Neutron Dosimetry II

7/31/2019 Neutron Dosimetry II

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Neutron Interactions and

Dosimetry II

Paired Dosimeters

Calibration of the Low-Neutron-Sensitivity Dosimeter


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Separate Measurement of Neutron and -Ray

Dose Components by Paired Dosimeters

If a mixed n + field is measured by means of twodosimeters having different values ofB/A, Eq. (2)can then be applied to each one and solvedsimultaneously to obtainD andDn, so long asBandA have known values

The best dosimeter pair is a TE-plastic ionchamber containing TE gas (for whichB/A 1) tomeasure the total n + dose, and anonhydrogenous dosimeter having as little neutronsensitivity as possible to measure the dose

Ideally this dosimeter should measure only-rays


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Dosimeters with Comparable Neutron

and -Ray Sensitivities (B/A 1) A-150 TE plastic ion chambers (B/A 1)

Rossi TE proportional counter (B/A 1)

Tissue-equivalent plastic calorimeters (B/A 1)

Aqueous chemical dosimeters (B


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Gamma-Ray Dosimeters with Relatively

Low Neutron Sensitivity (B


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Non-hydrogenous Ion Chambers

Graphite-walled ion chambers through which CO2gas is flowed at 1 atm have the advantage of being

low in atomic number, thus avoiding overresponsefor low-energy rays due to the photoelectric

effect

However, the discrimination against neutrons is

only moderate, withB/A 0.30 at 15 MeV for a0.3 cm3 cylindrical chamber, decreasing gradually

as the neutron energy is increased


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Non-hydrogenous Ion Chambers

Somewhat better neutron discrimination canbe achieved with a magnesium chamber

containing argon, because of the decrease inthe energy transferred to the heavier nuclei

by neutron elastic scattering

For a 2.4-cm3 spherical Mg-Ar chamber theB/A value for 14.8-MeV neutrons is about

0.17


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Thermoluminescent Dosimeters

7LiF (TLD-700) and CaF2:Mn TLDs both haveB/A values comparable to that of the Mg-Ar ion

chamber Thus either of these TLDs can be employed as the

neutron-insensitive member of the paired-

dosimeter method

7LiF, at least, has been shown to have aB/A valuethat is nearly proportional to the energy of the fast

neutrons below 15 MeV


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A LiF (TLD-100) or 6LiF (TLD-600) TLD can beemployed as an indirect fast-neutron dosimeter bycoupling it with a large moderating mass, forexample, by wearing it in a personnel badge on thebody

The incident fast neutrons become thermalized bymultiple elastic collisions in the body and some ofthem diffuse back out to the dosimeter

This is called an albedo dosimeterbecause itsreading depends on the ability of the body toreflect the thermalized neutrons


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Since 6LiF and LiF (containing natural lithiumwith 7% 6Li content) both are sensitive to rays

also, it is usually necessary to provide a secondTLD in the dosimeter package that is insensitive to

thermal neutrons

Both dosimeters in the pair require -ray

calibration, as their -ray sensitivities are seldomidentical


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An alternative to using 7LiF as a separate -ray dosimeter in the albedo package is

offered by the fact that LiF and 6LiF showan extra TLD glow peak at about 250300C, produced by the thermal-neutron dose

deposited by the secondary -particle andthe triton


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X-Ray Film

Nuclear-track emulsions are thick enough to allowfast neutrons to scatter protons elastically, and toallow them to spend their energy internally inproducing chemically developable tracks

An x-ray film has an emulsion thickness of 25mg/cm2, which is comparable to the range of a 1-MeV proton

If the film is sandwiched between Pb foils to keepout protons from the films surroundings,B/A canbe reduced to even lower levels than thoseexhibited by 7LiF


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Miniature G-M Counters

A miniature stainless-steel G-M counterwith a high-Z filter to flatten the energy

dependence of the -ray response has beenfound to have the lowestB/A ratio of any

known -ray dosimeter: approximately 0.02

for 15-MeV neutrons, decreasing graduallywith decreasing neutron energy


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Calibration of a Tissue-Equivalent

Ion Chamber for n + Dosimetry The -ray calibration factorA is first obtained

from a 60Co -ray beam for which the free-spaceexposure rate is known

The absorbed dose at the center of an equilibriumsphere of tissue, 0.52 g/cm2 in radius, for a free-space exposureX(C/kg) at the same location, isgiven (in grays) by

tissTCPE

eneq

airair

( ) (3)c

WD K A X

e


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Ion Chamber for n + Dosimetrywhere 1.003,

Aeq = attenuation of photons in

penetrating to the center of the tissue sphere 0.988,

= 33.97 J/C, and

= the ratio of mass energy absorptioncoefficients for tissue/air, 0.0293/0.0266 =1.102

tiss

en air/

air

/W e


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Ion Chamber for n + Dosimetry Eq. (3) thus reduces to

