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High level γ-dosimetry using CaSO4: Dy phosphor with high dy-concentration
Inrrmorionul Journul n/ Applied Rudiution und lsofopes Printed in Great Britain. Ail rights reserved
OOZO-708X/81/080553-06SO2.00/0 Copyright 0 1981 Pergamon Press Ltd
Vol. 32. pp. 553 to 558. 1981
High Level y-Dosimetry Using CaS04 : Dy Phosphor With High Dy-concentration BHUWAN
CHANDRA,
R. C. BHATT
and S. J. SUPE
Division of Radiological Protection, Bhabha Atomic Research Centre, Trombay, Bombay-400 Q35,India (Received22 September 1980; in revised
form
19 January 19~31)
It has been observed that the thermoluminescent traps in CaSO,:Dy phosphor with a high concentration of activator (2.0 mol% Dy) compared to that of normal samples (containing 0.05 mol% Dy) show a better stability and a lesser tendency towards saturation to y-radiation. This effect has been observed for the main dosimetric peak (s 225°C) as well as for the high temperature peak (- 390°C). Thus by using 390°C TL peak in CaS04:Dy (2.0mol’%)we were able to make high level y dose measurements in the range from 2 x 10’ to 3 x lo6 Gy. The 390°C TL peak in CaS04:Dy (2.0 mol%) samples increases non-linearly with dose and does not show any tendency towards saturation at least up to 3 x lo6 Gythe dose level studied. However, the corresponding high temperature peak in the normal samples (0.05 mol% Dy) shows saturation in its TL response above a ydose of 1.18 x lo6 Gy. In addition, a high temperature TL peak at 572°C which is only observed for the high activator concentration sample (as reported in our earlier work”“) increases non-linearly with dose and does not show saturation up to the y-dose of 3 x lo6 Gy. Investigations on photo-transferred TL of high temperature peaks as a function of y-dose were also carried out for both the types of samples. Introduction RECENTLY,OBERHOFER(‘)has reviewed the use of common TLD phosphors in the measurement of high level y-radiation doses (lo*-lo6 Gy) in chemical technology (polymerization, vulcanization of rubber, cracking of hydrocarbons), food-processing, sterilization of medical products, materials testing etc. From the review it is seen that in addition to calorimetric methods, chemical dosimeters such as Fricke dosimeter, Ceric sulphate dosimeter, Oxalic acid system and organic dyes in the form of liquid solutions are most often used in such applications. In general, chemical dosimetry methods require extra effort for obtaining accurate results and are usually expensive for routine applications in which a large number of dose measurements have to be made.“’ A less expensive alternative has been the use of thermoluminescent dosimeters. However, at such high doses the use of a TLD phosphor gets somewhat complicated because the TL response of the dosimetric peak (- 200°C) of most of the common TL phosphors saturates at about a dose of lo3 Gy. In order to overcome this difficulty several new techniques have been suggested by a number of researchers. These techniques involve the measurement of: (i) optical density changes in single crystals of LiF,(2*3’(ii) photoluminescence in LiF,t4’ (iii) high temperature TL glow peaks in LiF,“.@ and (iv) photo-transferred TL (PTTL) in CaS04:Dy and CaSO,:Tm phosphors.‘7-gi Conflicting results about the TL response characteristics of the high temperature glow peaks in LiF and PTTL peaks in CaSO, phosphors have, however, appeared in these reports. A.R.,. 32/g--s
For applications to high level y-dosimetry we have investigated: (i) the y-response of the main dosimetric peaks in CaS04:Dy (0.05 mol%) and CaS04:Dy, (2.0 mol%), (ii) the y response of the high temperature residual TL (RTL) glow peaks after a high y-dose and a post-annealing at 300°C for 1 h, (iii) the P’TTL from the repopulated shallow traps in the two CaSO,:Dy samples, and (iv) the y response of 572°C peak in CaSO,:Dy (2.0 mol%) phosphor.
