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TL light self-absorption: implications for studies on the relative TL efficiency as a function of linear energy transfer
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1979 Phys. Med. Biol. 24 832 (http://iopscience.iop.org/0031-9155/24/4/019) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 146.201.208.22 This content was downloaded on 02/10/2015 at 03:48
Please note that terms and conditions apply.
832
Correspondence
quantitativelyexactwithrespect to its total density; that is, a region of reconstructed density enhanced with respect to the original must be counterbalanced elsewhere in theimage by a region of relatively reduced reconstructed density. Two classes of filter cannot be normalised for quantitative imaging in the above way: that with G = 00 and that with G = 0. Simple back-projection ( F ( k )= 1) is an example of the former class, the infinite gain arising from the infinitely extended positive 'skirt' created by the convolution of the original density with I r 1-l (to which the method is equivalent). Zero-gain filters must generate regions of negative density in order to give a vanishing total density; this may yet prove to be an interesting class, since it may be possible to construct such filters to generate negative density areas only outside a finite r-space region, within which near-exact positive densities are reconstructed!
26 February 1978
S. LEEMAN, Department of Medical Physics, Royal Postgraduate Medical School and Hammersmith Hospital, London W12 OHS, U.K. REFERENCE
GORE, J. C . , and TOFTS,P. S., 1978, Phys. Med. Biol., 23, 1176.
TL Light Self-absorption: Implications for Studies on the Relative TL Efficiency as a Function of Linear Energy Transfer THE EDITOR, Sir, Many studies have been reported which investigate the thermoluminescent (TL) production efficiency q as a function of Linear Energy Transfer (LET) in LiF (Majborn, Botter-Jensen and Christensen 1977). I n most of these studies TL production the reference radiation hasbeen s°Co or 13'Cs gamma rays and the efficiency is defined as the number of TL photons per rad a t a n arbitrary LET compared with the number of TL photons per rad for the same mass of TL material at the dose averaged LET delivered by the reference radiation. The LET is varied by irradiation with protons, alphas, low energy electrons, various heavy ion beams or thermal neutrons. Most of the thermal neutron data have been shown (Horowitz, Ben Shahar, Mordechai, Dubi and Pinto 1977) to be in error because of either neglect or inadequatecalculation of the thermal neutron self-shielding factor. This situation has been corrected by the publication of Monte Carlo calculated thermalneutron self-shielding factorsfor various geometrical configurations of 6LiFandnaturalLiF dosemeters (Horowitz and Dubi 1976, Horowitz, Freeman and Dubi 1978). An additional
Correspondence
833
source of error, sometimes neglected (Woodley and Johnson 1965, Kastner, Hukkoo, Oltman and Dayal 1967, Suntharalingam and Cameron 1969, Harvey and Townsend 1971, Barber, Moore and Hutchinson 1975, Lasky and Moran TLD. The reference 1977), is TL photon self-absorption intheirradiated radiation, s°Co or 13'Cs gamma rays, irradiates the TLD volume homogeneously whereas the high LET radiation of the charged particle radiation field penetrates only a very small fraction of the dosemeter volume. For example, the range of a 5 MeV alpha in LiF is only 16 pm. It follows, therefore, that the self-absorption will begreaterinthe case of the gamma-irradiated TLD, resulting in an overestimate in the TL production efficiency. The degree of the error is to a certain extent dependent on the geometry of the light collection mechanism of the TLD reader. By limiting the light collection to direct illumination of the photomultiplier photocathode by the TLD we have made this geometry effect negligible (with, however, a consequent loss of reader sensitivity). Another benefit of limiting todirect illuminationis that the light flux more closely approximates a parallel beam.In thecase when the irradiated TLD is a powder it is trivial experimentallyto side-step the self-absorption error (Jahnert 1971) by simply mixing the surface-irradiated powder thoroughly so that the TL centres become homogeneously distributed throughout the TLD volume as is the case with the high energy gamma irradiation. I n measurement of 7 using single crystals or hot-pressed chips the self-absorption must be taken into account. Majborn et al. (1977) measured the attenuation coefficients by comparing the amount of TL detected when alpha-irradiated dosemeters were read with the irradiated side up (facing the photomultiplier) and down respectively. Assuming a parallel TL photon flux and minimal light scattering, the unattenuated light output, Io,is given by the Beer-Lambert expression as
