate the microscopic distribution of energy loss in irradiated matter is determination of the dose distributions versus lineal energy, yd(y), by the pulse height analysis of signals from tissue-equivalent proportional counters (TEPCs) The counters usually simulate small spherical volumes - most often of 1 pm in diameter. There are a few reports only on the application of TEPCs for measurements at nanometer levels and up to now the results are inconclusive^80'82). The distributions obtained with the recombination chambers refer to smaller volumes with effective diameter of about 70 nm. This causes that our results are interesting for nanodosimetry and some radiobiological investigations. By the other hand they cannot be directly compared with the dose distributions versus lineal energy obtained with TEPCs. Nevertheless, in few figures of this Section, our results are shown together with yd(y) distributions. Such presentation is given for qualitative comparison and discussion.
Radiation fields of isotopic neutron sources
Measurements were mostly performed using the REM-2 chamber in calibration laboratory at 1AE Swierk in reference radiation fields of 137Cs, 252Cf, 239Pu-Be and 241Am-Be sources. Measurements for 238Pu-Be neutron source were performed in calibration room of Radiation Protection Group at CERN
The californium source has been exposed either free-in-air, or surrounded with filters, to obtain modified fields of mixed neutron + gamma radiation, with different neutron spectra and gamma-to-total dose ratios. Nearly spherical paraffin and iron filters with 10 cm wall thickness have been used. Dose distributions versus restricted LET (with A»500 eV), obtained for this radiation fields are shown in Figure 5-4.
The iron filter shifts the fast neutron energy spectrum towards lower values and seriously decreases the gamma dose. This is reflected in LAd(L^ distribution (Figure 5-4b) by a strong increase of the energy deposition in LA range between 50 and 100 keV pm"1, for the source in the iron filter comparing with the bare one (Figure 5-4a).
The paraffin filter, absorbing and thermalising neutrons, increases the thermal neutron flux, seriously decreasing the fast neutron flux (with small changes in its energy spectrum shape) and decreases the neutron dose. Also this facts are
reflected in the obtained LA distribution (Figure 5-4c).
La (keV lum"1)
Figure 5-4. Dose distribution versus restricted LET for 252Cf radiation source: (a) the source exposed free-in-air; (b) the source in 10 cm spherical iron filter; qualitatively (c) the source in 10 cm spherical paraffin filter.
55
Table 5-1. Values of low LET dose fractions and of quality factors calculated from the absorbed dose distributions, LAd(LA) measured with the REM-2 recombination chamber.
Neutron source andfilter (10 cm)
qow (%) ®Y (%)(reference valued59)
Qicrtoi QlCRPeo
252Cf bare 38 35 ±2 6.8 11iron 12 10 ± 1 10.1 18
241Am-Be74 70 ±9 4.0 624 24 ±2 6.5 9
239Pu-Be 19 22 ±5 6.5 9.5
The La distribution obtained for 238Pu-Be source is shown in Figure 5-5. The measurements were performed in CERN's old calibration room<43) and are presented here together with the dose distribution versus lineal energy.
The lineal energy spectrum lias been determined by Aroua et al.(8-' using a TEPC of HANDI<83'84) type, exactly in the same position in the radiation field, where we performed measurements with the recombination chamber.
In Table 5-2 we compared the distribution from Figure 5-5 with the distributions determined in radiation field of 239Pu-Be source. All the measurements were performed using the same recombination chamber, but filled with two different gas mixtures. Similarly as in investigation of ambient dose equivalent response, we used the mixture of methane with 5% of nitrogen (MN) at pressure of 1 MPa and the methane based tissue-equivalent (TE) gas at pressure of 0.45 MPa The reference values of the photon component were equal to 19% and 22% for 238Pu-Be and 239Pu-Be sources, respectively.
Table 5-2. Dose distributions versus restricted LET determined in radiation fields of 238Pu-Be and239Pu-Be sources, using the REM-2 recombination chamber with different filling gas mixtures.
I.A interval keVpm"'
Fraction of absorbed dose deposited in the specified interval of L^
238Pu-Be, TE gas 239Pu-Be, TE gas 239Pu-Be, MN gaslow LET 0.17 0.20 0.1920- 50 0.73 0.70 0.6650-100 0.11 0.10 0.15
0.4 -
0.2 -
Figure 5-5. Absorbed dose distributions for 238Pu-Be neutron source: histogram - the distribution versus restricted LET, L^fL^ measured with the REM-2 recombination chamber; solid line: - the distribution versus lineal energy, yd(y) for 1 pm site measured with HANOI TEPC<85X
With both gas mixtures we obtained correct values of the photon component. The high LET part of the distribution seems to be slightly shifted to higher LET values, when the MN mixture is used, however, the shift is within the resolution ability of the fitting procedure. Similarly as for 252Cf and 241Am-Be, we did not observe the ionisation events with LA >100 keV pm'1.
56
In study of our method we repeated some measurements using the in-phamom recombination chamber of FI type filled with TE gas at a pressure of 0.8 MPa (for description of the chamber see Appendix A). The comparison of the results obtained with REM-2 and FI chambers is shown m Table 5-3.
Table 5-3. Fractions of absorbed dose deposited in the specified interval of LA as determined using REM-2 or FI recombination chambers for mixed neutron + gamma radiations of 252Cf and 241Am-Be.
La interval keV-pm*1
Chamber REM-2 Chamber FI
2»Cf 241 Am-Be 252Cf 241Am-Below LET 0.38 0.24 0.37 0.2820- 50 0.22 0.62 0.27 0.650 - 100 0.40 0.14 0.31 0.08
200 - 500 0.05 0.04
Although the REM-2 and FI chambers are drastically different, the obtained distributions are similar. The only considerable difference is that a small contribution (of about 5%) was found for the LA interval from 200 to 500 keV pm'1 when the measurements were performed with the F-l chamber. In our earlier workC46) we suggested, that this difference may be caused by the difference in gas pressure. Up to now this hypothesis has not been confirmed.
The distributions obtained by REM-2 chamber in the fields of 252Cf and 241 Am-Be sources do not differ considerably from yd(y) distributions measured by TEPCs'86\ however a contribution of few per cent from events with y higher than 100 keV-pnv1 was observed for 2,2Cf in y spectrum and is not pronounced in our La distribution. Shift of the maximum in LAd(LA) distribution of paraffin moderated 252Cf source comparing with the bare source is also observed in yd(y) spectrum^86-87) when the bare and D20 moderated sources are compared.
Radiation fields of monoenergetic neutrons
Measurements of saturation curves of REM-2 recombination chamber were performed at the same conditions at Physikalisch Technische Bundesanstalt (PTB) in Braunschweig as it was described in Section 4.1. Twelve monoenergetic neutron beams with energies ranging from 75 keV to 19 MeV have been investigated. The chamber was filled with MN gas mixture. Additionally, the chamber filled with TE gas was used in beam of 13.9 MeV neutrons. For this neutron energy we did not observe anv significant differences caused by change of the gas (see Table 5-4), however there is a small shift to higher LET values when the MN gas is used. The effect is similar to those observed for 239Pu-Be source.
Table 5-4. Fractions of absorbed dose determined for 13.9 Me V neutrons using REM-2 chamber filled with two gas mixtures.
La interval keV-gnr1
Fractions of absorbed dose
TE gas MN gaslow LET 0.19 0.2020- 50 0.76 0.7250 - 100 0.04 0.08
300 - 1000 0.01 0
57
First LAd(LA) distributions obtained by our recombination method in fields of monoenergetic neutrons are shown in Figure 5-6.
r*rv trtvif ■
1.0
0.5
0.0
1.0
0.5
0.0
1.0doilts
>>V o.o
rep-—r—r-rrvrriy—"i—rrmn
75 keV
i
■""< 1.0f”^ 0.5
0.0
1.0
0.5
0.0
1.0
....y i * -=144 keV i
T^TTYryTTp i. *'L i «t i it^uUAI
/ > nmipi 0.9 MeV
n-rmir r i i . rru
n''A-'A/A,
uu*uL-™«™ujuts«S*l~t~jL,ajj.uYrttiy,...... v.-f.rrif'—-vrivvTT^'—-rrmiii
2.5 MeV
0 5 rjw 0 •—"• -j
10 100
"i rmui| ■" i . » . vivij' * wit
: 4.5 MeV I
M
. j.rr7iiit^ '” « 1 J ii.i nib[ ’ 1 *r^~rTfw,
I 5.0 MeV !
,« d • uu»" v y "i k imi|....... i ■ 1 ■vt'iiii
i 13.9 MeV :
rmra’1' fT - -—-v14.8 MeV I
Tsviiij ...................... > i iim18 MeV
TTrmj— nnv«|-— v r > rii’i.)—i—mini
: 19 MeV !
L and y, keV jam
Figure 5-6. Dose distributions versus restricted LET for monoenergetic neutrons determined by REM-2 recombination chamber filled with MN (methane-nitrogen) gas mixture (light grey histograms) and with TE gas (grey histograms). Solid lines - >d(y) spectra for 200 nm diameter sites located at 1 cm depth in the ICRU sphere calculated for 15 MeV and 20 MeV neutrons by Schuhmacher(89X
58
Figure 5-6 assembles all the L^d(L^) distributions determined by recombination chamber in fields of monoenergetic neutrons. Before discussion of the results we want to underline some serious doubts concerning the distributions obtained for neutrons of the lowest energies (75 keV- 0.5 McV) The main goal of the investigations performed in these radiation fields was determination of the ambient dose equivalent response of the chamber. At the time of measurements we were not able to achieve the experimental conditions ensuring precise determination of energy loss distributions. The measurements were performed at relatively low dose rate, when the dark current of the chamber constituted about 5% of the measured signal for 0.5 MeV and 144 keV neutrons and up to 15% for 75 kcV neutrons. Moreover, fluctuations of the dark current were rather large. The dark current was subtracted from the ionisation current with accuracy sufficient for determination of H*(10), but the fitting procedure used for microdosimetric analysis requires much higher accuracy The subtracting procedure could greatly influence the resulting shape of the energy loss distributions. We had also too few points of the saturation curves as the ionisation current values were measured at 8 to 10 collecting voltages only, in this situation it is not unlikely that the obtained distributions might be significantly disturbed.
On the other hand, for all the three neutron fields of the lowest energies the obtained L^d(L^) distributions are systematically shifted towards higher LA values, comparing with published yd(y) distributions. Up to now we are not able to explain such result but we didn't want to remove the results from presentation as they may indicate a limitation of our method.
For the neutrons of energies from 0.5 to 5 MeV we observe a gradual shift of the high LET peak towards lower La values with increasing neutron energies what obviously reflects a decrease in LA of recoil protons.
Starting from the neutron energy of 13.9 MeV the spectra exhibit the significant low LET component which is caused by the fast recoil protons. The dose contributions from a-particles and heavy recoils results in some shift of the high LET peak towards higher LA values. A dose contribution from ionisation events with La>100 keVgnrr1 is observed for 14.8 MeV neutrons only.
The distributions for 14.8 and 19 MeV neutrons are presented in Figure 5-6 together with the yd(y) spectra calculated by Schuhmacheri89'. As our measurements for neutrons with energies above 0.5 MeV were performed in the neutron beams with very low photon component, therefore for these energies the low LET intervals in Figure 5-6 have been depicted as ranging from 2 keVgnr1 to 10 keVpm"1 for better qualitative comparison with the calculated yd(y) distributions. The change of the lower limit of the low LET interval is only formal, as the fitting procedure results in the dose component 'Z>low attributed to "low LET radiation" and there is no information on the lower end of the LA range. The choice of the upper end of the low LET interval is also arbitrary up to 20 keVpm1. Usually, the change of this limit to a L, value results in unchanged value of <D|0W and zero value of the dose component attributed to the LA interval between Lt and 20 keVgnr1. The only exception are neutrons of 14.8 MeV. In this case we can clearly observe that the peak of the L^dfL^ distribution is around 20 keV pm '.
