- Email: [email protected]
Application of XANES in gamma dosimetry
Applied Radiation and Isotopes 149 (2019) 48–51
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Application of XANES in gamma dosimetry P.K. Sahani
a,d,∗
, A.K. Sinha
b,d
c
T
b
, G. Haridas , M.N. Singh , T.A. Puntambekar
a
a
Beam Diagnostics & Coolant Systems Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, Madhya Pradesh, India Synchrotrons Utilization Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, Madhya Pradesh, India Health Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India d Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, Maharashtra, India b c
H I GH L IG H T S
gives a direct evidence of Dy transition to Dy on gamma irradiation of CaSO :Dy. • XANES under XANES spectrum is found to increase linearly with absorbed dose. • Area • XANES is proposed as a potential dosimetry technique in high dose exposure. 3+
2+
4
A R T I C LE I N FO
A B S T R A C T
Keywords: Synchrotron radiation Thermoluminescence XANES Gamma dosimetry
The thermoluminescence material, CaSO4:Dy, is widely used for the dosimetry of ionizing radiation due to its high sensitivity, low fading and wide dose range from μGy to few tens of gray. However, its application is limited at high doses due to non-linear and saturation effects. In this paper, X-ray Absorption Near Edge Structure (XANES) studies at the Dy L3-edge have been carried out on CaSO4:Dy discs exposed to gamma doses in the range 0–1000 Gy. The results show an increase in white line in XANES spectra with gamma dose. Structural change in CaSO4:Dy also has been studied using X-Ray Diffraction (XRD) and has found no structural change up to 1000 Gy. The study indicates that XANES can be used as an alternative dosimetry technique and is useful in the evaluation of absorbed dose in the case of accidental exposure to high radiation in a radiation facility or during a radiological accident.
1. Introduction
overall uncertainty of the three element TLD badge is ± 12% (Pradhan et al., 1999). The dose linearity of CaSO4:Dy is up to ∼30 Gy (Yamashita et al., 1971; Mckeever, 1985) and the dose estimation using TL glow in the linear region is relatively easy, as one can apply the calibration factor obtained from exposure of the phosphor to a standard radioactive source. Beyond ∼30 Gy, dose estimation using the glow curve is difficult due to nonlinear and saturation effects and is prone to inaccuracies (Mckeever, 1985). At higher doses, wide variations in the shapes of glow curves due to nonlinear growth and shift of high-temperature shoulder have been reported (Srivastava and Supe, 1979). Attempts have been made to estimate the correct dose in the supralinear region (> 30 Gy) but with limited success (Pradhan and Bakshi, 2006). Thus estimation of high dose using CaSO4:Dy through conventional TL technique is prone to errors. The TL mechanism from CaSO4:Dy is well understood and explained in the redox model by Nambi et al. (Nambi et al., 1974). According to this model, Dy3+ present in CaSO4:Dy phosphor acts as an electron trap centre and the anion radicals SO4−, SO3−, SO2−, O3− etc. act as hole
Thermoluminescence (TL) phosphor materials have been widely used in radiation dosimetry of ionizing radiation for many decades. CaSO4:Dy, having high sensitivity and low fading is a good choice and is widely used in many radiation facilities for personnel and environmental dosimetry across the world. For instance, personnel dosimetry in India is carried out by TL dosimetry using CaSO4:Dy discs, where Teflon (PTFE) is used as a binder to the phosphor in the ratio 3:1. Dysprosium (Dy) acts as the dopant and is added to CaSO4 matrix at 0.05 mol% (Vohra et al., 1980; Bakshi et al., 2010). Three CaSO4:Dy PTFE TL discs are placed under metal filter, perspex filter and open window inside the TLD badge to determine dose in the mixed field of high and low energy gamma rays and beta particles. Depending on the ratio of readings from the three discs (under metal filter, under perspex filter and open window), dose values are assigned using appropriate algorithm for the radiation types (Pradhan et al., 2002). The uncertainty in the TL sensitivity of CaSO4:Dy discs is within ± 8% and the ∗
Corresponding author. Beam Diagnostics & Coolant Systems Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, Madhya Pradesh, India. E-mail address: [email protected] (P.K. Sahani).
https://doi.org/10.1016/j.apradiso.2019.04.010 Received 2 August 2018; Received in revised form 21 January 2019; Accepted 5 April 2019 Available online 09 April 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 149 (2019) 48–51
P.K. Sahani, et al.
