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CaSO4:Dy microphosphor for thermal neutron dosimetry
Journal of Luminescence 170 (2016) 226–230
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
CaSO4:Dy microphosphor for thermal neutron dosimetry Mahesh S. Bhadane a, Nandkumar Mandlik b, B.J. Patil c, S.S. Dahiwale a, K.R. Sature a, V.N. Bhoraskar a, S.D. Dhole a,n a
Microtron Accelerator Laboratory, Department of Physics, Savitribai Phule Pune University, Pune 411007, India Department of Physics, Fergusson College, Savitribai Phule Pune University, Pune 411007, India c Department of Physics, Abasaheb Garware College, Pune 411004, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2015 Received in revised form 2 September 2015 Accepted 9 October 2015 Available online 22 October 2015
Dysprosium-doped calcium sulphate (CaSO4:Dy) microphosphor was synthesized by acid recrystallization method and its thermoluminescence (TL) properties irradiated with thermal neutrons was studied. Structural and morphological characteristics have been studied using X-ray diffraction and SEM which mainly exhibits a orthorhombic structure with particle size of 200 to 250 mm. Moreover, thermal neutron dosimetric characteristics of the microphosphor such as thermoluminescence glow curve, TL dose–response have been studied. This microphosphor powder represents a TL glow peak (Tmax) centered at around 240 °C. The TL response of CaSO4:Dy microphosphor as a function of thermal neutron fluence is observed to be very linear upto the fluence of 52 1011 n/cm2 and further saturates. In addition, TL glow curves were deconvoluted by computerized glow curve deconvolution (CGCD) method and corresponding trapping parameters have been determined. It has been found that for every deconvoluted peak there is change in the order of kinetics. Overall, the experimental results show that the CaSO4:Dy microphosphor can have potential to be an effective thermal neutron dosimetry. & 2015 Elsevier B.V. All rights reserved.
Keywords: 252 Cf FLUKA code Microphosphor Thermoluminescence CGCD
1. Introduction Thermoluminescence (TL) is a well known technique which commonly used for the dose measurement of ionizing radiations. Now a days, TL has increased their applications and uses like retrospective dosimetry [1], personal, clinical, environmental monitoring [2], medicine, glow curve analysis, biological related fields, high-level photon and neutron dosimetry with TLD materials etc. [3]. The neutron dosimetry is not as simple as the dosimetry of gamma radiations and other radiations because of the numerous direct and indirect energy transfer processes involved and the variation of the reaction cross sections with energy [1,4]. Neutron has wide range of energy spectrum, among them neutron dosimetry for thermal neutron is an important. Because, thermal neutron has less energy and favorable uses for medical purposes as well as nuclear research laboratories, nuclear power stations, reactors and industries [5,6]. Thermal neutron response of every TL materials depends on the capture cross section of the elements of TL materials. However, CaSO4:Dy is reported on the basis of mixed field dosimetry [7] and found an additive and no permanent damage to the γ-ray sensitivity observed after irradiating n
Corresponding author. Tel.: þ 91 2025692678; fax: þ 91 2025691684. E-mail address: [email protected] (S.D. Dhole).
http://dx.doi.org/10.1016/j.jlumin.2015.10.025 0022-2313/& 2015 Elsevier B.V. All rights reserved.
maximum thermal neutron fluence. Basically, the thermal neutron interacts with phosphor materials mainly through scattering and absorption processes i.e. Inelastic scattering, Elastic scattering, Non-elastic scattering and Capture process. However, in the present case, inelastic and non-elastic scattering processes are most probable as they form (n, γ) and (n, α) reactions which emits prompt γ and α particles responsible to have dosimetric change due to thermal neutrons. In the past few years, considerable amount of work is involved in search of new microcrystalline phosphor materials with better TL sensitivity and dosimetric properties. CaSO4:Dy is one of the high sensitive microphosphor for gamma dosimetry and has been studied by lakshmanan et al. [8] using co-precipitation technique. CaSO4:Dy and other microcrystalline phosphors are widely been used as a TLD materials and are useful for low dose measurements [9]. CaSO4:Dy is the most interesting phosphor activated with Dysprosium, because of the considerable stability. It also enhances the luminescence efficiency of phosphor. Therefore, aim of this work is to synthesize the Dy doped CaSO4 microcrystalline phosphor by acid recrystallization method and irradiated with thermal neutrons. The irradiation studies of this kind of phosphor with thermal neutron is rarely reported in the literature [10,11]. The characterization such as XRD, SEM, FTIR and thermoluminescence
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(TL) properties of thermal neutron irradiated CaSO4:Dy powder are studied for structural, morphological and chemical reformation. Information of trapping parameters through deconvolution is very much useful for the analysis of TL glow curve. Therefore, a computerized glow curve deconvolution (CGCD) program is applied to analyze the nature of traps [12,13] generated due thermal neutrons in microphosphor. Deconvolution of TL glow curve is an important and many trapping parameters such as trap depth, order of kinetics, frequency factor and maximum peak temperature can be calculated. Computerized (CGCD) Deconvolution is done using Kitis function [14,15] and applied to microcrystalline CaSO4:Dy TL glow curves for analysis. Fig. 2. FLUKA simulated fission spectrum of
2. Experimental details 2.1. Materials preparation Acid re-crystalization method was used to synthesized CaSO4: Dy microphosphor. The desired impurity of Dy (0.1 mol%) in the form of Dy2O3 (Sigma Aldrich) and CaSO4 2H2O (AR grade) were dissolved in proper ratio of concentrated sulfuric acid in a rounded bottom flask, which was connected to a sealed condenser system. The acid was then distilled out in a flask connected to the condenser. The solution was heated at about 337 °C (boiling point of H2SO4) to evaporate the acid completely. After cooling, the phosphor thus obtained in microcrystalline form was repeatedly washed with double distilled water to remove the acid traces and dried in an oven at 100 °C up to 4 h. The phosphor thus obtained in powder form was crushed and sieved to obtain grains ranging from 100 to 200 μm in size. Finally, the microcrystalline powder was annealed at 700 °C for 2 h in a quartz boat under nitrogen atmosphere and quenched by taking the boat out of the furnace and placing it on a metal block [16]. 2.2. Thermal neutron chamber and irradiation In the present study, samples were irradiated using 252Cf based thermal neutron source [17] having a flux of 106 n/cm2/s and half life 2.645 years. The schematic of the thermal neutron facility is shown in Fig. 1. The 252Cf irradiator presents cylindrical geometry with a coaxial stainless steel filter fitted in the center of the cylindrical chamber. The 252Cf is covered by paraffin wax as a moderator which filters out the high energy neutrons and converts to thermal neutrons. The irradiation of the sample was done in the outer shell of the ring which receives constant flux of thermal neutrons. The sample rod was used to keep and take out the samples from irradiation chamber. Moderator tank of paraffin wax is surrounded by borated paraffin to shield the neutrons. It is also covered by thin lead shielding to attenuate the undesirable gamma rays emitted from the 252Cf source and still remains at the sample position with small amount. Therefore, to avoid these gamma contamination during thermal neutron irradiation, set of
252
Cf moderated thermal neutron.
