DOSIMETRIC CHARACTERISTICS OF THE NOVEL LiF:Mg,Cu,Ag PHOSPHOR

DOSIMETRIC CHARACTERISTICS OF THE NOVEL LiF:Mg,Cu,Ag PHOSPHOR Abstract In this study, the preparation procedure and dosimetric characteristic of LiF activated by Mg, Cu and Ag were investigated for the first time. It was observed that with the thermal annealing at 240°C for 10 min the sensitivity of the novel LiF:Mg,Cu,Ag powder was more than that of the TLD-100 powder. Also, the thermoluminescence intensity decreased with the increase of annealing temperature from 240 to 300°C. The glow curve structure of this sample that is similar to TLD-100H shows a mean peak at ~243°C along with four overlapping peaks. Also, the absorbed dose response increases linearly at low dose ranges. Other thermoluminescence characteristics of this phosphor such as low residual signal, low fading rate and good stability at repeatability and reusability make it useful for the dosimetric applications. INTRODUCTION Thermoluminescence (TL) dosimetry materials have attracted much attention because they have potential applications in various fields of dosimetry, especially in diagnostic radiology, radiation protection purposes and medical dosimetry(1, 2). The TL sensitivity and glow curve structure of these materials are strongly dependent on impurity type, impurity concentration and thermal treatment(3–5). Hence, a lot of research is being undertaken to develop more advanced TL material. Amongst various TL materials, LiF phosphor is the well-known due to its tissue equivalence. LiF doped with different impurities (e.g. Ti, Mg, Cu, P, Si, Na) gave rise to a new era in the field of dosimetry. Excellent dosimetric phosphors such as LiF:Mg,Ti, LiF:Mg,Cu, LiF:Mg,Cu,P, LiF:Mg,Cu,Si and LiF:Mg,Cu,Si,Na were produced by researchers(6–10). Comprehensive studies about the preparation and dosimetric characteristics of these samples are well reported in the literature. The purpose of these studies is to improve the dosimetric properties of the samples. These phosphors are found to be promising material for dosimetric applications due to the linear TL response over a wide dose range, good tissue equivalency, low fading and good stability under heat treatment(11–17). Among these materials, LiF:Mg,Cu,P is the most sensitive TL dosimeter material. It was first investigated by Nakajima et al.(2) and was prepared in different forms, such as powder, chip, film, tube and polycrystalline hot-pressed chip. Two disadvantages of this material are the loss of sensitivity when exposed to annealing temperatures higher than 240°C, and a relatively high residual signal(4–6, 18). There may well be a need to develop new material that has the valuable properties of LiF:Mg,Cu,P and eliminates its drawbacks. So, the changing of the third impurity in LiF:Mg,Cu,X (X: P, Si, Na) namely X impurity replacement instead of Ag impurity was chosen as the subject of this work. This helped produce a newly developed LiF:Mg,Cu,Ag material in our laboratory for the first time. In the present study, we have investigated some dosimetric characteristics of this phosphor such as optimum annealing temperature and time, suitable preheat temperature, sensitivity, absorbed dose response, fading and reusability. MATERIALS AND METHODS In order to prepare the LiF:Mg,Cu,Ag phosphor, LiF powder was mixed in water with Mg, Cu, Ag dopants in compound forms of MgF2 (0.2 mol%), CuCl2 (0.05 mol%) and Ag2O (0.1 mol%), respectively. The mixture was dried at 100 ± 1°C for 1 h in an AMERITECH oven model 47 900. After drying the mixture in a platinum crucible, it was melted in an electric furnace at 1005 ± 2°C for 30 min under 6 l min−1 Nitrogen gas flow. The sample was taken out of the oven after melting and then rapidly cooled to room temperature with an electric fan. The obtained poly-crystallized material was powdered and sieved into different grain sizes between 74 and 177 μm. Before each irradiation and TL reading, the prepared sample was annealed at 240°C for 10 min. Gamma irradiations of samples were performed by a 137Cs source with an absorbed dose of 57 mGy (absorbed dose rate of 2.37 mGyh−1) and 60Co source at absorbed doses in the range of 0.5–100 Gy at room temperature. The TL measurements were recorded on a Harshaw TLD reader 4500 with a maximum readout temperature of 300°C and a 5°C s−1 heating rate under