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UID ATU 37675002 DVR 0005886TU Handelsgericht Wien Bankverbindung: Bank Austria-Creditanstalt AG Konto 51429141901 BLZ 12000 BIC BKAUATWW IBAN AT141200051429141901
Heavy charged particle thermoluminescent
efficiency of differently doped CaF2:Tm and
LiF:Mg,Ti phosphors with respect to 60Co
gamma irradiation
C. Gösselsberger, M. Hajek & M. Fugger
AIAU 28605
May 2008
2
Contents
Abstract ................................................................................................................................... 4
1. Thermoluminescence (TL) [1] ......................................................................................... 5
a) TL in History ................................................................................................................ 5
b) Theory of thermoluminescence .................................................................................... 6
c) Applications of TL ........................................................................................................ 8
d) Physical basics of dosimetry [2] ................................................................................... 9
Absorbed Dose .................................................................................................................. 9
Ionizing radiation .......................................................................................................... 10
Interactions between photons and irradiated matter .................................................. 10
Interactions between electrons and irradiated matter ................................................ 11
Linear Energy Transfer (LET) ...................................................................................... 11
Integral dose .................................................................................................................. 12
Detector .......................................................................................................................... 13
Ionization chambers ...................................................................................................... 13
e) TL-efficiency [3] .......................................................................................................... 14
2. HIMAC (Heavy Ion Medical Accelerator in Chiba) [4] ................................................ 15
3. Materials and Methods ................................................................................................. 18
a) TL-DAT-II ................................................................................................................... 18
b) TL-Dosimeter types .................................................................................................... 19
TLD-300: ........................................................................................................................ 19
TLD-600: ........................................................................................................................ 19
TLD-700: ........................................................................................................................ 19
MTT-7: ............................................................................................................................ 19
c) Glow-Curve measurements ....................................................................................... 20
d) Monte – Carlo Simulations ........................................................................................ 22
SRIM .............................................................................................................................. 22
3
Calculation Example: T6/02, BF=89.12 mm, nominal dose: 150.3 mGy .................... 23
e) Co-60 Theratron ......................................................................................................... 25
Calculation of the absorbed dose .................................................................................. 27
Example: Irradiation of T6/01 with Co-60-Theratron ................................................. 28
f) TLD-Software (glowcurve analysis) .......................................................................... 28
4. Results ............................................................................................................................ 31
a) Simulation and calculation of the LET and the actual dose .................................... 31
b) Efficiency and LET of TLD-300 ................................................................................. 32
c) Efficiency and LET of TLD-600 ................................................................................. 34
d) Efficiency and LET of TLD-700 ................................................................................. 36
e) Efficiency and LET of MTT-7 .................................................................................... 38
5. Discussion and Outlook ................................................................................................. 41
Acknowledgements ............................................................................................................... 41
References ............................................................................................................................. 42
4
Abstract
Thermoluminescence detectors (TLDs) have been used as powerful tools for radiation dose measurements for longer than half a century. Thus, the behavior of TLDs for monitoring γ - radiation is perfectly known. Due to the fact that heavy ion accelerators are used in medicine for cancer therapy and the patients’ radiation consumption has to be controlled, it is necessary to examine the efficiency of TLDs on heavy charged particles (HCP). Within this study the efficiency of different phosphors (TLD-300, TLD-600, TLD-700, purchased from Thermo Fisher Scientific Inc. (former Harshaw Chemical Co.) and MTT-7 purchased from TLD Poland) irradiated by HIMAC in Chiba, Japan with He-ions (150 MeV/amu) and Fe-ions (500 MeV/amu) respectively, are obtained. Furthermore the different TL responses to HCPs are presented.
5
1. Thermoluminescence (TL) [1]
Some insulators as Li store energy when they are exposed to ionizing radiation. This is done by bringing electrons in energetic higher stages. This energy is released in form of visible light by heating the crystal. The amount of light released is proportional to the amount of radiation received (dose).
a) TL in History The first scientifically recorded observation of thermoluminescence was made in 1663 by Robert Boyle. He wrote about a “glimmering light” from a diamond by “taking it to Bed with me, and holding it a good while upon a warm part of my Naked Body”. He also stimulated the emission of visible light by more conventional heating methods as placing the crystal on hot iron, by friction and from a candle. In 1676 Elsholtz observed a similar effect from the mineral fluorspar. Early explanations were that the heat was directly
converted to visible light. Oldenburg wrote 1676 about the thermoluminescence of a phosphor comes “from the Fire itself”. In 1726 Du Fay thought, that thermoluminescence occurred due to sulfur that actually burned during the process of the luminescence. But later he described that this emission of the visible light is nothing else than delayed phosphorescence.
