Radiation Protection Dosimetry (2008), Vol. 132, No. 1, pp Advance Access publication 20 August 2008

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1 Radiation Protection Dosimetry (2008), Vol. 132, No. 1, pp Advance Access publication 20 August 2008 doi: /rpd/ncn215 HIGH-SENSITIVE CaSO 4 :Dy THERMOLUMINESCENT PHOSPHOR SYNTHESIS BY CO-PRECIPITATION TECHNIQUE A. R. Lakshmanan 1,2, *, M. T. Jose 1 and O. Annalakshmi 1 1 Radiological Safety Division, Safety Group, Indira Gandhi Centre for Atomic Research, Kalpakkam , Tamil Nadu, India 2 Present address: Saveetha Engineering College, Thandalam, Chennai , India Received May revised July , accepted July This paper describes the successful development of the co-precipitation technique for the preparation of a high-sensitive dysprosium-doped calcium sulphate (CaSO 4 :Dy) thermoluminescent dosimetry (TLD) phosphor with dosimetric glow peak at 2308C which is a desired development in the field of radiation protection dosimetry. The main advantages of this coprecipitation technique over the conventional recrystallisation technique of phosphor preparation are: (i) preparation time is very less; (ii) quantity of sulphuric acid evaporated is insignificant; (iii) higher TL sensitivity 220% more than the present material; (iv) better glow curve structure; (v) lesser glow peak shift and better linearity to gamma and (vi) uniform crystal morphology and lower grain size all grains are mostly cuboidal in shape, quite uniform and small (average size about 25 mm), suitable for manufacturing teflon discs in as-prepared form. Optimum values of various parameters in the method of preparation for a batch of 20 g CaSO 4 :Dy phosphors to obtain maximum TL sensitivity, with favourable glow curves are studied. INTRODUCTION Thermoluminescence (TL) has found wide application in personnel monitoring of radiation workers and environmental radiation monitoring due to its reliability, sensitivity and commercial availability. Rare earth-doped alkaline earth metal sulphate phosphors are used as dosimetry material for monitoring. Among them CaSO 4 :Dy/Tm phosphor developed by Yamashita et al. (1) is quite popular due to its high sensitivity, stability and low cost. Presently, this phosphor is prepared by the recrystallisation technique in which gypsum (CaSO 4.2H 2 O) (1) or calcium nitrate (CaNO 3 ) (2) along with activator (dysprosium oxide - Dy 2 O 3 ) is dissolved in hot concentrated sulphuric acid (H 2 SO 4 ) completely and then totally evaporated to dryness at about 3258C either in open or using a distillation unit (3) in a fume hood. This technique requires the evaporation of 1 l of concentrated H 2 SO 4 to obtain 100 g of dosimetric quality CaSO 4 :Dy phosphor. Another technique recently developed by Lakshmanan (4,5) with calcium carbonate as raw material instead of gypsum also needs evaporation of 150 ml of H 2 SO 4 and 450 ml of hydrochloric acid to dryness to obtain 100 g of CaSO 4 :Dy, although this phosphor is 50% more sensitive than those prepared by the conventional methods. All other techniques (solid-state diffusion (6,7), co-precipitation, etc. (8 10) ) did not give the desired glow curve structure and/or the desired TL sensitivity. Results obtained so far show that the complex defect responsible for the *Corresponding author: arunachalamlakshmanan@yahoo. com dosimetric peak in CaSO 4 :Dy around 2008C could be created only in H 2 SO 4 media, and that too at elevated temperatures, which indicate that high activation energies are required for the creation of the above defect. Materials described in References 2 and 4 need evaporation of large volume of acids, which is tedious, cumbersome and yield corrosive acid vapour, and materials in References 6 10 either yield undesired TL glow curve structure or poor TL sensitivity. The main objective of the present work is to make CaSO 4 :Dy TL dosimetry (TLD) grade phosphor with high sensitivity as well as the desired glow curve structure involving evaporation of minimum quantity of H 2 SO 4 using the simple co-precipitation technique. It is for the first time, a high-sensitive CaSO 4 :Dy TLD phosphor is synthesised with desired sensitivity (20% higher than the conventional method) and glow curve structure by co-precipitation technique. TL properties of this phosphor is studied in detail and compared with TL phosphor prepared with the conventional recrystallisation method. EXPERIMENTAL DETAILS CaSO 4 :Dy phosphors used for the comparative studies were prepared through conventional recrystallisation by Rao et al. (3) and new co-precipitation methods. Raw materials and some of the processing steps (e.g. initial dissolution in hot H 2 SO 4, quantity of acid used for dissolution and the final sintering at 7008C) are common in recrystallisation and the new (co-precipitation) methods of synthesis. The various steps involved in the co-precipitation recipe are Downloaded from at Indira Gandhi Centre for Atomic Research on October 21, 2013 # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oxfordjournals.org