If (Q)TE is the charge (C) produced in theTE ion chamber when it is given the same -

irradiation that depositsD (Gy) in the

tissue sphere, then

37.1 GyD X

TE

TE (C/Gy)Q

AD


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Ion Chamber for n + Dosimetry The absorbed doseD in muscle tissue can

be related to the dose (D )TE in the TE

plastic chamber wall under TCPEconditions by

TETCPE

enTE TE TE TE

TE

tissTE TE

Q Q D Q

A D DD D


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Ion Chamber for n + Dosimetry The B-G relation, assumed to be valid here,

allows one to write

Substituting gives

TE

TE

TE / /g g

Q V

D W e S

TE

enTE TE

tiss / /g g

VA

W e S


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Ion Chamber for n + Dosimetry The neutron calibration factorBTE for the

TE ion chamber can next be expressed in a

form similar to that ofATE:

CPETETE TE TE TE

TE tiss

TE TE

n n n n

n

n n n n

Q Q D QB F

D D D D


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Ion Chamber for n + Dosimetry Applying the B-G relation to the neutron

case:

Now substituting gives

TE

TE

TE / /

n

nn n gg

Q V

D W e S

TE

TE TEtiss

/ /n

n n gg

VB F

W e S


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Ion Chamber for n + Dosimetry (B/A)TE for the TE chamber is the ratio

The compositions of the TE gas and TE-plastic wall are sufficiently similar that the

stopping power ratios are both close to

unity, as is their ratio

TEtiss

TE

en TEtissTE TE

/ /

/ /

g g

n

n g n g

W e SBFA W e S


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Ion Chamber for n + Dosimetry The ratio is also nearly unity, since

differences in the carbon and oxygen content inthe gas and wall have no effect

These elements have practically identical en/values over the wide range of-ray energies wherethe Compton effect dominates

Therefore, for the TE-gas-filled TE-plasticchamber:

tiss

en TE/

TE

tissTE

/

/

g

n

n g

W eBF

A W e


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Ion Chamber for n + Dosimetry The value of the ratio is obtained

from tables such as those in Appendix F,

entered at the appropriate neutron energyfor A-150 plastic and ICRU muscle

The reciprocal of the W -ratio has been

computed as a function of neutron energyby Goodman and Coyne for methane-based

TE gas

TE

tissnF


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Calibration of the Low-Neutron-

Sensitivity Dosimeter In principle one could use

to calculateB/A for a graphite-CO2 or Mg-Ar ionchamber to be employed in the paired-dosimetermethod

The resultingB/A value so obtained is seldomaccurate enough to be useful, especially where the-ray content is fairly low

TEtiss

TE en

TEtiss

TE TE

/ /

/ /

g g

n

n g n g

W e SBF

A W e S


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Sensitivity Dosimeter The most practical approach to determiningB/A is

an experimental one employing a narrow neutron

beam of the desired spectrum The method makes use of a Pb filter to remove the-ray contamination from the beam, while passing

most of the neutrons, which have a smaller

attenuation coefficient

Secondary radiation produced in the filter escapesfrom the narrow beam


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Sensitivity Dosimeter A previously calibrated TE chamber is used to

calibrate the beam in terms of neutron tissue doseDn

The low-neutron-sensitivity dosimeter (x) forwhich the value of (B/A)x is to be determined isgiven an identical irradiation, yielding the readingQx

Bx is simply equal to Qx/Dn, assumingD to bezero

Ax for that dosimeter is obtained from a60Co -ray

exposure


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Sensitivity Dosimeter In practice one does not know the degree to

which the beam is initially contaminated

with radiation, how much Pb filtration isneeded to purify the beam adequately, orhow much of the -ray contamination mayhave come from elsewhere than the beam

portGamma rays from the face of the shield would,

for example, not be removed by a beam filter


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Sensitivity Dosimeter A solution to this problem was devised which uses

the narrow-beam Pb-filtration method for

determining (B/A)x The neutron beam was generated by 35-MeV

deuterons on Be; its average energy was 15 MeV

It was collimated by a 2-cm hole through a large

Benelex (pressed wood) shield, as shown in the

following diagram


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Sensitivity Dosimeter The dosimeters were a TE-plastic-TE-gas

chamber and an air-filled graphite chamber

The three beam filtrations chosen were openbeam, 7.6-cm Pb, and a steel plug 66 cm

long filling the entire bore hole

The six measurements and responseequations are listed in the following table


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Sensitivity Dosimeter was the absorbed dose at the measurement

point in the open beam due to rays coming out of

the beam port was the dose contributed by rays from

elsewheremostly H-capture rays emitted fromthe face of the Benelex shield

Dn is the open-beam neutron dose, is that withthe Pb filter, and that with the plug in place

aD

sD

nDn

D


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Sensitivity Dosimeter This experimental approach to determining (B/A)x

for a low-neutron-sensitivity dosimeter provides a

value that is consistent with the (B/A)TE of thetissue-equivalent chamber with which it is

compared, and is relevant to the neutron spectrum

of the beam used

The method works as well with TLDs, G-Mcounters or other nonhydrogenous dosimeters as it

does with ion chambers


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Sensitivity Dosimeter Narrow-beam geometry is required for this

calibration procedure

The beam must be narrow enough, and themeasurement location distant enough from the

filters, so that significant amounts of secondary

radiation from the filters cannot reach the

dosimeters The method therefore requires a collimatable

beam of neutrons

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Neutron Dosimetry II