He made experiments with quartz and showed, that the effect of luminescence could be repeated by putting the crystal into light. The heat only stimulates the emission but is
Fig. 1: Luminescence
6
not its cause. Probably the first time the word “thermoluminescence” was published in literature was 1895 by Wiedemann & Schmidt. Trowbridge & Burbank found in 1898 that after the heating of fluorite the thermoluminescence can be reexcited by exposing it to X-rays. Rutherford, Chadwick and Ellis made a lot of studies of radiation-induced thermoluminescence. In 1935 Lyman was one of the first using thermoluminescence in dosimetry. He used CaSO4:Mn to detect ultra-violet radiation from the light of a spark. It was recognized by Urbach in 1930 that the temperature at which thermoluminescence occurred was related to the electron trap depth. Randall & Wilkins and Garlick & Gibson calculated equations for the thermoluminescence glow peaks. From this point on thermoluminescence grew in popularity as a technique for obtaining information on trap distributions.
b) Theory of thermoluminescence
The emission of light lasts a characteristic time cτ after the absorption of the radiation
and this parameter allows to distinguish between fluorescence ( cτ < 10-8s) and
phosphorescence ( cτ >10-8s). In fluorescence the emission of light is taking place simultaneously with the absorption of radiation and immediately after stopping the radiation no more light is emitted. In phosphorescence on the other hand there is a delay between the radiation absorption and the emission of light. Thus, thermoluminescence is a kind of long-period phosphorescence: minutes < cτ < 94.6 10⋅ years. The intensity of
phosphorescence emission is given by 0( ) exp( / )I t I t τ= − . At constant temperature the decay is thus a simple exponential decay.
In general luminescence emission is explained by the transfer of energy from radiation to the electrons of the solid. Electrons can absorb the photons energy and are transferred from the ground state to an excited state. When an electron is falling back to the ground state a luminescence photon is emitted.
7
Thermoluminescence can be described using the electronic band structure of an insulator. Radiation excites an electron from the ground state (a) in the valence band to an excited state in the conduction band (b). The electron can now fall back to the valence band or - and this is important for thermoluminescence - fall into a trap in the bandgap (forbidden zone) (c). In this trap the electron can stay for minutes up to several 109 years. To the same time an electron of an occupied state of the bandgap (it is occupied because all the states beneath the Fermi level are full with electrons) (d) fills the hole in the valence band (e). The hole is moving up to the activator trap (f). Putting energy to the insulator (e.g. heating) the electron of the trap can be excited to the conduction band again and from there it can recombine with the hole in the activator trap (f). Within this action thermoluminescence light is emitted. The probability for this recombination is much higher than a falling back of the electron to the trap (c).
When analyzing thermoluminescence in crystals the temperature is increased in a linear way. Depending on the trap depth the electrons are recombining at certain temperatures and the photodetector will measure an intensity of the emitted light. The normal way to display thermoluminescence data is to plot luminescence intensity as a function of temperature – known as “glowcurve”.
Fig. 2: Thermoluminescence model - electronic band structure
8
The trap depth at (2) is the smallest one, so temperatures around 100°C are enough to start emptying the trap. This is Peak (2) in the glowcurve. When temperature increases also the traps with a larger depth ((3) to five (5)) make peaks (3) to (5) in the glowcurve (Fig. 3).
c) Applications of TL
Radiation dosimetry. The absorption of radiation increases the level of thermoluminescence observed from a specimen. The radiation brings the electrons to the traps. The absorption of heat from the environment on the other hand releases electrons from their traps. Thus, the intensity of thermoluminescence is the result of a competition between trap filling by radiation and trap emptying by thermal excitation. At a given temperature of irradiation, the intensity of thermoluminescence is proportional to the absorbed radiation dose. This led Daniels et altera to use thermoluminescence for radiation dosimetry. The first proper application was the measuring of radiation following an atomic weapon test using LiF as crystal for dosimetry. LiF was found to be a good material for radiation dosimetry because of its high sensitivity. LiF was also used to measure the received dose of cancer patients which were treated with radioactive isotopes (by Daniels, Boyd & Saunders, 1953). The patients swallowed small LiF pellets
Fig. 3: typical glowcurve for LiF
9
which were analyzed after the pellets left the digestive system again. It was also realized that some of the unpredictable behaviors of LiF can be offset by different annealing procedures. The first commercial thermoluminescence dosimeter is the TLD-100 which consist of LiF doped with approximately 170 mole ppm Mg and 10 mole ppm Ti. This material is still the most popular dosemeter in use today.