2 CASO 4 :Dy THERMOLUMINESCENT PHOSPHOR SYNTHESIS described below chronologically: (1) Dissolution of CaSO H 2O (MERCK make) and Dy 2 O 3 (LOBA make) in hot concentrated H 2 SO 4 ; (2) Addition of distilled water containing trace quantity of nitric acid at room temperature (RT) into the hot solution kept at 2508C drop by drop using the separating funnel at a rate of 30 drops per min for initial 100 ml. The remaining mixture was added at a higher rate till the completion of the precipitation process. Precipitation of phosphor grains at the bottom of the glass vessel can be seen as the acid gets diluted. The water acid addition reaction is exothermic but it is violent only in the initial stages i.e. during the addition of initial drops of water but it does not cause spillage or any sputtering as long as the water is added slowly; (3) After the complete addition of acidified water, the resultant solution was left undisturbed overnight until the CaSO 4 :Dy crystals settle down; (4) The next day, separate the CaSO 4 :Dy crystals from the solution by decanting or using a ceramic micro filter of pore size 5 mm and dried it by controlled heating to remove the traces of water and acid sticking to the precipitate; (5) The phosphor obtained was mildly hand ground in an agate mortar to disperse some of the agglomerated grains and sintered at 7008C for 1 h in air in a muffle furnace in a quartz crucible; and (6) The phosphor was then passed through 75 mm sieve to separate out the grains of size above 75 mm. However, unlike the case with the conventional process, which yields mostly grains of size above 75 mm, the present technique yields predominantly (.90%) phosphor grains of size below 75 mm. The process flow diagram of the new co-precipitation technique used for the synthesis of CaSO 4 :Dy phosphor is shown in Figure 1. Optimum values of various parameters in the new co-precipitation method of preparation for a batch of 20 g CaSO 4 :Dy phosphors to obtain maximum TL sensitivity, with favourable glow curves are studied. TL measurements were carried using Nucleonix TL reader, at a linear heating rate of 108C/s. All TL sensitivity comparisons are made with area measurements from 120 to 3508C. For photoluminescence (PL) measurement at RT, a Fluorolog Jobin Yvon- Spex spectrofluorimeter was used. For gamma irradiations, a 60 Co gamma-cell with a dose rate of 360 Gy.h 21 was used. The photographs of phosphor Figure 1. Flow diagram of co-precipitation process to prepare CaSO 4 :Dy TLD phosphor. 43

3 grains were taken in a scanning electron microscope after it was spread on an aluminium holder over which gold was deposited. The ESEM XL30 (Philips make) instrument using secondary electron image at the accelerating potential of 10 kv was used for this purpose. Master Sizer particle size analyser employing laser scattering is used to study the particle size distribution. RESULTS AND DISCUSSION Characterisation of phosphor Grain size and morphology Figure 2a shows the scanning electron micrograph pattern of the new phosphor prepared by co-precipitation in as-prepared form without any sieving or grinding. It reveals that in the as-prepared stage itself, all grains are mostly cuboidal in shape and quite uniform and small (average size about 25 mm), whereas the conventional phosphor grains prepared by recrystallisation method are mostly above 75 mm in size and need to be ground for dosimetric A. R. LAKSHMANAN ET AL. applications. The ground conventional phosphor grains are quite non-uniform in size as well as nonuniform in morphology as seen in Figure 2b. Figure 3 shows the particle size distribution of the new phosphor in as-prepared form as measured by Master Sizer particle size analyser employing laser scattering. The average particle size in this case was found to be around 25 mm. Table 1 shows the weight percent of the phosphor according to their size after sieving. It shows that more than 90% of the co-precipitated phosphor has size below 75 mm. TL sensitivity and particle size distribution pattern variations of phosphors made in different batches (20 g each) are within 15%. The new phosphor is suitable for manufacturing teflon discs in as-prepared form, whereas the presently used conventional phosphors are mostly above 75 mm and has to be ground for dosimetric applications. The X-ray diffraction pattern of the new phosphor and the phosphor prepared by recrystallisation along with the standard JCPDS are shown in Figure 4. The diffraction data show that the product obtained by both the methods is characteristic of CaSO 4. Figure 2. (a) Phosphor by co-precipitation (in as-grown condition). (b) Phosphor by recrystallisation (after grinding). 44