Materials and Methods CaS04:Dy phosphors were prepared by us by following the method described by YAMASHITA et a1.(‘o) In all our experiments, phosphor grains in the size range 75-210~ were used. The samples were irra‘diated in plastic capsules of 4 mm wall-thickness. The radiation doses mentioned in this paper are absorbed ‘doses in tissue (1 Gy = 100 rad). For y-irradiations, a 6oCo y-cell having a dose rate of 5 x 103Gy h-l (calibrated with a Fricke chemical dosimeter system) was used. For PTTL and high temperature RTL measurements all the y-irradiated samples were annealed at 300°C for 1 h in an air oven in which the temperature was controlled to within f2”C. For UV irradiation, a standard 36 W pen-ray quartz UV lamp was used. For UV exposure, about 20mg of the y dosed and 300°C1 h treatedphosphor was spread out uniformly over a 9 cm’ area on an aluminium planchet and exposed to UV radiation for 10 min by keep ing the UV lamp at a distance of 20cm from the sample. The resulting UV exposure was well within ‘the saturation UV-exposure for PTTL for the above phosphor. After UV exposure, the TL glow _.curves of
553
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B. Chandra et al.
all the samples were recorded in the temperature range from 25 to 600°C by using a reader system described by CHANDRA and BHAIT.(’ ‘) Since for TL measurements the weights of the TL samples used were not the same, the TL glow peak intensities are not to be intercompared in Figs 1 and 2. Results Figures la and b show the TL glow curves of CaS04:Dy (0.05 mol%) and CaSO_,:Dy (2.0 mol%) for a dose level of 5 x 10’ Gy. The heating rates used for recording these glow curves are also shown. It can be seen that, in addition to the dosimetric peak (~22S’C) present in both the phosphors, there are two high temperature peaks-in CaSO*:Dy (O.O5mol%) at 412C and 490°C and in CaSG,:Dy (2.0mol%) at 412°C and 545°C. The 490°C TL peak in CaSO,:Dy (0.05 mol%) is observed from a minimum detectable level at 1.24 x lo5 Gy. The 545°C TL peak in CaS04:Dy (2.0mol%) is observed from a minimum detectable level at 104Gy. As the dose is increased from lo4 Gy to 3 x 10” Gy, the position of this peak shifts from 572 to 510°C. Figures 2a and 2b show the PTTL glow curves for CaSO,:Dy
(0.05 mol%) and CaSO,:Dy (2.0mol%) samples respectively, exposed to 5.0 x 10’ Gy, post-annealed at 3OO”C-1h and then exposed to UV for 10min. Figure 3 shows the effect of activator concentration on the growth of 572°C peak in CaS04:Dy samples. This peak is found to have a Dy-concentration threshold of 0.5 mol% and has a maximum intensity in the range l-2 mol% of Dy. Figure 4 shows the growth of TL/Gy vs dose for the dosimetric peaks in CaS04:Dy (0.05 mol%) and CaSO,:Dy (2.0mol%), respectively. It can be seen that in both of these phosphors, TL/Gy vs dose curve falls below the value of 1 for dose in excess of 6 x lo3 Gy. Furthermore, it is seen that beyond 6 x 1O’Gy the TL/Gy vs dose curve of CaSO,:Dy (2.0 mol%) declines relatively slowly in comparison to that of CaS04 :Dy (0.05 mol%). Figure 5 shows the growth of PTTL at the dosimetric peak for CaS04:Dy (0.05 mol’/J as a function of UV (253.7 nm) exposure time. The dose given was 1O’Gy and the post-annealing treatment given was 3OOC-1 h. It can be seen that the PTTL intensity of the dosimetric peak saturates after 1OOmin of UV irradiation. The high temperature RTL intensity slowly decreasing as a function of UV irradiation time 600
6
(a)
Time W
(b)
FIG. 1. Typical TL
glow
curves of: (a) CaSO.+:Dy (0.05 mot%), and (b) CaSO,:Dy given was 5.0 x 10’ Gy.
(2.0mol%). Dose
High level y-dosimetry using CaSO,
(4
6’
(W
FIG.
2. PTTL glow curves ofz (a) CaSO,:Dy (0.05 mol%), and (b) CaS04:Dy (2.0 mol%) samples irradiated to 5.0 x 10’ Gy. Post-annealing treatment was 300%lh and UV exposure for P’ITL was 10 min.
555
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B. Chandra
o(
,
05
(
1.0 Dy
Coneentratii
,
2
et al.
1616010’6
d ,
y-Dose
5
in Ca SD4 43y
(Gy)
( mol % 1
FIG. 3. Effect of activator concentration
on the growth ofi 572°C TL peak in CaSO,:Dy phosphor.