High energy stray radiation fields
Measurements were performed at CERN, in stray radiation fields outside the shielding of the H6 beam of the Super Proton Synchrotron, as a part of the experiment described with more details in our earlier report^90) and in Appendix B of this work.
The beam of 120 GeV/c positively charged particles (protons and pious) was directed onto a cylindrical copper target, placed either below a 40 cm thick iron roof or under a 80 cm thick concrete roof. For each target location the marked measurement positions were available on the roof top. The centre of the chamber was placed at a height of 25 cm over the base plane of the shielding.
59
The neutron energy spectrum^91) for the concrete shield had two main peaks - one in the 100 keV to 1 MeV region, and the second around 100 MeV. The spectrum for the iron shield was much softer and the peak between 100 keV and 1 MeV was much higher in intensity, relative to the peak close to 100 MeV (see Figure B-5 in Appendix B)
It was found by the microdosimetric analysis of the saturation curved90,47), that the low-LET componentpredominates absorbed dose, for both radiation fields investigated and equals 90% for the concrete roof and 73% for the iron one. The above values are in good agreement with the results of measurements, performed using the TEPCs of the HANOI type^92). Microdosimetric distributions determined for the two shielding roofs show significant differences (Figure 5-7). The main peaks of the high-LET part of these distributions are around 50 keV/pm and 100 keV/pm for concrete and iron roofs respectively. This difference influences significantly the ICRP60 quality factor value, which is more than 3 times higher for the iron roof (Q=6.5), than for the concrete one (Q=2). The contribution of low LET radiation to the ambient dose equivalent is about half of the total value for the concrete shield, compared with 11% for the iron shield.
Figure 5-7. Dose distribution versus restricted LET (filled histograms) and versus lineal energy (solid line)(93'92) for stray radiation fields at CERN: (a) concrete shield, (b) iron shield.
High-energy proton and neutron medical beams
A small recombination chamber of FI type was used for determination of microdosimetric distributions of energy deposition in therapy beams of the 660 MeV phasotron in the Joint Institute for Nuclear Research
0.3
Figure 5-8. Dose distribution versus restricted LET for p(660 MeV) + Be neutrons (a) free-in-air,(b) in phantom.
in Dubna (Russia). The medical facility in Dubna has five cabins for cancer therapy with beams of high-energy protons, pions and neutrons^94). Our measurements were performed for the beam of p(660 MeV) + Be neutrons, with mean energy of about 350 MeV and for 200 MeV proton beam
Two series of measurements were performed for the neutron beam. The first - free-in-air (the wall thickness of the chamber equals to 0.6 g/cm2) and the second at depth of 10.5 cm in a polymelhylmethacrylite (PMMA) phantom, i.e. at the depth of broad maximum of the absorbed dose. Considerable differences in microdosimetric distributions were observed (see Figure 5-8).
As can be seen from Figure 5-8 the low LET contribution in the phantom was higher than free-in-air and the high LET part of the
60
distribution was shifted towards lower LET values.Conditions in the phantom were much closer to charged particle equilibrium, so a liigher contribution
from high-energy protons could be expected, comparing with free-in-air conditions. Moreover, the distributions measured free in air were stronger influenced by neutrons of much lower energies than the mean value. For such neutrons the maximum of the absorbed dose lays at the depth close to the wall thickness of the chamber. Both factors increase the peak around ICO keV/gtn in the d(L^) distribution for the free in air position.
The microdosimetric distributions obtained for protons with incident energy of 200 MeV are presented in Figure 5-9
As expected, only a small contribution from high LET radiation has been found for the proton beam, for which 97% of the absorbed dose is deposited with low-LET.
A further feature of the LAd(LA) distribution is the existence of events having a liigher LET than the maximum for protons. This is due to production of higher LET secondary charged particles by nuclear inelastic scattering.
5.4. DISCUSSION AND CONCLUSIONS CONCERNING THE MICRODOSIMETRIC APPROACH
In previous Subsection we presented the experimentally obtained distributions of energy deposition. Since the saturation curves of recombination chamber are smooth, so our procedure has rather low resolution. It gives the value of the low LET dose component, indicates the position of the high LET peak and provides some information on the width of the distribution. In almost all the figures above we kept the same limits of the LA intervals adopted for fitting procedure, in order to facilitate comparison of the results. In fact, the position of the peak can be sometimes better detected by varying the interval limits.
The characteristic feature of our distributions is that for monoenergetic neutrons with energies 2.5 MeV and above we do not observe any contribution due to ionisation events with LA> 100 keVpm'1. The only exception are neutrons of 14.8 MeV, where we can see a small contribution due to events with LA above 300 keVpm"1. By the other hand small contributions due to such events were rather well detected in fields of high energy neutrons and protons. Therefore, the edge of the distributions at 100 keV-pm'1 should not be simply explained by low resolution of the method only.
A possible explanation may be the difference between restricted and unrestricted LET of charged particles. The ionisation events with lineal energy y between 100 and 300 kcV-pnr1 in 1 pm site are attributed mainly to a particles from nonelastic reactions of neutrons with carbon and oxygen. Restricted LET (A=100 eV) value^95"97) of majority of the a particles is below 100 keV pm'1.
Another point concerns comparison of the LAd(LA) distributions obtained by the recombination chamber with the yd(y) distributions. As mentioned above, we have no data, on the yd(yj distributions in 70 nm sites. The direct comparison with other, usually much larger, site sizes is not possible, so only some qualitative conclusions can be made. Moreover, there is a question about the scaling factor. We would like to remind here that the basic quantity for recombination method is the local ion density p and the LAd(LA) distributions are obtained by applying the scaling factor L0. For presentation of the energy deposition distributions we adopted L0=3.5 keV-pm'1. This value was close to the restricted dose mean LET. When we
ss.&jM—f. * A.4
L , keV pm
Figure 5-9. Dose distribution versus restricted LET for 200 MeV protons.
61
want to express p as proportional to lineal energy (p=y/y0) we have to remember that the site with diameter of about 70 nm should be considered. Dose mean lineal energy for such site size, determined for ,37Cs by variance-covariance method(98) is of about 4. Therefore, the scaling factor y0 should be probably higher thanL0.
It is generally known, that both the lineal energy in 1 pm sites and unrestricted LET are not suitable to describe the track structure at nanometre level. More adequate description of the meaningful track parameters can be achieved in terms of restricted LETf") and mean free path ionisation^96). Our parameter p (local ion density) can be directly related to these two quantities.
As already mentioned, our method is seriously limited by its low resolution, much worse than can be obtained by proportional counters. Nevertheless, one should remember that high resolution is not always necessary for radiobiological investigations. By the other hand the method can provide an experimental information on energy deposition in volumes much smaller than 1 pm. Up to now, these distributions can only be assessed with approximated computational methods, however, recent research on the gas detector physics^100,101) and advances in technology suggest possibility to develop in future microdosimetric instruments with nanometre resolution.
Returning to the recombination methods and to the practical point of view we can state that the information on radiation quality, derived from the energy deposition distributions is somewhat more accurate comparing with the simplified method described in Section 4. Therefore, the distributions can be successfully used for radiation protection purposes, however the method of their determination is not suitable yet for routine practice.
62
6. CONCLUSIONS
Recombination methods were invented almost contemporaneously with the 1CRU/TCRP concept of radiation quality factor, expressed in terms of LET. Since then recombination chambers and related methodology have been considerably developed, following development and new concepts of dosimetry, radiation protection, microdosimetry and nanodosimetry. In this work we briefly presented this evolution of recombination methods from the early experimental findings to advanced microdosimetric approach. In our opinion, however, the possibilities of the methods based on the phenomenon of initial recombination of ions are still far from exhaustion. We see the future of these methods in the fact that most of the initial changes in irradiated biological systems are caused by interactions between oppositely charged ions from the same track. Hence, initial recombination, by its very nature, can probably reflect the radiation quality - sometimes better than phenomena that occur in tissue-equivalent proportional counters, where the polarity of ions plays no role.
Methods presented in this work are mainly based on a simple theoretical model with few important assumptions derived from experimental observations. In fact, it is in this woik that the model and its assumptions have been clearly formulated for the first time. With some theoretical considerations and experiments described in Section 2 we gained a better understanding of the physical meaning of the parameters of the model and we were able to estimate the range of validity of the assumptions. Throughout all the work we presented several arguments to confirm the consistency of our approach and agreement with the experimental data. Nevertheless, we still have to deal with a simplified model rather than a good theory of initial recombination.
Sections 3 and 4 of this work are of more practical nature, it was shown that the recombination chambers can be used for direct determination of dose equivalent quantities in mixed radiation fields of practically unlimited composition and energy range. The relative simplicity and reliability of measurements are the additional practical advantages of the recombination methods.
Two important goals were achieved in last years. It was shown that a properly designed recombination chamber fulfils satisfactorily the requirements of a detector for an area monitor with dose equivalent response nearly independent on neutron energy. A new method was developed for determination of quality factor according to the recent recommendations of 1CRP.
The recombination chamber is used as a single instrument and this is an additional advantage of the chamber, compared with many-detector systems. In several cases many-detector systems show a somewhat higher value of ambient dose equivalent in comparison with the recombination chamber. This is easily to explain considering that detectors designed to be sensitive to one kind of radiation only, are also partly sensitive to other kinds of radiation. Therefore, the total ambient dose equivalent derived from readings of many-detector systems usually overestimates the true value of H*(10).
Recombination chambers can be considered as an alternative to TEPC-instnunents or remmeters. There is a large variety of such instruments and comparison with different types may lead to somewhat different conclusions. For this reason we did not include any such comparison here. Nevertheless, in our opinion the ambient dose equivalent response of the recombination chamber is generally less dependent on neutron energy than that of most known TEPC systems^02'105) and of commercial neutron dose rate meters^106). We expect that for most of the realistic radiation fields with a broad neutron energy spectrum, our instrument should give similar response as the best TEPC-systems.
The novel recombination method to determine the energy deposition distributions, presented in Section 5, is still under investigation. Our approach was based on comparison of saturation curve determined in a mixed radiation field with those measured in a reference gamma radiation field. We also
63
found a fitting procedure providing relatively stable results. Despite rather low resolution of the method, the obtained energy deposition distributions qualitatively reflect the main properties of the track structure expected for sites with diameter of about 70 nm in tissue .
This work and scientific interest of the author are pointed mainly at the physical aspects of recombination methods. Nevertheless, it is always a great satisfaction when the developed concepts and corresponding experimental techniques can be applied in practice. So, to conclude this work we would like to mention that prototypes of two new instruments based on recombination methods have been recently designed and are now under investigation in the Institute of Atomic Energy. The first one is a self-contained measuring system for in-flight and low-level dosimetry^107) and the second is a computer-controlled ambient dose equivalent meter for routine use in radiation protection^108).
The author hopes that the overview of the recombintion methods given here will contribute also to further development of these methods.
64
APPENDIX ARECOMBINATION CHAMBERS - CONSTRUCTION AND EXPERIMENTAL SET UP
Recombination chambers are high-pressure tissue-equivalent ionisation chambers, designed and operated in such a way that ion collection efficiency is governed by initial recombination of ions. Usually they are parallel-plate ionisation chambers filled with a tissue-equivalent (TE) gas mixture under a pressure 0.5 - 1 MPa. The spacing between TE electrodes is of the order of millimetres At larger spacing the volume recombination seriously limits the maximum dose rate at which the chamber car, be properly operated. Multielectrode chambers with electrodes connected in parallel are used in order to increase both the chamber sensitivity and the mass of the electrodes, when the chamber is intended to simulate the ICRU sphere.
The recombination chambers were first designed in 1962*20'22) with the aim of building a detector having a LET dependent response, that can simulate the dependence of radiation quality factor on LET. Since that time both the detector design and methods of measurements were considerably developed. Several types of recombination chambers have been designed for measurements in stray radiation fields*21,109), for particle beam investigations*110), for in-phantom measurements*110), for personnel dosimetry*111) and for direct determination of quality factor*112). Twinned chamber system"-21) and a differential system*21,109) are particularly suitable if dose rate is not stable.