Age determination. The known relationship between thermoluminescence intensity and absorbed radiation dose can be used in age determination. E.g. pottery: the pot loses all (most) trapped electrons because of the heating when manufactured. Thus, the geological age of the pot is set to zero when newly formed. When the dose rate of the
habitat is known the age can be calculated by: __
absorbed doseagedose rate
= .
d) Physical basics of dosimetry [2]
Absorbed Dose The radiation caused effects are up to the kind of radiation and the interaction between radiation and irradiated subject. The absorbed dose is calculated as the quotient of absorbed energy and the mass of the absorbing medium.
[ ][ ]
absorbed energy Jabsorbed dosemass of the absorbing medium kg
=
The absorbed dose is given in Gray (Gy) and often named just dose. In clinical uses the dose is always relating to water, because water is nearly equivalent to human tissue. Another advantage in using the water dose is the ability to easily measure it using ionization chambers. The absorbed dose is measured in a defined area; all interactions outside this area must not be included in the measurements.
Fig. 4: Definition of the absorbed dose
10
E.g. a photon is totally absorbed in a certain area and its energy is released to an electron. The electron obtains this energy as kinetic energy and ionizes other atoms or molecules along its path. In this example the electron leaves the volume which is used to calculate the dose. All energy transfers taking place outside this volume (dotted line) are not taken into account by the determination of the absorbed dose.
The actual transferred energy is a stochastic value because every microscopic energy transfer underlies random fluctuations. Many processes together follow physical laws and thus are not random anymore. E.g. 1 Gy creates in a water cubic of 1 cm³ about 1015 ionizations.
The exact definition of the absorbed dose contains the stochastic behavior of microscopic processes. It is defined as the mean value of the absorbed energy in a certain small mass divided by this small mass. The mass (volume) has to be small to get the dose in a mathematical point.
0
limm
Dmε
→=
Ionizing radiation There are two types of ionizing radiation: direct ionizing radiation and indirect ionizing radiation. Direct ionizing radiation contains of charged particles like electrons, protons, α - particles, …, which are ionizing and exciting atoms and molecules along their path. Indirect ionizing radiation (photons, neutrons,…) needs two steps to ionize atoms or molecules: 1) charged particles are released 2) these particles can now ionize other atoms or molecules like primary charged particles.
To describe the first step of the energy transfer of indirect ionizing radiation KERMA (kinetic energy released per unit mass) is introduced:
0
lim tr
m
EKm→
=
KERMA is the converted energy per unit mass.
Interactions between photons and irradiated matter The most important effects of photon-matter interactions are the Photoelectric Effect, the Compton Effect and the Pairing Effect. The weakening of the photon radiation can be described by:
( ) (0) exp( )N z N zµ= ⋅ − ⋅
11
Where N(z) is the amount of photons after passing a material with weakening coefficient µ and a thickness of z. The weakening coefficient is often dependent on energy.
Interactions between electrons and irradiated matter When electrons are pervading matter they interact with atoms and molecules on their way and leave parts of their kinetic energy. The electrons are slowed down in matter by: 1) Inelastic collisions with mantle electrons ionization or excitation of the atom and 2) creation of bremsstrahlung (photons) by acceleration of electrons.
The stopping power is defined as the mean value of the energy loss of an electron along a certain distance (unit: J/m):
dESdx
=
The total stopping power is the sum of the collision stopping power and the stopping power through creation of bremsstrahlung.
col radS S S= +
Linear Energy Transfer (LET) LET (linear energy transfer) is a concept for ionizing radiation and the rate at which the ionizing particle loses energy as it passes through a material. A high-LET particle loses its kinetic energy rapidly as it passes a material and a low-LET particle will come much longer distances while losing the same amount of energy.
12
Calculating the energy transfer e.g. in biological cells it is important to know the part of the stopping power which is correlated with a local energy absorption. To it belong the ionization and excitation of atoms as well as production of δ -electrons with energy lower than a specific value ∆ .
( )dELETdx∆ ∆=
When the specific ∆ becomes ∞ the LET becomes colS . The LET is used to describe the type of radiation.
Integral dose Since all forms of energy absorption through ionizing radiation is resulting from different interactions of electrons the absorbed dose can be written in integral form as:
max
0( )
colE
m Em
SD dEρ
= Φ∫
colS is the energy-dependent collision stopping power in a medium m with the density ρ and EdEΦ is the fluence of the electrons in the interval E and E+dE.
