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Fast-neutron response of sensitized LiF TLD-700
0191-278X/87 $3.00+ 0.00 Pergamon Journals Ltd.
Nucl. Tracks Radiat. Meas., Vol. 13, Nos 2/3, pp. 111-113, 1987 Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain
F A S T - N E U T R O N RESPONSE OF SENSITIZED LiF TLD-700 A. S. PRADHAN Division of Radiological Protection, Bhabha Atomic Research Centre, Bombay-400 085, India (Received 9 September 1986; in revised form 18 December 1986) Abstract--Sensitization was found to significantly reduce the fast-neutron sensitivity of LiF TLD-700 relative to that of gamma-rays. This is best suited for dosimetry of gamma-rays in the mixed fields of neutrons and gammas. The TL sensitivity of sensitized dosimeters does not change for several cycles of re-use. In the use of a two-peak method for simultaneous measurement of neutron and gamma-ray doses, although both the linearity and the intensity of high temperature (270°C) peak are significantly increased, a severe reduction in its relative fast-neutron sensitivity poses a serious problem.
1. I N T R O D U C T I O N IN MIXED fields of neutrons and gamma-rays, 7LiF thermoluminescence dosimeter (TLD) is commonly used to measure the gamma-ray content because its neutron response is much smaller than that of its gamma-ray response. However, one has to correct for the signal induced by fast neutrons when the gammaray and the fast neutron absorbed doses are of the same order. Recently, it has been demonstrated (Pradhan et al., 1985) that a two-peak method of LiF TLD-700 offers an attractive tool for the simultaneous measurement of both the fast neutron gammaray absorbed doses in cyclotron-produced neutron beams. This is because the relative fast neutron sensitivity of its glow peak at 270°C is very high compared with the sensitivity of its glow peak at 200°C. However, the supralinearity of 270°C peak beyond 0.15 Gy of gamma-rays and beyond 2 Gy of fast neutrons (Fig. 1) poses some problems in the use of the two-peak method in mixed fields having a gamma-ray content higher than 0.15 Gy. The other factor causing limitation is the low intensity of the 270°C peak. In the present study gamma-ray-induced sensitization was studied to improve the characteristics of TLD-700 for its use in mixed field dosimetry of fast neutrons and gamma-rays.
1.48 Gy min -~ (89 Gy h -l) at the diaphragm. The dose rate of neutron therapy unit was 0.56 Gy (total absorbed dose, DT) per 100 pulses (about 1 Gy in 4min at 125 cm from the target) at 0.8 cm depth in a tissue equivalent A-150 plastic phantom for a field size of 10 x 10 cm 2. Thermal neutron contamination of the neutron beam was expected to be very small, however, no measurements were made to find out the exact amount of thermal neutron contamination of the beam. The gamma-ray content in the neutron beam was 4.7% of D r at 0.8cm depth in a tissue equivalent phantom. The phantom consisted of tissueFast neutron absorbed dose (Gy) 100 I
102
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EXPERIMENTAL
7LiF (0.007% 6LiF and 99.993% 7LiF) and TLD700 ribbons (3.2 × 3.2 × 0.39mm 3) were procured from Harshaw Chemical Company, U.S.A. The T LD reader (Harshaw 2000 unit coupled with an on-line desk-top computer), the 6°Co gamma-ray teletherapy unit and the d(14) + Be fast neutron (E~ = 5.8 MeV, obtained by bombarding Be target by 14 MeV deuterons) facility (CIRCE) used in the present study are described elsewhere (Rassow et al., 1984). The 6°Co gamma-ray dose rate of the teletherapy unit was 0.35Gy min -1 at 60cm from the diaphragm and N.T.