trapping sites. On gamma irradiation, electrons and holes get trapped in the respective trapping centres and Dy3+ reduces to Dy2+ on electron trapping. During thermal excitation, the holes get released from the anion radical sites and recombine with trapped electrons at Dy2+ sites to form excited Dy3+ states. These excited Dy3+ states emit characteristic luminescence photons on returning to the ground state, which constitutes the glow curve as a function of temperature. Morgan and Stobe (Morgan and Stoebe, 1989), based on fluorescence studies on Dy3+, has shown that the wavelength of photons (340 nm) used to excite Dy3+ is the same as the thermoluminescence emission from pure CaSO4. Thus they suggested that the TL emission from CaSO4:Dy is due to the energy transfer from an unidentified recombination centre to a Dy3+ site leading to its excited state and hence the final luminescence is due to the transition to its ground state. In the present work, we propose X-ray Absorption Near Edge Structure (XANES) at Dy L3-edge as a tool for the estimation of dose up to 1000 Gy from CaSO4:Dy. Moreover, the direct experimental evidence is seen in the XANES spectrum for Dy3+ to Dy2+ transition on gamma ray irradiation confirming the redox model of TL emission. The observed increase in the area of XANES spectra with increasing dose provides a useful technique for estimation of high dose. X-ray diffraction has also been used to study the changes in crystal structure on high gamma dose exposure and is also reported.
Fig. 1. XANES spectra of reference standard (Dy2O3) and CaSO4:Dy samples obtained by gamma irradiation at different dose levels (0, 180, 500, 680 and 1000Gy).
obtained from the image plate is in the form of circular rings (DebyeScherrer rings) and are converted into conventional diffraction pattern (Intensity versus 2θ) using Fit2D software (Hammersley et al., 1994). NIST standard (LaB6) was used for measuring the wavelength of the incident radiation and the distance between the sample and detector, accurately.
2. Materials and methods CaSO4:Dy TL materials in powder and disc form were annealed at 400 °C for 1 h (Bhattacharyya et al., 2006; Bakshi et al., 2007; Singh et al., 2014a). [Earlier studies on phase transition in CaSO4:Dy due to thermal treatment have shown no change in the crystal structure for the annealing temperature of 400 °C, however its phase change from orthorhombic to tetragonal structure starts only after 800 °C (Bakshi et al., 2006, 2008). Since we have carried out annealing at 400 °C, we don't expect any phase change.] After annealing these TL materials were exposed to absorbed dose in the range 180–1000 Gy in a gamma irradiation chamber (BRIT-India make, Gamma Chamber-900). The irradiation chamber consists of Cobalt-60 source pencils in a cylindrical arrangement capable of delivering the dose rate of 850 Gy/h. TL materials were placed in the sample holder at the top of the gamma chamber and moved to the source location through a motorized driving mechanism. X-ray Absorption Near Edge Structure (XANES) measurements at Dy L3-edge of the control and irradiated TL materials were carried out at Angle Dispersive X-ray Diffraction beamline (BL-12) of Indus-2 synchrotron source at Raja Ramanna Centre for Advanced Technology, Indore, India. Since CaSO4:Dy material contains only 0.05 mol% Dy, x-ray fluorescence mode was chosen for the absorption measurement. X-ray diffraction (XRD) also was carried out on CaSO4:Dy powder at the same beamline to study the structural changes in it if any, due to exposure to high dose. For XANES measurement, CaSO4:Dy PTFE discs were used as these are the actual dosimeters used for personnel dosimetry in India. However XRD is performed on CaSO4:Dy powder as it is not possible to get good XRD signal from CaSO4:Dy with PTFE binder due to the amorphous background signal, which is quite significant in comparison with CaSO4:Dy powder. BL-12 uses Si (111) based double crystal monochromator (DCM) for the selection of required energy. The white synchrotron spectrum (full spectrum from Indus-2) is incident on the DCM, which delivers photons with energy resolution (ΔE/E) of less than 10−3 in 5–25 keV energy range. An ionization chamber is installed before the experimental station to measure the incident flux of synchrotron radiation on the sample. At the experimental station, monochromatic photons from the DCM were incident on the phosphor samples during XANES and XRD measurements. For XANES measurement, a Vortex Silicon Drift Detector (SDD) was used to detect the fluorescence from the samples, excited with synchrotron radiation. For the XRD measurement, an image plate area detector (Mar 345) was used. The XRD pattern
