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Further studies on the dosimetric characteristics of LiF:Mg,Cu,Si—A high sensitivity thermoluminescence dosimeter (TLD)
Radiation Measurements 43 (2008) 446 – 449 www.elsevier.com/locate/radmeas
Further studies on the dosimetric characteristics of LiF:Mg,Cu,Si—A high sensitivity thermoluminescence dosimeter (TLD) J.L. Kim ∗ , J.I. Lee, A.S. Pradhan, B.H. Kim, J.S. Kim Health Physics Department, Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong, Daejeon 305-600, Republic of Korea
Abstract Recent development of LiF:Mg,Cu,Si exhibiting high TL sensitivity (55 times that of TLD-100 LiF:Mg,Ti) and insignificant higher temperature peak leading to negligible residual TL signal has provided a TLD for pragmatic replacement of both LiF:Mg,Ti and LiF:Mg,Cu,P in personal dosimetry. LiF:Mg,Cu,Si withstood readout temperature up to 300 ◦ C. For the reader annealing using a maximum readout temperature and clamping at 260 ◦ C, no significant change in the TL sensitivity and glow curve structure was observed for more than 100 reuse cycles of exposures (5.5 mGy) and readout. TL emission spectrum LiF:Mg,Cu,Si was found to have three emission bands peaking at 355, 385 and 440 nm similar to that of LiF:Mg,Cu,P but differing in the relative intensities. Negligible fading, insensitivity to room light and absence of the effect of pre- or post-storage annealing on the response and the glow curve structure makes it a strong contender to be opted for personal dosimetry applications. © 2007 Published by Elsevier Ltd. Keywords: TLof LiF:Mg,Cu,Si; Fading; Reusability; Emission spectrum
1. Introduction High sensitivity (about 20–100 times that of LiF:Mg,Ti) and near tissue equivalent LiF:Mg,Cu,P TLD (Miljanic et al., 2002) has attracted the attention of doisimetric community since its discovery (Nakajima et al., 1979). The main problems associated with the use of LiF:Mg,Cu,P had been the loss of TL sensitivity by heating beyond 240 ◦ C and a high residual TL signal which increased with each readout cycle. A recent study (Luepke et al., 2006) has shown that in the commercially available LiF:Mg,Cu,P TLDs an increase of 1 ◦ C in the annealing temperature above 240 ◦ C continues to reduce the TL sensitivity significantly. Also, another recent study (Moscovitch et al., 2006) demonstrated that a thermal treatment needed for the encapsulation of LiF:Mg,Cu,P in Teflon for making a commercial TLD badge reduces the TL sensitivity by more than a factor of 2. In spite of all round efforts the world over to improve its dosimetric properties, replacement of the most widely used LiF:Mg,Ti by LiF:Mg,Cu,P in TLD personal dosimetry has ∗ Corresponding author. Fax: +82 42 868 8609.
E-mail address: [email protected] (J.L. Kim). 1350-4487/$ - see front matter © 2007 Published by Elsevier Ltd. doi:10.1016/j.radmeas.2007.10.045
remained full of apprehensions. This convinced us that it is necessary to make fresh efforts of changing the dopants and the preparation procedure to develop a high sensitivity LiF TLD material to suit the needs of personal dosimetry. Consequently, an optimized method of preparation of LiF:Mg,Cu,Si has been developed (Lee et al., 2006). The major features of the new method of preparation were (1) the avoidance of use of Na and P dopants, (2) the use of method of melting rather than limiting to sintering at temperature below the melting point and (3) the adoption of a final dual-step thermal treatment to improve the glow curve structure. This paper presents some of the important dosimetric aspects, namely, reusability, fading, effect of exposure to room light and TL emission spectrum. 2. Material and methods Different preparation procedures and varying concentrations dopants were tried (Lee et al., 2005). Consequently, an optimized method with dopant concentrations of 0.45 mol% of MgSO4 .7H2 O, 0.025 mol% of CuSO4 .5H2 O and 0.9 mol% of SiO2 and the use of method of melting for activation was
J.L. Kim et al. / Radiation Measurements 43 (2008) 446 – 449
arrived at (Lee et al., 2006). A dual-step thermal treatment at 300 ◦ C for 10 min followed by 260 ◦ C for 10 min was developed (Lee et al., 2007) as a final step of the preparation procedure for suppressing the higher temperature tail of the main dosimetric glow peak and improving the glow curve structure. Final dosimeters were in the shape of discs of thickness 0.8 mm and diameter 4.5 mm. The TL readouts were taken on a Harshaw 4500 TLD reader system (under the recommended flow rate of N2 gas) with optimized time and temperature profiles (TTP) for readouts using a heating rate of 10 ◦ C s−1 . Most of the equipment, including 137 Cs gamma-ray sources, a Studsvik 90 Sr–90 Y beta-ray reference irradiator (Model no.- 6527), oven and the set up for recording the emission spectra, were the same as described elsewhere (Lee et al., 2005, 2006, 2007). Individual calibration factors were established for the disc to disc variation and a minimum of four TLDs were used for each experimental point. The precision in the measurements was estimated to be better than ±3% (1). For the comparison, commercially available GR-200 LiF:Mg,Cu,P TLD discs were procured.