Nitrogen gas. For easy handling of dosimeter powder, it was dispersed into aluminum plates and put it on the reader-planchet. In this case, the glow curves confront with a forward shift at about 40°C. For comparison between samples, all the obtained intensities were normalized by the mass. All measurements were carried out at least five times and then the average of the experimental data was calculated. The average standard deviation of all measurements was 1.09. RESULT AND DISCUSSION Dosimetric characteristics Optimum annealing temperature and time To determine the optimum annealing temperature, the LiF:Mg,Cu,Ag samples were heated at different temperatures from 240°C to 300°C for 10 min in the Ameritech oven model 47 900. Then, samples were rapidly cooled to room temperature and irradiated with the 137Cs source. The readouts were made 1 h after irradiation at a heating rate of 5°C s−1. Figure 1 shows the TL relative intensity of this sample for annealing temperatures between 240 and 300°C. As observed from Figure 1, the maximum intensity appears at 240°C. By applying annealing temperatures higher than 240°C, the TL intensity decreases due to thermal damage to trap and the dissolution of the Mg precipitated phase. The TL intensity at 300°C is ~28.23% lower than that at 240°C. Hence, the annealing temperature is an important factor, as it causes large variations in TL sensitivity. This result is in agreement with the results of other studies(4, 5, 18). Figure 1. View largeDownload slide The effect of various annealing temperatures on TL intensity of LiF:Mg,Cu,Ag for 10 min. Figure 1. View largeDownload slide The effect of various annealing temperatures on TL intensity of LiF:Mg,Cu,Ag for 10 min. The influence of the annealing time at 240°C temperature for different intervals from 10 to 50 min is shown in Figure 2. The figure shows that the maximum sensitivity was at 10 min. Moreover, there is no significant degradation in the TL intensity as the heating time is increased. Therefore, the optimum annealing temperature and time for the reuse of LiF:Mg,Cu,Ag material can be suggested as 240°C for 10 min. Figure 2. View largeDownload slide The effect of annealing time on TL sensitivity of LiF:Mg,Cu,Ag at 240°C annealing temperature. Figure 2. View largeDownload slide The effect of annealing time on TL sensitivity of LiF:Mg,Cu,Ag at 240°C annealing temperature. Optimum preheat temperature To have a stable dosimeter in the long-term dose measurements, the preheat treatment is an important condition that eliminates the low temperature glow peaks. Figure 3 shows the effect of various preheat temperatures on the TL intensity from 80 to 150°C for 10 min on the response of LiF:Mg,Cu,Ag sample after irradiation with the 137Cs source (heating rate 5°C s−1). As it can be seen, the sensitivity of the LiF:Mg,Cu,Ag sample decreases for preheat temperatures lower than 100°C or higher than 100°C. Therefore, the optimum preheat temperature is 100°C that produces the highest TL intensity. Figure 3. View largeDownload slide The effect of preheat temperature on TL sensitivity of LiF:Mg,Cu,Ag. Figure 3. View largeDownload slide The effect of preheat temperature on TL sensitivity of LiF:Mg,Cu,Ag. Sensitivity The sensitivity of the LiF:Mg,Cu,Ag powder was compared to the TLD-100H standard powder and TLD-100 powder using the 137Cs gamma source with the similar condition. Figure 4 depicts the glow curves of the LiF:Mg,Cu,Ag and TLD-100H powder, both recorded at a linear heating rate of 5°C s−1 after the same annealing procedure for 10 min at 240°C. The glow curve structure of the sample prepared in this study is similar to the TLD-100H sample. The LiF:Mg,Cu,Ag sample exhibits four TL peaks at ~156, 221, 265°C and the main peak at 243°C. The glow curve of the TLD-100H sample also shows the same peaks but shifted ~6°C to lower temperatures. Also, it can be observed from the figure that the TL sensitivity of the LiF:Mg,Cu,Ag sample is ~3.5 less than that of the TLD-100H sample but, it is still about seven times more than that of the TLD-100 phosphor. Figure 4. View largeDownload slide Typical glow curves of the LiF:Mg,Cu,Ag, TLD-100H and TLD-100 powder. The TLD-100H curve is multiplied by 0.29 for better comparison. Figure 4. View largeDownload slide Typical glow curves of the LiF:Mg,Cu,Ag, TLD-100H and TLD-100 powder. The