4 CASO 4 :Dy THERMOLUMINESCENT PHOSPHOR SYNTHESIS Figure 3. Particle size distribution of new phosphor in as-prepared form (Master Sizer). Table 1. Particle size distribution of the phosphor (as-prepared form) by sieving. Batch no. Weight percent of the phosphor with particle size,75 mm mm.250 mm Study on TL property The TL glow curve structure (Figure 5) and sensitivity of the new phosphor are better than the presently used one prepared by recrystallisation. The unwanted low and high temperature peaks were absent in the new phosphor while the desired dosimetric peak near 2008C is more intense (by about 20% than that of the recrystallised phosphor). Less pronounced high temperature peaks near 3008C in new phosphor reduce the TL non-linearity with radiation dose in CaSO 4 :Dy phosphor. For the new phosphor, the glow peak shift is relatively minor with an increasing dose. The dosimetric peak shifts from 228 to 2458C with the increasing dose up to 2 k Gy and further increase in the dose shifts the peak back to 2288C (Figure 6). In contrast, the conventional phosphor showed a higher peak shift the dosimetric peak shifts from 220 to 2608C with the increase of gamma-ray dose up to 50 Gy and further increase in the dose shifts the peak back to 2208C. The growth of these high temperature TL peaks is attributed to the formation of complex deep traps during the growth of CaSO 4 :Dy crystals during recrystallisation process in excess H 2 SO 4 medium. The formation of these traps is minimised in co-precipitation technique since the phosphor crystal formation process is quite quick in this case. The shift in glow curve shape with radiation dose could create problems in dosimetry in radiotherapy or phototransferred TL in UV dosimetry in the form of uncertainty in dose estimation due to the shift in the region of interest (i.e. integrated glow curve area), variable residual dose, etc. The reduced glow peak shift with increase in dose in new phosphor is a result of reduced contribution of the high temperature peak. The dose response characteristics of the new phosphor are compared with that of the conventional phosphor in Figure 7. The supra-linear response with gamma-ray dose is slightly less in the co-precipitated phosphor as expected when compared with the phosphor obtained by recrystallisation. At low doses, which are in the range of interest for radiation protection and individual dosimetry, the response of the new phosphor is similar to the conventional phosphor. The minimum measurable level of dose with the new phosphor is better by a factor of two compared with the conventional phosphor made through recrystallisation. The supra-linearity in CaSO 4 :Dy has been attributed earlier to the competition between different traps in charge-carrier trapping during irradiation (11). TL fading studies showed almost similar amount of post-irradiation fading in new and conventional phosphors for storage at RT. 45

5 A. R. LAKSHMANAN ET AL. Figure 4. Comparison of XRD pattern of CaSO 4 :Dy phosphors prepared by new and conventional methods with standard JCPDS file of CaSO 4 ( ). Figure 5. Comparison of TL glow curves of new phosphor with conventional phosphor. Figure 6. TL glow curves of new CaSO 4 :Dy phosphor at various gamma-ray doses: 14.7 mgy (a), 519 mgy (b), 3 Gy (c), 27 Gy (d), 152 Gy (e), 2530 Gy (f) and 9.62 kgy (g). The intensities of different glow curves have not been normalised and hence are not to be inter compared. 46