13/2-3~-B
111
to-Z
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6°Co gamma ray absorbed dose (Gy) Fig. 1. TL response of the two peaks of TLD-700: 6°Co gamma-rays (A) 200°C glow peak and (B) 270°C glow peak; and d(14)+ Be (E. = 5.8 MeV) fast neutrons (C) 200°C glow peak and (D) 270°C glow peak. For the curve (B) the TL on the Y axis is to be multiplied by 10-L
112
A. S. P R A D H A N
equivalent A-150 plastic plates ( 2 5 x 2 5 c m 2) of different thicknesses (Rassow et al., 1984). During neutron irradiation the TLDs were sandwiched between the plates of A-150 plastic phantom so that the 0.8 cm thick plate faced the neutron beam and the remaining plates of the phantom provided the backing material. During 6°Co gamma-ray irradiation, the TLDs were sandwiched between two perspex sheets each of thickness 0.5 cm. For TL readout the computer was programmed to record the heights of glow peaks at 20ffC and 270°C. The heating rate used was 3 . 5 C s 1. Using this heating rate the precision in the TL readouts was comparable with that achieved by using peak area method. The peak height method was adopted in order to minimize the contribution of one glow peak in the other due to an overlapping region in the temperature range 200°C-270°C. The readouts were taken about 24 h after the irradiation. During annealing the TL D ribbons were contained in an aluminium (2 mm thick) tray. The unsensitized ribbons were annealed in an oven with reproducible heating and cooling cycles such that a temperature of 4 0 0 C was attained 17min after introducing the sample into the oven. The temperature (400°C) was maintained for 1 h following which the TLDs were cooled down to 100°C in 15 min by a current of air. The samples were maintained at 100°C for 2 h and then cooled down to room temperature (22°C). An individual calibration factor was determined for each peak of each detector from the readouts of the ribbons irradiated to the same absorbed dose. For any irradiation a minimum of five ribbons (placed adjacent to each other) were used. For sensitization, the TLD-700 ribbons were irradiated to 4 x 102 Gy of 6°Co gamma-rays and were then annealed at 340°C for 1 h in another air oven. For annealing of sensitized TLDs, the samples in the aluminium tray were introduced in the oven maintained at 340°C. After a preset time (1 h or 15 min) they were taken out of the oven and were allowed to cool down to room temperature by natural cooling. For re-use the sensitized ribbons were given an annealing treatment of 340:~C for 15 min. 3. RESULTS AND D I S C U S S I O N Figure 2 shows the glow curves of sensitized and unsensitized TLD-700 ribbons. Figure 3 shows the
102
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io-2 2 5 io-[ 2 5 io0 6°Co gamma ray absorbed dose (Gy)
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Fig. 3. 6°Co Gamma-ray response of unsensitized and sensitized TLD-700: unsensitized, (A) 200°C glow peak and (B) 270°C glow peak; and sensitized, (C) 200°C glow peak and (D) 270°C glow peak. For the curve (B), the TL on the Y axis is to be multiplied by 10-~. plot of glow peak height vs 6°Co gamma-ray absorbed dose in tissue. Both the 200°C and 270°C peaks of sensitized ribbons grow linearly with the gamma-ray absorbed dose up to 3 Gy. Thus, by sensitization the linearity of not only the 200°C peak (Jones, 1980) but also of the 270°C peak has been significantly improved. Table 1 compared the relative fast neutron sensitivity (ratio of TL/Gy of neutrons and 6°Co gamma-rays) of unsensitized and sensitized ribbons. The sensitized ribbons exhibit significantly reduced relative fast neutron sensitivity. The re-usability of the sensitized ribbons is shown in Fig. 4. It can be seen that the response of the 200°C peak of sensitized ribbons does not reduce significantly on re-use while the response of the 270°C peak reduces by about 10%. Table 2 shows the increase in TL sensitivity to gamma-rays and neutrons, by the sensitization. The increase in TL sensitivity 'sensitization' is more for gamma-rays than for neutrons. This results in reducing the relative fast neutron sensitivity and the reduc-
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Fig. 2. Glow curves of 0.38 mm thick TLD-700 ribbons irradiated to 1.0 Gy of fast neutrons (gamma-ray content 4.7%) and 0.10 Gy of 6°CO gamma-rays: (a) unsensitized, (b) sensitized.