TLD-100H curve is multiplied by 0.29 for better comparison. Residual signal The residual signal defined as the second TL readout which can be quantitatively given as the percentage ratio of the second to the first readout with the same reading conditions. This can be observed during the readout heating, when all the traps related to high temperature peaks cannot be emptied(8, 9). In the case of LiF:Mg,Cu,Ag this may occur for the readout heating at 240°C. The residual signals of the TLD-100H and LiF:Mg,Cu,Ag powders are compared at a heating rate of 5°Cs−1, after irradiation with the 137Cs gamma source. It was observed that the residual TL signals for TLD-100H and LiF:Mg,Cu,Ag are 0.05 and 0.06%, respectively. Both have low residual signals due to the lower intensity of the high temperature peak compared to the main peak Dose response The response of LiF:Mg,Cu,Ag as a function of the absorbed dose was investigated in the range of 0.5 to 100 Gy using a 60Co gamma source to irradiate five samples for each value of the absorbed dose (Figure 5). It can be seen that the TL absorbed dose response is linear up to 15 Gy, indicating the suitability of this material for environmental, personal and clinical dosimetry applications. Moreover, its high sensitivity permits to measure the low absorbed dose values. Figure 6 shows the absorbed dose response function, f (D), up to 100 Gy. From this figure, it is seen that the response function is linear up to 15 Gy and followed by a sub-linear (nonlinear) region. The normalized dose response function is defined as follows:(1)  f(D)=F(D)/DF(Dl)Dl (1)where, F (D) is the absorbed dose response at a dose D and Dl is a low dose at which the absorbed dose response is linear. Figure 5. View largeDownload slide Dose response of LiF:Mg,Cu,Ag powder with the 60Co gamma source from 0.5 to 15 Gy. Figure 5. View largeDownload slide Dose response of LiF:Mg,Cu,Ag powder with the 60Co gamma source from 0.5 to 15 Gy. Figure 6. View largeDownload slide Dose response function (f (D)) of the LiF:Mg,Cu,Ag powder with 60Co gamma source. Figure 6. View largeDownload slide Dose response function (f (D)) of the LiF:Mg,Cu,Ag powder with 60Co gamma source. Repeatability For the application of a TL material, it is important to check the stability and repeatability. The reproducibility of the LiF:Mg,Cu,Ag sample was examined in two stages: TL response repeatability for frequent use (anneal–radiation–readout cycle) and TL response repeatability for preparation (preparation–anneal–radiation–readout cycle). The assessment of reusability of LiF:Mg,Cu,Ag was repeated several times in the same manner according to the procedure of IES 1066(19). Figure 7 shows the results of frequent use of the LiF:Mg,Cu,Ag sample. As observed from the figure, the sensitivity of LiF:Mg,Cu,Ag after several anneal–radiation–readout cycles does not change significantly. The TL response repeatability for the preparation procedure was investigated by repeating the production procedure of the LiF:Mg,Cu,Ag TL material under the same conditions. It can be seen from this figure that the TL sensitivity changes smoothly after several times preparation repeatability, namely the preparation–anneal–radiation–readout cycle. The slight changes in the TL sensitivity seem to be a result of experimental errors. The variation coefficient of the TL response repeatability for frequent use and TL response repeatability for preparation was 5.7 and 6.5%, respectively. These results are in accordance with the procedure of IES 1066 and indicate good reusability of the LiF:Mg,Cu,Ag material in the personal and environmental dosimetry. Figure 7. View largeDownload slide The reproducibility of the LiF:Mg,Cu,Ag sample for the frequent used and preparation. Figure 7. View largeDownload slide The reproducibility of the LiF:Mg,Cu,Ag sample for the frequent used and preparation. Fading effect The fading effect on the TL sensitivity of LiF:Mg,Cu,Ag powder was studied by storing the irradiated samples at room temperature and in darkness during 50 days. As it can be seen in Figure 8, no significant fading was observed after 30 days. Nevertheless, a slight fall is noticeable with storage time, dropping to ~86% of the initial intensity by 50 days after irradiation. Thus, the low fading obtained is an important factor for radiation dosimetry. Figure 