3. Results & discussion The normalized XANES spectra of CaSO4:Dy samples exposed to different gamma doses in the range 180–1000 Gy along with the spectrum of standard Dy2O3 is shown in Fig. 1. The XANES spectra have been measured in fluorescence mode. The fluorescence counts is first normalized with I0 (the incident flux) to get αμ(E), where μ(E) is the absorption coefficient and α is a constant. Then the absorption coefficient below the absorption edge is normalized to 0 and above the absorption edge (∼30 eV above) is normalized to 1 (Singh et al., 2014b). The region of above 30 eV is chosen because at this energy multiple scattering is minimum and EXAFS oscillations are almost absent. In the spectra, there are three regions marked as p(pre-edge), s(shoulder) and w(white line). The pre-edge is attributed to +2 oxidation state of the absorbing atom (in this case Dy). It may be mentioned that in the case of transition metal oxides, the pre-edge is because of 1s-3d quadrupole (QP) transition (Singh et al., 2014c). The QP transition is normally forbidden but is allowed due to hybridization between transition metal 3d and oxygen 2p states. It can be observed from the spectra that the pre-edge feature is appearing for CaSO4:Dy whereas, Dy2O3 does not show any such feature. The spectra after subtraction of the Dy2O3 spectrum is shown in Fig. 2, where the pre-edge feature of CaSO4:Dy is prominently seen. This pre-edge is attributed to the presence of Dy2+ formed by the reduction of Dy3+ during gamma irradiation. The observed peak positions are in agreement with the white line and pre-edge peaks observed in KxDy@C82, where the variation of valence from Dy3+ to Dy2+ is attributed to the intercalation of K metal into Dy@C82 crystal (Kubozono et al., 2003). It can be noted that there is a conspicuous increase in pre-edge intensity with increase in gamma dose, suggestive of the increase in the concentration of Dy2+ as the gamma dose is increased. It is also to be noted that there is no absorption edge (pre-edge) corresponding to Dy2O3. Thus these observations give direct experimental evidence that Dy3+ gets converted to Dy2+ on gamma exposure, which is in agreement with the redox model of TL emission from CaSO4:Dy phosphors proposed by Nambi et al. (Nambi et al., 1974). Another important observation is that the XANES spectra shown in Fig. 2 indicate a white line at 7791 eV, whose area under the curve is 49
Applied Radiation and Isotopes 149 (2019) 48–51
P.K. Sahani, et al.
Table 1 Comparison of actual and the estimated dose from XANES study. Sample No.
True dose (Gy)
Estimated dose (Gy)
#1 #2 #3 #4 #5
0 180 500 680 1000
0 174.9 ± 0.09 382.6 ± 0.19 668.3 ± 0.33 1040 ± 0.52
Fig. 2. Relative XANES spectra of CaSO4:Dy samples with respect to standard Dy2O3 sample.
found to increase with increasing dose up to 1000 Gy. The white line in L3 edge spectra represents the transition from 2p3/2 state to the continuum state. The increase in white line intensity shows that on gamma irradiation, in addition to Dy3+ → Dy2+ transition, additional localized states are also generated in the continuum. This observation gives a clear indication that XANES spectrum of irradiated phosphors can provide information of absorbed dose from gamma irradiated TL materials. The net area under XANES spectra attributed to gamma ray irradiation is obtained by subtracting the area under zero dose sample from the area under the spectra of the gamma exposed CaSO4:Dy discs. The net areas, thus obtained, are plotted as a function of the absorbed dose in Fig. 3. The absorbed dose is found to be linearly proportional to the area under the curve and is represented by the following empirical relation. Absorbed dose (Gy) = k (Area)
Fig. 4. XRD pattern of un-irradiated and irradiated CaSO4:Dy samples.
high gamma dose where TL technique is not suitable. Another advantage of XANES technique is that the spectra can be repeatedly obtained unlike the TL technique, where the information of the absorbed dose will be lost after the first TL readout. However, the technique may not be suitable for low dose, where the TL technique works fine. So XANES is proposed as a complementary technique in case of accidental high dose exposure. The XRD pattern of CaSO4:Dy samples, studied to look for any structural changes, are shown in Fig. 4 and are compared with the standard data (JCPDS Card no. 37–1496). The orthorhombic phase of CaSO4 crystal with lattice parameters a = 6.993, b = 7.001, c = 6.241 A0 and space group Bmmb for CaSO4:Dy matches with the JCPDS Card no. 37–1496. Thus the XRD spectra indicate that the orthorhombic phase of CaSO4:Dy is retained even at the dose level up to 1000 Gy. However, differences in relative intensities of various Bragg peaks are observed. This may be due to the preferred orientation of some peaks, indicating structural recrystallization on gamma irradiation. This is also likely to affect the electronic structure, a conclusion supported by XANES measurements.
