ARTICLE IN PRESS Radiation Physics and Chemistry 76 (2007) 1523–1526 www.elsevier.com/locate/radphyschem Effect of post-irradiation thermal treatments on the stability of gamma-irradiated glass dosimeter K. Faraha,, A. Kovácsb, A. Mejria, H. Ben Ouadac a Laboratoire de Radiotraitement, Centre National des Sciences et Technologie Nucléaires, 2020 Sidi-Thabet, Tunisia Departement of Radiation Safety, Institute of Isotopes, Hungarian Academy of Sciences, Konkoly Thege Miklós út 29-33, 1121 Budapest, Hungary c Département de Physique, Laboratoire de Physique et Chimie des Interfaces, Faculté des Sciences de Monastir, 5000 Monastir, Tunisia b Abstract A commercial window glass has been investigated as a routine high dose dosimeter for gamma irradiation. The irradiated samples showed rapid fading at room temperature immediately after irradiation. This short-term rapid fading was followed by a slow fading at long-term. This strong initial fading is a problem for dosimetry purposes. However, when the dosimeter is measured at the same time interval after irradiation, it maintains proportionality to dose. Calibration curves have to be used for different time intervals after irradiation. In order to improve post-irradiation stability dosimeters were submitted to different post-irradiation thermal treatments from (20) up to 150 1C. After that, optical absorbance measurements were carried out up to 2 months at room temperature. The heating at 150 1C for 20 min was found to be the most suitable procedure for the removal of unstable entities responsible for the initial rapid fading. Due to these heat treatments, variation of response was found almost negligible 24 h after irradiation for several months. Calibration curves demonstrated the applicability of this glass as routine dosimeter in the dose range of 0.5–90 kGy. r 2007 Elsevier Ltd. All rights reserved. Keywords: Window glass dosimeter; Post-irradiation thermal treatments; Fading behaviour 1. Introduction The ionizing radiation produces electron–hole pairs which individually become trapped at various defect sites in the glass structure. Consequently, new optical absorption bands may appear. The ionizing radiation-induced colour centres in some commercial glasses have been found suitable for certain applications in radiation processing dosimetry (Zheng et al., 1988; Engin et al., 2006). Their use does not require special preparation. These dosimeters are recyclable, rigid, chemically inert, cheap and independent of humidity and dose rate (Zheng et al., 1988; Engin et al., 2006). The main difficulty presented by glass dosimeters is the undesirable strong initial fading. Many authors overcame this problem by taking measurements for the most adequate time intervals after irradiation (Rodrigues Jr. et al., 2002; Engin et al., 2006). In order to solve this problem in our work, the glass dosimeters were submitted to Corresponding author. Tel.: +216 22 25 70 98; fax: +216 71 53 74 10. E-mail address: k.farah@cnstn.rnrt.tn (K. Farah). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.02.065 different post-irradiation thermal treatments from 20 up to 150 1C and the effect of this treatments on the fading behaviour of the dosimeters was studied up to 2 months after irradiation at room temperature. 2. Materials and methods The glass samples were obtained from the same glass sheets purchased from the local market and were cut into pieces of 11  30  3 mm dimensions for optical measurements. The chemical composition of the glass samples was determined by using the Prompt Gamma Activation Analysis technique (Anderson et al., 2004) at the Budapest Neutron Centre resulting in wt%: 68.52 SiO2, 13.77 Na2O, 8.19 CaO, 4.34 MgO, 1.003 Al2O3, 0.588 K2O, 0.105 Fe2O3 and about 3.5% of other components. The irradiations were made at the Tunisian semiindustrial 60Co gamma irradiation facility at a dose rate: 2 kGy/h. Calibration was carried out at the 60Co calibration facility of the Egyptian Reference High Dose Laboratory at the dose rate of 4.78 kGy/h at controlled ARTICLE IN PRESS 1524 K. Farah et al. / Radiation Physics and Chemistry 76 (2007) 1523–1526 temperature of 34 1C. The traceability was established with alanine/EPR dosimetry system in terms of absorbed dose to water traceable to the National Physical Laboratory, UK. Optical absorption spectra were taken with a Perkin Elmer spectrophotometer (Lambda 20) in the range of 350–800 nm. Genesis 5 spectrophotometer and Käfer MFT 30 thickness gauge were used to measure the specific absorbance (i.e. absorbance of the dosimeter divided by the thickness of the dosimeter) changes produced in glass. An electrical furnace and a freezer were used to reach the desired post-irradiation storage temperatures. 