experiments were performed to estimate the actual contribution of the gamma dose. For this CaSO4:Dy phosphor covered with aluminum foil was kept in a 5 mm cylindrical plastic vial filled up with the boric powder. The boric powder was used to stop the thermal neutron and aluminum foil was used to stop the alpha particles generated from boron. In addition, a pure CaSO4:Dy sample was kept for irradiation to see the cumulative dose effects. The dose separation method was adopted to estimate the pure thermal neutrons. 2.3. FLUKA simulation of Cf-252 moderated based thermal neutron source and neutron energy spectrum To determine the energy spectrum of moderated Cf-252 source, a FLUKA simulation was carried out. Cf-252 is a natural isotope which emits fast neutrons through fission process. The geometry of the thermal neutron chamber (shown in Fig. 1) was designed in FLUKA. It is a general purpose Monte Carlo based particle simulation code and can simulate almost 80 different particles and estimate the energy spectrum in various kind of geometries. The estimated neutron spectrum at the sample position is shown in Fig. 2. The neutron spectrum is having peaked at energy 0.025 eV to 0.030 eV which corresponds to thermal neutrons. 2.4. Structural characterizations To confirm the formation of the compound, X-ray diffraction pattern was studied for structural analysis at room temperature for microphosphor samples using Cu-target (Cu-Kα ¼1.54 Å) on Bruker AXS D8 Advance X-ray Diffractometer. SEM image was obtained using Scanning Electron Microscope, “JEOL JSM-6360A (FEI, Netherlands),” operated at 20 kV. The FTIR spectrophotometer (JASCO FT/IR 6100) was used to record the spectrum of the sample in the range 4000 cm 1 to 400 cm 1 to know the probable formation of the chemical bonds.TL glow curve measurements of the microphosphor were recorded on a Nucleonix TLD Reader (Model 1009I) taking 5 mg of sample each time; at an constant heating rate of 5 °C s 1.
3. Results and discussion 3.1. Structural and morphological characteristics:
Fig. 1. Schematic of the cylindrical type diation chamber.
252
Cf moderated thermal neutron irra-
Fig. 3 shows XRD pattern of the CaSO4:Dy microphosphor irradiated with thermal neutrons at different fluences from 6 1011 n/cm2 to 52 1011 n/cm2. It is observed from the figure that, there is no shift in the peak but intensity of the peak decreases with increase in the neutron fluence. This is because of the thermal neutron produces prompt alpha particles through
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Fig. 5. FTIR spectra of microcrytstalline CaSO4:Dy phosphor irradiated at a) 6 1011 n/cm2, b) 13 1011 n/cm2, c) 26 1011 n/cm2, d) 39 1011 n/cm2 and e) 52 1011 n/cm2 thermal neutron fluences.
Fig. 3. X-ray diffraction pattern of microcrytstalline CaSO4:Dy irradiated with different thermal neutron fluences.
Fig. 6. TL glow curve of CaSO4:Dy microphosphor irradiated with thermal neutrons at the fluences from 6 1011 n/cm2 to 60 1011 n/cm2.
Fig. 4. SEM image of CaSO4:Dy as prepared microphosphor.