(1)
Where k is a constant and its value is found to be 166. The true dose and the estimated dose obtained from equation (1) for the samples are given in table 1. The empirical relation gives a reasonable fit for estimation of absorbed dose up to 1000 Gy. Therefore the XANES technique is proposed as a method for estimation of dose from TL materials exposed to
4. Summary and conclusions CaSO4:Dy is a widely used, high sensitive TL material in the field of radiation dosimetry. The phosphor is having dose linearity up to 30 Gy and shows supralinearity followed by saturation at higher doses. The dose quantification using conventional TL technique for high dose exposure is error-prone due to nonlinear response, the contribution from high temperature peaks and saturation effects. In the present study, CaSO4:Dy disc samples are exposed to gamma doses up to 1000 Gy and XANES & XRD spectra of the samples are studied. From the studies, it is concluded that: a) Pre-edge in XANES spectra of CaSO4:Dy samples gives direct experimental evidence of Dy3+ transition to Dy2+ on gamma irradiation, as proposed in the redox model.
Fig. 3. Relative areas under XANES spectra as a function of absorbed dose. 50
Applied Radiation and Isotopes 149 (2019) 48–51
P.K. Sahani, et al.
b) The area under the XANES spectrum attributed to gamma irradiation is found to increase linearly with absorbed dose and hence the technique is a potential tool for estimation of absorbed dose from TL samples exposed to high gamma dose. c) As the information of absorbed dose is not lost during XANES measurement, the spectrum can be obtained even after the first measurement unlike in TL where after the first readout, the information is lost. d) The XRD pattern of the exposed CaSO4:Dy indicated that the crystal retains the orthorhombic phase and hence no structural changes are taking place on gamma irradiation up to1000 Gy.
subjected to post preparation high temperature thermal treatment. J. Phys. D Appl. Phys. 41, 025402. Bakshi, A.K., Chatterjee, S., Palani Selvam, T., Dhabekar, B.S., 2010. Study on the response of thermoluminescent dosemeters to synchrotron radiation: experimental method and Monte Carlo calculations. Radiat. Protect. Dosim. 140 (Issue 2), 137–146. Bhattacharyya, D., Bakshi, A.K., Ciatto, G., Aquilanti, G., Pradhan, A.S., Pascarelli, S., 2006. Extended X-ray absorption fine structure (EXAFS) study of CaSO4:Dy phosphors. Solid State Commun. 137, 650–653. Hammersley, A.P., Svensson, S.O., Thompson, A., 1994. Calibration and correction of spatial distortions in 2D detector systems. Nucl. Instrum. Methods Phys. Res. 346, 312–321. Kubozono, Y., Takabayashi, Y., Shibata, K., Kanbara, T., Fujiki, S., Kashino, S., Fujiwara, A., Emura, S., 2003. Crystal structure and electronic transport of Dy@C82. Phys. Rev. B 67, 115410. Mckeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press. Morgan, M.D., Stoebe, T.G., 1989. Optical absorption and luminescent processes in thermoluminescent CaSO4:Dy. J. Phys.: Condens. Matter l, 5773–5781. Nambi, K.S.V., Bapat, V.N., Ganguly, A.K., 1974. Thermoluminescence of CaSO4 doped with rare earths. J. Phys. C Solid State Phys. 7, 4403–4415. Pradhan, A.S., Bakshi, A.K., 2006. Glow peak temperature, supralinearity and LET dependence of TLDs-correlation studies. Radiat. Protect. Dosim. 119 (1–4), 276–279. Pradhan, A.S., Bakshi, A.K., Srivastava, K., Bihari, R.R., 1999. Parameters of large scale production of personnel monitoring TLD cards based on CaSO4:Dy. Radiat. Protect. Dosim. 85 (1–4), 67–70. Pradhan, A.S., Adtani, M.M., Varadharajan, G., Bakshi, A.K., Srivastava, K., Bihari, R.R., 2002. Hand Book on the Use of TLD Badge Based on CaSO4:Dy Teflon TLD Discs for Individual Monitoring. BARC Technical Report No. BARC/2002/E/025. . Singh, A.K., Menon, S.N., Dhabekar, B., Kadam, S., Chougaonkar, M.P., Babu, D.A.R., 2014a. An alternative method for immediate dose estimation using CaSO4:Dy based TLD badges. Nucl. Instrum. Methods Phys. Res. B 339, 42–45. Singh, H., Sinha, A.K., Singh, M.N., Tiwari, P., Phase, D.M., Deb, S.K., 2014b. Spectroscopic and structural studies of isochronally annealed cobalt oxide nanoparticles. J. Phys. Chem. Solids 75, 397–402. Singh, H., Sinha, A.K., Ghosh, H., Singh, M.N., Rajput, P., Prajapat, C.L., Singh, M.R., Ravikumar, G., 2014c. Structural investigations on Co3-xMnxTeO6 ; (0 < x < 2); High temperature ferromagnetism and enhanced low temperature anti-ferromagnetism. J. Appl. Phys. 116, 074904. Srivastava, J.K., Supe, S.J., 1979. Gamma radiation induced changes in the glow curve of CaSO4: Dy TLD phosphor. Radiat. Eff. 45, 13–18. Vohra, K.G., Bhatt, R.C., Chandra, B., Pradhan, A.S., Lakshmanan, A.R., Shastry, S.S., 1980. A personnel dosimeter TLD badge based on CaSO4:Dy teflon TLD discs. Health Phys. 38, 193–197. Yamashita, T., Nada, N., Onishi, H., Kitamura, S., 1971. Calcium sulfate activated by Thulium or Dysprosium for thermoluminescence dosimetry. Health Phys. 21 (August), 295–300.