FIG. 4. Relative values of TL/Gy of CaSO.,:Dy (0.05 mol%) and CaS04:Dy (2.0 mo&,) as a function of y-dose.
is also indicated in the figure. After 1OOmin of UV exposure the RTL intensity decreases by a factor of 1.78 from its initial value. Figure 6 shows the growth of PTTL peak ( - 225°C) and the high temperature (RTL) peaks as a function of dose in CaS04:Dy (0.05 mol%). It can be seen that PTTL increases non-linearly up to 5 x lo5 Gy and beyond this dose it declines. At 5 x 10’ Gy the PTTL response is 11 times its value at 2 x 10’ Gy. The first high temperature RTL peak (-WC) grows nonlinearly with dose and saturates beyond a dose of 1.18 x lo6 Gy. The second high temperature RTL peak, which is detectable at a dose level of
1.24 x lo5 Gy continues to grow non-linearly with dose up to the dose level studied (3 x lo6 Gy). As the dose is increased from 1.7 x lo2 Gy to 3 x lo6 Gy the PTTL peak shifts from its position at 251°C to 197°C and the main RTL peak temperature increases from 400 to 421°C. Figure 7 shows the growth of PTTL peak ( - 225°C) and the high temperature RTL peaks as a function of y-dose in CaS04:Dy (2.0molx). It can be seen that the PTTL increases non-linearly up to a dose of 5 x lo5 Gy beyond which it declines. At 5 x lo5 Gy PTTL response is 10 times that at 2 x lo2 Gy. However, the high temperature RTL peaks at 390 and
200
? f
5 I50
d g
‘i d loo
E z C i
50
=
0 0
50
25 Ultmvblrt
75
125
wpeswu ( min 1
FIG. 5. Growth of PTTL of dosimetric peak (a), and decay of RTL peak (b), as a function of UV exposure time.
High level y-dosimetry
using
557
CaS04
2107
G!S!?&Y_(oIM~~I 170-
.
PTTL(25O%fmdd
y-Dow (Gy) FIG. 6. Relative TL intensities of the glow peaks in CaSO,: Dy (0.05 mol%) as a function of pre-gamma
dose. Multiplication factor indicated on the curves was applied to bring the intensity within the scale.
572°C grow non-linearly with dose and do not appear to saturate at least up to the highest dose used (3 x 106Gy). As the dose increases from 1.7 x 10’ Gy to 3 x lo6 Gy the PTTL peak shifts from 242 to 197°C and the main RTL peak temperature shifts from 390 to 428°C. However, in going from 104Gy to 3 x lo6 Gy the temperature of the second RTL peak decreases from 572 to 510°C. Only the PTTL of the second RTL (572°C) for CaS04:Dy (2.0mol%) samples was studied by annealing the dosed phosphor at 430°C for 30min and subsequently giving a UV exposure of 10 min each to 43o”Cd h treated samples. It has been found that the intensity of PTTL (at the dosimetric peak) for 572°C peak is less by a factor of about 60 as compared to the FTTL of the first RTL peak (- 390°C).
160-
Discussion It was observed that the thermoluminescent traps in CaS04:Dy phosphor containing 2.0 mol% concentration of Dy activator show a better stability and a lower tendency towards saturation against y-radiation dose as compared to the samples containing normal Dy concentration (0.05 mole/,). This effect was observed for the main dosimetric peak ( - 225°C) (Fig. 4) as well as for the high temperature TL peak (390°C) (Figs 6 and 7). This property of CaS04:Dy (2.0 mol%) phosphor was utilised by us for high level ydosimetry in the dose range from 10’ to 3 x lo6 Gy using 390°C TL peak. The dose vs TL response of CaSO,:Dy (2.0mol%) for 390°C peak increases non-linearly and does not show saturation at least up to a dose of 3 x lo6 Gy. However, the corresponding RTL peak
l
coso.Gyt2.ormt XI PlNt24o~Padt~
0 A
RTLaGo-cperk~ RTL (572 ‘c paok)
y-Ooso CGy)
FIG. 7. Relative TL intensities of the glow peaks in CaSO,:Dy (2.0mol%) as a function of pre-y-dose. Multiplication factor indicated on the curves was applied to bring the intensity within the scale.