(n, α) reaction which may be responsible to induce defects in the phosphor. The concentration of defects increases with increase in the neutron fluence which eventually decreases the periodic lattice ordering of the phosphor results into a decrease in the intensity of the XRD peaks. Fig. 3 shows, the XRD patterns of as prepared CaSO4:Dy microphosphor and is compared with the standard data (JCPDS Card no. 37-1496). From the data it is found that the pristine sample is in orthorhombic phase with a ¼6.993, b¼7.001 and c ¼6.241 for micro CaSO4:Dy which matches very well with the JCPDS Card no. 37-1496. The shape and size of these particles have been observed using the SEM image shown in Fig. 4. Corresponding histograms are also plotted in the inset of image. It is observed from histograms that the maximum number of particle is having the size of 200–250 mm. 3.2. FTIR analysis The FT-IR spectra of the thermal neutron irradiated samples of CaSO4:Dy microcrystalline is shown in Fig. 5. Normally, sulphate contains two S¼O and two S–O bonds. Actually, the four S–O bonds are equivalent. The 1090 cm 1 peak is due to the S–O stretching mode. Like any other bonds, sulphate bonds can bend, giving rise to one or two bonds normally in the 610 to 680 cm 1
range [18,19]. These bands are seen in the FTIR spectrum. It is worth noticing that the bending bands are sharper than the stretching bands. This is commonly observed in inorganic infrared spectra. These prominent peaks confirm the formation of sulphate bonding in the samples. The groups of peaks near 2000 cm 1 shown in Fig. 5 are overtones. The characteristic bands around 3000 cm 1 and 1600 cm 1 are ascribed to atmospheric water vapor, since KBr readily absorbs moisture in the air and this indicates that the prepared sample consists of a certain amount of moisture [18,19]. The peaks around 3000 cm 1 diminishes with irradiation, which indicates the water content decreases with neutron fluence. Fortunately, these undesirable peaks do not affect the identification of the substances involved in this experiment due to different absorption positions of water and the possible product. 3.3. Thermoluminescence glow curve TL glow curves (through dose separation) of microphosphor powder samples irradiated with thermal neutron fluence varying from 6 1011 n/cm2 to 60 1011 n/cm2 (glow curves a–f) is shown in Fig. 6. The thermoluminescence glow curve (TL) for assynthesized CaSO4:Dy microphosphor did not show any peak. But, when the microphosphor irradiated with thermal neutrons at a fluence of 6 1011 n/cm2, the TL glow curve exhibits peak at 240 °C and further the intensity of the glow peak increases with increase in the neutron fluence. However, there is no change in the peak position and shape.
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3.5. Analysis of glow curves and calculation of trapping parameters by CGCD fitting by Kitis functions The Computerized Glow Curve Deconvolution (CGCD) fitting in CaSO4:Dy microcrystalline material has been carried out using glow curve deconvolution (GCD) functions (Eqs. (1) and (2)), suggested by Kitis [14,22], for general and first order kinetics glow curves, respectively. For kinetic analysis the experimentally obtained TL glow curve is fitted with CGCD method and is given below For General order: " b E T Tm T2 2kT E T Tm I ðT Þ ¼ I m bðb 1Þ exp exp ðb 1Þ 2 1 kT T m E kT T m Tm Fig. 7. TL response of CaSO4:Dy microphosphor irradiated with thermal neutrons at different fluences from 6 1011 n/cm2 to 60 1011 n/cm2.
For dosimetric purposes, a thermoluminescent phosphor is expected to show the features such as, relatively a simple glow curve (no interfering glow peaks), high TL sensitivity, long term stability of the stored information at room temperature (namely low fading); good linearity of the TL signal in the specific useful range of radiation dose; effective atomic number Zeff close to that of the biological tissue (in order to deal with a tissue equivalent material) etc [20]. CaSO4:Dy microphosphor is very sensitive and also exhibits a single major peak at 240 °C for thermal neutron irradiation. Moreover, the higher temperature glow peak advantageous since higher temperature peak has lesser fading. Therefore, CaSO4:Dy microphosphor is good material for TLD. 3.4. TL response Fig. 7 shows the behavior of TL response with respect to thermal neutrons fluence. The TL response is calculated as TL signal, normalized to the mass of the CaSO4:Dy microphosphor powder samples. Initially, the assessment of the TL sensitivity from the slope of the growth linear curve which is nearly equal to one (¼ 0.987). Therefore, response of CaSO4:Dy microphosphpr sample linearly increases with thermal neutron fluence. Beyond the thermal neutron fluence of 52 1011 n/cm2, the TL response of the microphosphor saturates. Due to increase in the thermal neutron fluence a saturation effects occur because the distances between neighboring TC (trapping centre)/LC(luminescent centre) complex decrease and the TC/LC complex begins to merge and overlap. The overlapping regions do not contribute to additional TL since they do not result in additional trapping charge carriers due to the full occupancy of the available trapping centers and luminescence centers. In case of microparticles, the surface to volume ratio is large which results in a higher surface barrier energy of the microparticles. This also explains the high TL sensitivity of the microphosphor [13]. On increasing the thermal neutron fluence, the energy density crosses the threshold value of the surface barrier and thus a large number of defects are produced in the microphosphors. The number of defects created in the micro phosphor keeps on increasing with thermal neutrons till saturation is obtained. However, more significant to note here is that the microphosphor starts to lose its linearity at a relatively fluence of 52 1011 n/cm2. The microphosphor loses its linearity much possibly due to the radiation damage which results in amorphisation of the phosphor. However since the damage due to radiation in micromaterials is high [21], the microphosphors continue to show linearity even at low doses in the case of thermal neutron.
þ 1 þ ðb 1Þ
2kT m E
b b 1
ð1Þ
For First order: " # E T Tm T2 E T Tm 2kT 2kT m 2 exp 1 I ðT Þ ¼ I m exp 1 þ kT T m kT T m E E Tm ð2Þ Here, I (T) is the TL intensity at temperature T (K), Im, the maximum peak intensity, Tm, is the temperature corresponding to maximum peak intensity Im, E, trap depth or the thermal activation energy (eV) needed to free the trapped electrons, b, order of kinetics , k, Boltzmann's constant (8.6 10 5 eV K 1). The frequency factor s is obtained from the following equations: For general order: βE E exp s¼ ð3Þ kT m kT 2m 1 þ ðb 1Þ2kTE m For first order: βE E s ¼ 2 exp kT m kT m
ð4Þ
There are two very important dosimetric applications of CGCD fitted glow curve i.e one is improvements in precision and minimum measurable dose, and other one is improvements in dose reestimation capability. Fig. 8 shows the curve fitting technique which is capable of evaluating all the glow peaks in a multi-peak glow curve of actual CaSO4:Dy microphosphor. Therefore, deconvoluted of TL glow curves of CaSO4:Dy exposed to 39 1011 n/cm2 is taken as a typical case. The figure also shows the experimental and theoretically deconvoluted TL curve as well as multi-curve glow peak of three trapping parameters fit such as order of kinetics
Fig. 8. Comparison between the experimental and the theoretically fitted glow curves of CaSO4:Dy microparticles exposed to 39 1011 n/cm2 of thermal neutronfluence. Deconvoluted single fitted curves, a, b, c and d are also shown along with the residue inset curve.