Acknowledgements The authors would like to thank Dr. P. A. Naik, Director, Raja Ramanna Centre for Advanced Technology (RRCAT) for his encouragement and support in this work. We are thankful to Dr. Sanjay Kher, Head, Fibre Sensors Lab, RRCAT for providing support in radiation exposure of samples at gamma chamber. We express our sincere gratitude to Shri A. C. Thakurta, Director, Electron Accelerator Group, RRCAT and the Indus operation staff for providing synchrotron beam for the experiment. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apradiso.2019.04.010. References Bakshi, A.K., Pradhan, A.S., Tyagi, A.K., Kher, R.K., Bhatt, B.C., 2006. Correlation of phase transition with the change in TL characteristics in CaSO4:Dy phosphor-Effect of thermal treatment. Radiat. Protect. Dosim. 119 (1–4), 139–142. Bakshi, A.K., Jha, S.N., Olivi, L., Phase, D.M., Kher, R.K., Bhattacharyya, D., 2007. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy studies of CaSO4:Dy thermoluminescent phosphors. Nucl. Instrum. Methods Phys. Res. B 264, 109–116. Bakshi, A.K., Patwe, S.J., Bhide, M.K., Sanyal, B., Natarajan, V., Tyagi, A.K., Kher, R.K., 2008. Thermoluminescence, ESR and x-ray diffraction studies of CaSO4:Dy phosphor
51
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Application of XANES in gamma dosimetry P.K. Sahani
a,d,∗
, A.K. Sinha
b,d
c
T
b
, G. Haridas , M.N. Singh , T.A. Puntambekar
a
a
Beam Diagnostics & Coolant Systems Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, Madhya Pradesh, India Synchrotrons Utilization Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, Madhya Pradesh, India Health Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India d Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, Maharashtra, India b c
H I GH L IG H T S
gives a direct evidence of Dy transition to Dy on gamma irradiation of CaSO :Dy. • XANES under XANES spectrum is found to increase linearly with absorbed dose. • Area • XANES is proposed as a potential dosimetry technique in high dose exposure. 3+
2+
4
A R T I C LE I N FO
A B S T R A C T
Keywords: Synchrotron radiation Thermoluminescence XANES Gamma dosimetry
The thermoluminescence material, CaSO4:Dy, is widely used for the dosimetry of ionizing radiation due to its high sensitivity, low fading and wide dose range from μGy to few tens of gray. However, its application is limited at high doses due to non-linear and saturation effects. In this paper, X-ray Absorption Near Edge Structure (XANES) studies at the Dy L3-edge have been carried out on CaSO4:Dy discs exposed to gamma doses in the range 0–1000 Gy. The results show an increase in white line in XANES spectra with gamma dose. Structural change in CaSO4:Dy also has been studied using X-Ray Diffraction (XRD) and has found no structural change up to 1000 Gy. The study indicates that XANES can be used as an alternative dosimetry technique and is useful in the evaluation of absorbed dose in the case of accidental exposure to high radiation in a radiation facility or during a radiological accident.