8. View largeDownload slide Fading in the LiF:Mg,Cu,Ag powder for a period of 50 days. Figure 8. View largeDownload slide Fading in the LiF:Mg,Cu,Ag powder for a period of 50 days. Comparison of LiF:Mg,Cu,Ag with other samples Table 1 depicts the comparison of the most relevant dosimetric characteristics of LiF:Mg,Cu,Ag with TLD-100 (LiF:Mg,Ti) and TLD-100H (LiF:Mg,Cu,P). It is observed that dosimetric characteristics of LiF:Mg,Cu,Ag are the same to TLD-100H. Also, detection threshold of this new dosimeter was investigated according to IEC 1066(19) and found to be 5 μGy. Table 1. Comparison of the dosimetric characteristics of LiF:Mg,Cu,Ag with other samples. Dosimetric characteristics  LiF:Mg,Cu,Ag  TLD-100H  TLD-100  Sensitivity  7  24  1  Annealing temperature  240°C (10 min)  240°C (10 min)  400°C (1 h) and 100°C (2 h)  Main peak  243°C  237°C  227°C  Repeatability  Good  Good  Good  Response to low dose  Linearly  Linearly  Linearly  Dosimetric characteristics  LiF:Mg,Cu,Ag  TLD-100H  TLD-100  Sensitivity  7  24  1  Annealing temperature  240°C (10 min)  240°C (10 min)  400°C (1 h) and 100°C (2 h)  Main peak  243°C  237°C  227°C  Repeatability  Good  Good  Good  Response to low dose  Linearly  Linearly  Linearly  CONCLUSION The new LiF:Mg,Cu,Ag material has been prepared by a melting method. The study of the influence of various annealing temperatures from 240 to 300°C and heating times from 10 to 50 min demonstrated that by shifting the annealing temperature from 240 to 300°C, the TL intensity was reduced and the 10 min time is optimum at 240°C annealing temperature. It was observed that the TL sensitivity of this sample is about seven times higher than that of the TLD-100 powder. The absorbed dose response increases linearly from 0.5 to 15 Gy and it is sub-linear above 15 Gy. The low residual signal, fading and repeatability results show that this sample can be used for dosimetry applications as a powder. REFERENCES 1 Bos, A. J. High sensitivity thermoluminescence dosimetry. Nucl. Instrum. Methods Phys. Res. B  184, 3– 28 ( 2001). Google Scholar CrossRef Search ADS   2 Nakajima, T., Murayama, Y., Matsuzawa, T. and Koyano, A. Development of a new highly sensitive LiF thermoluminescence dosimeter and its applications. Nucl. Instrum. Methods  157, 155– 162 ( 1978). Google Scholar CrossRef Search ADS   3 Shoushan, W. The dependence of thermoluminescence response and glow curve structure of LiF(Mg, Cu, P) TL material on Mg, Cu, P dopant concentration. Radiat. Prot. Dosim.  25, 133– 136 ( 1988). Google Scholar CrossRef Search ADS   4 Tang, K., Fan, H., Cui, H., Zhu, H. and Liu, Z. Further study on the influence of thermal treatments on the glow curve structure in LiF:Mg,Cu,P (GR-200A). Radiat. Meas.  63, 1– 5 ( 2014). Google Scholar CrossRef Search ADS   5 Bilski, P., Budzanowki, M. and Olko, P. Dependence of LiF:Mg,Cu,P (MCP-N) glow curve structure on dopant composition and thermal treatment. Radiat. Prot. Dosim.  69, 187– 198 ( 1997). Google Scholar CrossRef Search ADS   6 Kolotilin, V. V., Hokhrekov, V. I., Tarasova, L. M. and Zakhriapin, S. B. A high sensitivity LiF:Mg,Cu,P thermoluminescent dosimeter. Nucl. Tracks Radiat. Meas.  21, 169– 171 ( 1993). Google Scholar CrossRef Search ADS   7 Gonzalez, P. R., Quiroz, M. C., Azorin, J., Furetta, C. and Avila, O. Improvement in the preparation method of LiF:Mg,Cu,P thermoluminescent phosphor. J. Appl. Sci.  5, 1408– 1411 ( 2005). 8 Bhatt, B. C., Shinde, S. S. and Bhatt, R. C. Comparative dosimetric studies of three LiF TL phosphors. Radiat. Prot. Dosim.  27, 21– 27 ( 1989). 9 Bilski, P. Lithium fluoride: from LiF:Mg,Ti to LiF:Mg,Cu,P. Radiat. Prot. Dosim.  100, 199– 206 ( 2002). Google Scholar CrossRef Search ADS   10 Yang, B., Lu, Q., Wang, S. and Townsend, P. D. Studies on thermoluminescence spectra and thermal stability of LiF:Mg,Cu, LiF:Mg,Cu,P and LiF:Mg,Cu,Si. Nucl. Instrum. Methods Phys. Res. B  239, 171– 178 ( 2005). Google Scholar CrossRef Search ADS   11 Nakajima, T., Murayama, Y. and Matsuzawa, T. Preparation and dosimetric properties of a highly sensitive LiF thermoluminescence dosimeter. Health Phys.  36, 79– 82 ( 1979). Google Scholar PubMed  12 Kim, J. L., Lee, J. I., Pradhan, A. S., Kim, B. H. and Kim, J. S. Further studies on the dosimetric characteristics of LiF: Mg, Cu, Si. A high sensitivity thermoluminescence dosimeter. Radiat. Meas.  43, 446– 449 ( 2008). Google Scholar CrossRef Search ADS   13 Kim, H. J., Chung, W. H., Doh, S. H., Chu, M. C., Kim, D. S. and Kang, Y. H. Thermoluminescence dosimetric properties of LiF:Mg,Cu,Na,Si. J. Korean Phys. Soc.  22, 415– 420 ( 1990). 