The problem of design of recombination chambers and dosimetric devices based on recombination methods, although very important, is outside the scope of this work Therefore, in the following we will describe briefly only those chambers, that were used in the experiments described in tliis work. More information can be found in the referenced papers and in our recent overview*113).
A.1. RECOMBINATION CHAMBERS
REM-2 recombination chamber
The cross section of the recombination chamber of REM-2 type is shown in Figure A.l.-l. The chamber contains 25 parallel-plate tissue-equivalent electrodes. The central rod connects 12 collecting electrodes. The polarising electrodes are connected alternately to one of two side rods and thus form two sets of electrodes. These sets can be connected either to the same collecting voltage (summation mode) or to different voltages of opposite polarity (differential mode).
The electrodes are 12 cm in diameter and 3 mm thick. The distance between electrodes is 7 mm. Total mass is equal to 6.5 kg. The effective wall thickness of the chamber is equivalent to about 1.8 cm of tissue and the gas cavity volume is of about 1800 cm-. The chamber is enclosed in 1.2 mm thick duraluminium envelope and covered with the heat-insulating layer made with foamed polystyrene. BNC connectors are used as voltage outlets and a special outlet was constructed for connection of the electrometer cable.
The chamber is usually filled, up to about 1 MPa, with a gas mixture consisting of methane and 5% (by weight) of nitrogen. At such pressure the recombination voltage U4 is close to 50 V.
65
The dose calibration factor N, defined as
N =,D (io);
has the value of about 400 pC/Gy, when *-^Cs source is used for calibration
(A-1)
In-phantom chamber type F I
The in-phantom F-l chamber (for historical reasons called sometimes KR-13 chamber*110)) has three parallel-plate tissue-equivalent electrodes, 34 mm in diameter. The distance between electrodes is equal to 1.75 mm (see Figure A-2). Total diameter of the chamber equals to 62 mm. Sensitivity of the chamber is of about 350 nC/Gy and operational dose rate range is from 10-$ up to 100 Gy/min.
Figure A-2. Cross section of the in-phantom chamber of FI type
Pen-like chambers of T5 and G5 type
The chambers of T5 and G5 type form a pair of penlike ionisation chambers (see Figure A-3). Both chambers are 115 mm long and 18 mm in diameter.
The T5 chamber was designed for determination of dose components at high dose rates (up to 500 Gy/h). The electrodes of the chamber are made with tissue-equivalent material, and the chamber is filled with TE gas up to the pressure of 0.2 - 0.8 MPa.
The G5 chamber is a graphite ionisation chamber filled with C02 up to 3 MPa.
The distance between electrodes is of 2.3 and 2 mm for T5 and G5 chambers respectively. Both chambers are enclosed in 0.3 mm thick aluminium envelope (see Figure A-3).
Figure A-3. Cross section of penlike recombination chamber of T5 type.
A.2. EXPERIMENTAL SET-UP AND MEASURING PROCEDURE
It was mentioned above that the REM-2 chamber contains two sets of polarising electrodes (see Figure A-l) and can be used either in summation or in differential mode.
Differential mode of measurements
The differential mode(2,'109) was used to measure directly the dose equivalent (rate). In this mode the near-saturation voltage Us (usually Us=1200 V) is applied to one set of polarising electrodes, while the recombination voltage UR of opposite polarity is applied to the other set of electrodes (see Fig. A-4).
66
The differential current measured by an electrometer, is equal to /'S-/'R, so the measuring system might be calibrated directly in units of dose equivalent rate:
HsD Q. ~z Q, =/s1 *R
0.04
lS
0.04(A-2)
The differential mode is particularly suitable, if dose rate is not stable. However, the sensitivity of the chamber to the absorbed dose is rather low, as the measured current zs-zR is much smaller than /s (e g. zs-zR = 0.04 Zg when Q4=l).
Figure A-4. Principle circuit of dose equivalent meter based on differential recombination chamber.
Summation mode of operation
The basic mode of REM-2 chamber operation in the experiments described in this work was the summation mode. Here, both sets of electrodes are connected to the same polarising voltage and different values of the voltage are applied to the whole chamber consecutively. In case of unstable beam conditions, this requires the use of a monitoring sy stem, but the chamber sensitivity to the absorbed dose is much higher than for differential mode. Moreover, it is possible to determine both the dose rate and quality factor separately and even when measuring the whole saturation curve to obtain the microdosimetric information as described in Section 5.
Electronic circuit
Usually, recombination methods involve measuring of ionisation current as a function of the collecting voltage. A schematic diagram of the automated measurement system is shown in Figure A-5.
Printeroutput
REM-2
CTRL
Figure A-5. Measurement system with REM-2 recombination chamber.
67
Our system^114> consists of the ionisation chamber, PC micro-computer (laptop), the analogue controlled high-voltage supply (ZSWN-2, designed and manufactured in 1AE), electrometer (Keithley 642) and digital voltmeter (Solartron 7071). The electrometer and voltmeter are controlled through IEEE-488 bus, and the HV-supply through the digital to analogue converter (DAC) card. During the setting time of the collecting voltage the input of the electrometer have to be shortened, otherwise the input overload may occur. A special computer controlled electronic circuit has been built into the electrometer for this purpose.
In dependence on conditions of an experiment we can measure either ionisation current or electrical charge collected during certain time on externally connected capacitor.
Software
Special software for measurement control and data acquisition has been created by the author of this work. The early version of the software has been described in IAE Report*114). Several improvements have been introduced to the software since that time, however the general algorithm of measuring cycle remains the same.
The program sets a high voltage to be supplied to the chamber, according to the sequence given by the user. A so-called reference sequence can be chosen optionally.
After the change of collecting voltage the program starts the measurements with some delay, needed for stabilisation of ionisation current.
The appropriate number of readings of the ionisation current value is recorded at given voltage in order to reduce the statistical error.
The number of readings at one voltage, the time period between the readings and the time for stabilisation of ionisation current are specified by the user.
After the start of the program the whole measurement cycle (from the beginning to the end of the voltage sequence) is performed automatically mid may be repeated a specified number of times.
All the data are recorded consecutively to the disk, so in case of any failure only the last few data will be lost. The program can be interrupted from the keyboard at any desired moment. During the measurements the results are displayed and optionally printed.
Accuracy of the results.
The accuracy of the measurements performed with recombination chambers should be determined mainly on the basis of the reproducibility of the experimentally determined saturation curves. In Table A -1 we present an example of a set of parameters of measurements and accuracy of the results obtained for the measurements carried out in radiation fields of isotopic sources l37Cs and 252Cf.
The saturation curves were determined for the collecting voltage sequences consisting of 54 values in the range from ±5 V up to ± 1300 V. Ionisation current for the two sources differed by two orders of magnitude.
Increasing the number of readings of ionisation current at certain collecting voltage we can obtain the value of the current with very low standard deviation. For our REM-2 chamber and 50 readings the standard deviation of single measured value is usually of order of 0.02%. However, we observed in our experiments, that the reproducibility of the ion collection characteristic is often worse, then expected from the experimentally obtained values of standard deviation. This is caused mainly by not well investigated effects of charge memory in the chamber Therefore, we usually report the accuracy of ionisation current measurements of about 0.2%.
68
Table A.2.-1 Example of a set of parameters of measurements and accuracy of the results obtained.
General information
Radiation source
Distance between chamber and source
Range of the electrometer
137Cs
1 mIQ'* A
252Cf
0.5 m
lO"10 A
Control parameters for measurementsNumber of readings at one voltage 50 50
Time period between readings 1 s 5 s
Time for stabilisation of ionisation current 60s 100 s
Number of repetitions of measurements of saturation curves 6 6
Accuracy of the measurementsStandard deviation for one measured value (50 readings) <0 02% <0.05%
Short time reproducibility of the saturation curve (i.e. difference between values obtained at the same collecting voltage but in <0.2% <0.25%repeated cycles of measurements)Long time reproducibility of the saturation curve (i.e. difference between values obtained in two series of measurements <0.2% <0.25%performed with time interval of 3 months)
Reproducibility of our results is illustrated in Figure A-6, where we present the saturation curves determined in three series of measurements performed with time intervals of several months
1.00
0.98
5S 0.94
•I 0.92
5g 0.90 g£ 0.88
0.8610 100
*****
September 1993 March 1995 August 1995
1000Collecting voltage, V
Figure A-6. Saturation curves determined by REM-2 recombination chamber in three series of measurements performed with long time intervals, as indicated in the Figure.
69
Some remarks on measuring procedure
Advanced recombination methods requires high accuracy of measurements. Therefore the specialattention should be paid to some side effects, which can disturb the measurements. Few practical advice arebriefly presented below<48).
1) Dark current of the chamber should be determined at few collecting voltages, especially at the highest voltage applied.Dark current exists also without irradiation. It contains such components as a leakage through insulators, a gaseous discharge at high electrical field strength, an own input current of electrometer, a charge displacement due to stress in cables and insulators etc. Very important components of the dark current are the components caused by a drift and variation of the voltage supplying the chamber and also by temperature dependence of interelectrode capacitance. These components are proportional to the voltage applied (U), to the capacity between the chamber electrodes (C) and to the rate of their change. In practice dark current limits the operational range of the chamber for low dose rates. In normally working REM-2 chamber the dark current is of order 10*14 A.
2) The measurements should be performed at both positive and negative polarities of collecting voltages and obtained values of ionisation current should be averaged over polarities. This procedure can reduce an influence of electrical charges collected from side volumes of the chamber, connectors, cables and preamplifier of the electrometer if placed in radiation field. It also reduces the charge memory effect related to the electrical charge remaining on non conducting elements in the chamber long time after irradiation and changing the effective value of the electrical field strengths between the electrodes.It is recommended to use the same procedure during the calibration in the reference gamma field.
3) There is a charge memory effect caused by electrical charge collected on insulators and on nonconducting parts of the TE-electrode surface. The fast-memory effect (representative time less than 10 min) and the slow one (representative time about 3 days) are distinguished. Practically the charge memory effect limits the lowest value of the collecting voltage. In REM-2 chamber we usually use voltage of above 5 V.
4) Ionisation current does not stabilise immediately after alteration of the voltage applied to the chamber. The time needed for stabilisation has to be determined for given chamber and electronic circuit. Sometimes, the stabilisation time longer then usual is required, if measurements are performed at low dose rate and full stabilisation of dark current is needed.
5) High stability of voltage supply is required. Very important component of the dark current is caused by a drift and variation of the voltage supplying the chamber. This component is proportional to the chamber capacity and to the rate of change of collecting voltage, dU/dt.
6) Lack of saturation at highest voltage applied. The problem of determination of saturation current in high pressure ionisation chambers is not fully solved yet. More details on this subject can be found in Subsection A 3, of this Appendix.
7) Volume recombination of ions. It is important at high dose rates, in particular for pulsed radiations, if the spacing between electrodes is not small enough. The measurements should be performed at two or more dose rates, if available. This facilitates estimation of a systematic error caused by the volume recombination and makes possible to introduce a compensating corrections, if necessary (see Subsection A.4. of this Appendix).
70
8) Ion collection efficiency is dependent on temperature of the filling gas. In precise measurements the temperature of the chamber should be measured and corrections should be introduced when the temperature during the measurements in investigated field differs considerably from the temperature, which had been recorded during calibration of the chamber.
9) Some attention should be paid to possible external interference transferred by main, by magnetic, electromagnetic and acoustic fields as well as by floor vibration etc.
10) In case when dose rate changes in time the use of monitor is required. Usually the best monitor is another TE ionisation chamber, similar to the recombination one.