1. Introduction
overall uncertainty of the three element TLD badge is ± 12% (Pradhan et al., 1999). The dose linearity of CaSO4:Dy is up to ∼30 Gy (Yamashita et al., 1971; Mckeever, 1985) and the dose estimation using TL glow in the linear region is relatively easy, as one can apply the calibration factor obtained from exposure of the phosphor to a standard radioactive source. Beyond ∼30 Gy, dose estimation using the glow curve is difficult due to nonlinear and saturation effects and is prone to inaccuracies (Mckeever, 1985). At higher doses, wide variations in the shapes of glow curves due to nonlinear growth and shift of high-temperature shoulder have been reported (Srivastava and Supe, 1979). Attempts have been made to estimate the correct dose in the supralinear region (> 30 Gy) but with limited success (Pradhan and Bakshi, 2006). Thus estimation of high dose using CaSO4:Dy through conventional TL technique is prone to errors. The TL mechanism from CaSO4:Dy is well understood and explained in the redox model by Nambi et al. (Nambi et al., 1974). According to this model, Dy3+ present in CaSO4:Dy phosphor acts as an electron trap centre and the anion radicals SO4−, SO3−, SO2−, O3− etc. act as hole
Thermoluminescence (TL) phosphor materials have been widely used in radiation dosimetry of ionizing radiation for many decades. CaSO4:Dy, having high sensitivity and low fading is a good choice and is widely used in many radiation facilities for personnel and environmental dosimetry across the world. For instance, personnel dosimetry in India is carried out by TL dosimetry using CaSO4:Dy discs, where Teflon (PTFE) is used as a binder to the phosphor in the ratio 3:1. Dysprosium (Dy) acts as the dopant and is added to CaSO4 matrix at 0.05 mol% (Vohra et al., 1980; Bakshi et al., 2010). Three CaSO4:Dy PTFE TL discs are placed under metal filter, perspex filter and open window inside the TLD badge to determine dose in the mixed field of high and low energy gamma rays and beta particles. Depending on the ratio of readings from the three discs (under metal filter, under perspex filter and open window), dose values are assigned using appropriate algorithm for the radiation types (Pradhan et al., 2002). The uncertainty in the TL sensitivity of CaSO4:Dy discs is within ± 8% and the ∗
Corresponding author. Beam Diagnostics & Coolant Systems Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, Madhya Pradesh, India. E-mail address: [email protected] (P.K. Sahani).
https://doi.org/10.1016/j.apradiso.2019.04.010 Received 2 August 2018; Received in revised form 21 January 2019; Accepted 5 April 2019 Available online 09 April 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 149 (2019) 48–51
P.K. Sahani, et al.
trapping sites. On gamma irradiation, electrons and holes get trapped in the respective trapping centres and Dy3+ reduces to Dy2+ on electron trapping. During thermal excitation, the holes get released from the anion radical sites and recombine with trapped electrons at Dy2+ sites to form excited Dy3+ states. These excited Dy3+ states emit characteristic luminescence photons on returning to the ground state, which constitutes the glow curve as a function of temperature. Morgan and Stobe (Morgan and Stoebe, 1989), based on fluorescence studies on Dy3+, has shown that the wavelength of photons (340 nm) used to excite Dy3+ is the same as the thermoluminescence emission from pure CaSO4. Thus they suggested that the TL emission from CaSO4:Dy is due to the energy transfer from an unidentified recombination centre to a Dy3+ site leading to its excited state and hence the final luminescence is due to the transition to its ground state. In the present work, we propose X-ray Absorption Near Edge Structure (XANES) at Dy L3-edge as a tool for the estimation of dose up to 1000 Gy from CaSO4:Dy. Moreover, the direct experimental evidence is seen in the XANES spectrum for Dy3+ to Dy2+ transition on gamma ray irradiation confirming the redox model of TL emission. The observed increase in the area of XANES spectra with increasing dose provides a useful technique for estimation of high dose. X-ray diffraction has also been used to study the changes in crystal structure on high gamma dose exposure and is also reported.
Fig. 1. XANES spectra of reference standard (Dy2O3) and CaSO4:Dy samples obtained by gamma irradiation at different dose levels (0, 180, 500, 680 and 1000Gy).
obtained from the image plate is in the form of circular rings (DebyeScherrer rings) and are converted into conventional diffraction pattern (Intensity versus 2θ) using Fit2D software (Hammersley et al., 1994). NIST standard (LaB6) was used for measuring the wavelength of the incident radiation and the distance between the sample and detector, accurately.