45 kGy 31 kGy 15 kGy 13 kGy 10 kGy 7 kGy 3 kGy 0.8 kGy 0.133 kGy Absorbance (a.u.) 0.6 0.4 0.0 400 450 500 550 600 3.1. Optical absorption spectra of gamma-irradiated window glass Optical absorption spectra of the irradiated glass samples were measured against nonirradiated sample in order to obtain the net-induced changes of absorption. Fig. 1 shows the effect of increasing doses between 0.13 and 45 kGy on the absorption spectra of glass dosimeter in the region from 350 to 800 nm. The radiation-induced colour was brown and two optical absorption bands were found at 410 and 600 nm. The intensities of the overall absorption spectra increased progressively with increasing doses. 3.2. Thermal stability 0.2 350 3. Results and discussion 650 700 750 800 Wavelength (nm) Fig. 1. Optical absorption spectra for the gamma-irradiated glass samples in the dose range 0.13–45 kGy. 3.2.1. Fading characteristics at room temperature Five replicate glass dosimeters were irradiated with 60Co gamma rays to 30 kGy. The absorbance changes were followed up to 535 days. After each measurement glass samples were stored in the dark at room temperature. Fig. 2 presents the fading behaviour of the 410 nm absorption band at room temperature for the long-term and the short-term period. A strong fading can be observed in the first 9 days followed by a slow fading up to 535 days. The 600 nm absorption band showed similar behaviour. The data in Fig. 2. were fitted using first-order kinetics based on the data measured after the first 10 days storage. The coefficient of correlation (R2) was 0.99. The stability of radiation-induced colour centres at room temperature was mainly controlled by the initial strong fading process. This initial fading process seems to follow neither a simple 8 Experimental data First order decay kinetic fit on data after 9 days Specificabsorbance at 410 nm(cm-1) Specific absorbance at 410 nm (cm-1) 7 6 5 4 3 2 8 Experimental data for the first 9 days Combination of second and first order decay kinetic fit Second order decay kinetic fit First order decay kinetic fit 7 6 5 4 3 2 0 1 2 4 6 8 10 12 Post-irradiation time(days) 0 200 400 600 800 1000 Post-irradiation time (days) Fig. 2. Post-irradiation fading and kinetics of room temperature response of glass samples irradiated at 30 kGy. Inset: kinetics of the fading behaviour at room temperature for the short-term period. (Specific absorbance: absorbance of the dosimeter divided by the thickness of the dosimeter). ARTICLE IN PRESS 1525 K. Farah et al. / Radiation Physics and Chemistry 76 (2007) 1523–1526 first-order nor second-order kinetics, but it can be well described by the sum of second- and first-order decay kinetics fit (y ¼ aebx+c/(1+cdx), where b is the firstorder and c is the second-order decay rate constant. Sheng et al. (2002) gave evidence that the long-term fading process of colour centres induced by X-rays in soda-lime silicate glass was dominated by first-order kinetics, while both the first- and the second-order kinetics played role in the short-term fading process. effective for the removal of unstable entities responsible for the initial strong fading. The standard deviation of glass dosimeters response measurements is about 0.5% (1s) within the first 2 h after irradiation. After the heat treatment performed at 150 1C, the response decay of irradiated glass dosimeters is about 8% between the first 24 h and 20 days. This means that glass dosimeters can be evaluated either within the first 2 h or just after 1 day after irradiation and heating. 3.2.2. Effect of post-irradiation heat treatments on the room temperature fading The effect of post-irradiation heat treatments on the glass response fading was studied in the temperature range of 60–150 1C using sets of three glass samples. After gamma irradiation with 30 kGy absorbed dose, dosimeter sets were immediately submitted to the different heat treatments for 20 min, which was found to be the best treatment time, and stored after irradiation in the dark at room temperature. Optical absorbance measurements were carried out up to 2 months. The specific absorbance values were normalized to the first measurements taken 5 min after the heat treatments. The results are presented in Fig. 3 and Table 1. The best results have been obtained with heat treatments at 150 1C (20 min). This procedure is very 3.2.3. Effect of low-temperature storage on the thermal fading Fig. 4. shows the thermal fading of glass samples stored at low temperature. Two sets of three glass samples were irradiated to 30 kGy and then kept in the freezer immediately after irradiation. One set was stored at (5) 1C and the other one at (20) 1C. Measurements were taken up to 70 days. After the first 24 h an approximate 5% reduction of response was observed for the two sets (Table 1). 