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Relative thermoluminescent yield of heavy charged particles: Theory and experiment
Nuclear Instruments and Methods 175 (1980) 29-30 O North-Holland Publishing Company
Part III. Radial~on effects on solid state dosimeters RELATIVE THERMOLUMINESCENT EXPERIMENT *
Y I E L D O F H E A V Y C H A R G E D P A R T I C L E S : T H E O R Y AND
Yigal S. HOROWlTZ and John KALEF-EZRA **
Physics Department, Ben Gurion University of the Negev, Beersheva, Israel
The relative thermoluminescent yield of 4 MeV alpha particles to 4.1 keV X-raysin LiF-TLDwas calculated using a modified track structure model and compared with experimental measurements. Good quantitative agreement is obtained between theory and experiment.
1. Introduction
2. Theory
The use of thermoluminescent dosimeters (TLDs) in diverse radiation fields such as mixed neutrongamma and heavy charged particles (HCP) is hampered by the problem of the translation of the TL signal to units of absorbed dose. This problem arises because the TL signal produced per imparted energy is a function of ionization density. Previously, many workers in the field have chosen to assume that the relative TL production efficiency, rL (defined as the TL signal/imparted energy by the radiation field in question divided by the TL signal/imparted energy by a reference radiation, both at low absorbed doses) is a nearly universal function of linear energy transfer (LET) and have folded the r/-LET function with the dose-LET distribution in order to calibrate the TL signal for a particular radiation field. It is for this reason that many groups [1] have studied the TL-LET response of various materials in the expectation of establishing a class of TL materials with a universal T L - L E T relationship. Unfortunately, Horowitz et al. [2,3], have recently established the non-universality of the T L - L E T dependence in various batches of commercial LiF and Li2B407 TLDs. Moreover, conventional track structure theory (TST) calculations [4,5] have predicted significantly different values of r? for particles of the same LET but different charge and mass.
In the present paper we report our initial investigations regarding the degree of applicability of TST to the prediction of HCP induced TL in LiF-TLD. In TST the relative TL response, ~, to HCPs (widely believed to arise from recombination of liberated charge carriers by the primary HCP and/or from saturation of activator sites) is treated in a 3-d model whose main premise is that the density of liberated charge carriers around the track of the HCP is the only parameter that governs the dependence of 77 on particle type and energy. It follows that ~ij of an HCP with initial energy E0 should be given by R oo
f f o
l, E) 2,r at
o
,Tij -
,
f o
(1)
Di(r, l, E) 27rr dr dl o
where D(r, l,E) is the absorbed dose of the ejected charge carriers around the HCP track (of average track length R in the TLD), i,j define the HCP and the electron reference radiation respectively and the dose-TL response f(D) is defined as the ratio of the TL signal/imparted energy at a particular absorbed dose, D, from the reference radiation field to the TL signal/imparted energy at low dose from the same radiation field. The following points deserve special emphasis: 1) In conventional TST, f(D) is generated, by 6°Co gamma rays. Since f(D) is strongly dependent on radiation quality and furthermore, since the average energy expended per ion pair formed is energy
* Partially supported by the United States-Israel Bi-National Science Foundation. ** In partial fulfillment of the requirements for the Ph.D. degree in Physics. 29
HI. RADIATIONEFFECTS
Y.S. Horowitz-Ezra / Relative TL yield
30
Table 1 Heavy charged particle induced TL in LiF and BeO Theoretical
Experimental
Material
HCP
Energy (MeV)
f(D)
LiF
c~
4
3H-betas
Beo
160
166
9 kVeff. X-rays
HCP
Energy (MeV)
n
0.56
a
4
0.54 -+0.04
2.27
12C ZONe 4°Ar
125 208 416
2.06 _+0.14 2.25 ± 0.38 2.20 ± 0.22
dependent for low energy electrons it follows that f(D) should be generated by electrons of initial spectrum (as far as possible) similar to that of the initial energy spectrum ejected by the HCP. Since the maximum energy of a free electron ejected by a 4 MeV alpha particle cannot exceed about 2 keV we cannot properly use f(D) generated by 6°Co with Compton electrons o f initial energies greater by two orders of magnitude. We have therefore used 3H betas and unfiltered 20 kVp X-rays to generate f(D). 2) Previous attempts to apply TST to the calculation of HCP TL yields [4,5] have employed approximate analytic models for the calculation o f D(r, l, E). We employ, however, recent calculations o f D(r, E) with appropriate scaling factors based on ionization chamber measurements in tissue equivalent gas [6]. 3) There are other important advantages to.generating f(D) with low energy electrons: approximate matching of the volumes irradiated by the HCP and the reference radiation, elimination of the possibility o f distortion o f f ( D ) at high doses due to TL self-absorption and reduction by many orders o f magnitude o f the lattice defects created by the reference radiation.