2. Materials and methods CaSO4:Dy TL materials in powder and disc form were annealed at 400 °C for 1 h (Bhattacharyya et al., 2006; Bakshi et al., 2007; Singh et al., 2014a). [Earlier studies on phase transition in CaSO4:Dy due to thermal treatment have shown no change in the crystal structure for the annealing temperature of 400 °C, however its phase change from orthorhombic to tetragonal structure starts only after 800 °C (Bakshi et al., 2006, 2008). Since we have carried out annealing at 400 °C, we don't expect any phase change.] After annealing these TL materials were exposed to absorbed dose in the range 180–1000 Gy in a gamma irradiation chamber (BRIT-India make, Gamma Chamber-900). The irradiation chamber consists of Cobalt-60 source pencils in a cylindrical arrangement capable of delivering the dose rate of 850 Gy/h. TL materials were placed in the sample holder at the top of the gamma chamber and moved to the source location through a motorized driving mechanism. X-ray Absorption Near Edge Structure (XANES) measurements at Dy L3-edge of the control and irradiated TL materials were carried out at Angle Dispersive X-ray Diffraction beamline (BL-12) of Indus-2 synchrotron source at Raja Ramanna Centre for Advanced Technology, Indore, India. Since CaSO4:Dy material contains only 0.05 mol% Dy, x-ray fluorescence mode was chosen for the absorption measurement. X-ray diffraction (XRD) also was carried out on CaSO4:Dy powder at the same beamline to study the structural changes in it if any, due to exposure to high dose. For XANES measurement, CaSO4:Dy PTFE discs were used as these are the actual dosimeters used for personnel dosimetry in India. However XRD is performed on CaSO4:Dy powder as it is not possible to get good XRD signal from CaSO4:Dy with PTFE binder due to the amorphous background signal, which is quite significant in comparison with CaSO4:Dy powder. BL-12 uses Si (111) based double crystal monochromator (DCM) for the selection of required energy. The white synchrotron spectrum (full spectrum from Indus-2) is incident on the DCM, which delivers photons with energy resolution (ΔE/E) of less than 10−3 in 5–25 keV energy range. An ionization chamber is installed before the experimental station to measure the incident flux of synchrotron radiation on the sample. At the experimental station, monochromatic photons from the DCM were incident on the phosphor samples during XANES and XRD measurements. For XANES measurement, a Vortex Silicon Drift Detector (SDD) was used to detect the fluorescence from the samples, excited with synchrotron radiation. For the XRD measurement, an image plate area detector (Mar 345) was used. The XRD pattern
3. Results & discussion The normalized XANES spectra of CaSO4:Dy samples exposed to different gamma doses in the range 180–1000 Gy along with the spectrum of standard Dy2O3 is shown in Fig. 1. The XANES spectra have been measured in fluorescence mode. The fluorescence counts is first normalized with I0 (the incident flux) to get αμ(E), where μ(E) is the absorption coefficient and α is a constant. Then the absorption coefficient below the absorption edge is normalized to 0 and above the absorption edge (∼30 eV above) is normalized to 1 (Singh et al., 2014b). The region of above 30 eV is chosen because at this energy multiple scattering is minimum and EXAFS oscillations are almost absent. In the spectra, there are three regions marked as p(pre-edge), s(shoulder) and w(white line). The pre-edge is attributed to +2 oxidation state of the absorbing atom (in this case Dy). It may be mentioned that in the case of transition metal oxides, the pre-edge is because of 1s-3d quadrupole (QP) transition (Singh et al., 2014c). The QP transition is normally forbidden but is allowed due to hybridization between transition metal 3d and oxygen 2p states. It can be observed from the spectra that the pre-edge feature is appearing for CaSO4:Dy whereas, Dy2O3 does not show any such feature. The spectra after subtraction of the Dy2O3 spectrum is shown in Fig. 2, where the pre-edge feature of CaSO4:Dy is prominently seen. This pre-edge is attributed to the presence of Dy2+ formed by the reduction of Dy3+ during gamma irradiation. The observed peak positions are in agreement with the white line and pre-edge peaks observed in KxDy@C82, where the variation of valence from Dy3+ to Dy2+ is attributed to the intercalation of K metal into Dy@C82 crystal (Kubozono et al., 2003). It can be noted that there is a conspicuous increase in pre-edge intensity with increase in gamma dose, suggestive of the increase in the concentration of Dy2+ as the gamma dose is increased. It is also to be noted that there is no absorption edge (pre-edge) corresponding to Dy2O3. Thus these observations give direct experimental evidence that Dy3+ gets converted to Dy2+ on gamma exposure, which is in agreement with the redox model of TL emission from CaSO4:Dy phosphors proposed by Nambi et al. (Nambi et al., 1974). Another important observation is that the XANES spectra shown in Fig. 2 indicate a white line at 7791 eV, whose area under the curve is 49
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Table 1 Comparison of actual and the estimated dose from XANES study. Sample No.