A.3. DETERMINATION OF SATURATION CURRENT
As the collecting potential of an ionisation chamber in a radiation field increase, the current increases until it approaches the saturation current for the given radiation intensity. The saturation current is reached when all ions formed in the chamber are collected at the electrodes. Such saturation is usually not achievable for high-pressure ionisation chambers, as the maximum voltage is limited by electron multiplication and sometimes by increasing dark current Tlieiefore, saturation current has to be determined by an extrapolation procedure.
The most common approach to determination of saturation current in case of initial recombination is the use of the following approximating relation:
1 _ a 1
where isis the saturation current, /'(U) is ionisation current at voltage U and a is a constant.
(A-4)
To obtain the saturation current, the l/i'(U) function should be plotted against 1/U. Linear extrapolation to the zero value of 1/U gives the reciprocal of the saturation current value.
The relation (A-4) is valid only in the voltage range close to saturation and can be derived both from Jaffd and Lea theories of initial recombination^115’30). Basic equations of these theories, given in Section 2.1 can be rewritten in the following form:
Jaffd theory: l.LV"/ /s 8ttD
(A-5)
1 1 aLea theory: - = —+------ t-—P(y) (A-6)
/ /s 4(2ti) bD
buXwhere y = -p— (A-7)
V2D
To obtain the saturation current, the 1 li function should be plotted against ey 2KQ(y2 12) or against
P(y) function. However, the information required to calculate these functions is not available for all secondaries produced by neutrons and not available for TE gas. For high voltages, however, both of the functions asymptotically behave as -Jn /X (see Figure A-7). Therefore the equation (A-4) is commonly applied to derive the saturation current.
71
and P(y) functions
The range of validity of the approximation depends on kind and on pressure of the filling gas in the chamber, as well as on kind and energy of radiation. Therefore, no general rules have been formulated concerning the minimum voltage value and special attention should be paid to the proper choice of the extrapolation range.
It is relatively easy to achieve the region close to saturation for gamma radiation and small corrections for lack of saturation can be successfully derived. An example of such extrapolation is shown in Figure A-8, for the saturation
curve of REM-2 chamber irradiated with 137Cs radiation source. For this chamber lack of saturation at voltage of 1000 V equals to 0.4%.
Determination of saturation current is much more difficult when the chamber is irradiated with neutrons. Approximated relation (A-4) is valid for "mono LET" radiation, only. Neutrons always have relatively broad LET spectrum and the uncertainty of the method is considerable.(see Figure A-9). In our opinion much better results can be obtained by microdosimetric analysis, as it was, ... . . Figure A-8. Extrapolation to saturation current foresen in ec ion REM-2 chamber irradiated with I37Cs radiation source.
Error bars of 0.2% are marked.
> M 'l| 1 I J-- -‘-I
0.8 -
Neutron energy, MeV
d 1.2 -
"1.1 -• 0.9 MeV neutrons
* 14.8 MeV neutrons
0.000 0.010
Figure A-9. Extrapolation to saturation current for REM-2 chamber irradiated with 0.9 MeV and 14.8 MeV monoenergetic neutrons
Figure A-10. Ion collection efficiency for neutrons of different energy at collecting voltage of 1000 V applied to the REM-2 chamber.
72
A.4. THE EFFECT OF VOLUME RECOMBINATION
It is highly desirable for recombination methods to operate the chamber in such a way that the effect of volume recombination is negligible. However, it is not always possible to avoid the volume recombination, especially when relatively low values of collecting voltage are applied at high dose rates
Since the effects of back-diffusion to electrodes can be neglected in all the cases considered in this work, so the output current i from an ionisation chamber is reduced from the saturation current is by initial recombination and by volume recombination only. As it has been already mentioned initial and volume recombination are phenomena between positive and negative ions that result from a single ionising particle and those resulting from different ionising particles, respectively. When the dose rate is not very high the both phenomena occur consecutively and the total collection efficiency in an ionisation chamber can be expressed as:
f=f,fv (A-8)
M. Zielczynski suggested^116) that in such a case the two kinds of recombination can be distinguished on the basis of their radically different dependence on dose rate (the initial recombination (fz) does not depend on the dose rate and the volume recombination (fv) can be considered as proportional to the dose rate). The degree of initial recombination^34) can be derived from measurements at two dose rates D,and D2. Small corrections can be then introduced to account for volume recombination, as it is illustrated in Figure A-l 1.
The collection efficiency fv for a parallel-plateionisation chamber, with the electrodes separated by a distance d, is expressed as(117):
fv = 2(l + J) + j$2 )'1 (A-9)
= m2 ——(A-10) V U2
m-(a/ck+k_)12 (A-11)
where r. is the saturation current, V is the volume of the chamber, U is the collecting voltage, a is the recombination coefficient and k, and k„ are the mobilities of positive and negative ions, respectively.
1.0
S'.yO
I 0.8
ISS. 0.7
r 4 4 < I t W V | < i 111 « V-i
|A-b2
I 10 100Collecting voltage (V)
Figure A-l 1. Illustration of the method for determination of volume recombination from measurements of saturation curves of the recombination chamber at two dose rates D, and D2.
When the volume recombination is small (£2« 1) the equation A-9 can be approximated by:
fv = l-l^2 (A-12)
The value of the parameter m2 is proportional to the gas pressure, so the above equation can be written as:
(A-13)
a4 m2 d4 P V
(A-14)
73
and then, according to the Figure A-11 one can have:
M = f . dfv = f (A-15)
8(U,/sl) = f.[l-fv(U,/sl)] (A-16)
(A-17)
The values of correction factor 8 were determined at different gas pressures using two ionisation chambers of similar construction - one was a tissue equivalent chamber of REM-2 type and the second an aluminium chamber filled with CO- The measured values of 8 were usually below 5% of the value of total ion collection efficiency. Considering results of measurements at different collecting voltages we could state that the value of the parameter a was constant in the investigated range of the gas pressure (from 0.2 up to 1 MPa). This experimental result confirmed validity of the assumption given by equation (A-8) over the working range of our recombination chambers.
In the approach presented above we assumed that the value of m does not depend on collecting voltage applied to the chamber. There are some experimental results (e g. measurements of Niatetf118) and later of Takata and Matiullah'119-), which show some decrease of m with increasing value of collecting voltage. In our opinion the effect is likely due to generally known phenomenon of dependence of the ion mobility on the ion age. Such an effect is negligible in all the applications presented in this work , as correction factors 8 are used for very narrow range of collecting voltages only.
74
APPENDIX BRADIATION FIELDS IN THE VICINITY OF HIGH-ENERGY ACCELERATORS
The problem of dosimetry near high-energy accelerators involves two different subjects. The first is dosimetry of stray radiation fields, mostly those behind shields. This subject is referred here. The second subject is in beam dosimetry, which will be referred in Appendix C.
In this Appendix we will present maybe the most important and spectacular field of applications of recombination methods. This is, the determination of dose equivalent in complex high-energy radiation fields with unknown composition and energy spectrum. Such fields are of special interest for radiation protection, as they exist in several regions behind shields, where people may have access. Here the radiation is usually a mixture covering a wide energy range greatly depending on shielding thickness.
Recombination chambers are specially suitable for determination of dose equivalent in complex field, as the chambers can be supposed to have a true response to any mixture of radiation, with accuracy sufficient for radiation protection. Measurement of RIQ at different thickness of material provides information about establishing charged particle equilibrium in shields and in other materials irradiated by heavy particles.
It is not possible to describe here all the measurements performed with recombination chambers in different accelerator centres. Therefore, we will present briefly the historical overview of the most important experiments, and then we will pay more attention to our recent measurements in reference stray radiation fields at CERN.
The first large scale application of recombination methods was determination of quality factors in vicinity of high energy accelerators, performed in mid 60-ties almost simultaneously by Baarli and Sullivan!120) at CERN and by Zielczynski and co-workers at JINR in Dubna(121).
Zielczynski and co-workers!121) measured quality factor values in the building of 10 GeV proton synchrotron at JINR in Dubna. The obtained quality factor values were in the range between 3 and 10, in dependence on location in the building (see Figure B-l).
5.7 Iz*
Figure B-l. Values of quality factor determined by recombination method in the building of 10 GeV proton synchrotron at JINR in Dubna!121).
75
Baarli and Sullivan*120) investigated radiation fields at four different locations near the CERN PS machine. They stated that about half of the dose was due to fast neutrons. Thermal neutrons, gamma rays and high energy particle radiation together contributed approximately as much as the fast neutrons. The observed quality factor values ranged from 2.8 to 14.
Later, the recombination chambers have been used in several accelerator centres over the world.The radiation environment of high energy electron accelerator at Frascati has been examined by Pszona
et al*122) and by Lucci et al*123). The mean quality factor for most of the investigated places was found to be between 2 and 5. The component of radiation, that was responsible for such high values of the quality factor, were neutrons generated through (y, n) processes with shielding materials and accelerator structures.
The REM-2 recombination chamber, developed by Zielczynski, lias been used by Cossairt and co- workers*124*126) at Fetmilab. to determine the quality factor of radiation fields in which neutrons were the main component. Consistency was found between the value of Q so determined and the value derived from detailed knowledge of the radiation field in particular the neutron spectrum.
Several series of intercomparative dose equivalent measurements have been performed around the high energy proton accelerators at CERN*12T) and at the Institute of High Energy Physics in Serpukhov*128'129). Summary of the results and their comparison with the results obtained by proportional counter was given by Hbfert and Stevenson*130) and later included to the overview paper of Thomas and Stevenson*131). The overall conclusion from these measurements was that the values of dose equivalent agreed within better than a factor of two in all the inter comparisons. The quality factors obtained were reasonable within the qualitative understanding of the radiation field composition.
Recombination chambers have been also used for determination of the dosimetric characteristics of neutron reference fields. Such fields are intended for routine neutron dosimeters calibration at conditions, that are similar to working conditions near high energy particle accelerators or during the flights at high altitudes It is obvious, that physical and dosimetric parameters of the reference fields should be determined with maximum achievable accuracy.
In 1992 an intercomparison experiment was organised at the Department of Radiation Safety and Radiation Research at JINR in Dubna, with the aim to determine the main characteristics of reference neutron fields*71). The measurements were carried out in the soft reference field based on 660 MeV phasotron and in two reference fields based on 252Cf in polyethylene moderators with diameters 12.7 and 29.2 cm. The main characteristics of the JINR neutron reference fields, such as neutron fluences, spectra, dose equivalent and kerma rates have been measured by different methods. The REM-2 chamber was used in this experiment for determination of the dose equivalent rate and the tissue kerma rate
B.l MEASUREMENT OF DOSE EQUIVALENT IN REFERENCE HIGH-ENERGY STRAY RADIATION FIELDS
In 1993 and 1994 several series of the international intercomparison experiment on measurement of dose equivalent in relativistic stray radiation fields were organised at CERN. In this Section we will present the main results of our measurements performed during this experiment. Some more details on measuring and calibration procedure can be found in our earlier published reports*132,90). About 25 different techniques were used*133) during the intercomparison experiments but the complete results has not been published yet. Therefore, our data presented here are mainly compared with the already available results of the CERN- based groups.
76
Conditions of the experiment
CERN reference fields of high-energy particles have been provided outside shielding configurations of the H6 beam in the SPS North Experimental Hall. The fields have similar composition and spectra as those measured on board of civil aircraft and may serve as test and calibration fields foi developments of the radiation protection instrumentation*133).
The beam, of 120 GeV/c or 205 GeV/c positively charged particles (protons and pions extracted from the SPS accelerator) was directed onto a cylindrical copper target (50 cm long and 7 cm in diameter).The target was placed either in the position under a 40 cm thick iron roof shield (called iron position) or under an 80 cm thick concrete roof shield (called concrete position). The beam was shielded from the side by a concrete wall with the thickness of 80 cm for the concrete position and of 160 cm for the iron position*133).