Fig. 5: High and low LET values, same applied energy
13
It is often necessary to convert the absorbed dose to different materials. E.g. the ionization chamber is filled with air and one need the absorbed dose in water for clinical uses. The dose-conversion-factor f is calculated as follows:
max
max
0
0
( )
( )
colE
Ewater
colE
Eair
S dEf
S dE
ρ
ρ
Φ=
Φ
∫
∫.
Or easier:
( )col
waterair
Sfρ
= .
Detector There are three things one has to bear in mind using a detector to measure radiation:
• The detector states a disturbance of the radiation field around the measuring point.
• The detector measures an absorbed dose belonging to the interactions of the detector material.
• The absorbed dose has to be converted to the right reference material.
The sensor should be constructed in a way to do not disturb the surrounding medium. The sensors which are calibrated by authorized institutes get a calibration factor to calculate the absorbed dose as follows:
D M N= ⋅
M is the measured value and N the calibration factor.
Ionization chambers The ionization chambers used for clinical issues are measuring the absorbed dose in water and are calibrated with a Co-60 radiation source at a distance of 100 cm at a temperature of 293.15 K and air pressure of 101.325 kPa.
In normal use of these ionization chambers the calibration conditions are not fulfilled. Thus, different correction factors ki have to be calculated.
01
( )m
w ii
D k N M M=
= ⋅ ⋅ −∏
M0 is the displayed value at zero radiation.
14
An example for a correction factor is the one for temperature and air pressure which is calculated as follows:
0
0p
pTkT p
= ⋅ .
e) TL-efficiency [3]
Thermoluminescence detectors (TLDs) have been used as powerful tools for radiation dose measurements for longer than half a century. Thus, the behavior of TLDs for monitoring γ - radiation is perfectly known. Due to the fact that heavy ion accelerators are used in medicine for cancer therapy and the patients’ radiation consumption has to be controlled, it is necessary to examine the efficiency of TLDs on heavy charged particles (HCP).
The absolute or intrinsic TL-efficiency, α , of a phosphor is defined as the ratio of the mean energy emitted as TL light, 0ε , to the mean energy applied to the TL phosphor by the irradiation field, ε :
0εαε
= .
The determination of this intrinsic efficiency is very complex and can be barely detected. Thus, the relative TL-efficiency, i.e. the TL signal produced per unit dose and unit mass by the radiation under study (in this study: He and Fe ions) with respect to the TL signal per unit dose and unit mass produced by a reference radiation (in this study: Co-60 γ -rays) is determined. The relative TL efficiency ,HCP γη is given by
,//
HCP HCPHCP
R DR Dγ
γ γ
η =
where HCPR and Rγ are the TL responses per unit mass for the radiation under study
(HCP) and the reference radiation (γ ) at dose levels HCPD and Dγ , respectively.
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2. HIMAC (Heavy Ion Medical Accelerator in Chiba) [4]
The National Institute of Radiological Sciences (NIRS) started charged particle therapy in 1994 with the world’s first heavy ion medical accelerator HIMAC. By March 2007 more than 3000 patients have been treated using HIMAC. The charged particles (carbon atoms) have an advantage of about three times the RBE (relative biological effectiveness) compared to gamma rays, and an OER (oxygen enhancement ratio), where a smaller value is better, of about half that of gamma rays.
Fig. 6: Heavy Ion Medical Accelerator in Chiba
16
It is also important that the radiation used in therapy causes less damage to healthy tissues. Therefore, it is necessary to know how the radiation dose will distribute in the different tissues. Both gamma ray and fast neutron beams are strongest near the surface of the human body and are attenuated in deeper areas of the tissues. When treating a deep cancer a charged particle beam can yield good results because the relative dose is much higher after several centimeters of tissue.
Fig. 7: Relative biological effectiveness left and oxygen enhancement ratio right of various radiation types
Fig. 8: Distribution of depth versus dose for various types of radiation in the living body
17
The peak of the charged particle beam can be focused on the location of the tumor, where the beam becomes dramatically intense. Before and after this region the applied energy is weak and therefore preserves the healthy tissue.
The HIMAC complex consists of heavy-ion accelerators and four irradiation ports, and can accelerate heavy ions up to approximately 80% of the speed of light. HIMAC is used for cancer treatments from 7 a.m. to 7 p.m. on weekdays and provides heavy-ion beams for fundamental research at night and on weekends. These research projects involve biology, chemistry and physics as well as medical physics and result in over sixty papers a year.