I
= Io
1-exp(-pd) Pd
where I is the attenuated light output (measured with the alpha irradiated surface down), d the is thickness of the dosemeter and p is the optical absorption coefficient. The correction factor, I/Io,was found to be 0.9 for Harshaw LiF, TLD-700hot-pressed chips (d = 0.9 mm)and 0.61 forown-make Li,B,O, tablets (d = 0.8 mm) which translate to p(LiF) = 2.4 cm-l and p(Li,B,O, = 13.5 cm-l. To our knowledge no other measurements of p for hot-pressed LiF or Li,B,O, have been reported in the literature. m7e therefore considered it of considerable interest to carry out an additional measurement of p for Lip, especially because of the continuing interest in the TL-LET properties of this material. A slight advantage of our method (using slow neutron irradiation) is that the TL photon self-absorption occurs in irradiated material (contrary t o the method described by Majborn wherein the self-absorption occurs in unirradiatedmaterial), so that we have also investigated the possibility that irradiation-inducedlightabsorptioncentresbringabouta significant dose dependence of p. Within experimental error (3%, 1 SD) over the dose range
Correspondence
834
from 0 to 500 rad we observe no dose dependence in IjI' (see eqn (2)). TL-LET studies are usually confined to thisdose range to avoid supralinearity corrections in 7 due to thevariation of the supralinearity factor withLET. In our method TLD-BOO hot pressed LiF chips areirradiatedwithparallel-beam, monoenergetic slow neutrons of two different energies, 13.7 meV and 81.0 meV, from a neutron diffractorneter. Details of the irradiation facility have been given in a previous publication (Horowitz et a1 1977). We have also measured the TL emission spectrum following neutron, gamma and alpha irradiation and the spectra are identical within experimental error, so that variations in the emission spectra do not play a part in the self-absorption corrections. Because the capture cross-section for the 6Li(n,a ) T reaction follows the l / w law very closely, the exponential depth dose distributions for the two neutron energies will be significantly different. For equal absorbed doses, the ratio of the TL light measured at the two neutron energies will be given by -I =
I'
C(C'+p) l-exp[-(C+p)d] C ' ( C + p ) l-exp[-(C'+p)d]
(assumingagainparallel TL photon flux), where C,C' arethe macroscopic cross-sections at the two neutron energies. We and for the TLD-600 chips in our possession p = 1.8 in fairly good agreement with the value of 2.4 reported by Majborn et al. (1977). It is, of course, not our intention to imply that all LiF chips have identical optical absorption coefficients ; however, the reasonably similar values reported herein and by Majborn et al. increases confidence in this method with which we take TL self-absorption into account in TL production efficiency measurements as a function of LET. Y. S. HOROWITZ, I. FRAIER and J. KALEFEZRA, Department of Physics, Ben Gurion University of the Negev, Beersheva, 1 December 1978 Israel REFERENCES BARBER, D. E., MOORE,R.,and HUTCHINSON, T., 1975, Health Phya., 28, 13. HARVEY,J. R.,and TOWNSEND, S., 1971, in Proc.ThirdInt.Conf. o n Luminescence Dosimetry, Risco, Denmark, p. 1015. HOROWITZ, Y. S., BEN SHAHAEL, B., MORDECHAI,S., DUBI, A., and PINTO,H., 1977, Radiat. Res., 69, 402. HOROWITZ, Y. S.,and DUBI,A., 1976, P h y s . M e d . B i o l . , 21, 976. HOROWITZ, Y. S., FREEMAN, S., and DUBI,A., 1979, Nucl. Instrum. Meth., 160, 313-315. KASTNER, J., HUKKOO, R., OLTMAN, B. G., and DAYAL, Y., 1967, Radiat. Res., 32, 625. LASKY,J. R., and " I R A N , P. R.,1977, Phys. Med. Biol., 22, 852. MAJBORN,B., B0TTER-JENSEN, L.,and CHRISTENSEX, F., 1977, in P r o c . F i f t h I n t . COnf. o n Luminescence Dosimetry, 860 Paulo, Brazil, p. 124 and references therein. JAHNERT, B., 1971, in Proc. Third Int. Conf. o n Luminescence Dosimetry, Riso, Denmark, p. 1031. SUNTHARALINGAM, N., and CAXERON,J. R.,1969, Phys. Med. Biol., 14, 397. WOODLEY, R. G., and JOHNSON, N. M., 1965, in Proc. First Int. Conf. o n Luminescence Dofiimetry, Stanford, Calif., p. 502.