For each target location two sets of measurement positions were available on both the roof top and nearthe side wall of the shielding. Special grids of
Beam
direction
13 14 15 16
9 10 11 12
5 6 7UB
8
1 2 3 4
Top grid
Target
1 2 3 4
5 6 7 8
9 10 11 12
Side grid
Figure. B-2. Grid system showing the top and side measurement positions for both concrete and iron shielding configurations
50 x 50 cm were marked and labelled as T1 to 116 positions on the roof and SI to S8 in side positions, as shown in Figure B-2. Our REM-2 chamber was placed in the centre of the measurement position, usually vertically. In the positions on the top shielding the centre of the chamber was placed at the height of 25 cm over the base plane of the roof shielding.
The time characteristic of the field revealed the pulse structure of the beam with the duration time of 2 s and repetition time of 14.4 s, as sketched in Figure B-3.
Flat top
2 Beamb extra- >8 8 a , etion b
Pulse cycle
Figure. B-3. Evolution of the magnetic field in a pulse duration. During beam extraction the radiation intensity structure follows the variations of the magnetic field (200 MHz).
The beam intensity was monitored with a Precision Ionisation Chamber (PIC) in the beam line located at the end of the beam pipe upstream of the target positions. The PIC count was the primary signal for the normalisation of all measurements. In order to provide some redundancy for normalising the results between different irradiations, two Studsvik Model 5210A Rem Counters were placed in the two of the grid positions on the top shield (T4 and T9).
The neutron energy spectra were similar to those calculated earlier*91) for the beam of 205 GeV/c positively charged particles (see Figure B-4). Generally, the spectrum for the concrete shield is much harder than for iron one. The characteristic broad peak in the 100 keV to 1 MeV region dominates in the spectrum for the iron shield. The second peak, in the energy region close to 100 MeV, is of similar shape for both concrete and iron shield. However, this peak is weakly pronounced in the spectrum for the iron shield and constitutes a large fraction of the spectrum for concrete shield.
77
t----- ro 200—16 iron shield
T6 concrete shield
Neutron energy, MeV
16 concrete shield
K)"4 B 3 B"2 B'1 B°
Neutron energy, MeV
Figure B-4. Neutron fiuence spectra outside the top iron and concrete shielding of the CERN-CEC Reference Field Facility, as calculated with the FLUKA program^91), normalised to 106 protons at 205 GeV/c in the incident beam. The original figure given by Roesler and Stevenson^91) was plotted by the author of this work in the linear scale for the fiuence axis.
Measurement of dose equivalent
Our measurements of dose equivalent were performed with the beam of particles with momentum of 120 GeV/c. Ionisation current of the REM-2 chamber was determined by measuring electrical charge collected on externally connected capacitor during certain number of accelerator pulses. All the values were normalised to one count of the Precision Ionisation Chamber (PIC) used for the beam monitoring (1 PIC count is equivalent approximately to 2104 beam particles). Ambient dose equivalent and quality factor values, according to the ICRP-21 definition, were determined for five measurement positions.
Main dosimetnc results are displayed in Table B.-l for all the measurement positions in order to show the obtained values of the quality factor (Q4) and the range of the measured H*(10) values Number of PIC counts/s, given in the Table, is the average of the monitor count rate for the time of measurements in certain position. Accuracy of the results is of about 10%.
Table B-l. Dosimetric results for the H6S93 experiment normalised to one count of the monitor^90) (Q and H*(10) are evaluated according to ICRP-21).
Location Position PIC count/s q4 H*(10) pSv/PIC count
Top iron 6 608 4.3 1380Top iron 5 600 4.1 1210Side iron 2 711 3.7 33Top concrete 6 613 1.7 364Side concrete 4 517 3.25 304
The highest values of quality factors, exceeding 4 Sv/Gy, were obtained on the iron roof shielding. Such result could be expected and might be attributed to significant contribution from neutrons with energies between 0.1 and 1 MeV. Q4 measured behind the thick side wall of the iron location was slightly higher, comparing with those for the thin wall near the concrete location. Surprisingly low value of Q4=1.7 was obtained for T6 position on the concrete roof shielding. This result will be discussed later, taking into account the muon component of the absorbed dose.
In order to obtain more detailed information on the radiation fields we determined saturation curves of the chamber, placed in positions number 6, both on concrete and iron shields. From these curves the distri-
78
buttons of absorbed dose versus LET were determined. The distributions are shown in Section 5 (see Figure 5-7), and here we will present only the dosimetric results derived from the distributions. We were able to determine the contribution of low-LET radiation to the absorbed dose, as well as dose fractions associated with few, arbitrary chosen LET compartments. The quality factor QlCRP^0 has been calculated from these data, using the Q(L) dependence given in ICRP Report 60. Results of the calculations are shown in Table B-2 in comparison with the results obtained using the TE proportional counter of HAND! type^92X
Table B-2. Comparison of main dosimetric results, obtained by HANDI-TEPC system^92* and by REM-2 recombination chamber, for top concrete and top iron locations.
Location T6 concrete position T6 iron position
HANDI REM-2 HANDI REM-2D, (pGy/PIC) 203.5 214 323.5 321
Diow let (pGy/PIC count) 17.3.4 190 227.9 230
Hicrp-21 (pSv/PIC count) 546.9 364 1340 1380
QlCRP-21 2.69 1.7 4.14 4.3
Hlow LET (ICRP-21) (pSv/PIC count) 194.3 190 255.7 230
hicrp-60 (pSv/PIC count) 577.5 410 1736 2090
QlCRP-60 2.84 19 5.37 6.5
H|ow let (ICRP-60) (pSv/PIC) 173.4 190 227.9 230
It can be seen from the Table B-2, that both methods result in very close values of the absorbed dose and of the low-LET contribution to the absorbed dose. Very good agreement (within 3%) is also observed for the top iron position, when the values of HICRP.21 and Qicrp-2i are compared.
For the top concrete position we noticed much larger difference of Q[CRP,.21 determined by both methods (about 50%). This difference can be explained by the influence of muon component. Our measurements were performed at lower beam intensity than the measurements of the CERN group, therefore the muon contribution was higher in our measurements.
The second reason, of much smaller importance, is the difference in construction of the detectors and in their effective wall thickness. REM-2 chamber is much larger than HANDI TEPC. Rather large mass of the chamber influences the radiation field and may increase the contribution of low-LET radiation because of (n,y) reactions occurring in the chamber itself. However, as was mentioned in the Section 4, we expect that the scattering properties of the chamber and the interactions in the chamber walls should be similar as in the ICRU sphere , so our data should be close to H*(10).
Determination of muon component of the absorbed dose
The low-LET fraction of the high-energy reference fields includes contribution from the muon- background. During the series of measurements performed in September 1993 and described above, some of our data were collected in the position T6 concrete for the conditions of "closed" beam (i.e. when the beam was generated, but a collimator was closed before the copper target). The radiation background, mostly due to high energy muons, was clearly visible in such conditions. Absorbed dose rate from this background was estimated to 7x10 * Gy/s (normalised to the effective measurement time 2 s of beam extraction in 14.2 s cycle), which was about 50% of the total absorbed dose for the "open" beam(90>. It was also determined that the value of Q4 of high-energy muons was equal to Q4=1±0.15.
79
In May 1994 we performed the second series of measurements with the same shielding conditions as in 1993, but with 205 GeV/c beam This time our measurements were aimed mainly at estimation of the muon background.
For this work the simple calculation of muon contribution to the absorbed dose has been performed. The value of Q4 was expressed as:
Q4 = <D(11 +(1 -<Dm)<3t (B-l)
where is the muon contribution to the absorbed dose and QT is quality factor of target radiation.The muon background strongly influenced the value of the quality factor and did not depend of settings
of the beam collimator before the target. Therefore, the measured values of the quality factor were dependent on the intensity of radiation from the target. The measurements with REM-2 chamber were performed in position T8 on the concrete shielding at two different intensities (750 and 3350 monitor counts/radiation pulse). We observed that the value of the absorbed dose per monitor count decreased with increasing intensity and the measured values of the quality factor, Q4, were 3.1 and 4.1 respectively.
The value of the muon background calculated from the difference between Q4 values was equal to 3x10 * Gy/s. After subtraction of the muon background the dosimetric parameters of the target radiation were determined (see Table B-3). The obtained values of the ambient dose equivalent and of the quality factor, as well as the value of the muon background contribution are in good agreement with the appropriate values reported by the CERN group(134\
Table B-3. Dosimetric values for target radiation measured with REM-2 chamber and HANOI Tepc<134) in May 1994 on top concrete in position T8 (normalised to one count of the monitor).
Device D H* Qtotal Dhigh-LET H*high LET Qhigh-LET
(pGy/PIC) (pSv/PIC) (pGy/PIC) (pSv/PIC)
REM-2 chamber 115 495 4.3 37 410 11
HANOI TEPC<m> 123 512 4.17 38 407 10.6
Final remarks
In conclusion of this Appendix we would like to emphasise our strong belief that recombination chamber is relatively simple but powerful tool for radiation protection dosimetry in the vicinity of high energy accelerators.
In the experiment described above our REM-2 ionisation chamber was operated in a special way. Instead of differential mode, which is routinely used at CERN, the summation mode was chosen for chamber operation. Instead of measuring current, we measured the electrical charge, which was collected on the external capacitor during certain number of radiation pulses. Both polarities of collecting voltages were applied sequentially and the readings were appropriately averaged over the polarities. The recombination chamber, operated in such way, was working very stable. Reproducibility of the results obtained in several series of repeated measurements was better than 0.5%.
The use of the summation mode increased the sensitivity of the chamber, with respect to the differential mode. Thus, it was possible to determine the dose equivalent even in the side iron position, behind the thick concrete wall, where the absorbed dose rate was about 25 pGy/h.
Additionally, operating the chamber in the summation mode we were able to estimate the implications of new ICRP recommendations, due to change of Q(L) relation. For the top concrete location this change caused an increase of the dcse equivalent by 12% For the top iron location the influence of the new recommendation was stronger and the dose equivalent increased by about 57%.
80
APPENDIX CAPPLICATIONS OF RECOMBINATION METHODS IN RADIOTHERAPY
The problem of dosimetry for radiotherapy differs in many aspects from those for radiation protection. The main differences concern higher dose rates used for the therapy, different interpretation of radiation quality and the requirement of high accuracy of the determination of the absorbed dose and its spatial distribution in a phantom.
In 1987 Mijnheer et al.!135) came to the conclusion that the absorbed dose to be delivered to a patient should be determined with the accuracy not worse than 3.5%. Such accuracy is not always achievable yet, e g. accurate dosimetry of high energy neutron beams is still the challenging task. In this Appendix we want to show that recombination method can be used as a tool for better evaluation of the absorbed dose and radiation quality of high-energy particle beams.
In radiation therapy with low-LET radiations the absorbed dose is a sufficiently good quantity to predict the biological effect. However, with the development of neutron and high-LET particle therapy, tire need of determination of radiation quality became evident. Relative biological effectiveness, RBE, of high-LET particles not only reaches relatively high values but varies with the neutron energy and with the depth in irradiated body. For example RBE of fast neutrons may reach the value of about 3 in comparison to photons, when the dose levels and biological endpoints relevant for therapy are considered'136).
The expected values of RBE directly influence the treatment planning and, thus, the same accuracy is required on the RBE as on the absorbed dose level. Moreover, the oxygen enhancement ratio, OER, which plays important role in radiotherapy, is also dependent on LET!137).
Some years ago it was recognised that for high LET radiotherapy the microdosimetry could play the most important role in providing a method for specifying the radiation quality . Series of microdosimetric measurements were performed for different beams of fast neutrons (with energy up to 65 MeV) using a tissue equivalent proportional counter (TEPC). The results were compared in overview papers of Wambersee et. al.(136 138) and of Pihet et al.!139).
Our measurements were performed at medical facility at Joint Institute on Nuclear Research (JINR) in Dubna for high-energy neutron and proton beams. First microdosimetric results obtained for these beams using the recombination method are described in Section 7 of this work. Up to now, however, we have not enough experimental data to consider relevance of our microdosimetric approach for radiotherapy. Therefore in this Appendix we want to present applications based on the concept of RJQ. only.