14 Zha, Z., Wang, S., Shen, W., Zhu, J. and Cai, G. Preparation and characteristics of LiF:Mg,Cu,P thermoluminescent material. Radiat. Prot. Dosim.  47, 111– 118 ( 1993). Google Scholar CrossRef Search ADS   15 Rahman, M. S., Lee, J. I., Kim, J. L. and Cho, G. Dosimetric properties of the newly developed LiF:Mg,Cu,Si TL material. J. Sci. Res.  5, 25– 31 ( 2013). Google Scholar CrossRef Search ADS   16 Lee, J. I., Yang, J. S., Kim, J. L., Pradhan, A. S., Lee, J. D., Chung, K. S. and Choe, H. S. Dosimetric characteristics of LiF:Mg,Cu,Si thermoluminescent materials. Appl. Phys. Lett.  89, 094110 ( 2006). Google Scholar CrossRef Search ADS   17 Doh, S. H., Chu, M. C., Chung, W. H., Kim, H. J., Kim, D. S. and Kang, Y. H. Preparation of LiF(Mg,Cu,Na,Si) phosphor and its thermoluminescent characteristics. Korean Appl. Phys.  2, 425– 443 ( 1989). 18 Cai, G. G., Fesquet, J., Dusseau, L., Martini, M., Meinardi, F., Huang, B. L., Tang, K. Y., Beteill, D. and Gasiot, J. Thermoluminescence of LiF:Mg,Cu,P (GR-200A)TLD after annealing between 200 and 400°C. Radiat. Prot. Dosim.  65, 163– 166 ( 1996). Google Scholar CrossRef Search ADS   19 IEC (International Electrotechnical Commission) 1066 International Standard (thermoluminescence dosimetry systems for personal and environmental monitoring) (1991). © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

DOSIMETRIC CHARACTERISTICS OF THE NOVEL LiF:Mg,Cu,Ag PHOSPHOR

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Abstract

Abstract In this study, the preparation procedure and dosimetric characteristic of LiF activated by Mg, Cu and Ag were investigated for the first time. It was observed that with the thermal annealing at 240°C for 10 min the sensitivity of the novel LiF:Mg,Cu,Ag powder was more than that of the TLD-100 powder. Also, the thermoluminescence intensity decreased with the increase of annealing temperature from 240 to 300°C. The glow curve structure of this sample that is similar to TLD-100H shows a mean peak at ~243°C along with four overlapping peaks. Also, the absorbed dose response increases linearly at low dose ranges. Other thermoluminescence characteristics of this phosphor such as low residual signal, low fading rate and good stability at repeatability and reusability make it useful for the dosimetric applications. INTRODUCTION Thermoluminescence (TL) dosimetry materials have attracted much attention because they have potential applications in various fields of dosimetry, especially in diagnostic radiology, radiation protection purposes and medical dosimetry(1, 2). The TL sensitivity and glow curve structure of these materials are strongly dependent on impurity type, impurity concentration and thermal treatment(3–5). Hence, a lot of research is being undertaken to develop more advanced TL material. Amongst various TL materials, LiF phosphor is the well-known due to its tissue equivalence. LiF doped with different impurities (e.g. Ti, Mg, Cu, P, Si, Na) gave rise to a new era in the field of dosimetry. Excellent dosimetric phosphors such as LiF:Mg,Ti, LiF:Mg,Cu, LiF:Mg,Cu,P, LiF:Mg,Cu,Si and LiF:Mg,Cu,Si,Na were produced by researchers(6–10). Comprehensive studies about the preparation and dosimetric characteristics of these samples are well reported in the literature. The purpose of these studies is to improve the dosimetric properties of the samples. These phosphors are found to be promising material for dosimetric applications due to the linear TL response over a wide dose range, good tissue equivalency, low fading and good stability under heat treatment(11–17). Among these materials, LiF:Mg,Cu,P is the most sensitive TL dosimeter material. It was first investigated by Nakajima et al.