FAST NEUTRON
R E S P O N S E O F S E N S I T I Z E D LiF TLD-700
Table 1. Relative fast neutron sensitivity* of 200°C and 270°C glow peaks of unsensitized and sensitized TLD-700 ribbons Type of TLD-700 ribbons
Relative fast neutron sensitivitivity 200°C 270°C
Unsensitized Sensitized
0.147 0.074
Table 2. Effect of sensitization on the TL response of 200°C and 270°C glow peaks of TLD-700 ribbons to 6°Co gamma-rays and d(14)+Be neutrons (E, = 5.8 MeV) Sensitization factor* 200°C 270°C
Radiation
1.26 0.207
*Relative fast neutron sensitivity is the ratio of TL/Gy (Gy in tissue) of fast neutrons to that of TL/Gy (Gy in tissue) of 6°Co gamma-rays. For determination of TL/Gy of 6°Co gamma-rays an irradiation of only 0.1 Gy (in tissue) was used in order to avoid correction for the supralinearity of the 270°C glow peak of unsensitized ribbons. For the estimation of TL/Gy of neutrons, the responses were corrected for the contributions due to gamma-ray content of the neutron beam, tion is very high for the 270°C peak as compared with that of 200°C peak. By sensitization, the difference in the relative fast neutron sensitivities of the 200°C and 270°C is reduced considerably. The large difference in the relative fast neutron sensitivities of the two peaks of TLD-700 is a desirable feature of the two-peak method for simultaneous measurement of neutron and gamma-ray absorbed doses in mixed fields. Thus, the sensitization does not help in the use of the two-peak method of TLD-700, though both the linearity and intensity of the 270°C peak are remarkably improved. However, the reduction in the relative fast neutron sensitivity of the most widely used dosimetric peak (at 200°C) of TLD-700, by sensitization, significantly improves its capability of gamma-ray absorbed dose measurement in mixedfields of neutrons and gamma-rays. This is because in the mixed-field dosimetry one needs a photon sensitive dosimeter which should preferably have as low a sensitivity to neutrons as possible (ICRU-26, 1977), to measure the associated gamma-ray content. Therefore, the sensitized TLD-700 ribbons are best suited for dosimetry of gamma-rays in mixed neutron-
113
6°Co Gamma-rays d(14) + Be neutron
5.66 2.95
25.00 3.45
*Sensitization factor is the ratio of TL response of sensitized (irradiated to 4 × 102 Gy of 6°Co gammarays and then annealed at 340°C for 1 h) ribbons to that of unsensitized ribbons for a 6°Co gamma-ray absorbed dose of 0.1 Gy (in tissue) and fast neutron absorbed dose of 1 Gy (in tissue).
photon fields. They have the additional advantage of photon energy independence (Jones, 1980) so that they can be used in mixed fields which may even have low-energy photons, where use of high Z~g T L D s such as CaF2 and CaSO4 becomes problematic. Also the sensitized T L D s offer a simplicity of annealing for re-use as there is no significant change in their T L sensitivity due to annealing. The unsensitized LiF T L D s are sensitive to thermal treatments during annealing and great care has to be taken to follow the established annealing procedures. In some situations, the use of a pair of sensitized and unsensitized TLD-700 ribbons could provide some information about both the gamma-ray and neutron absorbed dose content of mixed fields because of the difference in the relative fast neutron sensitivities of their 200°C glow peaks. Acknowledgements--Thanks are due to the Alexander von Humboldt Foundation of West Germany for the financial assistance during the course of this work. The author is indebted to Professor J. Rassow of Universitaets-Klinikum, Essen, West Germany, for providing the experimental facilities and for his interest in the work.
REFERENCES -J I,--
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3
4
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Fig. 4. Variation in the response of TL peaks at 200°C and 270°C of sensitized TLD-700 ribbons with a number of re-use cycles (annealing of 340°C for 15 min, was used after every cycle).
1CRU Report-26 (1977) Neutron Dosimetry for Biology and Medicine. Bethesda, Md 20814, USA. Jones A. R. (1980) The application of sensitized lithium fluoride TLDs to personnel and environmental dosimetry. Nucl. Instrum. Meth. 175, 145-146. Pradhan A. S., Rassow J., Meissner P. and Olthoff-Muenter K. (1984) Fast neutron thermoluminescent response of CaF 2:Tin (TLD-300): influence of encapsulating material. Radiat. Protect. Dos. 9, 269-272. Pradhan A. S., Rassow J. and Meissner P. (1985) Dosimetry of d(14)+ Be neutrons with the two-peak method of LiF TLD-700. Phys. Med. Biol. 30, 1349-1354, Rassow J., Temme A., Baumhoer W. and Meissner P. (1984) Dosimetrical verification of calculated total and gamma ray absorbed dose distribution, D T and D o for fast neutron therapy. Strahlentherapie 160, 168-179.