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Table 1 Summary of obtained values for different peaks of a microphosphor irradiated at thermal neutron fluence of 39 1011 n/cm2. Sample name
Peak
Peak Temp. Tm (°C)
Peak Temp. Tm (K)
Order of Kinetics (b)
Trap depth E (eV)
Frequency factor S (s 1)
39 1011 n/cm2
a b c d
240 190 154 380
513 463 427 653
1.84 1.50 1.00 1.48
0.99 1.00 0.50 0.70
1.0 109 1.9 1010 1.2 105 2.2 107
(b), activation energy (E) and frequency factor (S) respectively and determined parameters mentioned in Table 1 by using Kitis et al. [14,23,24] method. The following Eq. (5) shows goodness of the fitting, i.e. figure of merit (FOM) is judged objectively and in percent defined by [25], FOM ¼
Ji X jYj YðXjÞj Jf
A
100
ð5Þ
where Ji¼initial temperature in the fit region, Jf ¼final or ending temperature in the fit region, Yj¼PMT tube current at temperature j, Y(Xj) ¼Value of the function at channel j and A¼area under the peak, i.e., integral of the fit function between Ji and Jf. Hence, it is found to be around 1.15%. This shows that experimental and theoretical glow curves are in good agreement and very much overlapping on each side. The residue curve of the theoretically fitted TL glow curve is shown in the inset of Fig. 8. The calculated trapping parameters for microcrystalline CaSO4:Dy microphosphor irradiated at a thermal neutron fluence of 39 1011 n/cm2 is summarized in Table 1. In this study, the nature of TL glow curve of microcrystalline CaSO4:Dy is essentially similar to that of the corresponding microcrystalline sample. The wide range of activation energies for the microcrystalline material narrows to 0.50 eV (Table 1). This extension in the trap levels in case of microcrystalline material are attributed to the difference in the crystal field effects. This could be due to the widening of the band gap of microcrystalline material. Due to widening of the band gaps, the energy levels of the doped impurities also get reorganized. Deconvolution of all TL glow curves of micro CaSO4:Dy irradiated at different thermal neutron fluences has been performed, but for simplicity deconvolution is shown only for the sample irradiated by 39 1011 n/cm2 of 252Cf moderated based thermal neutron fluence.
4. Conclusions In this study CaSO4:Dy microphosphor material has been synthesized using the acid re-crystalization method. The microphosphor was irradiated by thermal neutrons at different fluences and their XRD, SEM, FTIR, and TL analysis have been carried out. These characteristic confirms the orthorhombic structure, formation of microparticles and possible bond appearance. The TL result showed that the sensitivity and the relative intensity of the glow peaks are strongly dependent on the Dy present in the phosphor materials. In case of exposure with thermal neutrons the TL signal linearly increases with the neutron fluence and the lowest detectable fluence has been found to be around 6 1011 n/cm2 and
FOM
1.15%
the maximum upto 60 1011 n/cm2. The TL glow curve of the microphosphor shows a single glow curve and a dosimetric in nature. Moreover, using the CGCD method glow curves are deconvoluted and they are perfectly matching with experimental TL curve. These results confirm that microphosphor could be promising for retrospective thermal neutron dosimetry. However, this newly developed neutron dosimeter can be used as a routine TLD badge system in the mixed field of gamma and neutrons for radiation workers engaged in nuclear fuel cycle operation and other applications.
References [1] M. Marrale, A. Longo, A. Bartolotta, S. Basile, M.C. D’Oca, E. Tomarchio, G.A.P. Cirrone, F. Di Rosa, F. Romano, G. Cuttone, M. Brai, Nucl. Instrum. Methods B 292 (2012) 55. [2] N.A. Larsen, B.G. Markey, S.W.S. Mckeever, Radiat. Meas. 27 (1997) 295. [3] M. Oberhofer, A. Scharmann, in: Courses Ispra (Ed.), Applied Thermoluminescence Dosimetry, Adam Higher Ltd., Bristol, 1979 (Chapter 12–15). [4] J.A. Douglas, UKAEA, Harwell. Atom. Ener. Res. Establ., The applications of TL materials in neutron dosimetry, 1978. [5] Gad Shani, Radiation Dosimetry: Instrumentation and Methods, Second Ed., CRC Press, Boca Raton, London, 2000. [6] A. Triolo, M. Marrale, M. Brai, Nucl. Instrum. Methods B 264 (2007) 183. [7] K. Ayyangar, A.R. Lakshmanan, B. Chandra, K. Ramadas, Phys. Med. Biol. 19 (5) (1974) 665. [8] A.R. Lakshmanan, M.T. Jose, O. Annalakshmi, Radiat. Prot. Dosim. 132 (2008) 42. [9] U. Madhusoodanan, M.T. Jose, A. Tomita, W. Hoffmann, A.R. Lakshmanan, J. Lumin. 82 (1999) 221. [10] M.D. Morgan, M.A. LaCombe, T.G. Stoebe, Radiat. Prot. Dosim. 6 (1–4) (1983) 321. [11] B.C. Bhatt, A.R. Lakshmanan, S.S. Shinde, R.C. Bhatt, Radiat. Prot. Dosim. 