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My IOPscience
TL light self-absorption: implications for studies on the relative TL efficiency as a function of linear energy transfer
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1979 Phys. Med. Biol. 24 832 (http://iopscience.iop.org/0031-9155/24/4/019) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 146.201.208.22 This content was downloaded on 02/10/2015 at 03:48
Please note that terms and conditions apply.
832
Correspondence
quantitativelyexactwithrespect to its total density; that is, a region of reconstructed density enhanced with respect to the original must be counterbalanced elsewhere in theimage by a region of relatively reduced reconstructed density. Two classes of filter cannot be normalised for quantitative imaging in the above way: that with G = 00 and that with G = 0. Simple back-projection ( F ( k )= 1) is an example of the former class, the infinite gain arising from the infinitely extended positive 'skirt' created by the convolution of the original density with I r 1-l (to which the method is equivalent). Zero-gain filters must generate regions of negative density in order to give a vanishing total density; this may yet prove to be an interesting class, since it may be possible to construct such filters to generate negative density areas only outside a finite r-space region, within which near-exact positive densities are reconstructed!
26 February 1978
S. LEEMAN, Department of Medical Physics, Royal Postgraduate Medical School and Hammersmith Hospital, London W12 OHS, U.K. REFERENCE
GORE, J. C . , and TOFTS,P. S., 1978, Phys. Med. Biol., 23, 1176.
TL Light Self-absorption: Implications for Studies on the Relative TL Efficiency as a Function of Linear Energy Transfer THE EDITOR, Sir, Many studies have been reported which investigate the thermoluminescent (TL) production efficiency q as a function of Linear Energy Transfer (LET) in LiF (Majborn, Botter-Jensen and Christensen 1977). I n most of these studies TL production the reference radiation hasbeen s°Co or 13'Cs gamma rays and the efficiency is defined as the number of TL photons per rad a t a n arbitrary LET compared with the number of TL photons per rad for the same mass of TL material at the dose averaged LET delivered by the reference radiation. The LET is varied by irradiation with protons, alphas, low energy electrons, various heavy ion beams or thermal neutrons. Most of the thermal neutron data have been shown (Horowitz, Ben Shahar, Mordechai, Dubi and Pinto 1977) to be in error because of either neglect or inadequatecalculation of the thermal neutron self-shielding factor. This situation has been corrected by the publication of Monte Carlo calculated thermalneutron self-shielding factorsfor various geometrical configurations of 6LiFandnaturalLiF dosemeters (Horowitz and Dubi 1976, Horowitz, Freeman and Dubi 1978). An additional
Correspondence
833
source of error, sometimes neglected (Woodley and Johnson 1965, Kastner, Hukkoo, Oltman and Dayal 1967, Suntharalingam and Cameron 1969, Harvey and Townsend 1971, Barber, Moore and Hutchinson 1975, Lasky and Moran TLD. The reference 1977), is TL photon self-absorption intheirradiated radiation, s°Co or 13'Cs gamma rays, irradiates the TLD volume homogeneously whereas the high LET radiation of the charged particle radiation field penetrates only a very small fraction of the dosemeter volume. For example, the range of a 5 MeV alpha in LiF is only 16 pm. It follows, therefore, that the self-absorption will begreaterinthe case of the gamma-irradiated TLD, resulting in an overestimate in the TL production efficiency. The degree of the error is to a certain extent dependent on the geometry of the light collection mechanism of the TLD reader. By limiting the light collection to direct illumination of the photomultiplier photocathode by the TLD we have made this geometry effect negligible (with, however, a consequent loss of reader sensitivity). Another benefit of limiting todirect illuminationis that the light flux more closely approximates a parallel beam.In thecase when the irradiated TLD is a powder it is trivial experimentallyto side-step the self-absorption error (Jahnert 1971) by simply mixing the surface-irradiated powder thoroughly so that the TL centres become homogeneously distributed throughout the TLD volume as is the case with the high energy gamma irradiation. I n measurement of 7 using single crystals or hot-pressed chips the self-absorption must be taken into account. Majborn et al. (1977) measured the attenuation coefficients by comparing the amount of TL detected when alpha-irradiated dosemeters were read with the irradiated side up (facing the photomultiplier) and down respectively. Assuming a parallel TL photon flux and minimal light scattering, the unattenuated light output, Io,is given by the Beer-Lambert expression as