Nucl. Tracks Radiat. Meas., Vol. 13, Nos 2/3, pp. 111-113, 1987 Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain
F A S T - N E U T R O N RESPONSE OF SENSITIZED LiF TLD-700 A. S. PRADHAN Division of Radiological Protection, Bhabha Atomic Research Centre, Bombay-400 085, India (Received 9 September 1986; in revised form 18 December 1986) Abstract--Sensitization was found to significantly reduce the fast-neutron sensitivity of LiF TLD-700 relative to that of gamma-rays. This is best suited for dosimetry of gamma-rays in the mixed fields of neutrons and gammas. The TL sensitivity of sensitized dosimeters does not change for several cycles of re-use. In the use of a two-peak method for simultaneous measurement of neutron and gamma-ray doses, although both the linearity and the intensity of high temperature (270°C) peak are significantly increased, a severe reduction in its relative fast-neutron sensitivity poses a serious problem.
1. I N T R O D U C T I O N IN MIXED fields of neutrons and gamma-rays, 7LiF thermoluminescence dosimeter (TLD) is commonly used to measure the gamma-ray content because its neutron response is much smaller than that of its gamma-ray response. However, one has to correct for the signal induced by fast neutrons when the gammaray and the fast neutron absorbed doses are of the same order. Recently, it has been demonstrated (Pradhan et al., 1985) that a two-peak method of LiF TLD-700 offers an attractive tool for the simultaneous measurement of both the fast neutron gammaray absorbed doses in cyclotron-produced neutron beams. This is because the relative fast neutron sensitivity of its glow peak at 270°C is very high compared with the sensitivity of its glow peak at 200°C. However, the supralinearity of 270°C peak beyond 0.15 Gy of gamma-rays and beyond 2 Gy of fast neutrons (Fig. 1) poses some problems in the use of the two-peak method in mixed fields having a gamma-ray content higher than 0.15 Gy. The other factor causing limitation is the low intensity of the 270°C peak. In the present study gamma-ray-induced sensitization was studied to improve the characteristics of TLD-700 for its use in mixed field dosimetry of fast neutrons and gamma-rays.
1.48 Gy min -~ (89 Gy h -l) at the diaphragm. The dose rate of neutron therapy unit was 0.56 Gy (total absorbed dose, DT) per 100 pulses (about 1 Gy in 4min at 125 cm from the target) at 0.8 cm depth in a tissue equivalent A-150 plastic phantom for a field size of 10 x 10 cm 2. Thermal neutron contamination of the neutron beam was expected to be very small, however, no measurements were made to find out the exact amount of thermal neutron contamination of the beam. The gamma-ray content in the neutron beam was 4.7% of D r at 0.8cm depth in a tissue equivalent phantom. The phantom consisted of tissueFast neutron absorbed dose (Gy) 100 I
102
i00
IO I o,~ z "
-
~__ I0 -p 2.
EXPERIMENTAL
7LiF (0.007% 6LiF and 99.993% 7LiF) and TLD700 ribbons (3.2 × 3.2 × 0.39mm 3) were procured from Harshaw Chemical Company, U.S.A. The T LD reader (Harshaw 2000 unit coupled with an on-line desk-top computer), the 6°Co gamma-ray teletherapy unit and the d(14) + Be fast neutron (E~ = 5.8 MeV, obtained by bombarding Be target by 14 MeV deuterons) facility (CIRCE) used in the present study are described elsewhere (Rassow et al., 1984). The 6°Co gamma-ray dose rate of the teletherapy unit was 0.35Gy min -1 at 60cm from the diaphragm and N.T.