Experimental values of RIQ for the medical beams are shown and discussed in Subsection C.l. In Subsection C.2 we present the recombination method for determination of corrections to W-values, which improve the accuracy of determination of the absorbed dose by ionisation chambers. For the method it is not necessary to know the composition and the energy spectrum of particles ionising the chamber cavity. Moreover, the W-value determined by this method corresponds to a definite cavity size, which is just that needed for radiation therapy dosimetry. The accuracy of our method depends on the kind of radiation and may vary between 0.5 and 2%. Such accuracy is not provided by other methods
RIQ can be also used for calculation of certain quality-dependent parameters and correction factors, which appear in cavity theory of ionisation chambers!140). It has been also shown!-3) that Q15/0 1 can simulate the observed dependence on LET for human kidney cell survival.
81
C.l. DETERMINATION OF RIQ IN HIGH ENERGY PROTON AND NEUTRON BEAMS
High-energy neutron beams
The medical high energy neutron facility at JINR phasotron has been described in details in several reports^94-141’142). Briefly, the neutron beam is generated by steering the beam of 660 MeV protons onto a target. Targets made of beryllium, copper and lead can be used. The neutron energy spectra for different targets are shown in Figure C-V141X Field size between 2 and 15 cm2 may be obtained by appropriate collimator settings. The neutron beam is intended for use for radiation therapy of large, hypoxic tumours -
In experiments presented here the recombination chamber of FI type has been used to investigate the dependence of radiation quality on depth in phantom and on radial distance from the beam axis.
As described in Section 3, the RIQ has been introduced for purposes of radiation protection and its values does not reflect the values of RBE of medical beams. Moreover, every value of RBE may strictly be used only for the given biological system, and specified beam of radiation as well as dose level for the reference radiation. Under these restrictions, RIQ can be used as an indicator of radiation quality
when different beams and irradiation conditions are compared. Pihet et al. showed^39) the relative importance of small variations of the LET component between 50 and 150 keV/pm on the radiation quality of the beams. Q4 changes in this LET region from 9.3 to 16, so the recombination chamber is rather sensitive detector of such variations.
TTT'T’i"r‘tur,f"V ri" v "t j >i i k i
A Pb target ■ Be target o Cu target
A- JL.-A.-L-I. I . i‘ .A 1 1 J 1 1 * 1 1 1 1 1 1
Depth in phantom, cm H20
Figure C-2. Depth distribution of recombination index of radiation quality in water phantom for neutron beams generated by 660 MeV protons incident on three different targets^44).
The measurements of Q4 were made on three separate occasions - in 1967(l43>, in 1990 after reconstruction of the 660 MeV phasotron^144) and in 1993. The results obtained in 1990 for three different targets are shown in Figure C-2 in dependence on depth in water phantom. There are two significant features of this figure. The values of Q4 are similar for all the targets and progressively decrease with depth in the phantom, z. The curves representing Q4(z) for copper and beryllium targets differ less than 5%; somewhat larger difference up to about 15% is observed when the lead and beryllium targets are compared.
both independently and in combination with protons
'Vi"sio*o6 5 e
o0 100 200 300 400 500 600 700
Neutron Energy, MeV
Figure C-l. Neutron energy spectra of neutrons emitted from of 660 MeV medical facility at JINR Dubna, for three different targets.
82
The observed increase of Q4 for heavier targets is probably due to decrease of mean energy of neutrons incident on the phantom. Higher values of Q4 close to the front wall of the phantom merely reflects the absence of proton equilibrium and relatively higher heavy particle contribution per unit dose, comparing with larger depths.
Generally, beams ensuring higher LET are preferred for radiation therapy of hy poxic tumors. In case of the high-energy neutron beams considered above the differences in RIQ are small, in spite of considerably different energy spectra. Therefore, the main criterion for the choice of the target became the beam intensity and the beryllium target is the best from this point of view.
In Figure C-3 the Q4 and absorbed dose rate values are presented in dependence on depth in the phantom. The experimental values of Q4 are compared with those resulting from calculations of Serov*144). It can be seen that in the region of the broad maximum of the dose rate, i.e. between 10 and 25 cm, the values of Q4 are practically independent of the depth in phantom and equal to Q4 - 3±0 3.
■n"i" i 'll" ITT | '■n“rt-Lmrrr'| » r r-r-mn—rf-r-rr
Dose rate
I « t I , L..4-4 A. J■JL.J..A. * ■> 4 « ) >.■!...« 1 I
Depth in phantom, cm H^O
Figure C-3. Depth distribution of RIQ and absorbed dose rate in water phantom for neutrons from reaction p(660 MeV)+Be. A and o - Zielczyhski et al. 1967*143) and 19 90*144), V - ihis work. Solid line - calculations of Serov*144).
i" i i | rrrf
Dose rate
- 0.6
Distance from the beam axis, cm H O
Figure. C-4 Radial distribution of RIQ and absorbed dose rate at 15 cm in water phantom for neutrons from reaction p(660 MeV)+Be. In this Figure points represent the experimental values of RIQ determined by Zielczyhski and co-workers*144). The solid line shows the result of Monte Carlo calculations of RIQ and dotted lines show the range of accuracy of the calculations*144)Dose rate profile (dashed line in the Figure) has been determined experimentally by Abazov and co-workers*141).
83
High-energy proton beams
Medical facility at JINRt’45-146) includes four proton medical beams used for irradiation of deeply lying tumours with broad and narrow proton beams of various energy of 100, 130, 200 and 660 MeV. Protons of energies lower than 660 MeV are obtained by slowing down in carbon moderator.
One of the major features of protons is their characteristic depth dose distribution. For collimated beam of monoenergetic particles in the considered energy range this distribution is described by the Bragg curve. Therefore, the dose can be better focused on the tumour volume, comparing with other kinds of radiation, such as electrons, photons and neutrons. Protons have also sharp radial distribution, formed by collimator. The depth dose distribution of proton beam can be relatively easy modified in order to obtain the uniform dose distribution over some extended region^145).
In spite of uniform dose distribution the radiobiological effectiveness of protons may change with depth in tissue, particularly towards the end of the proton range. The recombination index of radiation quality may serve as a good indicator of such changes. Systematic measurements of depth distribution of Q4 in water phantom have been performed by Zielczynski et al. for protons of energy 660 MeV and of 260 MeV147X The results of these measurements are displayed in Figure C-5.
The value of Q4 for protons with energies of 660 MeV was found to be 1.8±0.3, almost independently of depth in phantom The similar conclusion was drawn by Zielczynski for 260 MeV protons. However, in opinion of the author of this work the observed variations of the Q4 values must not be neglected, as they reflect real changes of the mean LET of protons. In contrast to measurements made at 660 MeV, those at 260 MeV showed an initial small decrease in the value of Q4 with depth into water phantom, from about 1.5 at 2.5 cm depth to 1 at a depth of 22 cm, due to the pro
duction of secondary charged particles by nuclear inelastic scattering and absence of charged particle equilibrium at small distances from the phantom wall After that, the value of Q4 increases sharply at the limit of the primary protons range and reaches the value of 2.8 at a depth of 45 cm. The increase of Q4 has, however, no practical significance, as it is observed at depths greater than 22 cm, where the dose rate of the proton beam is already very low.
Recently, the value of Q4 has been also determined for 200 MeV proton beam. The measurements were performed free in air, as the main goal of the experiment was determination of dose versus LET distribution for high energy protons'4 ^ (for the results see Section 6). The value of Q4 determined at this occasion was equal to 1.2*0.2.
660tvfeV
260 MeV
Depth in phantom, cm H^O
Figure C-5. Depth distribution of recombination index of radiation quality Q4 in water phantom for protons with initial energy of 660 and 260 MeV.
84
C.2. ENERGY EXPENDED TO CREATE AN ION PAIR AS A FACTOR DEPENDENT ON RADIATION QUALITY
When using an ionisation chamber for determination of absorbed dcse, one needs to know the energy expended to create an ion pair, W, since the absorbed dose is defined in terms of energy deposited in matter. This quantity is often known with insufficient accuracy, which may lead to a considerable uncertainty of the absorbed dose value, especially in case of medical beams of high-energy heavy particles.
This Subsection contains the main part of our work of the same tilled48I and is based on idea of Zielczynski to correlate the value of W with recombination index of radiation quality, Q4, which is used here just as a LET dependent quantity, that can be measured in a phantom in the beam considered, by means of the same (or similar) ionisation chamber as used for the determination of the absorbed dose.
Usually W is related to the type and energy of particles ionising gas in the chamber cavity. In general case the composition and energy spectrum of particles depend on many factors like the position of point considered in the irradiated phantom, the beam size, the filters used and any oilier factors modifying the beam and its penetrating ability. The energy spectrum of particles ionising the chamber cavity is influenced also by the ionisation chamber itself, so it depends on material and size of the chamber.
Determination of the composition and energy spectrum of all the primary and secondary particles at different points in phantom for different beam dimensions, filters, etc. for definite ionisation chamber - by calculation or by measurements - is impractical, because of its uncertainties and time consuming. Usually a constant, arbitrary chosen value of W is taken. However, this simplification introduces an uncertainty to determination of the absorbed dose and also distorts such measurements as depth dose distributions, independence of the absorbed dose on the beam size, comparison of the dose values determined by different ionisation chambers etc.
Relation between W/Wy and LET
Ionisation chambers used for radiation therapy are usually calibrated in a standard field of gamma radiation from 60Co source. Therefore not the value of W, but a ratio W/W^ is needed for determination of absorbed dose in the investigated beam, Wy is the energy required to create an ion pair in the gas of the chamber irradiated in the standard gamma field. Although W-values are gas dependent, the ratios of W/W? can be considered as practically independent of the kind of gas, for most gases of interest !48\
On the basis of existing experimental and calculated data, we have made a hypothesis that W/Wy is correlated with the linear energy transfer of charged particles (LET), if the energy of the particles is higher than a certain threshold. The threshold energy approximately corresponds to the energy, for which there is a maximum of the stopping power for the type of particle considered. Residual ranges for particles with energy below the threshold do not exceed some micrometers of water.
Correlation between the energy expended to create an ion pair and the linear energy transfer can be roughly explained by the fact, that there is a limited number of molecules, which are close to the track of ionising particle. In the track of high LET particle the local ion density is high and a number of ionised molecules is comparable with total number of molecules touched by the particle track. Interactions of the particle with distant or once ionised molecules result in an increase of W.
Recently D. Harder (private communication, 1995) suggested, that the increase in W may be also caused by preferential recombination in tracks of high LET.
85
wWy
1.2
1.1
1.0
Figure C-6. WAVy for charged particles plotted against unrestricted LET: o electrons of E> 1 keV,
□ protons of 20 keV to 10 MeV, • a-particles of 0.8 MeV to 4 GeV, W C6+ ions of 6 to 48 MeV, A 08+ ions of 3.2-64 MeV, V 06+ ions 34.5 MeV. The curve is plotted according to the relation W=Wy[1+|3(Q4-1)] with p =0.0072.
The Figure C.2-1 shows values of WAV for different particles (with energy above the threshold) plotted against the unrestricted linear energy transfer (L). The W-values are taken from published data, mostly for TE-gas111’14915°). If no data were available for TE-gas, we used data for air (electrons/149) and for nitrogen (Q+6 ions/151). Values of Wy used are 29.3, 33.97 and 34.5 eV for TE-gas, air and nitrogen respectively. Where it was possible, we used the differential W-values for particles with range well exceeding 10 gg/cm2 of TE gas (which corresponds to a cavity size approximately equal to 8 mm) and the integral W-values for particles with lower energies Values of LET for electrons, protons and alpha particles have been taken from ICRU collision stopping power tables14), but for heavy particles - from calculations of Armstrong and Chandler1152).
Figure C-6 shows, that for practical needs the values of WAVy ratio for charged particles (with energy exceeding threshold) can be considered as an unequivocal function of LET.