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3. Materials and Methods
a) TL-DAT-II The labor-made read-out system TL-DAT-II consists of a heating chamber, a photodetector and a PC with a specific software. The chamber with the heating plate can
be evacuated after positioning the dosimeter chip. During the measurement this chamber is flooded with poor nitrogen (99.999% N) to prevent any artifacts from dust particles in the “normal” labor air. For all measurements of this project work the heating rate is set to 5°C per second. Depending on the dosimeter used the TL-DAT-II heats up to 400°C and 480°C respectively. Different filters can be assembled to the photodetector in order to do not exceed a maximum number of 20,000 counts. This is important because above this number the photodetector leaves its linearity.
Fig. 9: Dosimeter folder, heating plate and closure head
19
b) TL-Dosimeter types
TLD-300:
• CaF2:Tm
• Bake out: 1 hour at 400°C, slow cool down
• Manufacturer: Thermo-Fisher Scientific Inc. (former: Harshaw Chemical Co.)
TLD-600:
• LiF:Mg,Ti
• 300 ppm Mg, 11 ppm Ti
• Enriched with Li-6 ~ 95%
• Bake out: 1 hour at 400°C, slow cool down
• Manufacturer: Thermo-Fisher Scientific Inc. (former: Harshaw Chemical Co.)
TLD-700:
• LiF:Mg,Ti
• 300 ppm Mg, 11 ppm Ti
• Enriched with Li-7 ~ 99.99%
• Bake out: 1 hour at 400°C, slow cool down
• Manufacturer: Thermo-Fisher Scientific Inc. (former: Harshaw Chemical Co.)
MTT-7:
• LiF:Mg,Ti
• 50 ppm Mg, 120 ppm Ti
• Enriched with Li-7 ~ 99.99%
• Bake out: 1 hour at 400°C + 2 hours at 100°C, fast cool down
• Manufacturer: TLD-Poland, Krakau
20
c) Glow-Curve measurements
Irradiated chips. Four different dosimeter types (TLD-300, TLD-600, TLD-700 and MTT7) were used within this study. At HIMAC in Chiba, Japan different doses of heavy-ion radiation were applied to the dosimeter chips (i.e. 50 mGy and 150 mGy). The ions used were: He-150 (150 MeV/amu, completely ionized) and Fe-500 (500 MeV/amu). For each ion and each dosimeter type different binary filters (BF) from 0 mm to 139.27 mm were prepended.
Measurement. Every folder consists of four to six dosimeter chips of one type with identical irradiation data. Thus, one gets at least for data points for each irradiation situation.
TLD-600 and TLD-700 chips were measured as follows:
1) TL-DAT-II and the Computer are turned on. IR filter and NG-3 filter are applied to the photodetector. N flush is activated, water as coolant is turned on, high voltage for the photodetector is turned on, and vacuum pump is turned on. Setting the measurement parameters in the computer software.
2) A test run with no chip in the chamber is performed.
3) The first dosimeter chip is positioned in the middle of the heating plate and the closure head is put on again.
4) Starting to evacuate the chamber for about 10 seconds.
5) Flooding the chamber with poor nitrogen (the whole measurement). Make sure that the chip is not jumping away when N-flooding is turned on.
6) Setting the filename (e.g. He1-T600.01T) and the comments (e.g. HIMAC 2007/01, BF 0.00 mm, 150 mGy, IR, NG3)
7) Start to heat from ~20°C (room temperature) up to 480°C with an velocity of 5°C per second
8) After 480°C are reached the plate is cooled down again. At this moment the N-flooding can be turned off again.
9) Wait until the heating plate has 85°C or less before taking the chip out again (putting it to a cleaned copper-plate, because when the chip is too hot the polystyrene folder will melt).
10) Save the displayed glowcurve.
11) When the heating plate is cooled down to at least 45°C the next chip can be put in the heating chamber of the TL-DAT-II.
21
12) For each ion and for each chip-type at least one background-measurement should be performed. To do so set the background parameter to 80°C.
TLD-300 chips were measured as follows:
Analog to the measurement of TLD-600 and TLD-700 but:
• No IR-filter
• Maximum temperature: 400°C
MTT-7 chips were measured as follows:
Before doing steps 1) to 12) from above an annealing procedure is performed. For this all dosimeter chips are arranged on a special panel designed for the oven used for the annealing which is performed for thirty minutes at 120°C. This is important to erase peak number two, three and four to get a more exact value for the peak of interest (number five). After about one hour, when temperature is at least down to 45°C the dosimeter chips can be taken out of the oven and the measurements (see above) can be started. The only difference is, that there is no NG-3 filter attached to the photodetector.