B. Chandra et al.
558
in normal concentration samples shows saturation in its TL response beyond a y-dose of 1.18 x 106Gy. Earlier, we have reported’“’ a high temperature TL peak (C 572°C) for a dose above lo4 Gy, only in high concentration samples [CaS04:Dy (2.0 mol%)]. This peak increases non-linearly up to the dose studied (3 x lo6 Gy). Thus, the usable dose range of 572°C peak is from lo4 to 3 x lo6 Gy. However, its position in glow curve shifts from a temperature of 572 to 510°C as the dose is increased from lo4 to 3 x lo6 Gy. Another high temperature TL peak was observed at 490°C in CaS04:Dy (0.05 mol%) above a dose of 1.24 x lo5 Gy. This peak does not saturate in its TL response up to the dose level studied (3 x lo6 Gy). However, its TL intensity is comparatively low and the usable dose range is also short. Figures 6 and 7 show the FTTL characteristics of the high temperature residual (RTL) peaks in CaS04:Dy (0.05 mol%) and CaS04:Dy (2.0 mol%). It can be seen that the response increases non-linearly up to a dose of 5 x 10’ Gy in both the phosphors and beyond this dose it declines. This agrees well with the results of LAKSHMANAN and BHAI-T(~)for the normal concentration samples. However, Caldas and Mayhugh have reported (*) that the FITL response of high temperature TL peak in CaSO,:Dy (Harshaw, U.S.A.) grows with dose as a power of 0.55 up to at least lo6 Gy. Batch-to-batch variation has been found(‘) to be mainly responsible for this discrepancy; hence it is essential that in PTTL and RTL studies, the same batch of phosphor is used for calibration as well as for measurement of unknown y-doses. The decrease in the PTTL response beyond 6 x 1O’Gy could be explained as due to the damage of the dosimetric traps at such high doses. However, the extended usable range of PTTL samples (up to 6 x 1O’Gy) as compared to the TL response of the dosimetric peak (which declines beyond 6 x 10’ Gy) could be explained as due to the filling up of relatively small number of dosimetric traps during a small UV exposure (10min) for PTTL as compared to the filling up of these traps during y irradiation. Under proper conditions (by using an optimum fluence of UV), the PTTL technique can be employed repeatedly (with proper calibration) for the re-estimation of ydoses as long a~ the RTL peaks are not erased during the readout. Since the FTTL response characteristics change” ‘) appreciably with UV wavelength the same UV lamp should be used during calibration and dosimetry. Conclusion
The modified CaS04:Dy (2.0mol%) phosphor appears to be a promising dosimeter for high level
y-dosimetry in the range from 102 Gy to at least up to 3 x lo6 Gy. There is no other TL phosphor reported so far in the literature which can be used to measure ydoses higher than 106Gy. Caldas and Mayhugh have reported(*) that the PTTL response in CaS04:Dy (Harshaw, U.S.A.) grows with dose as a power of 0.55 up to at least i06 Gy. However, in the present work FTTL response in CaS04:Dy (0.05 mol%) and CaS04:Dy (2.0 mol%) was found to be less dependent on pre-y-dose and declines in intensity beyond 6 x lo5 Gy. However, our results on photo-TL studies agree well with those of LAKSHMANAN and BHAIT.‘~)The estimation of y-doses in reactor cores while using TL phosphors is somewhat difficult because the temperature during irradiation could be high and at an elevated temperature the main TL dosimetric peak (w 225°C) will show appreciable fading. In such situations, measurement of high temperature TL (-390°C) and PTTL will be useful in the estimation of gamma doses. This technique can be employed successfully as long as the temperature during irradiation is below 300°C. The second high temperature RTL peak in CaS04:Dy (2.0mol%) samples could be. utilised for dose measurements up to 3 x lo6 Gy even when the temperature during irradiations is as high as 430°C. Acknowledgements-We thank Dr K. G. Vohra for encouragement and helpful discussions during the course of this work.
References 1. OBERHOFER M. Aromkernenergie 31, 209 (1978). 2. VAGHANW. J. and MILLERL. 0. Hth Phys. 18, 578 (i9%3). 3. CLAFFYE. W., GORBICSS. G. and ATTIXF. H. Proc. 3rd Int. Con& on Luminescence Dosimetry, Riso Report 249 (AEC. Riso DenmarkA p. 756 (1971). 4. REGULLA D. F. Hth Phys. 22,491 (1972). 5. GOLDSIEINN., TOCHILIN E. and MILLERW. G. HIth Phys. 14, 159 (1968). 6. J.UN V. K., KATHURIAS. P. and GANGULYA. K. J. Phys. C. Solid. St. Phys. 7, 3810 (1974). 7. NAMBIK. S. V. and HIGASUMURAT. Proc. 3rd Int. Co@ on Luminescence Dosimenetry, Riso Report 249 (AEC, Riso, Denmark), p, 1107 (1971). 8. CALDASL. V. E. and MAYHUGHM. R. Hlth Phys. 31, 451 (1976). 9. LAK~HMANAN A. R. and BHATT R. C. Phys. Med. Biol. 24, 1258 (1979). 10. YAMASHITA T., NADA N., ONI~HIH. and KIT-A S. Proc.