447
Table 1 Comparison of relative TL and residual TL signal of LiF:Mg,Cu,Si after different annealing and readout procedures Readout cycle
Reader anneala
Oven annealb
Relative TL
Residual TL signal
Relative TL
Residual TL signal
0.002 – 0.005
1.00 1.00 –
0.0001 0.0001 –
One 1.000 Twenty 1.000 Hundred 0.99
by keeping the TLD in the TLD reader at 260 ◦ C for 4 s following the readout up to 260 ◦ C. b Annealing in an oven at 260 ◦ C for 5 min prior to exposure and readout up to 300 ◦ C in the TLD reader. a Annealing
structure. Except for the relative intensities of different glow peaks, the glow curve structure was similar to that of LiF:Mg,Cu,P (Lee et al., 2006). The dose response of the main dosimetric peak was linear from 1 Gy to 20 Gy of 137 Cs gamma rays.
3. Results and discussion
3.1. Reusability
Fig. 1 shows the glow curves of the LiF:Mg,Cu,Si sintered discs subjected to different heat treatments during preparation. A thermal treatment (as a final step of the preparation procedure) of 300 ◦ C for 10 min followed by 260 ◦ C for 10 min was found to yield the best results and was termed as dual-step of thermal treatment (Lee et al., 2007). The TL sensitivity of LiF:Mg,Cu,Si was 55 times that of LiF:Mg,Ti, TLD-100 and about 10% higher than LiF:Mg,Cu,P. Quality assurance parameters of handling and processing the chemicals and the steps of heat treatment were found to be crucial. About 700 dosimeters were made out of five batches. Batch to batch variation in the TL sensitivity was within 10% with almost the same glow curve
20
In personnel dosimetry applications, one of the important requirements is to readout a very large number of dosimeters in the shortest possible time. Since the exposures encountered in any cycle of use (monthly or quarterly) are to result in the doses much below a few mSv (annual limit 20 or 50 mSv), it is usually preferred that the readout cycle in the reader should also offer the required annealing for resetting the dosimeter for the subsequent reuse. For such reuses, a readout cycle using a heating rate of 10 ◦ C s−1 and clamping the maximum readout temperature at 260 ◦ C with a hold of 4 s was opted. No change in the TL sensitivity (Table 1) and the glow curve structure (Fig. 2) were observed for the reuse of more than 100 cycles of exposures (5.5 mGy) and readout. The dual-step thermal treatment of 300 ◦ C for 10 min followed by 260 ◦ C for 10 min as a part of the preparation procedure was found unsatisfactory for subsequent reuse cycles. This treatment was found to reduce the TL sensitivity if used repeatedly as an annealing treatment for the reuse. For the reuse in the cases of higher exposures, etc. an annealing of 260 ◦ C for 5 min was found to be sufficient (Table 2). One of the important result is that the glow curve structure is not affected by different cycles of anneal, exposures and readouts by using different annealing treatments.
10
3.2. Fading and pre- or post-storage annealing
50
TL Intensity (arb. unit)
40
4
300/260
3
300
2
260
1
No annealing
30
0 50
100
150 200 Temperature (°C)
250
300
Fig. 1. Glow curves of the LiF:Mg,Cu,Si discs subjected to 1—No treatment, 2.260 ◦ C for 10 min (260), 3.300 ◦ C for 10 min (300) and 4.300 ◦ C for 10 min followed by 260 ◦ C for 10 min (300/260) thermal treatments after the sintering of the discs during preparation.