25 (1) (1988) 37. [12] N. Salah, P.D. Sahare, S.P. Lochab, P. Kumar, Radiat. Meas. 41 (2006) 40. [13] N. Salah, P.D. Sahare, A.A. Rupasov, J. Lumin. 124 (2007) 357. [14] G. Kitis, J.M. Gomez-Ros, J.W.N. Tuyn, J. Phys. D: Appl. Phys. 31 (1998) 2636. [15] A.M. Sadek, H.M. Eissa, A.M. Basha, G. Kitis, J. Lumin. 146 (2014) 418. [16] P.D. Sahare, S.V. Moharil, J. Phys. D: Appl. Phys. 23 (1990) 567. [17] N.S. Rajurkar, A.K. Jha, R.P. Bhadane, J. Radional. Nucl. Chem. Lett. 186 (5) (1994) 393. [18] Brian C. Smith, Quatitative Spectroscopy: Theory and Practice, Academic Press, An Elsevier Science Imprint, USA, 2002. [19] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, A John Wiley & Sons, Hoboken, New Jersey, 1986. [20] J. Azorín, C. Furetta, A. Scacco, Phys. Status Solidi A 9 (1993) 1. [21] S.R. Shinde, A. Bhagwat, S.I. Patil, S.B. Ogale, G.K. Mehta, S.K. Date, G. Marest, J. Magn. Magn. Mater. 186 (1998) 342. [22] Vasilis Pagonis, George Kitis, Claudio Furetta, Numerical and Practical Exercises in Thermoluminescence, Springer Science þ Business Media, Inc., New York, 2006. [23] G. Kitis, V. Pagonis, Radiat. Meas. 43 (2008) 737. [24] R. Chen, J. Electrochem. Soc. 116 (1969) 1254. [25] N.W. Eddy, Nucl. Instrum. Methods 145 (1943) 389.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
CaSO4:Dy microphosphor for thermal neutron dosimetry Mahesh S. Bhadane a, Nandkumar Mandlik b, B.J. Patil c, S.S. Dahiwale a, K.R. Sature a, V.N. Bhoraskar a, S.D. Dhole a,n a
Microtron Accelerator Laboratory, Department of Physics, Savitribai Phule Pune University, Pune 411007, India Department of Physics, Fergusson College, Savitribai Phule Pune University, Pune 411007, India c Department of Physics, Abasaheb Garware College, Pune 411004, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2015 Received in revised form 2 September 2015 Accepted 9 October 2015 Available online 22 October 2015
Dysprosium-doped calcium sulphate (CaSO4:Dy) microphosphor was synthesized by acid recrystallization method and its thermoluminescence (TL) properties irradiated with thermal neutrons was studied. Structural and morphological characteristics have been studied using X-ray diffraction and SEM which mainly exhibits a orthorhombic structure with particle size of 200 to 250 mm. Moreover, thermal neutron dosimetric characteristics of the microphosphor such as thermoluminescence glow curve, TL dose–response have been studied. This microphosphor powder represents a TL glow peak (Tmax) centered at around 240 °C. The TL response of CaSO4:Dy microphosphor as a function of thermal neutron fluence is observed to be very linear upto the fluence of 52 1011 n/cm2 and further saturates. In addition, TL glow curves were deconvoluted by computerized glow curve deconvolution (CGCD) method and corresponding trapping parameters have been determined. It has been found that for every deconvoluted peak there is change in the order of kinetics. Overall, the experimental results show that the CaSO4:Dy microphosphor can have potential to be an effective thermal neutron dosimetry. & 2015 Elsevier B.V. All rights reserved.
Keywords: 252 Cf FLUKA code Microphosphor Thermoluminescence CGCD
1. Introduction Thermoluminescence (TL) is a well known technique which commonly used for the dose measurement of ionizing radiations. Now a days, TL has increased their applications and uses like retrospective dosimetry [1], personal, clinical, environmental monitoring [2], medicine, glow curve analysis, biological related fields, high-level photon and neutron dosimetry with TLD materials etc. [3]. The neutron dosimetry is not as simple as the dosimetry of gamma radiations and other radiations because of the numerous direct and indirect energy transfer processes involved and the variation of the reaction cross sections with energy [1,4]. Neutron has wide range of energy spectrum, among them neutron dosimetry for thermal neutron is an important. Because, thermal neutron has less energy and favorable uses for medical purposes as well as nuclear research laboratories, nuclear power stations, reactors and industries [5,6]. Thermal neutron response of every TL materials depends on the capture cross section of the elements of TL materials. However, CaSO4:Dy is reported on the basis of mixed field dosimetry [7] and found an additive and no permanent damage to the γ-ray sensitivity observed after irradiating n
Corresponding author. Tel.: þ 91 2025692678; fax: þ 91 2025691684. E-mail address: [email protected] (S.D. Dhole).
http://dx.doi.org/10.1016/j.jlumin.2015.10.025 0022-2313/& 2015 Elsevier B.V. All rights reserved.