13/2-3~-B
111
to-Z
10-2
t 2
L 5
J i0-1
t 2
I 5
I i00
6°Co gamma ray absorbed dose (Gy) Fig. 1. TL response of the two peaks of TLD-700: 6°Co gamma-rays (A) 200°C glow peak and (B) 270°C glow peak; and d(14)+ Be (E. = 5.8 MeV) fast neutrons (C) 200°C glow peak and (D) 270°C glow peak. For the curve (B) the TL on the Y axis is to be multiplied by 10-L
112
A. S. P R A D H A N
equivalent A-150 plastic plates ( 2 5 x 2 5 c m 2) of different thicknesses (Rassow et al., 1984). During neutron irradiation the TLDs were sandwiched between the plates of A-150 plastic phantom so that the 0.8 cm thick plate faced the neutron beam and the remaining plates of the phantom provided the backing material. During 6°Co gamma-ray irradiation, the TLDs were sandwiched between two perspex sheets each of thickness 0.5 cm. For TL readout the computer was programmed to record the heights of glow peaks at 20ffC and 270°C. The heating rate used was 3 . 5 C s 1. Using this heating rate the precision in the TL readouts was comparable with that achieved by using peak area method. The peak height method was adopted in order to minimize the contribution of one glow peak in the other due to an overlapping region in the temperature range 200°C-270°C. The readouts were taken about 24 h after the irradiation. During annealing the TL D ribbons were contained in an aluminium (2 mm thick) tray. The unsensitized ribbons were annealed in an oven with reproducible heating and cooling cycles such that a temperature of 4 0 0 C was attained 17min after introducing the sample into the oven. The temperature (400°C) was maintained for 1 h following which the TLDs were cooled down to 100°C in 15 min by a current of air. The samples were maintained at 100°C for 2 h and then cooled down to room temperature (22°C). An individual calibration factor was determined for each peak of each detector from the readouts of the ribbons irradiated to the same absorbed dose. For any irradiation a minimum of five ribbons (placed adjacent to each other) were used. For sensitization, the TLD-700 ribbons were irradiated to 4 x 102 Gy of 6°Co gamma-rays and were then annealed at 340°C for 1 h in another air oven. For annealing of sensitized TLDs, the samples in the aluminium tray were introduced in the oven maintained at 340°C. After a preset time (1 h or 15 min) they were taken out of the oven and were allowed to cool down to room temperature by natural cooling. For re-use the sensitized ribbons were given an annealing treatment of 340:~C for 15 min. 3. RESULTS AND D I S C U S S I O N Figure 2 shows the glow curves of sensitized and unsensitized TLD-700 ribbons. Figure 3 shows the
102
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Fig. 3. 6°Co Gamma-ray response of unsensitized and sensitized TLD-700: unsensitized, (A) 200°C glow peak and (B) 270°C glow peak; and sensitized, (C) 200°C glow peak and (D) 270°C glow peak. For the curve (B), the TL on the Y axis is to be multiplied by 10-~. plot of glow peak height vs 6°Co gamma-ray absorbed dose in tissue. Both the 200°C and 270°C peaks of sensitized ribbons grow linearly with the gamma-ray absorbed dose up to 3 Gy. Thus, by sensitization the linearity of not only the 200°C peak (Jones, 1980) but also of the 270°C peak has been significantly improved. Table 1 compared the relative fast neutron sensitivity (ratio of TL/Gy of neutrons and 6°Co gamma-rays) of unsensitized and sensitized ribbons. The sensitized ribbons exhibit significantly reduced relative fast neutron sensitivity. The re-usability of the sensitized ribbons is shown in Fig. 4. It can be seen that the response of the 200°C peak of sensitized ribbons does not reduce significantly on re-use while the response of the 270°C peak reduces by about 10%. Table 2 shows the increase in TL sensitivity to gamma-rays and neutrons, by the sensitization. The increase in TL sensitivity 'sensitization' is more for gamma-rays than for neutrons. This results in reducing the relative fast neutron sensitivity and the reduc-
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Fig. 2. Glow curves of 0.38 mm thick TLD-700 ribbons irradiated to 1.0 Gy of fast neutrons (gamma-ray content 4.7%) and 0.10 Gy of 6°CO gamma-rays: (a) unsensitized, (b) sensitized.