Tables 3 and 4 show the fading of gamma ray induced TL signal of the main dosimetric glow peak of LiF:Mg,Cu,Si. It can be seen that for storage at room temperature up to a period of 3 months, no significant fading was observed (Table 3). For evaluating the influence of room light, the TLDs (covered with a very thin and transparent plastic) were kept at 110 cm under a pair of fluorescent tubes during laboratory working hours of 1 month. The samples were placed closed to
448
J.L. Kim et al. / Radiation Measurements 43 (2008) 446 – 449 Table 2 Reusablity on repeated exposures (10 mGy) and readout of LiF:Mg,Cu,Si
400 1 cycle 10 cycles 20 cycles
4
350 300
3
250
2
200 150
1
Readout cycle
Temperature (°C)
TL Intensity (arb. unit)
5
100
0 50
100 Channel number
150
50 200
a maximum
1 50 100
400 300
2
200 1
Temperature (°C)
TL Intensity (arb. unit)
(b)b
1.00 0.99 1.00 1.00 1.00 1.00 0.97 – –
1.00 1.01 1.02 1.02 1.01 1.01 1.01 1.01 1.00
readout temperature 300 ◦ C and oven annealing at 260 ◦ C for
500
3
100
0 0
(a)a
5 min. b Using maximum readout temperature of 260 ◦ C with a hold of 4 s (reader anneal).
600 4
1 2 3 4 5 10 20 40 100
Relative TL for different cycles used for reuse
50
100 Channel number
150
0 200
Fig. 2. Typical glow curves of LiF:Mg,Cu,Si subjected to different reusability procedures (not corrected for the disc to disc variation). Top figure: after one cycle, ten cycles and twenty cycles of reuse using oven annealing at 260 ◦ C for 5 min prior to exposure for maximum readout temperature of 300 ◦ C in the TLD reader. Bottom figure: after one cycle, fifty cycles and hundred cycles of reuse using clamping the maximum readout temperature at 260 ◦ C with a hold of 4 s in the TLD reader.
a glass window avoiding exposure to direct sunlight but were exposed to light from both the fluorescent tubes as well as from the usual day light. Table 4 shows that exposure to room light for a period of 30 days reduced the TL by not more than 12%. TLDs stored for a period of 6 months after annealing at 260 ◦ C for 10 min and exposed to gamma rays and those stored for a period of 6 months and there after annealed at 260 ◦ C for 10 min and exposed to gamma rays did not show any unexpected difference in their TL readouts. The readouts (corrected for the accumulation of the background signal) were within 2% (experimental error). 3.3. TL emission spectrum Fig. 3 shows the TL emission spectrum of LiF:Mg,Cu,Si and LiF:Mg,Cu,P. The TL emission of the dosimetric glow peaks exhibit a peak at about 384 nm in the case of LiF:Mg,Cu,Si and at about 368 nm in the case of LiF:Mg,Cu,P. The analysis of the emission spectrum (deconvolution) revealed three
Table 3 Fading of TL Signal of LiF:Mg,Cu,Si pellets exposed to 10 mGy of 137 Cs gamma rays and stored for different durations at room temperature (corrected for varying signal introduced by storage background) Duration of storage
Relative response
1h 1 day 3 days 10 days 30 days 60 days 90 days
1.000 1.001 0.997 0.960 0.973 0.968 0.957
Table 4 Effect of room light on the TL of LiF:Mg,Cu,Si pellets exposed to 10 mGy of 137 Cs gamma rays and stored in dark and under room light at room temperature for a duration of 30 days Storage duration and condition
Relative response
Freshly annealed and stored under dark at room temperature after exposure
1.00
Freshly annealed and stored under room light after exposure
0.88
emission bands peaking at 355, 385 and 440 nm in both the TLDs. The emission band peaking at 385 nm is predominant in the TL emission of the dosimetric glow peaks of both the TLDs. However, the emission band peaking at 355 nm was significantly higher for LiF:Mg,Cu,P (having significant higher temperature peak) than in LiF:Mg,Cu,Si. The TL emission of the higher temperature glow peak of LiF:Mg,Cu,P, shows an intense emission band peaking at about 356 nm although all the three emission bands at 355, 385 and 440 nm are present. The reduced TL emission at 355 nm in LiF:Mg,Cu,Si is evidently due to the minimization of the higher temperature tail of the dosimetric glow peak.