maximum thermal neutron fluence. Basically, the thermal neutron interacts with phosphor materials mainly through scattering and absorption processes i.e. Inelastic scattering, Elastic scattering, Non-elastic scattering and Capture process. However, in the present case, inelastic and non-elastic scattering processes are most probable as they form (n, γ) and (n, α) reactions which emits prompt γ and α particles responsible to have dosimetric change due to thermal neutrons. In the past few years, considerable amount of work is involved in search of new microcrystalline phosphor materials with better TL sensitivity and dosimetric properties. CaSO4:Dy is one of the high sensitive microphosphor for gamma dosimetry and has been studied by lakshmanan et al. [8] using co-precipitation technique. CaSO4:Dy and other microcrystalline phosphors are widely been used as a TLD materials and are useful for low dose measurements [9]. CaSO4:Dy is the most interesting phosphor activated with Dysprosium, because of the considerable stability. It also enhances the luminescence efficiency of phosphor. Therefore, aim of this work is to synthesize the Dy doped CaSO4 microcrystalline phosphor by acid recrystallization method and irradiated with thermal neutrons. The irradiation studies of this kind of phosphor with thermal neutron is rarely reported in the literature [10,11]. The characterization such as XRD, SEM, FTIR and thermoluminescence
M.S. Bhadane et al. / Journal of Luminescence 170 (2016) 226–230
227
(TL) properties of thermal neutron irradiated CaSO4:Dy powder are studied for structural, morphological and chemical reformation. Information of trapping parameters through deconvolution is very much useful for the analysis of TL glow curve. Therefore, a computerized glow curve deconvolution (CGCD) program is applied to analyze the nature of traps [12,13] generated due thermal neutrons in microphosphor. Deconvolution of TL glow curve is an important and many trapping parameters such as trap depth, order of kinetics, frequency factor and maximum peak temperature can be calculated. Computerized (CGCD) Deconvolution is done using Kitis function [14,15] and applied to microcrystalline CaSO4:Dy TL glow curves for analysis. Fig. 2. FLUKA simulated fission spectrum of
2. Experimental details 2.1. Materials preparation Acid re-crystalization method was used to synthesized CaSO4: Dy microphosphor. The desired impurity of Dy (0.1 mol%) in the form of Dy2O3 (Sigma Aldrich) and CaSO4 2H2O (AR grade) were dissolved in proper ratio of concentrated sulfuric acid in a rounded bottom flask, which was connected to a sealed condenser system. The acid was then distilled out in a flask connected to the condenser. The solution was heated at about 337 °C (boiling point of H2SO4) to evaporate the acid completely. After cooling, the phosphor thus obtained in microcrystalline form was repeatedly washed with double distilled water to remove the acid traces and dried in an oven at 100 °C up to 4 h. The phosphor thus obtained in powder form was crushed and sieved to obtain grains ranging from 100 to 200 μm in size. Finally, the microcrystalline powder was annealed at 700 °C for 2 h in a quartz boat under nitrogen atmosphere and quenched by taking the boat out of the furnace and placing it on a metal block [16]. 2.2. Thermal neutron chamber and irradiation In the present study, samples were irradiated using 252Cf based thermal neutron source [17] having a flux of 106 n/cm2/s and half life 2.645 years. The schematic of the thermal neutron facility is shown in Fig. 1. The 252Cf irradiator presents cylindrical geometry with a coaxial stainless steel filter fitted in the center of the cylindrical chamber. The 252Cf is covered by paraffin wax as a moderator which filters out the high energy neutrons and converts to thermal neutrons. The irradiation of the sample was done in the outer shell of the ring which receives constant flux of thermal neutrons. The sample rod was used to keep and take out the samples from irradiation chamber. Moderator tank of paraffin wax is surrounded by borated paraffin to shield the neutrons. It is also covered by thin lead shielding to attenuate the undesirable gamma rays emitted from the 252Cf source and still remains at the sample position with small amount. Therefore, to avoid these gamma contamination during thermal neutron irradiation, set of
252
Cf moderated thermal neutron.
experiments were performed to estimate the actual contribution of the gamma dose. For this CaSO4:Dy phosphor covered with aluminum foil was kept in a 5 mm cylindrical plastic vial filled up with the boric powder. The boric powder was used to stop the thermal neutron and aluminum foil was used to stop the alpha particles generated from boron. In addition, a pure CaSO4:Dy sample was kept for irradiation to see the cumulative dose effects. The dose separation method was adopted to estimate the pure thermal neutrons. 2.3. FLUKA simulation of Cf-252 moderated based thermal neutron source and neutron energy spectrum To determine the energy spectrum of moderated Cf-252 source, a FLUKA simulation was carried out. Cf-252 is a natural isotope which emits fast neutrons through fission process. The geometry of the thermal neutron chamber (shown in Fig. 1) was designed in FLUKA. It is a general purpose Monte Carlo based particle simulation code and can simulate almost 80 different particles and estimate the energy spectrum in various kind of geometries. The estimated neutron spectrum at the sample position is shown in Fig. 2. The neutron spectrum is having peaked at energy 0.025 eV to 0.030 eV which corresponds to thermal neutrons. 2.4. Structural characterizations To confirm the formation of the compound, X-ray diffraction pattern was studied for structural analysis at room temperature for microphosphor samples using Cu-target (Cu-Kα ¼1.54 Å) on Bruker AXS D8 Advance X-ray Diffractometer. SEM image was obtained using Scanning Electron Microscope, “JEOL JSM-6360A (FEI, Netherlands),” operated at 20 kV. The FTIR spectrophotometer (JASCO FT/IR 6100) was used to record the spectrum of the sample in the range 4000 cm 1 to 400 cm 1 to know the probable formation of the chemical bonds.TL glow curve measurements of the microphosphor were recorded on a Nucleonix TLD Reader (Model 1009I) taking 5 mg of sample each time; at an constant heating rate of 5 °C s 1.
3. Results and discussion 3.1. Structural and morphological characteristics:
Fig. 1. Schematic of the cylindrical type diation chamber.
252
Cf moderated thermal neutron irra-
Fig. 3 shows XRD pattern of the CaSO4:Dy microphosphor irradiated with thermal neutrons at different fluences from 6 1011 n/cm2 to 52 1011 n/cm2. It is observed from the figure that, there is no shift in the peak but intensity of the peak decreases with increase in the neutron fluence. This is because of the thermal neutron produces prompt alpha particles through
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Fig. 5. FTIR spectra of microcrytstalline CaSO4:Dy phosphor irradiated at a) 6 1011 n/cm2, b) 13 1011 n/cm2, c) 26 1011 n/cm2, d) 39 1011 n/cm2 and e) 52 1011 n/cm2 thermal neutron fluences.