2nd
Int.
Conf
on
Luminescence
Dosimetry,
USAEC-CONF 680920, p. 4 (1968). 11. BHUWANCHANDRAand BHATTR. C. Nucl. Instrum. Meth. 164, 571 (1979). 12. SUNTAC. M. and WATANAII~E S. J. Phys. D: A@. Phys. 9, 1271 (1976).
OOZO-708X/81/080553-06SO2.00/0 Copyright 0 1981 Pergamon Press Ltd
Vol. 32. pp. 553 to 558. 1981
High Level y-Dosimetry Using CaS04 : Dy Phosphor With High Dy-concentration BHUWAN
CHANDRA,
R. C. BHATT
and S. J. SUPE
Division of Radiological Protection, Bhabha Atomic Research Centre, Trombay, Bombay-400 Q35,India (Received22 September 1980; in revised
form
19 January 19~31)
It has been observed that the thermoluminescent traps in CaSO,:Dy phosphor with a high concentration of activator (2.0 mol% Dy) compared to that of normal samples (containing 0.05 mol% Dy) show a better stability and a lesser tendency towards saturation to y-radiation. This effect has been observed for the main dosimetric peak (s 225°C) as well as for the high temperature peak (- 390°C). Thus by using 390°C TL peak in CaS04:Dy (2.0mol’%)we were able to make high level y dose measurements in the range from 2 x 10’ to 3 x lo6 Gy. The 390°C TL peak in CaS04:Dy (2.0 mol%) samples increases non-linearly with dose and does not show any tendency towards saturation at least up to 3 x lo6 Gythe dose level studied. However, the corresponding high temperature peak in the normal samples (0.05 mol% Dy) shows saturation in its TL response above a ydose of 1.18 x lo6 Gy. In addition, a high temperature TL peak at 572°C which is only observed for the high activator concentration sample (as reported in our earlier work”“) increases non-linearly with dose and does not show saturation up to the y-dose of 3 x lo6 Gy. Investigations on photo-transferred TL of high temperature peaks as a function of y-dose were also carried out for both the types of samples. Introduction RECENTLY,OBERHOFER(‘)has reviewed the use of common TLD phosphors in the measurement of high level y-radiation doses (lo*-lo6 Gy) in chemical technology (polymerization, vulcanization of rubber, cracking of hydrocarbons), food-processing, sterilization of medical products, materials testing etc. From the review it is seen that in addition to calorimetric methods, chemical dosimeters such as Fricke dosimeter, Ceric sulphate dosimeter, Oxalic acid system and organic dyes in the form of liquid solutions are most often used in such applications. In general, chemical dosimetry methods require extra effort for obtaining accurate results and are usually expensive for routine applications in which a large number of dose measurements have to be made.“’ A less expensive alternative has been the use of thermoluminescent dosimeters. However, at such high doses the use of a TLD phosphor gets somewhat complicated because the TL response of the dosimetric peak (- 200°C) of most of the common TL phosphors saturates at about a dose of lo3 Gy. In order to overcome this difficulty several new techniques have been suggested by a number of researchers. These techniques involve the measurement of: (i) optical density changes in single crystals of LiF,(2*3’(ii) photoluminescence in LiF,t4’ (iii) high temperature TL glow peaks in LiF,“.@ and (iv) photo-transferred TL (PTTL) in CaS04:Dy and CaSO,:Tm phosphors.‘7-gi Conflicting results about the TL response characteristics of the high temperature glow peaks in LiF and PTTL peaks in CaSO, phosphors have, however, appeared in these reports. A.R.,. 32/g--s
For applications to high level y-dosimetry we have investigated: (i) the y-response of the main dosimetric peaks in CaS04:Dy (0.05 mol%) and CaS04:Dy, (2.0 mol%), (ii) the y response of the high temperature residual TL (RTL) glow peaks after a high y-dose and a post-annealing at 300°C for 1 h, (iii) the P’TTL from the repopulated shallow traps in the two CaSO,:Dy samples, and (iv) the y response of 572°C peak in CaSO,:Dy (2.0 mol%) phosphor.