J.L. Kim et al. / Radiation Measurements 43 (2008) 446 – 449
449
4. Conclusions 385
LiF:Mg,Cu,Si
Development of high sensitivity LiF:Mg,Cu,Si overcomes the drawbacks of LiF:Mg,Cu,P. LiF:Mg,Cu,Si exhibits an intense dosimetric peak with its TL emission peaking at 384 nm and a negligible higher temperature glow peak. The high TL sensitivity, improved thermal stability to withstand large numbers of exposure and readout cycles, negligible fading and insensitivity to exposures to room light provide a TLD material for pragmatic replacement of both LiF:Mg,Ti and LiF:Mg,Cu,P in personnel monitoring.
440
TL intensity (arb. unit)
355
References
385 LiF:Mg,Cu,P
356 440
LiF:Mg,Cu,P at high-temperature glow peak
356
385 440
300
350
400
450 500 Wavelength (nm)
550
600
Fig. 3. TL emission spectrum of dosimetric glow peak of LiF:Mg,Cu,Si (top figure), of dosimetric glow peak of LiF:Mg,Cu,P (middle figure) and higher temperature glow peak of LiF:Mg,Cu,P (bottom figure).
Lee, J.I., Kim, J.L., Chang, S.Y., Chung, K.S., Choe, H.S., 2005. On the role of the dopants in LiF:Mg,Cu,Na,Si thermoluminescent material. Radiat. Prot. Dosim. 115, 340–344. Lee, J.I., Yang, J.S., Kim, J.L., Pradhan, A.S., Lee, J.D., Chung, K.S., Choe, H.S., 2006. Dosimetric characteristic of LiF:Mg,CuSi thermoluminescent materials. Appl. Phys. Lett. 89, 094110. Lee, J.I., Kim, J.L., Yang, J.S., Pradhan, A.S., Kim, B.H., Chung, K.S., Choe, H.S., 2007. Dual-step thermal treatment for the stability of glow curve structure and the TL sensitivity of the newly developed LiF:Mg,CuSi. Radiat. Meas. 42, 597–600. Luepke, M., Goblet, F., Polvika, B., Siefert, H., 2006. Sensitivity loss of LiF:Mg,Cu,P thermoluminescence dosimeters caused by oven annealing. Radiat. Prot. Dosim. 121, 195–201. Miljanic, S., Ranogajec-Komor, M., Knezevic, Z., Vekic, B., 2002. Main dosimetric characteristics of some tissue-equivalent TL detectors. Radiat. Prot. Dosim. 100, 437–442. Moscovitch, M., John, T.J.St., Casata, J.R., Blake, P.K., Rotunda, J.E., Ramlo, M., Velbeck, K.J., Luo, L.Z., 2006. The application of LiF:Mg,Cu,P to large scale personnel dosimetry: current status and future directions. Radiat. Prot. Dosim. 119, 248–254. Nakajima, T., Murayama, Y., Matsuzaba, T., 1979. Preparation and dosimetric properties of highly sensitive thermoluminescence dosimeter. Health Phys. 36, 79–83.