3.2.4. Effect of thermal treatments on the fading behaviour The first- and second-order kinetics fading rate constants were calculated for each storage temperature to fit the short-term fading of the experimental data (Table 2). The results show that the fading behaviour of the glass samples is remarkably different for the various conditions of No heat treatment 60 °C 80 °C 100 °C 120 °C 150 °C 0.8 0.6 0.4 0.2 40 Room temperature 104 102 100 98 96 0 0 0 1.2 Normalized specific absorbance 1 Relative specific absorbance (%) Normalized specific absorbance 1.2 80 0.5 1 1.5 2 Post-irradiation time (hours) 120 2.5 160 Post-irradiation time (days) - 5 °C - 20 °C 1 0.8 0.6 0.4 0.2 0 0 Fig. 3. Post-irradiation fading at room temperature of glass samples after 30 kGy irradiation and different heat treatments for 20 min. Inset: behaviour of glass samples, heated at 150 1C for 20 min, within the first 2 h after the heat treatment. 20 40 60 Post-irradiation time (days) 80 100 Fig. 4. Thermal fading of glass samples stored at room and low temperature (5 and 20 1C) up to 70 days after an irradiation of 30 kGy. Table 1 Effect of thermal treatments on fading of glass samples after an irradiation of 30 kGy with gamma rays Storage time 24 h 20 days 60 days Reduction of relative specific absorbance (%) Room temperature 5 1C 20 1C 60 1C 80 1C 100 1C 120 1C 150 1C 25 54 65 5 40 66 5 39 65 17 49 60 16 45 54 15 35 45 12 29 31 7 15 19 *The samples heated in the temperature range of 60–150 1C were kept in the furnace for 20 min. ARTICLE IN PRESS 1526 K. Farah et al. / Radiation Physics and Chemistry 76 (2007) 1523–1526 Table 2 Fading time constants for temperature range from 20 to 150 1C T (1C) First-order kinetics t (h) Second-order kinetics k (h) Room temperature 5 20 60 80 100 130 150 189.6 1128 2184 169.5 168 110.4 6.25 3.15 30.9 60 117 7.5 0.43 0.024 0.0006 0 4. Conclusions The observations of our study indicate, that the postirradiation heating (150 1C for 20 min) of the commercial window glass reduces the radiation-induced absorbance and accelerates the recombination of the entities responsible for the initial strong fading. Therefore, the evaluation of the window glass dosimeters is suggested to be performed either within the first 2 h or 1 day after irradiation and heat treatment. This procedure does not affect the useful dose range and the reproducibility of measurements. This study demonstrated the necessity of heat treatment of window glass after irradiation and thus their applicability for routine dosimetric purposes with acceptable post-irradiation stability. 0.16 Specific absorbance (cm-1) glass dosimeters. In fact, by comparing the data obtained from a similar batch of samples irradiated to the same dose, but not heat treated, and measured at fixed time (24 h) after irradiation resulted in a 3% (1s) overall coefficient of variation. 0.14 0.12 0.1 0.08 0.06 0.04 Acknowledgements 0.02 0 0 20 40 60 80 100 120 Dose (kGy) Fig. 5. Dose response curve of glass dosimeters (410 nm; heated at 150 1C for 20 min) in the dose range of 0.5–90 kGy fitted with power law function. storage and thermal treatments. Post-irradiation heat treatments led to fast fading by accelerating the recombination of defects and the impurities diffusing into the glass matrix, while post-irradiation storage at low temperatures (5 and 20 1C) slows down the recombination processes. 3.3. Dose response curve The heating at 150 1C for 20 min was found to be the most suitable procedure to stabilize the irradiated glass dosimeter. The dose response curve taken after this procedure for the dose range of 0.5–90 kGy is shown in Fig. 5. This procedure did not affect the metrological properties (reproducibility and useful dose range) of the The authors wish to thank Dr. T. Belgya and Dr. Zs. Révay for the PGAA glass analysis and Dr. F. AbdelRehim for the calibration of the glass dosimeters. The authors wish also to thank Dr. P.G. Fuochi for his useful discussions and suggestions. References Anderson, D.L., Belgya, T., Firestone, R.B., Kasztovsky, Zs., Lindstom, R.M., Molnár, G.L., Révay, Zs., Yonezawa, C., 2004. Handbook of Prompt Gamma Activation Analysis. Kluwer Academic Publishers, Dordrecht, The Netherlands. Engin, B., Aydas, C., Demirtas, H., 2006. ESR dosimetric properties of window glass. Nucl. Instrum. Methods B 243, 149–155. Rodrigues Jr., Ary de, A., Caldas, L.V.E., 2002. Commercial plate window glass tested as a routine dosimeter at a gamma irradiation facility. Radiat. Phys. Chem. 63, 765–767. Sheng, J., Kadona, K., Yazawa, T., 2002. Fading behaviour of X-ray induced color centers in soda-lime silicate glass. Appl. Radiat. Isot. 57, 813–817. Zheng, Z., Hoenggui, D., Jie, F., Daochuan, Y., 1988. Window glass as a routine dosimeter for radiation processing. Radiat. Phys. Chem. 31, 419–423.