Fig. 10: Equipment for the annealing
22
d) Monte – Carlo Simulations Monte-Carlo (MC)-simulation is a stochastic process, where random experiments are often performed. The Law of large numbers is the justification for using MC simulations to calculate complicated mathematical and physical problems.
Within this project work MC-simulation software – SRIM 2008 – was used to calculate (simulate) the factual LET (linear energy transfer) and the factual dose in the dosimeter crystal.
SRIM SRIM calculates with the newest cross sections of the different ions and atoms. It consists of two main applications: “Stopping/Range Tables” and “TRIM Calculation”.
Calculating LET.
Using “Ion Stopping and Range Tables” one can enter an ion and its lowest and highest energy as well as the target material and the software will calculate the dedicated LETs and the projected ranges within this material.
Using “TRIM Calculation” one can enter ion data and the different target date (binary filter in mm H2O equivalent, ionization chamber – 1mm H2O equivalent, polystyrene from the dosimeter folder and finally the dosimeter crystal itself) and the software will simulate a certain amount of ions and calculate the dedicated delivered energies of the transmitted ions. Within this project work 500 ions were simulated respectively. The
Fig. 11: Irradiation assembly: LET and dose are calculated when entering the dosimeter crystal and when leaving it – the mean value of these values is finally calculated
23
mean value of these energies is calculated and set as lowest and highest ion energy in the “Range Tables” where finally the dedicated LET is calculated. This procedure is done first without the dosimeter itself at the target list LETin (LET when entering the dosimeter) and finally with the dosimeter itself LETout (LET when beam is leaving the dosimeter material again).With these two values the mean LET in the dosimeter can be calculated.
Calculating the dose.
The dosein and the doseout can be calculated by extrapolating the dose before and after the dosimeter in EXCEL. For this procedure the x mm polystyrene and the x mm LiF and CaF2 have to be converted to y mm H2O using the “Stopping Range Tables” for the different materials.
Calculation Example: T6/02, BF=89.12 mm, nominal dose: 150.3 mGy Starting Energy: He-150 means fully ionized He-ions with energy of 150MeV/amu, i.e. 150 1000 4.003 600450keV⋅ ⋅ = . In “Stopping and Range Tables” of SRIM the projected range for He-ions up to this energy are calculated. Furthermore the values of energy and range before and after the maximum range (this can be found on the HIMAC measurement protocol) are used to extrapolate the energy value dedicated to the maximum range. This energy is the starting energy of the He-150 ions. In our example the starting energy equals to 578190 keV. (This is the energy before entering the first binary filter. The difference to the 600450 keV is lost before reaching the first BF). This interpolation is done with MS EXCEL.
Fig. 12: SRIM calculation software: „Ion Stopping and Range Tables“ and „Trim calculation“
24
LET: To calculate the linear energy transfer in the dosimeter material “TRIM=Transmission of Ions in Matter” is used, where the ion energy is set to the calculated value from above. E.g. for LET calculation of an TLD-600 with BF 89.12 mm two steps are necessary: 1) Simulate the transmission of 500 ions through the ionization chamber and the binary filters as well as 1 mm polystyrene (folder) to get values for the ion energy at the beginning of the dosimeter. Use the mean value of the delivered energies to calculate the LETin using “Range Tables”. 2) like 1) but include the thickness of 0.89 mm of the dosimeter chip itself as an target layer same simulation of 500 ions delivered energy mean value LETout. The mean value of the LET this example equals to 3.4keV/µm.
Dose: The dose applied to the dosimeter after e.g. 89.12 mm BF is calculated by extrapolating the normalized values of the adjacent Gy/count values of the different ranges. There are also two values to be calculated: 1) the dose at the beginning of the dosimeter: the range is added as follows: BF, which is already stated in mm H2O (89.12) plus 1 mm H2O for the ionization chamber plus 1 mm polystyrene, which has to be converted to mm H2O using the range tables (therefore the range of He-ions in H2O and polystyrene are compared); 2) the dose at the end of the dosimeter: range = like 1) plus 0.89 mm LiF (converted to equivalent of mm H2O as above);
Fig. 13: TRIM simulation: Energy Loss versus Target Depth – the Bragg-Peak is clearly visible on the right hand side
25
Averaging the two values delivers the actual dose applied to the dosimeter chip. For this example the nominal dose is 150.3 mGy. The calculated actual dose adds up to 204.46 mGy.
e) Co-60 Theratron
After reading out the dosimeter chips with the TL-DAT-II all the chips comes into the oven for bake out to make them ready for use again. Without the bake out procedures the dosimeters’ sensitivity could change over 200%.