Further studies on the dosimetric characteristics of LiF:Mg,Cu,Si—A high sensitivity thermoluminescence dosimeter (TLD) J.L. Kim ∗ , J.I. Lee, A.S. Pradhan, B.H. Kim, J.S. Kim Health Physics Department, Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong, Daejeon 305-600, Republic of Korea
Abstract Recent development of LiF:Mg,Cu,Si exhibiting high TL sensitivity (55 times that of TLD-100 LiF:Mg,Ti) and insignificant higher temperature peak leading to negligible residual TL signal has provided a TLD for pragmatic replacement of both LiF:Mg,Ti and LiF:Mg,Cu,P in personal dosimetry. LiF:Mg,Cu,Si withstood readout temperature up to 300 ◦ C. For the reader annealing using a maximum readout temperature and clamping at 260 ◦ C, no significant change in the TL sensitivity and glow curve structure was observed for more than 100 reuse cycles of exposures (5.5 mGy) and readout. TL emission spectrum LiF:Mg,Cu,Si was found to have three emission bands peaking at 355, 385 and 440 nm similar to that of LiF:Mg,Cu,P but differing in the relative intensities. Negligible fading, insensitivity to room light and absence of the effect of pre- or post-storage annealing on the response and the glow curve structure makes it a strong contender to be opted for personal dosimetry applications. © 2007 Published by Elsevier Ltd. Keywords: TLof LiF:Mg,Cu,Si; Fading; Reusability; Emission spectrum
1. Introduction High sensitivity (about 20–100 times that of LiF:Mg,Ti) and near tissue equivalent LiF:Mg,Cu,P TLD (Miljanic et al., 2002) has attracted the attention of doisimetric community since its discovery (Nakajima et al., 1979). The main problems associated with the use of LiF:Mg,Cu,P had been the loss of TL sensitivity by heating beyond 240 ◦ C and a high residual TL signal which increased with each readout cycle. A recent study (Luepke et al., 2006) has shown that in the commercially available LiF:Mg,Cu,P TLDs an increase of 1 ◦ C in the annealing temperature above 240 ◦ C continues to reduce the TL sensitivity significantly. Also, another recent study (Moscovitch et al., 2006) demonstrated that a thermal treatment needed for the encapsulation of LiF:Mg,Cu,P in Teflon for making a commercial TLD badge reduces the TL sensitivity by more than a factor of 2. In spite of all round efforts the world over to improve its dosimetric properties, replacement of the most widely used LiF:Mg,Ti by LiF:Mg,Cu,P in TLD personal dosimetry has ∗ Corresponding author. Fax: +82 42 868 8609.
E-mail address: [email protected] (J.L. Kim). 1350-4487/$ - see front matter © 2007 Published by Elsevier Ltd. doi:10.1016/j.radmeas.2007.10.045
remained full of apprehensions. This convinced us that it is necessary to make fresh efforts of changing the dopants and the preparation procedure to develop a high sensitivity LiF TLD material to suit the needs of personal dosimetry. Consequently, an optimized method of preparation of LiF:Mg,Cu,Si has been developed (Lee et al., 2006). The major features of the new method of preparation were (1) the avoidance of use of Na and P dopants, (2) the use of method of melting rather than limiting to sintering at temperature below the melting point and (3) the adoption of a final dual-step thermal treatment to improve the glow curve structure. This paper presents some of the important dosimetric aspects, namely, reusability, fading, effect of exposure to room light and TL emission spectrum. 2. Material and methods Different preparation procedures and varying concentrations dopants were tried (Lee et al., 2005). Consequently, an optimized method with dopant concentrations of 0.45 mol% of MgSO4 .7H2 O, 0.025 mol% of CuSO4 .5H2 O and 0.9 mol% of SiO2 and the use of method of melting for activation was
J.L. Kim et al. / Radiation Measurements 43 (2008) 446 – 449
arrived at (Lee et al., 2006). A dual-step thermal treatment at 300 ◦ C for 10 min followed by 260 ◦ C for 10 min was developed (Lee et al., 2007) as a final step of the preparation procedure for suppressing the higher temperature tail of the main dosimetric glow peak and improving the glow curve structure. Final dosimeters were in the shape of discs of thickness 0.8 mm and diameter 4.5 mm. The TL readouts were taken on a Harshaw 4500 TLD reader system (under the recommended flow rate of N2 gas) with optimized time and temperature profiles (TTP) for readouts using a heating rate of 10 ◦ C s−1 . Most of the equipment, including 137 Cs gamma-ray sources, a Studsvik 90 Sr–90 Y beta-ray reference irradiator (Model no.- 6527), oven and the set up for recording the emission spectra, were the same as described elsewhere (Lee et al., 2005, 2006, 2007). Individual calibration factors were established for the disc to disc variation and a minimum of four TLDs were used for each experimental point. The precision in the measurements was estimated to be better than ±3% (1). For the comparison, commercially available GR-200 LiF:Mg,Cu,P TLD discs were procured.