Fig. 3. X-ray diffraction pattern of microcrytstalline CaSO4:Dy irradiated with different thermal neutron fluences.
Fig. 6. TL glow curve of CaSO4:Dy microphosphor irradiated with thermal neutrons at the fluences from 6 1011 n/cm2 to 60 1011 n/cm2.
Fig. 4. SEM image of CaSO4:Dy as prepared microphosphor.
(n, α) reaction which may be responsible to induce defects in the phosphor. The concentration of defects increases with increase in the neutron fluence which eventually decreases the periodic lattice ordering of the phosphor results into a decrease in the intensity of the XRD peaks. Fig. 3 shows, the XRD patterns of as prepared CaSO4:Dy microphosphor and is compared with the standard data (JCPDS Card no. 37-1496). From the data it is found that the pristine sample is in orthorhombic phase with a ¼6.993, b¼7.001 and c ¼6.241 for micro CaSO4:Dy which matches very well with the JCPDS Card no. 37-1496. The shape and size of these particles have been observed using the SEM image shown in Fig. 4. Corresponding histograms are also plotted in the inset of image. It is observed from histograms that the maximum number of particle is having the size of 200–250 mm. 3.2. FTIR analysis The FT-IR spectra of the thermal neutron irradiated samples of CaSO4:Dy microcrystalline is shown in Fig. 5. Normally, sulphate contains two S¼O and two S–O bonds. Actually, the four S–O bonds are equivalent. The 1090 cm 1 peak is due to the S–O stretching mode. Like any other bonds, sulphate bonds can bend, giving rise to one or two bonds normally in the 610 to 680 cm 1
range [18,19]. These bands are seen in the FTIR spectrum. It is worth noticing that the bending bands are sharper than the stretching bands. This is commonly observed in inorganic infrared spectra. These prominent peaks confirm the formation of sulphate bonding in the samples. The groups of peaks near 2000 cm 1 shown in Fig. 5 are overtones. The characteristic bands around 3000 cm 1 and 1600 cm 1 are ascribed to atmospheric water vapor, since KBr readily absorbs moisture in the air and this indicates that the prepared sample consists of a certain amount of moisture [18,19]. The peaks around 3000 cm 1 diminishes with irradiation, which indicates the water content decreases with neutron fluence. Fortunately, these undesirable peaks do not affect the identification of the substances involved in this experiment due to different absorption positions of water and the possible product. 3.3. Thermoluminescence glow curve TL glow curves (through dose separation) of microphosphor powder samples irradiated with thermal neutron fluence varying from 6 1011 n/cm2 to 60 1011 n/cm2 (glow curves a–f) is shown in Fig. 6. The thermoluminescence glow curve (TL) for assynthesized CaSO4:Dy microphosphor did not show any peak. But, when the microphosphor irradiated with thermal neutrons at a fluence of 6 1011 n/cm2, the TL glow curve exhibits peak at 240 °C and further the intensity of the glow peak increases with increase in the neutron fluence. However, there is no change in the peak position and shape.
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3.5. Analysis of glow curves and calculation of trapping parameters by CGCD fitting by Kitis functions The Computerized Glow Curve Deconvolution (CGCD) fitting in CaSO4:Dy microcrystalline material has been carried out using glow curve deconvolution (GCD) functions (Eqs. (1) and (2)), suggested by Kitis [14,22], for general and first order kinetics glow curves, respectively. For kinetic analysis the experimentally obtained TL glow curve is fitted with CGCD method and is given below For General order: " b E T Tm T2 2kT E T Tm I ðT Þ ¼ I m bðb 1Þ exp exp ðb 1Þ 2 1 kT T m E kT T m Tm Fig. 7. TL response of CaSO4:Dy microphosphor irradiated with thermal neutrons at different fluences from 6 1011 n/cm2 to 60 1011 n/cm2.
For dosimetric purposes, a thermoluminescent phosphor is expected to show the features such as, relatively a simple glow curve (no interfering glow peaks), high TL sensitivity, long term stability of the stored information at room temperature (namely low fading); good linearity of the TL signal in the specific useful range of radiation dose; effective atomic number Zeff close to that of the biological tissue (in order to deal with a tissue equivalent material) etc [20]. CaSO4:Dy microphosphor is very sensitive and also exhibits a single major peak at 240 °C for thermal neutron irradiation. Moreover, the higher temperature glow peak advantageous since higher temperature peak has lesser fading. Therefore, CaSO4:Dy microphosphor is good material for TLD. 3.4. TL response Fig. 7 shows the behavior of TL response with respect to thermal neutrons fluence. The TL response is calculated as TL signal, normalized to the mass of the CaSO4:Dy microphosphor powder samples. Initially, the assessment of the TL sensitivity from the slope of the growth linear curve which is nearly equal to one (¼ 0.987). Therefore, response of CaSO4:Dy microphosphpr sample linearly increases with thermal neutron fluence. Beyond the thermal neutron fluence of 52 1011 n/cm2, the TL response of the microphosphor saturates. Due to increase in the thermal neutron fluence a saturation effects occur because the distances between neighboring TC (trapping centre)/LC(luminescent centre) complex decrease and the TC/LC complex begins to merge and overlap. The overlapping regions do not contribute to additional TL since they do not result in additional trapping charge carriers due to the full occupancy of the available trapping centers and luminescence centers. In case of microparticles, the surface to volume ratio is large which results in a higher surface barrier energy of the microparticles. This also explains the high TL sensitivity of the microphosphor [13]. On increasing the thermal neutron fluence, the energy density crosses the threshold value of the surface barrier and thus a large number of defects are produced in the microphosphors. The number of defects created in the micro phosphor keeps on increasing with thermal neutrons till saturation is obtained. However, more significant to note here is that the microphosphor starts to lose its linearity at a relatively fluence of 52 1011 n/cm2. The microphosphor loses its linearity much possibly due to the radiation damage which results in amorphisation of the phosphor. However since the damage due to radiation in micromaterials is high [21], the microphosphors continue to show linearity even at low doses in the case of thermal neutron.