To perform a calibration measurement for each crystal the chips have to be exposed to a known radiation dose which should be similar the primary dose. For this study the irradiation for calibration was carried out using the Co-60 Theratron at the AKH in Vienna.
The alignment consists of two separate rooms. In room one the control systems for the radiation source and the display for the ionization chamber are located. In the second room stand the radiation source and the assembly of ionization chamber and dosimeter folder.
At the beginning of the irradiation procedure the light visor is activated to align the setup of ionization chamber (0.6 ccm) and dosimeter folder. The distance between irradiation source and dosimeter is about 1.5 meters.
Fig. 14: Theratron 780, AKH Vienna
26
After completing the setup this room has to be left and the door closed otherwise the irradiation cannot be started (part of the safety system).
At the beginning a test run was performed to find out the applied energy dose per time unit. To change the applied dose it was just necessary to change the irradiation duration and the distance folder – source remained fixed.
Then two dosimeter-folders containing chips that got a similar dose, were put besides the ionization chamber and the irradiation was started.
Fig. 15: Setup of ionization chamber and dosimeter folder
Fig. 16: Display unit for the ionization chamber, type: 2570/1 FARMER
27
The display unit for the ionization chamber was a FARMER type 2570/1. On the display an uncorrected value for the applied dose is shown. This value M has to be corrected by temperature and ambient pressure.
Calculation of the absorbed dose Co-60: energy of the photons: E=1.25 MeV
Mass-Energyabsorption-Coefficients for a photon-energy of 1.25 MeV (data: NIST Physics Reference Data 2008)
W kN f N= ⋅
W W pD N M k= ⋅ ⋅
K pK N M k= ⋅ ⋅
M… value on display of the ionization chamber
NW, NK … calibration factor for absorbed dose in water and air-KERMA respectively (for CO-60)
f… dose conversion factor
kp… correction factor for air pressure and temperature
The correction factor kp is calculated as described in the basics chapter above. The values at our calibration measurements at the Co-60-Theratron for temperature and air pressure were: T=295.15 K and p=100.11 kPa. The pressure has to be corrected itself: p h h= − ∆ ⋅ . The pressure p is the barometer value h minus a temperature depended
value ∆ multiplied by h. Using these values kp equals to 1.019.
Air: 2/ 2.666 10 ² /en cm gµ ρ −= ⋅
Water: 2/ 2.965 10 ² /en cm gµ ρ −= ⋅
Tissue (ICRU-44): 2/ 2.938 10 ² /en cm gµ ρ −= ⋅
28
Dose conversion factor: ( ) ( )
1.105 1.112
col enwater waterair air
Sf
f
µρ ρ
= ≅
= ≅is calculated using the following values:
( ) 1.829 ² /
( ) 1.655 ² /
( ) 1.818 ² /
colwater
colair
coltissue
S MeV cm g
S MeV cm g
S MeV cm g
ρ
ρ
ρ
= ⋅
= ⋅
= ⋅
Nk = 0.87 for Farmer 2570/1B (serial no. 1086) with chamber 2571 (serial no. 2178)
1.105 0.87 0.961.112 0.87 0.97
W k
W k
N f NN f N
= ⋅ = ⋅ == ⋅ = ⋅ =
using the two different f values.
Data as written on the calibration sheet of BEV: Nw=0.95 (15.12.2005).
1.105 1.019 0.87 0.9798W p kD f k N M M M= ⋅ ⋅ ⋅ = ⋅ ⋅ ⋅ = ⋅
Data as written on the calibration sheet of ARC (2007): Nk=0.85; Nw=0.94
0.9579wD M= ⋅
For this study the calibration data of ARC (Austrian Research Center) is used for calculating the absorbed doses at the Co-60-Theratron.
Example: Irradiation of T6/01 with Co-60-Theratron The dose of the T6/01 dosimeter folder at HIMAC irradiation is 151.65 mGy. For calibration it is very important to use similar doses.
During a 100 second test measurement a dose of M=169.5 mGy was displayed. This value was used to calculate the irradiation duration to get about 150 mGy.