Materials and Methods CaS04:Dy phosphors were prepared by us by following the method described by YAMASHITA et a1.(‘o) In all our experiments, phosphor grains in the size range 75-210~ were used. The samples were irra‘diated in plastic capsules of 4 mm wall-thickness. The radiation doses mentioned in this paper are absorbed ‘doses in tissue (1 Gy = 100 rad). For y-irradiations, a 6oCo y-cell having a dose rate of 5 x 103Gy h-l (calibrated with a Fricke chemical dosimeter system) was used. For PTTL and high temperature RTL measurements all the y-irradiated samples were annealed at 300°C for 1 h in an air oven in which the temperature was controlled to within f2”C. For UV irradiation, a standard 36 W pen-ray quartz UV lamp was used. For UV exposure, about 20mg of the y dosed and 300°C1 h treatedphosphor was spread out uniformly over a 9 cm’ area on an aluminium planchet and exposed to UV radiation for 10 min by keep ing the UV lamp at a distance of 20cm from the sample. The resulting UV exposure was well within ‘the saturation UV-exposure for PTTL for the above phosphor. After UV exposure, the TL glow _.curves of
553
554
B. Chandra et al.
all the samples were recorded in the temperature range from 25 to 600°C by using a reader system described by CHANDRA and BHAIT.(’ ‘) Since for TL measurements the weights of the TL samples used were not the same, the TL glow peak intensities are not to be intercompared in Figs 1 and 2. Results Figures la and b show the TL glow curves of CaS04:Dy (0.05 mol%) and CaSO_,:Dy (2.0 mol%) for a dose level of 5 x 10’ Gy. The heating rates used for recording these glow curves are also shown. It can be seen that, in addition to the dosimetric peak (~22S’C) present in both the phosphors, there are two high temperature peaks-in CaSO*:Dy (O.O5mol%) at 412C and 490°C and in CaSG,:Dy (2.0mol%) at 412°C and 545°C. The 490°C TL peak in CaSO,:Dy (0.05 mol%) is observed from a minimum detectable level at 1.24 x lo5 Gy. The 545°C TL peak in CaS04:Dy (2.0mol%) is observed from a minimum detectable level at 104Gy. As the dose is increased from lo4 Gy to 3 x 10” Gy, the position of this peak shifts from 572 to 510°C. Figures 2a and 2b show the PTTL glow curves for CaSO,:Dy
(0.05 mol%) and CaSO,:Dy (2.0mol%) samples respectively, exposed to 5.0 x 10’ Gy, post-annealed at 3OO”C-1h and then exposed to UV for 10min. Figure 3 shows the effect of activator concentration on the growth of 572°C peak in CaS04:Dy samples. This peak is found to have a Dy-concentration threshold of 0.5 mol% and has a maximum intensity in the range l-2 mol% of Dy. Figure 4 shows the growth of TL/Gy vs dose for the dosimetric peaks in CaS04:Dy (0.05 mol%) and CaSO,:Dy (2.0mol%), respectively. It can be seen that in both of these phosphors, TL/Gy vs dose curve falls below the value of 1 for dose in excess of 6 x lo3 Gy. Furthermore, it is seen that beyond 6 x 1O’Gy the TL/Gy vs dose curve of CaSO,:Dy (2.0 mol%) declines relatively slowly in comparison to that of CaS04 :Dy (0.05 mol%). Figure 5 shows the growth of PTTL at the dosimetric peak for CaS04:Dy (0.05 mol’/J as a function of UV (253.7 nm) exposure time. The dose given was 1O’Gy and the post-annealing treatment given was 3OOC-1 h. It can be seen that the PTTL intensity of the dosimetric peak saturates after 1OOmin of UV irradiation. The high temperature RTL intensity slowly decreasing as a function of UV irradiation time 600
6
(a)
Time W
(b)
FIG. 1. Typical TL
glow
curves of: (a) CaSO.+:Dy (0.05 mot%), and (b) CaSO,:Dy given was 5.0 x 10’ Gy.
(2.0mol%). Dose
High level y-dosimetry using CaSO,
(4
6’
(W
FIG.