(2) and was prepared in different forms, such as powder, chip, film, tube and polycrystalline hot-pressed chip. Two disadvantages of this material are the loss of sensitivity when exposed to annealing temperatures higher than 240°C, and a relatively high residual signal(4–6, 18). There may well be a need to develop new material that has the valuable properties of LiF:Mg,Cu,P and eliminates its drawbacks. So, the changing of the third impurity in LiF:Mg,Cu,X (X: P, Si, Na) namely X impurity replacement instead of Ag impurity was chosen as the subject of this work. This helped produce a newly developed LiF:Mg,Cu,Ag material in our laboratory for the first time. In the present study, we have investigated some dosimetric characteristics of this phosphor such as optimum annealing temperature and time, suitable preheat temperature, sensitivity, absorbed dose response, fading and reusability. MATERIALS AND METHODS In order to prepare the LiF:Mg,Cu,Ag phosphor, LiF powder was mixed in water with Mg, Cu, Ag dopants in compound forms of MgF2 (0.2 mol%), CuCl2 (0.05 mol%) and Ag2O (0.1 mol%), respectively. The mixture was dried at 100 ± 1°C for 1 h in an AMERITECH oven model 47 900. After drying the mixture in a platinum crucible, it was melted in an electric furnace at 1005 ± 2°C for 30 min under 6 l min−1 Nitrogen gas flow. The sample was taken out of the oven after melting and then rapidly cooled to room temperature with an electric fan. The obtained poly-crystallized material was powdered and sieved into different grain sizes between 74 and 177 μm. Before each irradiation and TL reading, the prepared sample was annealed at 240°C for 10 min. Gamma irradiations of samples were performed by a 137Cs source with an absorbed dose of 57 mGy (absorbed dose rate of 2.37 mGyh−1) and 60Co source at absorbed doses in the range of 0.5–100 Gy at room temperature. The TL measurements were recorded on a Harshaw TLD reader 4500 with a maximum readout temperature of 300°C and a 5°C s−1 heating rate under Nitrogen gas. For easy handling of dosimeter powder, it was dispersed into aluminum plates and put it on the reader-planchet. In this case, the glow curves confront with a forward shift at about 40°C. For comparison between samples, all the obtained intensities were normalized by the mass. All measurements were carried out at least five times and then the average of the experimental data was calculated. The average standard deviation of all measurements was 1.09. RESULT AND DISCUSSION Dosimetric characteristics Optimum annealing temperature and time To determine the optimum annealing temperature, the LiF:Mg,Cu,Ag samples were heated at different temperatures from 240°C to 300°C for 10 min in the Ameritech oven model 47 900. Then, samples were rapidly cooled to room temperature and irradiated with the 137Cs source. The readouts were made 1 h after irradiation at a heating rate of 5°C s−1. Figure 1 shows the TL relative intensity of this sample for annealing temperatures between 240 and 300°C. As observed from Figure 1, the maximum intensity appears at 240°C. By applying annealing temperatures higher than 240°C, the TL intensity decreases due to thermal damage to trap and the dissolution of the Mg precipitated phase. The TL intensity at 300°C is ~28.23% lower than that at 240°C. Hence, the annealing temperature is an important factor, as it causes large variations in TL sensitivity. This result is in agreement with the results of other studies(4, 5, 18). Figure 1. View largeDownload slide The effect of various annealing temperatures on TL intensity of LiF:Mg,Cu,Ag for 10 min. Figure 1. View largeDownload slide The effect of various annealing temperatures on TL intensity of LiF:Mg,Cu,Ag for 10 min. The influence of the annealing time at 240°C temperature for different intervals from 10 to 50 min is shown in Figure 2. The figure shows that the maximum sensitivity was at 10 min. Moreover, there is no significant degradation in the TL intensity as the heating time is increased. Therefore, the optimum annealing temperature and time for the reuse of LiF:Mg,Cu,Ag material can be suggested as 240°C for 10 min. Figure 2. View largeDownload slide The effect of annealing time on TL sensitivity of LiF:Mg,Cu,Ag at 240°C annealing temperature. Figure 2. View largeDownload slide The effect of annealing time on TL sensitivity of LiF:Mg,Cu,Ag at 240°C annealing temperature. Optimum preheat temperature To have a stable dosimeter in the long-term dose measurements, the preheat treatment is an important condition that eliminates the low temperature glow peaks. Figure 3 shows the effect of various preheat temperatures on the TL intensity from 80 to 150°C for 10 min on the response of LiF:Mg,Cu,Ag sample after