geometry identical to the TLD irradiation. Although 3H betas and 20 kVp X-rays were used to generate f(D) we chose to measure the TL signal per imparted energy for the low energy electrons using the X-ray spectrum generated by a 3H beta source (tritium absorbed in Ti) which was covered by a thin mylar foil (average energy of the X-rays was 4.1 keV). Calibration o f the X-ray fluence was carried out using a windowless, 4 mm thick, Si(li) detector also in identical geometrical configuration as the TLD irradiation. The good agreement between theory and experiment is shown in table 1. Substitutingf(D) = 1 in the supralinear region fields r / = 0.4 which illustrates the importance of the supralinear dose contribution. We have also calculated ~ for BeO irradiated by 12C and 2°Ne ions using experimental data from Tochilin et al. [7]. Given the imperfect matching of the experimental and theoretical parameters, we consider the agreement satisfactory. We tentatively conclude on the basis of our still limited data that modified TST with appropriate matching of the reference and HCP secondary electron spectra can be used to calculate HCP TL yields to good accuracy.
References 3. Results and discussion The TLDs employed were commercial bulb dosimeters (Harshaw). Since b o t h r7 and f(D) depend on the particular mapping o f the of the impurity concentrations and defects of the host TL materials, we always measure both o f these parameters in identical TLDs. The experimental measurement o f 77ij involves the measurement o f the TL signal per imparted energy for the HCP and the reference radiation. Alpha particle energy (4 MeV) and fluence calibrations were carried out using a Si surface barrier detector in
[1] Y. Furuta et al., Proc. 4th Int. Conf. on Luminescence dosimetry, Cracow, Poland (1974) p. 96 and references therein. [2] Y.S. Horowitz et al., Nucl. Instr. and Meth. 165 (1979) 27. [3] Y.S. Horowitz et al., Phys. Med. Biol. 24 (1979) 1268. [4] R. Katz et al., in Topics in radiation dosimetry, Suppl. 1 (ed. F. Attix; Academic Press, 1972) Ch. 6. [51 D. Zimmerman, Rad. Eft. 14 (1972) 81. [6~ M.N. Varma et at., Proc. 6th Symp. on Microdosimetry (eds. J. Booz and H. Ebert; Harwood Academic Publ., 1978) p. 227 and references therein. [71 E. Tochilin et al., Proc. 2nd Int. Conf. on Luminescence dosimetry, Conf. 680920 (1968) p. 424.
Part III. Radial~on effects on solid state dosimeters RELATIVE THERMOLUMINESCENT EXPERIMENT *
Y I E L D O F H E A V Y C H A R G E D P A R T I C L E S : T H E O R Y AND
Yigal S. HOROWlTZ and John KALEF-EZRA **
Physics Department, Ben Gurion University of the Negev, Beersheva, Israel
The relative thermoluminescent yield of 4 MeV alpha particles to 4.1 keV X-raysin LiF-TLDwas calculated using a modified track structure model and compared with experimental measurements. Good quantitative agreement is obtained between theory and experiment.
1. Introduction
2. Theory
The use of thermoluminescent dosimeters (TLDs) in diverse radiation fields such as mixed neutrongamma and heavy charged particles (HCP) is hampered by the problem of the translation of the TL signal to units of absorbed dose. This problem arises because the TL signal produced per imparted energy is a function of ionization density. Previously, many workers in the field have chosen to assume that the relative TL production efficiency, rL (defined as the TL signal/imparted energy by the radiation field in question divided by the TL signal/imparted energy by a reference radiation, both at low absorbed doses) is a nearly universal function of linear energy transfer (LET) and have folded the r/-LET function with the dose-LET distribution in order to calibrate the TL signal for a particular radiation field. It is for this reason that many groups [1] have studied the TL-LET response of various materials in the expectation of establishing a class of TL materials with a universal T L - L E T relationship. Unfortunately, Horowitz et al. [2,3], have recently established the non-universality of the T L - L E T dependence in various batches of commercial LiF and Li2B407 TLDs. Moreover, conventional track structure theory (TST) calculations [4,5] have predicted significantly different values of r? for particles of the same LET but different charge and mass.