True dose (Gy)
Estimated dose (Gy)
#1 #2 #3 #4 #5
0 180 500 680 1000
0 174.9 ± 0.09 382.6 ± 0.19 668.3 ± 0.33 1040 ± 0.52
Fig. 2. Relative XANES spectra of CaSO4:Dy samples with respect to standard Dy2O3 sample.
found to increase with increasing dose up to 1000 Gy. The white line in L3 edge spectra represents the transition from 2p3/2 state to the continuum state. The increase in white line intensity shows that on gamma irradiation, in addition to Dy3+ → Dy2+ transition, additional localized states are also generated in the continuum. This observation gives a clear indication that XANES spectrum of irradiated phosphors can provide information of absorbed dose from gamma irradiated TL materials. The net area under XANES spectra attributed to gamma ray irradiation is obtained by subtracting the area under zero dose sample from the area under the spectra of the gamma exposed CaSO4:Dy discs. The net areas, thus obtained, are plotted as a function of the absorbed dose in Fig. 3. The absorbed dose is found to be linearly proportional to the area under the curve and is represented by the following empirical relation. Absorbed dose (Gy) = k (Area)
Fig. 4. XRD pattern of un-irradiated and irradiated CaSO4:Dy samples.
high gamma dose where TL technique is not suitable. Another advantage of XANES technique is that the spectra can be repeatedly obtained unlike the TL technique, where the information of the absorbed dose will be lost after the first TL readout. However, the technique may not be suitable for low dose, where the TL technique works fine. So XANES is proposed as a complementary technique in case of accidental high dose exposure. The XRD pattern of CaSO4:Dy samples, studied to look for any structural changes, are shown in Fig. 4 and are compared with the standard data (JCPDS Card no. 37–1496). The orthorhombic phase of CaSO4 crystal with lattice parameters a = 6.993, b = 7.001, c = 6.241 A0 and space group Bmmb for CaSO4:Dy matches with the JCPDS Card no. 37–1496. Thus the XRD spectra indicate that the orthorhombic phase of CaSO4:Dy is retained even at the dose level up to 1000 Gy. However, differences in relative intensities of various Bragg peaks are observed. This may be due to the preferred orientation of some peaks, indicating structural recrystallization on gamma irradiation. This is also likely to affect the electronic structure, a conclusion supported by XANES measurements.
(1)
Where k is a constant and its value is found to be 166. The true dose and the estimated dose obtained from equation (1) for the samples are given in table 1. The empirical relation gives a reasonable fit for estimation of absorbed dose up to 1000 Gy. Therefore the XANES technique is proposed as a method for estimation of dose from TL materials exposed to
4. Summary and conclusions CaSO4:Dy is a widely used, high sensitive TL material in the field of radiation dosimetry. The phosphor is having dose linearity up to 30 Gy and shows supralinearity followed by saturation at higher doses. The dose quantification using conventional TL technique for high dose exposure is error-prone due to nonlinear response, the contribution from high temperature peaks and saturation effects. In the present study, CaSO4:Dy disc samples are exposed to gamma doses up to 1000 Gy and XANES & XRD spectra of the samples are studied. From the studies, it is concluded that: a) Pre-edge in XANES spectra of CaSO4:Dy samples gives direct experimental evidence of Dy3+ transition to Dy2+ on gamma irradiation, as proposed in the redox model.
Fig. 3. Relative areas under XANES spectra as a function of absorbed dose. 50
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b) The area under the XANES spectrum attributed to gamma irradiation is found to increase linearly with absorbed dose and hence the technique is a potential tool for estimation of absorbed dose from TL samples exposed to high gamma dose. c) As the information of absorbed dose is not lost during XANES measurement, the spectrum can be obtained even after the first measurement unlike in TL where after the first readout, the information is lost. d) The XRD pattern of the exposed CaSO4:Dy indicated that the crystal retains the orthorhombic phase and hence no structural changes are taking place on gamma irradiation up to1000 Gy.
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Acknowledgements The authors would like to thank Dr. P. A. Naik, Director, Raja Ramanna Centre for Advanced Technology (RRCAT) for his encouragement and support in this work. We are thankful to Dr. Sanjay Kher, Head, Fibre Sensors Lab, RRCAT for providing support in radiation exposure of samples at gamma chamber. We express our sincere gratitude to Shri A. C. Thakurta, Director, Electron Accelerator Group, RRCAT and the Indus operation staff for providing synchrotron beam for the experiment. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apradiso.2019.04.010. References Bakshi, A.K., Pradhan, A.S., Tyagi, A.K., Kher, R.K., Bhatt, B.C., 2006. Correlation of phase transition with the change in TL characteristics in CaSO4:Dy phosphor-Effect of thermal treatment. Radiat. Protect. Dosim. 119 (1–4), 139–142. Bakshi, A.K., Jha, S.N., Olivi, L., Phase, D.M., Kher, R.K., Bhattacharyya, D., 2007. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy studies of CaSO4:Dy thermoluminescent phosphors. Nucl. Instrum. Methods Phys. Res. B 264, 109–116. Bakshi, A.K., Patwe, S.J., Bhide, M.K., Sanyal, B., Natarajan, V., Tyagi, A.K., Kher, R.K., 2008. Thermoluminescence, ESR and x-ray diffraction studies of CaSO4:Dy phosphor
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