Using these values the T6/01 dosimeter folder was irradiated for 85 seconds and got an M=143.7 mGy. The absorbed dose equals to 0.9579 143.7 137.7wD mGy= ⋅ = (using the ARC data).
f) TLD-Software (glowcurve analysis)
For the glowcurve analysis the software TLD (WEGS Data) in version V 03.32 is used. All the glowcurves of one dosimeter type irradiated by the same HCP are opened in TLD.
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First of all a background fitting is performed using batch-processing. The part of the glowcurve where the blackbody radiation is visible is used to calculate a synthetic background which is finally subtracted (for TLD-600, TLD-700 and MTT-7 the interval 390°C-450°C is used, for TLD-300 the interval is set to 340°C-395°C)
At the same time a smoothing (21-point) is performed and the maximum of peak 5 is transferred to 220°C (for TLD-300 to 254°C). Then the peak height at 220°C (254°C) is used to calculate the absorbed dose.
The calibration factor is calculated by using the peak 5 height of the Theratron-irradiated dosimeter chips. For every dosimeter the calibration factor is calculated by
( )dosekcounts γ= . To get the absorbed dose of the HIMAC-irradiated crystal one can easily
calculate it by HCP HCPdose counts k= ⋅ . This is done for every single chip single chip calibration.
Fig. 17: glowcurve of TLD-700, irradiated with Fe500 before and after performing the background fitting, the smoothing and the transferring of the peak 5 maximum to 220°C
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As already described above the efficiency of the TLD material is ,//
HCP HCPHCP
R DR Dγ
γ γ
η = . Ri
are the TL response signals (counts). It is calculated by corresponding the measured doseHCP to the effective dose calculated using SRIM.
The dependence of the efficiency ,HCP γη on the LET, is shown in the results section using
graphs with bidirectional error bars. All graphs are drawn with Sigma-Plot in version 10.
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4. Results
a) Simulation and calculation of the LET and the actual dose
Fig. 18: LET and actual dose of different TL phosphors irradiated with He150 and Fe500, respectively
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b) Efficiency and LET of TLD-300
Fig. 19: LET values and efficiency of TLD-300, He-150
Fig. 20: LET values and efficiency of TLD-300, Fe-500
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Fig. 21: Efficiency for different LETs in water
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c) Efficiency and LET of TLD-600
Fig. 22: LET values and efficiency of TLD-600, He-150
Fig. 23: LET values and efficiency of TLD-600, Fe-500
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Fig. 24: Efficiency for different LETs in water
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Fig. 26: LET values and efficiency of TLD-700, Fe-500
d) Efficiency and LET of TLD-700
Fig. 25: LET values and efficiency of TLD-700, He-150
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Fig. 27: Efficiency for different LETs in water
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e) Efficiency and LET of MTT-7
Fig. 28: LET values and efficiency of MTT-7, He-150
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Fig. 29: LET values and efficiency of TLD-700, Fe-500
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Fig. 30: Efficiency for different LETs in water
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5. Discussion and Outlook
Within this study the efficiency of different phosphors (TLD-300, TLD-600, TLD-700, purchased from Thermo Fisher Scientific Inc. (former Harshaw Chemical Co.) and MTT-7 purchased from TLD Poland) irradiated by HIMAC in Chiba, Japan with He-ions (150 MeV/amu) and Fe-ions (500 MeV/amu) respectively, are obtained.
The results section presents the TL efficiency with respect to Co-60 for glow peak 5 for HCP radiation of different TLDs. The TL efficiency shows remarkable differences due to the irradiated HCPs. ,HCP γη is about 1 for He150 irradiated chips and beneath 1 for the
Fe500 irradiated chips (0.4-0.8). Furthermore the study shows a trend which reveals that the TL efficiency decreases with increasing LET.
The study shows how the TLD efficiency of different phosphors on HCPs can be measured, which is important for the clinical use. Thus, it is possible to control the patients’ radiation consumption during e.g. cancer therapy with heavy charged particles using different thermo luminescent phosphors. A main advantage in using TLDs is that they can be very small and do not need any current supply or cables during the measurements.
The results of this study justify also this special medical use of examined dosimeter types.
Acknowledgements
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References
[1] S.W.S. McKeever (1983) Thermoluminescence of solids, Cambridge University Press
[2] M. Wannemacher, J. Debus, F. Wenz (2006), Strahlentherapie, Springer
[3] T. Berger, M. Hajek (2007) TL-efficiency – Overview and experimental results over the years, Radiation Measurements, Elsevier
[4] Website of the National Institute of Radiological Sciences (NIRS): www.nirs.go.jp (himac-single.pdf)