I
= Io
1-exp(-pd) Pd
where I is the attenuated light output (measured with the alpha irradiated surface down), d the is thickness of the dosemeter and p is the optical absorption coefficient. The correction factor, I/Io,was found to be 0.9 for Harshaw LiF, TLD-700hot-pressed chips (d = 0.9 mm)and 0.61 forown-make Li,B,O, tablets (d = 0.8 mm) which translate to p(LiF) = 2.4 cm-l and p(Li,B,O, = 13.5 cm-l. To our knowledge no other measurements of p for hot-pressed LiF or Li,B,O, have been reported in the literature. m7e therefore considered it of considerable interest to carry out an additional measurement of p for Lip, especially because of the continuing interest in the TL-LET properties of this material. A slight advantage of our method (using slow neutron irradiation) is that the TL photon self-absorption occurs in irradiated material (contrary t o the method described by Majborn wherein the self-absorption occurs in unirradiatedmaterial), so that we have also investigated the possibility that irradiation-inducedlightabsorptioncentresbringabouta significant dose dependence of p. Within experimental error (3%, 1 SD) over the dose range
Correspondence
834
from 0 to 500 rad we observe no dose dependence in IjI' (see eqn (2)). TL-LET studies are usually confined to thisdose range to avoid supralinearity corrections in 7 due to thevariation of the supralinearity factor withLET. In our method TLD-BOO hot pressed LiF chips areirradiatedwithparallel-beam, monoenergetic slow neutrons of two different energies, 13.7 meV and 81.0 meV, from a neutron diffractorneter. Details of the irradiation facility have been given in a previous publication (Horowitz et a1 1977). We have also measured the TL emission spectrum following neutron, gamma and alpha irradiation and the spectra are identical within experimental error, so that variations in the emission spectra do not play a part in the self-absorption corrections. Because the capture cross-section for the 6Li(n,a ) T reaction follows the l / w law very closely, the exponential depth dose distributions for the two neutron energies will be significantly different. For equal absorbed doses, the ratio of the TL light measured at the two neutron energies will be given by -I =
I'
C(C'+p) l-exp[-(C+p)d] C ' ( C + p ) l-exp[-(C'+p)d]
(assumingagainparallel TL photon flux), where C,C' arethe macroscopic cross-sections at the two neutron energies. We and for the TLD-600 chips in our possession p = 1.8 in fairly good agreement with the value of 2.4 reported by Majborn et al. (1977). It is, of course, not our intention to imply that all LiF chips have identical optical absorption coefficients ; however, the reasonably similar values reported herein and by Majborn et al. increases confidence in this method with which we take TL self-absorption into account in TL production efficiency measurements as a function of LET. Y. S. HOROWITZ, I. FRAIER and J. KALEFEZRA, Department of Physics, Ben Gurion University of the Negev, Beersheva, 1 December 1978 Israel REFERENCES BARBER, D. E., MOORE,R.,and HUTCHINSON, T., 1975, Health Phya., 28, 13. HARVEY,J. R.,and TOWNSEND, S., 1971, in Proc.ThirdInt.Conf. o n Luminescence Dosimetry, Risco, Denmark, p. 1015. HOROWITZ, Y. S., BEN SHAHAEL, B., MORDECHAI,S., DUBI, A., and PINTO,H., 1977, Radiat. Res., 69, 402. HOROWITZ, Y. S.,and DUBI,A., 1976, P h y s . M e d . B i o l . , 21, 976. HOROWITZ, Y. S., FREEMAN, S., and DUBI,A., 1979, Nucl. Instrum. Meth., 160, 313-315. KASTNER, J., HUKKOO, R., OLTMAN, B. G., and DAYAL, Y., 1967, Radiat. Res., 32, 625. LASKY,J. R., and " I R A N , P. R.,1977, Phys. Med. Biol., 22, 852. MAJBORN,B., B0TTER-JENSEN, L.,and CHRISTENSEX, F., 1977, in P r o c . F i f t h I n t . COnf. o n Luminescence Dosimetry, 860 Paulo, Brazil, p. 124 and references therein. JAHNERT, B., 1971, in Proc. Third Int. Conf. o n Luminescence Dosimetry, Riso, Denmark, p. 1031. SUNTHARALINGAM, N., and CAXERON,J. R.,1969, Phys. Med. Biol., 14, 397. WOODLEY, R. G., and JOHNSON, N. M., 1965, in Proc. First Int. Conf. o n Luminescence Dofiimetry, Stanford, Calif., p. 502.