There are hundreds of published experimental data of W for different particles, which correspond well (within stated accuracy) with the Figure C-6, but for the clarity of the figure were omitted. This concerns, among other, very low LET particles, like g-mesons, and very high LET ions. It is obvious that W-values for beams of X and y radiation of any energy may be also related to their effective LET, practically equal to LET for electrons.
As shown in Figure C-6, WAV is nearly constant at L<3.5 keV/gm, it grows up rapidly between 10 and 100 keV/gm and then increases slowly at higher LET. Very similar dependence on LET is observed for the recombination index of radiation quality , Q4 (see Figure 3-3 and equations 3-10, 3-11). To be precise we should remember that Q4 depends on local ionisation density along the track of ionising particle, and only indirectly on LET. Nevertheless it can be approximately expressed as the following function of LET:
LQ4 =------------------- for L > L0
0.96Lg +0.04 L(C-l)
Q4 = 0 85+0.15L/LC forLsL0where L0 = 3.5 keV/gm.
On a search for correlation between WAVy and Q4 we decided to fit the data presented in Figure C-6 with the following function:
i—m-rrrq T-----rTTTTTT
^___ L.J*JU,XJUl I >.. 1.1 I III
LET, keV/um
86
(C-2)W = wr(i + p(Q4-1))where P is a fitted parameter and Q4 is given by relation (C-l).
Result of the fit is shown as the solid curve in Figure C.2-1. The fitted value of p is equal to:p = 0.0072 ± 0.0003
Figure C-6 does not show the data for neutrons. LET spectra of charged particles liberated by neutrons are broad. Neutrons of definite energy can not be characterised by a single value of LET, therefore they can not be introduced into the Fig. C-6. Nevertheless it is possible to determine single values of Q and Q4 for definite neutron energy.
Calculated values of W in TE-gas*150) and Q<153) (as defined in ICRP-21) are plotted in Figure C-7 versus neutron energy En. The calculations were performed for kerma conditions, therefore the results may approximately concern the
irradiation of small TE chamber placed in monoenergetic neutron beam without using a phantom.There is striking similarity of both curves, their correlation factor is equal to 0.97. Therefore, W(En) was
expressed as a function of Q(En) and fitted with a function similar to given by Equation (C-2). The fitted parameter p equals to 0.00722 ± 0.00005, and agrees very well with the p value obtained from the data given in Figure C-6.
The W vs. Q dependence given above was obtained for limited range of the neutron energy (0.1 MeV to 20 MeV). On the basis of available information we expect, however, that the dependence given by equation (C-2) can be used also for neutrons with the energy outside of this range. For high energy neutrons (20 MeV to 1.1 GeV) the relation (C-2) agree with calculations of W and Q, done by Morstin et a! although the accuracy of the calculations is not high enough to confirm unambiguously the validity of our relation. Thermal neutrons also fulfil the relation (C-2), as kerma in TE-gas is mostly due to 600 keV protons. The only case, when the relation (C-2) cannot be used, is a small ionisation chamber Eradiated free in air by neutrons of intermediate energies (approximately from 10 eV to 10 keV). This case is practically not important for the radiation therapy, because most often the chamber is placed in an externally irradiated phantom. In such situation the contribution of intermediate neutrons to the dose is usually negligible. Even if such neutrons are present in the beam the contribution from n-p and n-y reactions due to the neutron thermalization is much higher than from elastic collisions of intermediate neutrons. Although an estimation of possible contribution of intermediate neutrons may be necessary in certain beams, for which W-values would be determined using the relation (C-2).
Similarly the contribution to the absorbed dose due to the ends of ranges of charged particles should be estimated in case of irradiation with a beam of such particles. Usually in radiotherapy beams the contribution of residual ranges is small and the correction to the W-value is negligible, even in Bragg peaks created at definite depth. The correction can be estimated comparing the size of Bragg peak with the residual ranges of particles having energy below the threshold mentioned in the beginning of this paper.
Neutron energy, MeVFigure C-7. Energy per ion pair and quality factor as functions of neutron energy.
87
These ranges are equal to 0 5 pm of water for protons and to some micrometers for heavier ions. It is not necessary to introduce any corrections for the residual range of electrons.
Both corrections - due to residual ranges and due to intermediate neutrons - lead to a somewhat higher value of coefficient p than it was given in previous paragraph. It seems reasonable to round it up to the value of p = 0.008.
Practical application
Our method was used to determine the W values for the tissue equivalent chamber placed in a water phantom irradiated by the high energy neutron medical beam from the phasotron at JINR (Dubna). This was the same beam, that was already described in Subsection C. 1 of this Appendix.
Neutrons were obtained by bombarding the 36 cm thick beryllium target by 660 MeV protons. Mean energy of neutrons generated with this target was equal to 350 MeV. The beam size has been defined by a collimator, 10 cm in diameter. Distance from target to a 50 x 60 cm water filled phantom was equal to 9 m.
W-values have been calculated using the equation (C-2). The needed values of recombination index of radiation quality have been determined at different depths in the phantom using a parallel-plate recombination chamber of FI type (see Figure A-2). The obtained values of W are displayed in Figure C-8, together with the depth distribution of the absorbed dose, determined with the same ionisation chamber.
-T”r^pT'T,,T”T"f~r-i"r r'| "> i i i i \ i i > j i t", i
Dose rate
Energy expended per ion pair
10 20 Depth in phantom, cm H^O
Figure C-8. Energy per ion pair in TE gas and absorbed dose rate in water phantom irradiated by p(660 MeV) + Be neutrons
Maximum of the depth dose distribution in the phantom was observed at 15 cm. The accuracy of the W-value determined at this depth was estimated^148' to ±0.5%.
As it was already mentioned, the aim of the work described here was to improve the accuracy of determination of the WWy ratio. The above example of practical application shows, that our method can provide the W/W value with accuracy of about 0.5%. Generally, the accuracy of the method depends on kind of radiation and may van between 0.5 and 2%, that is still some improvement in comparison with other methods, which for high-energy neutrons provide the value of W/Wy with accuracy not better then 4%.
Our method is useful for beams of radiation other than X, y and electrons. It seems particularly suitable for neutrons, protons, pions, heavy ions and any new types of radiation beams, especially those for which the energy spectrum of secondary particles in a phantom is poorly known.
88
APPENDIX DTWIN-CHAMBER TECHNIQUE FOR DETERMINATION OF PHOTON CONTRIBUTION TO
THE ABSORBED DOSE
"Neutron insensitive" detectors
As the neutron fields are always accompanied by photons, which have different biological effectiveness than neutrons, it is usually necessary to determine separately the dose components due to neutrons, ©„, and due to photons, ®r The accuracy of separation of these components became very important, when fast neutrons have been applied for cancer therapy.
Generally, two dosemeters with different relative sensitivities to neutrons and photons are used for the evaluation of the component radiations. Photon detectors insensitive to neutrons do not exist, and therefore the neutron contribution to its response has to be taken into account.
According to ICRU Report 26*155) and European Protocol for Neutron Dosimetry for External Beam Therapy*156), the absorbed dose components can be computed by the use of the following simultaneous equations:
R,t = kt©n + ht©y (D-l)
R'u = ku#n + VZ)y (D-2)
In these equations the subscript t refers to the device measuiing total absorbed dose and the subscript u refers to the neutron insensitive detector. R't and R'u are the quotients of the response of the dosemeters by their sensitivity to the gamma rays used for calibration, (Dn and <Dy. are quotients of the neutron and photon dose to the total absorbed dose. Similarly, k, and k^ are the sensitivities of each dosemeter to neutrons, relative to the sensitivity to the gamma radiation used for calibration, and ht and h^ are the sensitivities of each dosemeter to the photons in the mixed fields, relative to the sensitivity to the gamma radiation used for calibration. In this case, sensitivity is defined as the quotient of the dosemeter response by the absorbed dose in tissue.
The Protocol for Neutron Dosimetry recommends the use of tissue equivalent ionisation chamber as a detector of total absorbed dose. The TE chamber is paired with neutron insensitive detector, such as G-M counter or non-hydrogenous ionisation chamber. The later is usually magnesium or aluminium chamber filled with argon (Mg-Ar or Al-Ar chamber) or graphite chamber filled with carbon dioxide (C-C02 chamber). Although, G-M counters have low neutron sensitivity, their signal strongly depends on photon energy, what may lead to considerable uncertainties in measurements of the absorbed dose. On the other hand, the non-hydrogenous ionisation chambers have higher neutron sensitivities, which are strongly dependent on neutron energy*157*160) and often not well known (see Figure D-l).
^ y”-y T
Mg-Ar
-C-CO
- C-CO, calc.
Ii.i. i A I 11
Neutron energy (MeV)
Figure D-l. Relative sensitivity k^ for C-C02 and Mg-Ar ionisation chambers (with continuously flowing gas), obtained from deconvolution of experimental data by Waterman etal.*157) and calculated by Rubach and Bichsel*159).
89
The neutron sensitivity’ of the C-C02 chamber can be considerably lowered by use of high-pressure chamber, operated in recombination regime^33). Our method makes use of the fact that the secondary charged particles, which are produced in the detector wall and gas by photons and neutrons, differ greatly in ionisation density along the particle track Therefore, recombination of ions in tracks of neutron secondaries will be much higher than in tracks of electrons resulting from interaction of photons with matter.
Influence of cavity size on neutron sensitivity of C-COz chamber
The use of ionisation chambers in dosimetry is based on Bragg-Gray theory, which relates the dose absorbed in a matter surrounding a gas cavity with ionisation in this cavity. The response of C-C02 chamber deviates from that derived from Bragg-Gray relation because of the chamber inhomogeneity and of
Spacing distance —o—0.2 cm
:s 0.3
> 0.2
E,„ MeVFigure D-2. Relative sensitivity k^ of C-C02 ionisation chamber (under atmospheric gas pressure) as a function of neutron energy for two spacing distances between electrodes, calculated by Makarewicz and Pszona(,61\ Upper edge of shadowed area represents the "infinitesimal" cavity. Lower edge - "infinite" cavity.
One of important conclusions, which can be deriv
trie finite size of cavity. This problem was carefully investigated by Makarewicz and PszonaG61) who used the Monte Carlo method for calculating the response of a parallel plate ionisation chamber irradiated by fast neutrons. Relative sensitivity k^ has been considered for two extreme cases, depending on the size of a cavity and range of charged secondaries:
- in the first case the ionisation current is generated by charged recoils from walls of the chamber - this corresponds to the "infinitesimal" cavity
- in the second case the ionisation current is generated by charged recoils from cavity - this corresponds to the "infinite" cavity.
As shown in Figure D-2 there is an evident influence of cavity size on relative sensitivity of the chamber. The shadowed area in the figure represents the possible range of changing of k^ depending on cavity size. The upper edge represents the ideal case of C-C chamber, the lower edge case of "infinite" C02 cavity, from this work, is that ionisation chamber with the
effective distance between electrodes (i.e. the distance multiplied by gas pressure) exceeding 1 cm can be considered as an "infinite" cavity for neutrons with energy up to 17 MeV. For such a cavity the relative sensitivity k^ can be calculated from the Bragg-Gray theory as:
(ku)sat = (Sc.CoA (”^)Y (n^-)n (D*3)
where: subscript y refers to gamma radiation used for calibration,(kJsai - is the relative sensitivity of the chamber at saturation,
(Sc.co2)y (lVHc)y
Kco2 andW7 and Wn
- is the ratio of average mass stopping power of the graphite wall relative to the gas- is the ratio of mass attenuation coefficients of tissue and graphite,
-are kerma factors for carbon dioxide and tissue respectively,- are the values of average energy expended to create an ion pair for gamma
radiation and neutrons respectively,
90
High-pressure ionisation chamber as a neutron insensitive, detector
For our high-pressure non-hydrogenous C-C02 ionisation chambers the condition of d-p/p0> 1 cm is always fulfilled, so in this work these chambers are considered as the "infinite" ones. When a chamber works under conditions of local recombination, an additional factor has to be introduced to the equation (D-3), namely a ratio of ion collection efficiencies for neutron and gamma radiations which is the key factor lowering the relative neutron sensitivity in our method:
k„ = (Sc.co,)?%)?(-K, Mr,
wn £r(D-4)
where: fn and f^ are ion collection efficiencies for neutrons and gamma radiation respectively.Our investigations#4-162) were aimed to determine the ku value for high-pressure C-C02 chamber and to
find the optimum conditions (gas pressure and electrical field strength) for operation of the chamber Direct, experimental determination of fn/fy (and so ky from D-4) is not possible, because of unavoidable presence of a gamma-ray component in any neutron field. For our chamber with very low ky almost all the ionisation current is due to gamma radiation, so the determination of fn would require an extremely high, practically unreachable accuracy of measurements.