2. PTTL glow curves ofz (a) CaSO,:Dy (0.05 mol%), and (b) CaS04:Dy (2.0 mol%) samples irradiated to 5.0 x 10’ Gy. Post-annealing treatment was 300%lh and UV exposure for P’ITL was 10 min.
555
556
B. Chandra
o(
,
05
(
1.0 Dy
Coneentratii
,
2
et al.
1616010’6
d ,
y-Dose
5
in Ca SD4 43y
(Gy)
( mol % 1
FIG. 3. Effect of activator concentration
on the growth ofi 572°C TL peak in CaSO,:Dy phosphor.
FIG. 4. Relative values of TL/Gy of CaSO.,:Dy (0.05 mol%) and CaS04:Dy (2.0 mo&,) as a function of y-dose.
is also indicated in the figure. After 1OOmin of UV exposure the RTL intensity decreases by a factor of 1.78 from its initial value. Figure 6 shows the growth of PTTL peak ( - 225°C) and the high temperature (RTL) peaks as a function of dose in CaS04:Dy (0.05 mol%). It can be seen that PTTL increases non-linearly up to 5 x lo5 Gy and beyond this dose it declines. At 5 x 10’ Gy the PTTL response is 11 times its value at 2 x 10’ Gy. The first high temperature RTL peak (-WC) grows nonlinearly with dose and saturates beyond a dose of 1.18 x lo6 Gy. The second high temperature RTL peak, which is detectable at a dose level of
1.24 x lo5 Gy continues to grow non-linearly with dose up to the dose level studied (3 x lo6 Gy). As the dose is increased from 1.7 x lo2 Gy to 3 x lo6 Gy the PTTL peak shifts from its position at 251°C to 197°C and the main RTL peak temperature increases from 400 to 421°C. Figure 7 shows the growth of PTTL peak ( - 225°C) and the high temperature RTL peaks as a function of y-dose in CaS04:Dy (2.0molx). It can be seen that the PTTL increases non-linearly up to a dose of 5 x lo5 Gy beyond which it declines. At 5 x lo5 Gy PTTL response is 10 times that at 2 x lo2 Gy. However, the high temperature RTL peaks at 390 and
200
? f
5 I50
d g
‘i d loo
E z C i
50
=
0 0
50
25 Ultmvblrt
75
125
wpeswu ( min 1
FIG. 5. Growth of PTTL of dosimetric peak (a), and decay of RTL peak (b), as a function of UV exposure time.
High level y-dosimetry
using
557
CaS04
2107
G!S!?&Y_(oIM~~I 170-
.
PTTL(25O%fmdd
y-Dow (Gy) FIG. 6. Relative TL intensities of the glow peaks in CaSO,: Dy (0.05 mol%) as a function of pre-gamma
dose. Multiplication factor indicated on the curves was applied to bring the intensity within the scale.
572°C grow non-linearly with dose and do not appear to saturate at least up to the highest dose used (3 x 106Gy). As the dose increases from 1.7 x 10’ Gy to 3 x lo6 Gy the PTTL peak shifts from 242 to 197°C and the main RTL peak temperature shifts from 390 to 428°C. However, in going from 104Gy to 3 x lo6 Gy the temperature of the second RTL peak decreases from 572 to 510°C. Only the PTTL of the second RTL (572°C) for CaS04:Dy (2.0mol%) samples was studied by annealing the dosed phosphor at 430°C for 30min and subsequently giving a UV exposure of 10 min each to 43o”Cd h treated samples. It has been found that the intensity of PTTL (at the dosimetric peak) for 572°C peak is less by a factor of about 60 as compared to the FTTL of the first RTL peak (- 390°C).
160-
Discussion It was observed that the thermoluminescent traps in CaS04:Dy phosphor containing 2.0 mol% concentration of Dy activator show a better stability and a lower tendency towards saturation against y-radiation dose as compared to the samples containing normal Dy concentration (0.05 mole/,). This effect was observed for the main dosimetric peak ( - 225°C) (Fig. 4) as well as for the high temperature TL peak (390°C) (Figs 6 and 7). This property of CaS04:Dy (2.0 mol%) phosphor was utilised by us for high level ydosimetry in the dose range from 10’ to 3 x lo6 Gy using 390°C TL peak. The dose vs TL response of CaSO,:Dy (2.0mol%) for 390°C peak increases non-linearly and does not show saturation at least up to a dose of 3 x lo6 Gy. However, the corresponding RTL peak
l
coso.Gyt2.ormt XI PlNt24o~Padt~
0 A
RTLaGo-cperk~ RTL (572 ‘c paok)
y-Ooso CGy)
FIG. 7. Relative TL intensities of the glow peaks in CaSO,:Dy (2.0mol%) as a function of pre-y-dose. Multiplication factor indicated on the curves was applied to bring the intensity within the scale.