irradiation with the 137Cs source (heating rate 5°C s−1). As it can be seen, the sensitivity of the LiF:Mg,Cu,Ag sample decreases for preheat temperatures lower than 100°C or higher than 100°C. Therefore, the optimum preheat temperature is 100°C that produces the highest TL intensity. Figure 3. View largeDownload slide The effect of preheat temperature on TL sensitivity of LiF:Mg,Cu,Ag. Figure 3. View largeDownload slide The effect of preheat temperature on TL sensitivity of LiF:Mg,Cu,Ag. Sensitivity The sensitivity of the LiF:Mg,Cu,Ag powder was compared to the TLD-100H standard powder and TLD-100 powder using the 137Cs gamma source with the similar condition. Figure 4 depicts the glow curves of the LiF:Mg,Cu,Ag and TLD-100H powder, both recorded at a linear heating rate of 5°C s−1 after the same annealing procedure for 10 min at 240°C. The glow curve structure of the sample prepared in this study is similar to the TLD-100H sample. The LiF:Mg,Cu,Ag sample exhibits four TL peaks at ~156, 221, 265°C and the main peak at 243°C. The glow curve of the TLD-100H sample also shows the same peaks but shifted ~6°C to lower temperatures. Also, it can be observed from the figure that the TL sensitivity of the LiF:Mg,Cu,Ag sample is ~3.5 less than that of the TLD-100H sample but, it is still about seven times more than that of the TLD-100 phosphor. Figure 4. View largeDownload slide Typical glow curves of the LiF:Mg,Cu,Ag, TLD-100H and TLD-100 powder. The TLD-100H curve is multiplied by 0.29 for better comparison. Figure 4. View largeDownload slide Typical glow curves of the LiF:Mg,Cu,Ag, TLD-100H and TLD-100 powder. The TLD-100H curve is multiplied by 0.29 for better comparison. Residual signal The residual signal defined as the second TL readout which can be quantitatively given as the percentage ratio of the second to the first readout with the same reading conditions. This can be observed during the readout heating, when all the traps related to high temperature peaks cannot be emptied(8, 9). In the case of LiF:Mg,Cu,Ag this may occur for the readout heating at 240°C. The residual signals of the TLD-100H and LiF:Mg,Cu,Ag powders are compared at a heating rate of 5°Cs−1, after irradiation with the 137Cs gamma source. It was observed that the residual TL signals for TLD-100H and LiF:Mg,Cu,Ag are 0.05 and 0.06%, respectively. Both have low residual signals due to the lower intensity of the high temperature peak compared to the main peak Dose response The response of LiF:Mg,Cu,Ag as a function of the absorbed dose was investigated in the range of 0.5 to 100 Gy using a 60Co gamma source to irradiate five samples for each value of the absorbed dose (Figure 5). It can be seen that the TL absorbed dose response is linear up to 15 Gy, indicating the suitability of this material for environmental, personal and clinical dosimetry applications. Moreover, its high sensitivity permits to measure the low absorbed dose values. Figure 6 shows the absorbed dose response function, f (D), up to 100 Gy. From this figure, it is seen that the response function is linear up to 15 Gy and followed by a sub-linear (nonlinear) region. The normalized dose response function is defined as follows:(1)  f(D)=F(D)/DF(Dl)Dl (1)where, F (D) is the absorbed dose response at a dose D and Dl is a low dose at which the absorbed dose response is linear. Figure 5. View largeDownload slide Dose response of LiF:Mg,Cu,Ag powder with the 60Co gamma source from 0.5 to 15 Gy. Figure 5. View largeDownload slide Dose response of LiF:Mg,Cu,Ag powder with the 60Co gamma source from 0.5 to 15 Gy. Figure 6. View largeDownload slide Dose response function (f (D)) of the LiF:Mg,Cu,Ag powder with 60Co gamma source. Figure 6. View largeDownload slide Dose response function (f (D)) of the LiF:Mg,Cu,Ag powder with 60Co gamma source. Repeatability For the application of a TL material, it is important to check the stability and repeatability. The reproducibility of the LiF:Mg,Cu,Ag sample was examined in two stages: TL response repeatability for frequent use (anneal–radiation–readout cycle) and TL response repeatability for preparation (preparation–anneal–radiation–readout cycle). The assessment of reusability of LiF:Mg,Cu,Ag was repeated several times in the same manner according to the procedure of IES 1066(19). Figure 7 shows the results of frequent