þ 1 þ ðb 1Þ
2kT m E
b b 1
ð1Þ
For First order: " # E T Tm T2 E T Tm 2kT 2kT m 2 exp 1 I ðT Þ ¼ I m exp 1 þ kT T m kT T m E E Tm ð2Þ Here, I (T) is the TL intensity at temperature T (K), Im, the maximum peak intensity, Tm, is the temperature corresponding to maximum peak intensity Im, E, trap depth or the thermal activation energy (eV) needed to free the trapped electrons, b, order of kinetics , k, Boltzmann's constant (8.6 10 5 eV K 1). The frequency factor s is obtained from the following equations: For general order: βE E exp s¼ ð3Þ kT m kT 2m 1 þ ðb 1Þ2kTE m For first order: βE E s ¼ 2 exp kT m kT m
ð4Þ
There are two very important dosimetric applications of CGCD fitted glow curve i.e one is improvements in precision and minimum measurable dose, and other one is improvements in dose reestimation capability. Fig. 8 shows the curve fitting technique which is capable of evaluating all the glow peaks in a multi-peak glow curve of actual CaSO4:Dy microphosphor. Therefore, deconvoluted of TL glow curves of CaSO4:Dy exposed to 39 1011 n/cm2 is taken as a typical case. The figure also shows the experimental and theoretically deconvoluted TL curve as well as multi-curve glow peak of three trapping parameters fit such as order of kinetics
Fig. 8. Comparison between the experimental and the theoretically fitted glow curves of CaSO4:Dy microparticles exposed to 39 1011 n/cm2 of thermal neutronfluence. Deconvoluted single fitted curves, a, b, c and d are also shown along with the residue inset curve.
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Table 1 Summary of obtained values for different peaks of a microphosphor irradiated at thermal neutron fluence of 39 1011 n/cm2. Sample name
Peak
Peak Temp. Tm (°C)
Peak Temp. Tm (K)
Order of Kinetics (b)
Trap depth E (eV)
Frequency factor S (s 1)
39 1011 n/cm2
a b c d
240 190 154 380
513 463 427 653
1.84 1.50 1.00 1.48
0.99 1.00 0.50 0.70
1.0 109 1.9 1010 1.2 105 2.2 107
(b), activation energy (E) and frequency factor (S) respectively and determined parameters mentioned in Table 1 by using Kitis et al. [14,23,24] method. The following Eq. (5) shows goodness of the fitting, i.e. figure of merit (FOM) is judged objectively and in percent defined by [25], FOM ¼
Ji X jYj YðXjÞj Jf
A
100
ð5Þ
where Ji¼initial temperature in the fit region, Jf ¼final or ending temperature in the fit region, Yj¼PMT tube current at temperature j, Y(Xj) ¼Value of the function at channel j and A¼area under the peak, i.e., integral of the fit function between Ji and Jf. Hence, it is found to be around 1.15%. This shows that experimental and theoretical glow curves are in good agreement and very much overlapping on each side. The residue curve of the theoretically fitted TL glow curve is shown in the inset of Fig. 8. The calculated trapping parameters for microcrystalline CaSO4:Dy microphosphor irradiated at a thermal neutron fluence of 39 1011 n/cm2 is summarized in Table 1. In this study, the nature of TL glow curve of microcrystalline CaSO4:Dy is essentially similar to that of the corresponding microcrystalline sample. The wide range of activation energies for the microcrystalline material narrows to 0.50 eV (Table 1). This extension in the trap levels in case of microcrystalline material are attributed to the difference in the crystal field effects. This could be due to the widening of the band gap of microcrystalline material. Due to widening of the band gaps, the energy levels of the doped impurities also get reorganized. Deconvolution of all TL glow curves of micro CaSO4:Dy irradiated at different thermal neutron fluences has been performed, but for simplicity deconvolution is shown only for the sample irradiated by 39 1011 n/cm2 of 252Cf moderated based thermal neutron fluence.
4. Conclusions In this study CaSO4:Dy microphosphor material has been synthesized using the acid re-crystalization method. The microphosphor was irradiated by thermal neutrons at different fluences and their XRD, SEM, FTIR, and TL analysis have been carried out. These characteristic confirms the orthorhombic structure, formation of microparticles and possible bond appearance. The TL result showed that the sensitivity and the relative intensity of the glow peaks are strongly dependent on the Dy present in the phosphor materials. In case of exposure with thermal neutrons the TL signal linearly increases with the neutron fluence and the lowest detectable fluence has been found to be around 6 1011 n/cm2 and
FOM
1.15%
the maximum upto 60 1011 n/cm2. The TL glow curve of the microphosphor shows a single glow curve and a dosimetric in nature. Moreover, using the CGCD method glow curves are deconvoluted and they are perfectly matching with experimental TL curve. These results confirm that microphosphor could be promising for retrospective thermal neutron dosimetry. However, this newly developed neutron dosimeter can be used as a routine TLD badge system in the mixed field of gamma and neutrons for radiation workers engaged in nuclear fuel cycle operation and other applications.
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