In the present paper we report our initial investigations regarding the degree of applicability of TST to the prediction of HCP induced TL in LiF-TLD. In TST the relative TL response, ~, to HCPs (widely believed to arise from recombination of liberated charge carriers by the primary HCP and/or from saturation of activator sites) is treated in a 3-d model whose main premise is that the density of liberated charge carriers around the track of the HCP is the only parameter that governs the dependence of 77 on particle type and energy. It follows that ~ij of an HCP with initial energy E0 should be given by R oo
f f o
l, E) 2,r at
o
,Tij -
,
f o
(1)
Di(r, l, E) 27rr dr dl o
where D(r, l,E) is the absorbed dose of the ejected charge carriers around the HCP track (of average track length R in the TLD), i,j define the HCP and the electron reference radiation respectively and the dose-TL response f(D) is defined as the ratio of the TL signal/imparted energy at a particular absorbed dose, D, from the reference radiation field to the TL signal/imparted energy at low dose from the same radiation field. The following points deserve special emphasis: 1) In conventional TST, f(D) is generated, by 6°Co gamma rays. Since f(D) is strongly dependent on radiation quality and furthermore, since the average energy expended per ion pair formed is energy
* Partially supported by the United States-Israel Bi-National Science Foundation. ** In partial fulfillment of the requirements for the Ph.D. degree in Physics. 29
HI. RADIATIONEFFECTS
Y.S. Horowitz-Ezra / Relative TL yield
30
Table 1 Heavy charged particle induced TL in LiF and BeO Theoretical
Experimental
Material
HCP
Energy (MeV)
f(D)
LiF
c~
4
3H-betas
Beo
160
166
9 kVeff. X-rays
HCP
Energy (MeV)
n
0.56
a
4
0.54 -+0.04
2.27
12C ZONe 4°Ar
125 208 416
2.06 _+0.14 2.25 ± 0.38 2.20 ± 0.22
dependent for low energy electrons it follows that f(D) should be generated by electrons of initial spectrum (as far as possible) similar to that of the initial energy spectrum ejected by the HCP. Since the maximum energy of a free electron ejected by a 4 MeV alpha particle cannot exceed about 2 keV we cannot properly use f(D) generated by 6°Co with Compton electrons o f initial energies greater by two orders of magnitude. We have therefore used 3H betas and unfiltered 20 kVp X-rays to generate f(D). 2) Previous attempts to apply TST to the calculation of HCP TL yields [4,5] have employed approximate analytic models for the calculation o f D(r, l, E). We employ, however, recent calculations o f D(r, E) with appropriate scaling factors based on ionization chamber measurements in tissue equivalent gas [6]. 3) There are other important advantages to.generating f(D) with low energy electrons: approximate matching of the volumes irradiated by the HCP and the reference radiation, elimination of the possibility o f distortion o f f ( D ) at high doses due to TL self-absorption and reduction by many orders o f magnitude o f the lattice defects created by the reference radiation.
geometry identical to the TLD irradiation. Although 3H betas and 20 kVp X-rays were used to generate f(D) we chose to measure the TL signal per imparted energy for the low energy electrons using the X-ray spectrum generated by a 3H beta source (tritium absorbed in Ti) which was covered by a thin mylar foil (average energy of the X-rays was 4.1 keV). Calibration o f the X-ray fluence was carried out using a windowless, 4 mm thick, Si(li) detector also in identical geometrical configuration as the TLD irradiation. The good agreement between theory and experiment is shown in table 1. Substitutingf(D) = 1 in the supralinear region fields r / = 0.4 which illustrates the importance of the supralinear dose contribution. We have also calculated ~ for BeO irradiated by 12C and 2°Ne ions using experimental data from Tochilin et al. [7]. Given the imperfect matching of the experimental and theoretical parameters, we consider the agreement satisfactory. We tentatively conclude on the basis of our still limited data that modified TST with appropriate matching of the reference and HCP secondary electron spectra can be used to calculate HCP TL yields to good accuracy.
References 3. Results and discussion The TLDs employed were commercial bulb dosimeters (Harshaw). Since b o t h r7 and f(D) depend on the particular mapping o f the of the impurity concentrations and defects of the host TL materials, we always measure both o f these parameters in identical TLDs. The experimental measurement o f 77ij involves the measurement o f the TL signal per imparted energy for the HCP and the reference radiation. Alpha particle energy (4 MeV) and fluence calibrations were carried out using a Si surface barrier detector in
[1] Y. Furuta et al., Proc. 4th Int. Conf. on Luminescence dosimetry, Cracow, Poland (1974) p. 96 and references therein. [2] Y.S. Horowitz et al., Nucl. Instr. and Meth. 165 (1979) 27. [3] Y.S. Horowitz et al., Phys. Med. Biol. 24 (1979) 1268. [4] R. Katz et al., in Topics in radiation dosimetry, Suppl. 1 (ed. F. Attix; Academic Press, 1972) Ch. 6. [51 D. Zimmerman, Rad. Eft. 14 (1972) 81. [6~ M.N. Varma et at., Proc. 6th Symp. on Microdosimetry (eds. J. Booz and H. Ebert; Harwood Academic Publ., 1978) p. 227 and references therein. [71 E. Tochilin et al., Proc. 2nd Int. Conf. on Luminescence dosimetry, Conf. 680920 (1968) p. 424.