B. Chandra et al.
558
in normal concentration samples shows saturation in its TL response beyond a y-dose of 1.18 x 106Gy. Earlier, we have reported’“’ a high temperature TL peak (C 572°C) for a dose above lo4 Gy, only in high concentration samples [CaS04:Dy (2.0 mol%)]. This peak increases non-linearly up to the dose studied (3 x lo6 Gy). Thus, the usable dose range of 572°C peak is from lo4 to 3 x lo6 Gy. However, its position in glow curve shifts from a temperature of 572 to 510°C as the dose is increased from lo4 to 3 x lo6 Gy. Another high temperature TL peak was observed at 490°C in CaS04:Dy (0.05 mol%) above a dose of 1.24 x lo5 Gy. This peak does not saturate in its TL response up to the dose level studied (3 x lo6 Gy). However, its TL intensity is comparatively low and the usable dose range is also short. Figures 6 and 7 show the FTTL characteristics of the high temperature residual (RTL) peaks in CaS04:Dy (0.05 mol%) and CaS04:Dy (2.0 mol%). It can be seen that the response increases non-linearly up to a dose of 5 x 10’ Gy in both the phosphors and beyond this dose it declines. This agrees well with the results of LAKSHMANAN and BHAI-T(~)for the normal concentration samples. However, Caldas and Mayhugh have reported (*) that the FITL response of high temperature TL peak in CaSO,:Dy (Harshaw, U.S.A.) grows with dose as a power of 0.55 up to at least lo6 Gy. Batch-to-batch variation has been found(‘) to be mainly responsible for this discrepancy; hence it is essential that in PTTL and RTL studies, the same batch of phosphor is used for calibration as well as for measurement of unknown y-doses. The decrease in the PTTL response beyond 6 x 1O’Gy could be explained as due to the damage of the dosimetric traps at such high doses. However, the extended usable range of PTTL samples (up to 6 x 1O’Gy) as compared to the TL response of the dosimetric peak (which declines beyond 6 x 10’ Gy) could be explained as due to the filling up of relatively small number of dosimetric traps during a small UV exposure (10min) for PTTL as compared to the filling up of these traps during y irradiation. Under proper conditions (by using an optimum fluence of UV), the PTTL technique can be employed repeatedly (with proper calibration) for the re-estimation of ydoses as long a~ the RTL peaks are not erased during the readout. Since the FTTL response characteristics change” ‘) appreciably with UV wavelength the same UV lamp should be used during calibration and dosimetry. Conclusion
The modified CaS04:Dy (2.0mol%) phosphor appears to be a promising dosimeter for high level
y-dosimetry in the range from 102 Gy to at least up to 3 x lo6 Gy. There is no other TL phosphor reported so far in the literature which can be used to measure ydoses higher than 106Gy. Caldas and Mayhugh have reported(*) that the PTTL response in CaS04:Dy (Harshaw, U.S.A.) grows with dose as a power of 0.55 up to at least i06 Gy. However, in the present work FTTL response in CaS04:Dy (0.05 mol%) and CaS04:Dy (2.0 mol%) was found to be less dependent on pre-y-dose and declines in intensity beyond 6 x lo5 Gy. However, our results on photo-TL studies agree well with those of LAKSHMANAN and BHAIT.‘~)The estimation of y-doses in reactor cores while using TL phosphors is somewhat difficult because the temperature during irradiation could be high and at an elevated temperature the main TL dosimetric peak (w 225°C) will show appreciable fading. In such situations, measurement of high temperature TL (-390°C) and PTTL will be useful in the estimation of gamma doses. This technique can be employed successfully as long as the temperature during irradiation is below 300°C. The second high temperature RTL peak in CaS04:Dy (2.0mol%) samples could be. utilised for dose measurements up to 3 x lo6 Gy even when the temperature during irradiations is as high as 430°C. Acknowledgements-We thank Dr K. G. Vohra for encouragement and helpful discussions during the course of this work.
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