447
Table 1 Comparison of relative TL and residual TL signal of LiF:Mg,Cu,Si after different annealing and readout procedures Readout cycle
Reader anneala
Oven annealb
Relative TL
Residual TL signal
Relative TL
Residual TL signal
0.002 – 0.005
1.00 1.00 –
0.0001 0.0001 –
One 1.000 Twenty 1.000 Hundred 0.99
by keeping the TLD in the TLD reader at 260 ◦ C for 4 s following the readout up to 260 ◦ C. b Annealing in an oven at 260 ◦ C for 5 min prior to exposure and readout up to 300 ◦ C in the TLD reader. a Annealing
structure. Except for the relative intensities of different glow peaks, the glow curve structure was similar to that of LiF:Mg,Cu,P (Lee et al., 2006). The dose response of the main dosimetric peak was linear from 1 Gy to 20 Gy of 137 Cs gamma rays.
3. Results and discussion
3.1. Reusability
Fig. 1 shows the glow curves of the LiF:Mg,Cu,Si sintered discs subjected to different heat treatments during preparation. A thermal treatment (as a final step of the preparation procedure) of 300 ◦ C for 10 min followed by 260 ◦ C for 10 min was found to yield the best results and was termed as dual-step of thermal treatment (Lee et al., 2007). The TL sensitivity of LiF:Mg,Cu,Si was 55 times that of LiF:Mg,Ti, TLD-100 and about 10% higher than LiF:Mg,Cu,P. Quality assurance parameters of handling and processing the chemicals and the steps of heat treatment were found to be crucial. About 700 dosimeters were made out of five batches. Batch to batch variation in the TL sensitivity was within 10% with almost the same glow curve
20
In personnel dosimetry applications, one of the important requirements is to readout a very large number of dosimeters in the shortest possible time. Since the exposures encountered in any cycle of use (monthly or quarterly) are to result in the doses much below a few mSv (annual limit 20 or 50 mSv), it is usually preferred that the readout cycle in the reader should also offer the required annealing for resetting the dosimeter for the subsequent reuse. For such reuses, a readout cycle using a heating rate of 10 ◦ C s−1 and clamping the maximum readout temperature at 260 ◦ C with a hold of 4 s was opted. No change in the TL sensitivity (Table 1) and the glow curve structure (Fig. 2) were observed for the reuse of more than 100 cycles of exposures (5.5 mGy) and readout. The dual-step thermal treatment of 300 ◦ C for 10 min followed by 260 ◦ C for 10 min as a part of the preparation procedure was found unsatisfactory for subsequent reuse cycles. This treatment was found to reduce the TL sensitivity if used repeatedly as an annealing treatment for the reuse. For the reuse in the cases of higher exposures, etc. an annealing of 260 ◦ C for 5 min was found to be sufficient (Table 2). One of the important result is that the glow curve structure is not affected by different cycles of anneal, exposures and readouts by using different annealing treatments.
10
3.2. Fading and pre- or post-storage annealing
50
TL Intensity (arb. unit)
40
4
300/260
3
300
2
260
1
No annealing
30
0 50
100
150 200 Temperature (°C)
250
300
Fig. 1. Glow curves of the LiF:Mg,Cu,Si discs subjected to 1—No treatment, 2.260 ◦ C for 10 min (260), 3.300 ◦ C for 10 min (300) and 4.300 ◦ C for 10 min followed by 260 ◦ C for 10 min (300/260) thermal treatments after the sintering of the discs during preparation.
Tables 3 and 4 show the fading of gamma ray induced TL signal of the main dosimetric glow peak of LiF:Mg,Cu,Si. It can be seen that for storage at room temperature up to a period of 3 months, no significant fading was observed (Table 3). For evaluating the influence of room light, the TLDs (covered with a very thin and transparent plastic) were kept at 110 cm under a pair of fluorescent tubes during laboratory working hours of 1 month. The samples were placed closed to
448
J.L. Kim et al. / Radiation Measurements 43 (2008) 446 – 449 Table 2 Reusablity on repeated exposures (10 mGy) and readout of LiF:Mg,Cu,Si
400 1 cycle 10 cycles 20 cycles
4
350 300
3
250
2
200 150
1
Readout cycle
Temperature (°C)
TL Intensity (arb. unit)
5
100
0 50
100 Channel number
150
50 200
a maximum
1 50 100
400 300
2
200 1
Temperature (°C)
TL Intensity (arb. unit)
(b)b
1.00 0.99 1.00 1.00 1.00 1.00 0.97 – –
1.00 1.01 1.02 1.02 1.01 1.01 1.01 1.01 1.00
readout temperature 300 ◦ C and oven annealing at 260 ◦ C for
500
3
100
0 0
(a)a
5 min. b Using maximum readout temperature of 260 ◦ C with a hold of 4 s (reader anneal).