use of the LiF:Mg,Cu,Ag sample. As observed from the figure, the sensitivity of LiF:Mg,Cu,Ag after several anneal–radiation–readout cycles does not change significantly. The TL response repeatability for the preparation procedure was investigated by repeating the production procedure of the LiF:Mg,Cu,Ag TL material under the same conditions. It can be seen from this figure that the TL sensitivity changes smoothly after several times preparation repeatability, namely the preparation–anneal–radiation–readout cycle. The slight changes in the TL sensitivity seem to be a result of experimental errors. The variation coefficient of the TL response repeatability for frequent use and TL response repeatability for preparation was 5.7 and 6.5%, respectively. These results are in accordance with the procedure of IES 1066 and indicate good reusability of the LiF:Mg,Cu,Ag material in the personal and environmental dosimetry. Figure 7. View largeDownload slide The reproducibility of the LiF:Mg,Cu,Ag sample for the frequent used and preparation. Figure 7. View largeDownload slide The reproducibility of the LiF:Mg,Cu,Ag sample for the frequent used and preparation. Fading effect The fading effect on the TL sensitivity of LiF:Mg,Cu,Ag powder was studied by storing the irradiated samples at room temperature and in darkness during 50 days. As it can be seen in Figure 8, no significant fading was observed after 30 days. Nevertheless, a slight fall is noticeable with storage time, dropping to ~86% of the initial intensity by 50 days after irradiation. Thus, the low fading obtained is an important factor for radiation dosimetry. Figure 8. View largeDownload slide Fading in the LiF:Mg,Cu,Ag powder for a period of 50 days. Figure 8. View largeDownload slide Fading in the LiF:Mg,Cu,Ag powder for a period of 50 days. Comparison of LiF:Mg,Cu,Ag with other samples Table 1 depicts the comparison of the most relevant dosimetric characteristics of LiF:Mg,Cu,Ag with TLD-100 (LiF:Mg,Ti) and TLD-100H (LiF:Mg,Cu,P). It is observed that dosimetric characteristics of LiF:Mg,Cu,Ag are the same to TLD-100H. Also, detection threshold of this new dosimeter was investigated according to IEC 1066(19) and found to be 5 μGy. Table 1. Comparison of the dosimetric characteristics of LiF:Mg,Cu,Ag with other samples. Dosimetric characteristics  LiF:Mg,Cu,Ag  TLD-100H  TLD-100  Sensitivity  7  24  1  Annealing temperature  240°C (10 min)  240°C (10 min)  400°C (1 h) and 100°C (2 h)  Main peak  243°C  237°C  227°C  Repeatability  Good  Good  Good  Response to low dose  Linearly  Linearly  Linearly  Dosimetric characteristics  LiF:Mg,Cu,Ag  TLD-100H  TLD-100  Sensitivity  7  24  1  Annealing temperature  240°C (10 min)  240°C (10 min)  400°C (1 h) and 100°C (2 h)  Main peak  243°C  237°C  227°C  Repeatability  Good  Good  Good  Response to low dose  Linearly  Linearly  Linearly  CONCLUSION The new LiF:Mg,Cu,Ag material has been prepared by a melting method. The study of the influence of various annealing temperatures from 240 to 300°C and heating times from 10 to 50 min demonstrated that by shifting the annealing temperature from 240 to 300°C, the TL intensity was reduced and the 10 min time is optimum at 240°C annealing temperature. It was observed that the TL sensitivity of this sample is about seven times higher than that of the TLD-100 powder. The absorbed dose response increases linearly from 0.5 to 15 Gy and it is sub-linear above 15 Gy. The low residual signal, fading and repeatability results show that this sample can be used for dosimetry applications as a powder. REFERENCES 1 Bos, A. J. High sensitivity thermoluminescence dosimetry. Nucl. Instrum. Methods Phys. Res. B  184, 3– 28 ( 2001). Google Scholar CrossRef Search ADS   2 Nakajima, T., Murayama, Y., Matsuzawa, T. and Koyano, A. Development of a new highly sensitive LiF thermoluminescence dosimeter and its applications. Nucl. Instrum. Methods  157, 155– 162 ( 1978). Google Scholar CrossRef Search ADS   3 Shoushan, W. The dependence of thermoluminescence response and glow curve structure of LiF(Mg, Cu, P) TL material on Mg, Cu, P dopant concentration. Radiat. Prot. Dosim.  25, 133– 136 ( 1988). 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Radiation Protection DosimetryOxford University Press

Published: Mar 1, 2018

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