FAST NEUTRON
R E S P O N S E O F S E N S I T I Z E D LiF TLD-700
Table 1. Relative fast neutron sensitivity* of 200°C and 270°C glow peaks of unsensitized and sensitized TLD-700 ribbons Type of TLD-700 ribbons
Relative fast neutron sensitivitivity 200°C 270°C
Unsensitized Sensitized
0.147 0.074
Table 2. Effect of sensitization on the TL response of 200°C and 270°C glow peaks of TLD-700 ribbons to 6°Co gamma-rays and d(14)+Be neutrons (E, = 5.8 MeV) Sensitization factor* 200°C 270°C
Radiation
1.26 0.207
*Relative fast neutron sensitivity is the ratio of TL/Gy (Gy in tissue) of fast neutrons to that of TL/Gy (Gy in tissue) of 6°Co gamma-rays. For determination of TL/Gy of 6°Co gamma-rays an irradiation of only 0.1 Gy (in tissue) was used in order to avoid correction for the supralinearity of the 270°C glow peak of unsensitized ribbons. For the estimation of TL/Gy of neutrons, the responses were corrected for the contributions due to gamma-ray content of the neutron beam, tion is very high for the 270°C peak as compared with that of 200°C peak. By sensitization, the difference in the relative fast neutron sensitivities of the 200°C and 270°C is reduced considerably. The large difference in the relative fast neutron sensitivities of the two peaks of TLD-700 is a desirable feature of the two-peak method for simultaneous measurement of neutron and gamma-ray absorbed doses in mixed fields. Thus, the sensitization does not help in the use of the two-peak method of TLD-700, though both the linearity and intensity of the 270°C peak are remarkably improved. However, the reduction in the relative fast neutron sensitivity of the most widely used dosimetric peak (at 200°C) of TLD-700, by sensitization, significantly improves its capability of gamma-ray absorbed dose measurement in mixedfields of neutrons and gamma-rays. This is because in the mixed-field dosimetry one needs a photon sensitive dosimeter which should preferably have as low a sensitivity to neutrons as possible (ICRU-26, 1977), to measure the associated gamma-ray content. Therefore, the sensitized TLD-700 ribbons are best suited for dosimetry of gamma-rays in mixed neutron-
113
6°Co Gamma-rays d(14) + Be neutron
5.66 2.95
25.00 3.45
*Sensitization factor is the ratio of TL response of sensitized (irradiated to 4 × 102 Gy of 6°Co gammarays and then annealed at 340°C for 1 h) ribbons to that of unsensitized ribbons for a 6°Co gamma-ray absorbed dose of 0.1 Gy (in tissue) and fast neutron absorbed dose of 1 Gy (in tissue).
photon fields. They have the additional advantage of photon energy independence (Jones, 1980) so that they can be used in mixed fields which may even have low-energy photons, where use of high Z~g T L D s such as CaF2 and CaSO4 becomes problematic. Also the sensitized T L D s offer a simplicity of annealing for re-use as there is no significant change in their T L sensitivity due to annealing. The unsensitized LiF T L D s are sensitive to thermal treatments during annealing and great care has to be taken to follow the established annealing procedures. In some situations, the use of a pair of sensitized and unsensitized TLD-700 ribbons could provide some information about both the gamma-ray and neutron absorbed dose content of mixed fields because of the difference in the relative fast neutron sensitivities of their 200°C glow peaks. Acknowledgements--Thanks are due to the Alexander von Humboldt Foundation of West Germany for the financial assistance during the course of this work. The author is indebted to Professor J. Rassow of Universitaets-Klinikum, Essen, West Germany, for providing the experimental facilities and for his interest in the work.
REFERENCES -J I,--
~£2oo*c
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3
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5
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Number of reuse cycle
Fig. 4. Variation in the response of TL peaks at 200°C and 270°C of sensitized TLD-700 ribbons with a number of re-use cycles (annealing of 340°C for 15 min, was used after every cycle).
1CRU Report-26 (1977) Neutron Dosimetry for Biology and Medicine. Bethesda, Md 20814, USA. Jones A. R. (1980) The application of sensitized lithium fluoride TLDs to personnel and environmental dosimetry. Nucl. Instrum. Meth. 175, 145-146. Pradhan A. S., Rassow J., Meissner P. and Olthoff-Muenter K. (1984) Fast neutron thermoluminescent response of CaF 2:Tin (TLD-300): influence of encapsulating material. Radiat. Protect. Dos. 9, 269-272. Pradhan A. S., Rassow J. and Meissner P. (1985) Dosimetry of d(14)+ Be neutrons with the two-peak method of LiF TLD-700. Phys. Med. Biol. 30, 1349-1354, Rassow J., Temme A., Baumhoer W. and Meissner P. (1984) Dosimetrical verification of calculated total and gamma ray absorbed dose distribution, D T and D o for fast neutron therapy. Strahlentherapie 160, 168-179.