600 4
1 2 3 4 5 10 20 40 100
Relative TL for different cycles used for reuse
50
100 Channel number
150
0 200
Fig. 2. Typical glow curves of LiF:Mg,Cu,Si subjected to different reusability procedures (not corrected for the disc to disc variation). Top figure: after one cycle, ten cycles and twenty cycles of reuse using oven annealing at 260 ◦ C for 5 min prior to exposure for maximum readout temperature of 300 ◦ C in the TLD reader. Bottom figure: after one cycle, fifty cycles and hundred cycles of reuse using clamping the maximum readout temperature at 260 ◦ C with a hold of 4 s in the TLD reader.
a glass window avoiding exposure to direct sunlight but were exposed to light from both the fluorescent tubes as well as from the usual day light. Table 4 shows that exposure to room light for a period of 30 days reduced the TL by not more than 12%. TLDs stored for a period of 6 months after annealing at 260 ◦ C for 10 min and exposed to gamma rays and those stored for a period of 6 months and there after annealed at 260 ◦ C for 10 min and exposed to gamma rays did not show any unexpected difference in their TL readouts. The readouts (corrected for the accumulation of the background signal) were within 2% (experimental error). 3.3. TL emission spectrum Fig. 3 shows the TL emission spectrum of LiF:Mg,Cu,Si and LiF:Mg,Cu,P. The TL emission of the dosimetric glow peaks exhibit a peak at about 384 nm in the case of LiF:Mg,Cu,Si and at about 368 nm in the case of LiF:Mg,Cu,P. The analysis of the emission spectrum (deconvolution) revealed three
Table 3 Fading of TL Signal of LiF:Mg,Cu,Si pellets exposed to 10 mGy of 137 Cs gamma rays and stored for different durations at room temperature (corrected for varying signal introduced by storage background) Duration of storage
Relative response
1h 1 day 3 days 10 days 30 days 60 days 90 days
1.000 1.001 0.997 0.960 0.973 0.968 0.957
Table 4 Effect of room light on the TL of LiF:Mg,Cu,Si pellets exposed to 10 mGy of 137 Cs gamma rays and stored in dark and under room light at room temperature for a duration of 30 days Storage duration and condition
Relative response
Freshly annealed and stored under dark at room temperature after exposure
1.00
Freshly annealed and stored under room light after exposure
0.88
emission bands peaking at 355, 385 and 440 nm in both the TLDs. The emission band peaking at 385 nm is predominant in the TL emission of the dosimetric glow peaks of both the TLDs. However, the emission band peaking at 355 nm was significantly higher for LiF:Mg,Cu,P (having significant higher temperature peak) than in LiF:Mg,Cu,Si. The TL emission of the higher temperature glow peak of LiF:Mg,Cu,P, shows an intense emission band peaking at about 356 nm although all the three emission bands at 355, 385 and 440 nm are present. The reduced TL emission at 355 nm in LiF:Mg,Cu,Si is evidently due to the minimization of the higher temperature tail of the dosimetric glow peak.
J.L. Kim et al. / Radiation Measurements 43 (2008) 446 – 449
449
4. Conclusions 385
LiF:Mg,Cu,Si
Development of high sensitivity LiF:Mg,Cu,Si overcomes the drawbacks of LiF:Mg,Cu,P. LiF:Mg,Cu,Si exhibits an intense dosimetric peak with its TL emission peaking at 384 nm and a negligible higher temperature glow peak. The high TL sensitivity, improved thermal stability to withstand large numbers of exposure and readout cycles, negligible fading and insensitivity to exposures to room light provide a TLD material for pragmatic replacement of both LiF:Mg,Ti and LiF:Mg,Cu,P in personnel monitoring.
440
TL intensity (arb. unit)
355
References
385 LiF:Mg,Cu,P
356 440
LiF:Mg,Cu,P at high-temperature glow peak
356
385 440
300
350
400
450 500 Wavelength (nm)
550
600
Fig. 3. TL emission spectrum of dosimetric glow peak of LiF:Mg,Cu,Si (top figure), of dosimetric glow peak of LiF:Mg,Cu,P (middle figure) and higher temperature glow peak of LiF:Mg,Cu,P (bottom figure).
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