We developed#4) a procedure, which provides a semitheoretical determination of the relative neutron sensitivity of the detector to a degree of accuracy that is ai least competitive with that of any other determination available to date.
As it has been already described in Section 2 the ion collection efficiency for neutrons can be expressed as:
d(L)
1 + -
L0 Wn
dLF(X,p)
(D-5)
The values of function F(X,p) can be determined experimentally by measurements of ion collection efficiency for alpha radiation, which simulates rather well the secondary particles generated by neutrons interacting with C02. As the LET spectrum of alpha radiation is narrow one can use an approximate relation:
fa - 1,+ L^F(X.p)
(D-6)
and by substitution:
f - _L f------- lik)-------DJi| L Wq 1-fqWn {a
LD
D(L)dL
• + (a(~ WnL W„ 1)
(D-7)
The value of expression % is not very different from unity, therefore at low values of fa , which isL Wa
always the case for this method, the above equation can be well approximated by:
fn ~ kx f-aWnWa
jHL)dL = ^ la. ElLm Wa
(D-8)
where LTn is so called track-average LET i.e. an average of (1ZL) over the dose versus LET distribution for neutrons in carbon dioxide.
91
Combining Equations (D-4) and (D-8) we have a final expression for relative neutron sensitivity:
ku (Sc,C02)y (^ )y ( k, W„ LTn f7 (D-9)
For convenience the above equation can be rewritten in the following form:
ku=k, k2(En)|*- (D-10)
where:k,=(Sc.co,),-^-(^ (D-ll)
(D-12)
The value of the factor k, does not depend neither of neutron energy nor of chamber operation conditions and can be calculated using the tabulated data#49"164). For radiation of 137Cs, which we used for calibration, the value of this factor is equal to k,= 1.066.
The value of k2 is a function of neutron energy. The ratio (KCo2/Kt) was calculated using the tabulated data#65). The value of La was calculated by Makarewicz#66), as a track average LET for 222Rn alpha particles in carbon dioxide. According to this calculations La = 102 MeV m2 kg'1. Values of for few neutron energies in energy range 0.1 MeV up to 19 MeV were determined from calculated D(L) distributions for carbon dioxide#4) (see Figure D-3).
r—v-T-7--r~i"T»i—r
0.0 0.5 !.0
JQQ III..*. ml it * ml 1 L 4»>nl \ —
0 5 10 15 20Neutron energy (MeV)
Figure D-3. Track average LET for monoenergetic neutrons in carbon dioxide#4).
—■— G-5 chamber - -a— REM-2AI chamber
40V,cm
X^i4-V/cm...
p [MPa]
Figure D-4. The ratio faIL as a function ofpressure#4).
The ratio of ion collection efficiencies (fa/fy) was determined#4) for two different non-hydrogenous chambers (see Figure D-4). The first, called REM-2A1, was of the same construction as REM-2 chamber (see Appendix A), but its electrodes were made of aluminium and the chamber was filled with C02. The values of fa for this chamber were determined for 222Rn alpha particles#9) (the experiment has been already mentioned in Section 2.4.) In the same experiment the f? was measured, in a field of 137Cs radiation source. The second chamber was a pen-like graphite chamber of G-5 type filled with C02. G-5 chamber is similar in construction to T-5 chamber described in Appendix A and is intended for beam dosimetry at high dose
92
rates. The values of fa for this chamber were calculated semitheoreucally, using the parameters resulting from the fit shortly described in Section 2.4. The values of f^ were measured for gas pressure up to 5.5 MPa.
For REM-2A1 chamber the best point of operation was chosen as p = 1 MPa, which is close to the possible maximum pressure for the chamber, and X» 150 V/cm. At such conditions the ratio fa/fy is of about 0.2. The REM-2A1 chamber is intended to work at low dose-rates, so usually the volume recombination can be neglected. It is not the case for G-5 chamber used at high dose rates. Because of volume recombination the applied voltage can't be too low and the choice of the voltage applied depends to some extent on the dose rate in investigated radiation field*34), as illustrated in Figure D-5 (see also Appendix A). The gas pressure should be of about 3 MPa, as further increase of gas pressure causes only small decrease of the fa/l^ ratio and the correction for volume recombination became unacceptable large. For most of practical cases the field strength should be of order 200 V/cm.
100 T
1 MPe
Dose rate (mGy/min)
Figure D-5. Minimum electrical field strength (in G-5 chamber), which ensures volume recombination below 1%.
p = 3.4 MPa
0 5 10 15Neutron energy (Me V)
Figure D-6. Relative neutron sensitivity ku of C-C02 chamber (of G-5 type) for optimum conditions.
The calculated values of ku for optimum conditions are shown in Figure D-6. As it can be seen the ky value for G-5 chamber is more than 10 times lower, comparing with C-C02 chambers operated at atmospheric gas pressure and high field strength (Figure D-1).
Experimental verification of neutron sensitivity function
The values of ky for ionisation chamber of G-5 type were determined experimentally in radiation fields of:— isotopic neutron sources: 239Pu-Be surrounded by 10 cm thick Fc filter
and 2$2Cf (bare),— cyclotron beam (12 MeV d + Be) with mean neutron energy of 5.5 MeV, and— 14 MeV neutron generator.Dose rate components were determined using the equation (D-2). The value of hu = 1 was used, basing
on calculated*34) energy dependence of hy. The chamber had been calibrated in reference field of 137Cs source, at the same values of supplying voltages, which were used for determination of ky
An example of experimental results is shown in Figure D-7 for mixed radiation field of 239Pu-Be source surrounded by 10 cm thick Fe filter. For this field the saturation curves of the chamber were measured at three values of filling gas pressure*34 162). Theoretical values of ku were calculated by integration of equation (D-10) over the neutron spectrum of 239Pu-Be and using the values of fa/fy calculated semitheoretically, as before (solid lines in the figure) The best fit to the measured saturation curves was
93
obtained for the value of the photon component of the absorbed dose equal to 10.6%. The points presented in the Figure D-7 were obtained assuming this value of the photon component.
Pu-Be + 10 cm Fe
p=1.03MPa
p=2.08MPa
p=3.05MPa p=3.05MPa-
Optimum conditions0.000
X (V/cm)
Figure D-7. Calculated (lines) and experimentally determined ratio of neutron-to-gamma sensitivity for C-C02 chamber of G-5 type, for radiation field of 239Pu-Be source in iron filter.
It was also shown*34*, that for the chamber operated at optimum conditions, Iq, do not exceed: 3% for the field of 14 MeV neutron generator, 1% for 5.5 MeV cyclotron beam, and 0.5% for 252Cf. The experimentally determined values of photon component to the absorbed dose, measured for several different neutron fields can be found in our earlier world162*.
The overall uncertainty of k^ was estimated to be about 20% and results mainly from the uncertainties in the mass stopping powers of heavy charged particles. The experimental values of fa/fy, used in calculation are also of limited accuracy. However, as the k^ values are very low, the above uncertainty introduces only small error to the value of the photon dose, determined with our method. The error can be assessed from the equation (D-2) as:
1 — ©vA<Dy =------ — Aku (D-13)
1 - ku
For radiotherapy beams the photon component ®y is generally in range from 5% to 20%. Therefore, if we even assume a constant value of k^-0.5% for beams with neutron energy up to 10 MeV and k^=l% for beams with neutron energy up to 15 MeV, the error introduced into the calculation of ©y usually does not exceed 1% and in most of the practical cases is less than 0.5%.
94
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ACKNOWLEDGEMENTS
This work is respectfully dedicated to Drs Mieczyslaw Zielc/ynski and Anthony H Sullivan - the two inventors and main animators of the recombination methods.
Firstly I owe thanks to my friend and teacher Prof. M. Zielczynski who over the years taught me the details of the recombination methods. His continuous enthusiasm and optimism created an atmosphere, in which I was able to develop my preliminary ideas into some methods that, I hope, can contribute to the field of mixed radiation dosimetry.
I had also the privilege of meeting Dr A. H. Sullivan to whom 1 wish to express here my sincere appreciation for our short but memorable discussions, for his help during measurements at CERN and for his generous hospitality.
Some experiments discussed in this work were performed at the Joint Institute for Nuclear Research in Dubna, in the Radiobiological Institute TNO in Rijswijk, in the Physikalisch Technische Bundesanstalt in Braunschweig and in CERN. I would like to express my gratitude for the invaluable help received in these laboratories.
Measurements at the medical facility of the JINR in Dubna were carried out with helpful support of several members of the Phasotron Laboratory . In particular, 1 would like to thank Dr E P. Cberevatenko for his assistance during the experiments as well as Mr S, V. Shvidkij for his skilled technical help in electronics.
I would like to express my very special gratitude to my colleagues from the Radiobiological Institute TNO for the way in which they friendly welcome me in their laboratories Over the year when I was working at the TNO this Institute became my second home. I would like especially to thank Dr J. Zoetelief and Prof. J. J. Brocrse who taught me, among others, the desperately needed self-dependence in my research. I also remember v ery well the pleasant daily cooperation of Mr A. C. Engels and Mr C J. Bouts and how they taught me to run and drive the TNO Van de Graff accelerator.
I am most grateful to the authorities of the Neutron Physics Division of Physikalisch Technische Bundesanstalt in Braunschweig for making available their irradiation facilities. I sincerely appreciate the interest of Dr W. Alberts to my work and his help in organisation of my measurements at PTB. Of great importance for me was the direct collaboration of Drs H J. Brede and S Gulabakke, who dedicated to me a lot of their time and experience. My appreciation is also extended to Dr H Schuhmacher for important discussions and for making available the detailed results of his calculations of microdosimetric spectra. I sincerely regret being not able to mention here all the people who greatly helped me during my short time stays in Braunschweig. The perfect "German" organisation in the best meaning of this word in connection with the great scientific potential of PTB and extremely friendly atmosphere provided me with some very important results in this work.
I would like to express my deep appreciation to Dr M. Htifert, Head of Radiation Protection Group at CERN for the excellent working conditions and efficient organisation of experiments as well as for giving me so much time from his busy schedule. I am very obliged to Mr R. Raffnsoe and to several people from CERN TIS RP staff for their friendly help in solving technical and bureaucratic problems.
I am truly grateful to Prof. D. Harder and to Dr E. R. Bartels for fruitful discussion and warm hospitality during my short but very effective visit to Gottingen
It was also very helpful for me to be encouraged by Dr H. G. Menzel.
105
Many thanks go to Dr M. Walig6rski for his helpful remarks and to Dr P. Olko who performed some microdosimetric calculations, cited in this work.
The work was completed at the Institute of Atomic Energy. I should like to express here my gratitude to the authorities of the IAE for looking with favour at my research activity.
In the period from 1990 to 1993 our investigations of the recombination methods were partly supported by the International Atomic Energy Agency under research contract No 6353/RB. I greatly appreciate the financial support of the Agency and the kind cooperation of the Project Officer Dr R Griffith.
I am also very grateful to the Polish State Committee of Scientific Research for financial support.
Finally, I want to use this opportunity to thank my husband Dr Andrzej Golnik. This work could have never been completed without his continues support, criticism and encouragement.
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