6 CASO 4 :Dy THERMOLUMINESCENT PHOSPHOR SYNTHESIS Figure 7. Relative TL response to gamma-ray doses of CaSO 4 :Dy phosphors prepared by recrystallisation and co-precipitation methods. Figure 8. Comparison of PL spectrum of new phosphor with conventional phosphor. PL spectra The PL spectra of CaSO 4 :Dy phosphor in Figure 8 show all the characteristic excitation and emission peaks of Dy 3þ in all the samples with almost same intensity ratio between different peaks. The TL efficiency is enhanced on annealing at 7008C due to efficient diffusion of Dy into the crystal volume. PL sensitivity, however, decreased during this annealing; thereby implying that PL arises mainly from crystal surface defects. Similar results have been reported in the earlier recipes as well (12). Optimisation of phosphor synthesis Effect of dysprosium concentration The variation in TL sensitivity of CaSO 4 :Dy with different concentrations of dopant (Dy) are studied. The maximum TL sensitivity was obtained with dopant concentration of 1 mol%. Figure 9 shows the percentage of Dy incorporated in the CaSO 4 crystals for various concentrations of Dy in the acid solution during co-precipitation, which is estimated from the neutron activation analysis (NAA). For comparison purpose, CaSO 4 :Dy (0.2 mol%) sample made by recrystallisation method was also tested by NAA and in this sample, all the Dy added was assumed to be incorporated into the CaSO 4 lattice. This comparison shows that in the co-precipitated sample, at 0.1 mol% dopant concentration level, nearly all the Dy added (actually about 90% as compared to the recrystallisation sample) seems to have been incorporated into the CaSO 4 lattice. Despite the fact that Dy 2 (SO 4 ) 3 has a high solubility in water nearly 5 g in 100 cc (at 0.1 mol% Dy concentration level, the amount of water present in solution is 280 times 47

7 A. R. LAKSHMANAN ET AL. Table 2. TL sensitivity of phosphor prepared with reused acid. H 2 SO 4 TL sensitivity Fresh 1.00 Reused once 0.97 Figure 9. Variation in the percentage of Dy incorporated in the CaSO 4 crystals for various concentrations of Dy in the acid bath during co-precipitation as estimated from the NAA. For comparison purpose, CaSO 4 :Dy (0.2 mol%) sample made by recrystallisation method was also tested by NAA and in this sample, all the Dy added was assumed to be incorporated into the CaSO 4 lattice. more than its solubility value), 90% of the Dy in the solution gets into the CaSO 4 lattice during co-precipitation as a result of the high solubility of Dy 3þ ions in CaSO 4 lattice. This is an amazing result and shows that there is a competition between two processes one being the solubility Dy 3þ ions in CaSO 4 lattice and the other is its solubility in the acid solution. The former dominates over the latter when the concentration of Dy 3þ ions added is quite low. Such competing solubility processes between a solid and a solution in a mixed phase have been recently found to be of great importance in trapping hydrogen ions in cavitation-induced micro/nano crystals which could eventually lead to their nuclear fusion (13). Figure 9 shows that the percentage of Dy getting into the CaSO 4 lattice in the co-precipitation process slowly decreases with increased Dy concentration in the acid solution. Although the actual amount of Dy getting into the CaSO 4 lattice increases nearly 10 times at 3 mol% when compared with that of 0.1 mol%, a proportional increase in the TL sensitivity is not observed with increasing Dy concentration as a result of concentration quenching. When the Dy concentration added is 1 mol%, the H 2 SO 4 solution (after evaporation of water) could be reused at least once without adding any more Dy. The product obtained with reused acid resulted in almost identical TL sensitivity as shown in Table 2. Alternatively, solutions containing lower concentration of Dy (e.g. 0.2 mol%) can be reused by adding the required quantity of Dy once again (as done in the recrystallisation technique) since the TL intensity variations are just within 10% in the Dy concentration range mol%. Figure 10. Variation in the TL intensity of CaSO 4: Dy phosphor with temperature of H 2 SO 4 bath at the starting of co-precipitation. Effect of temperature of the acid bath Figure 10 shows the variation in the TL sensitivity of the CaSO 4 :Dy phosphor with temperature of the acid solution bath at the starting of the co-precipitation process. The TL sensitivity increases by about 40% when the temperature of the acid bath was increased from 40 to 1258C as expected from the enhanced probability of incorporation of Dy 3þ ions in the CaSO 4 lattice at elevated temperatures in acid media. A further increase in temperature from 125 to 2508C resulted only in minor increase in TL sensitivity (10%). The change in the temperature of the acid bath with elapsed time during the precipitation of CaSO 4 :Dy crystals by the addition of water. At the initial stages, when the temperature of the acid solution is high, the reaction is highly exothermic and hence the water is added very slowly. As more and more water is added, the reaction becomes mild and as the acid gets diluted, the temperature of the solution also comes down. TL sensitivity variation with the amount of water and nitric acid content Acidified water (nitric acid) is used for the precipitation of CaSO 4 :Dy crystals from H 2 SO 4. The variation in the TL sensitivity of phosphor prepared 48

8 CASO 4 :Dy THERMOLUMINESCENT PHOSPHOR SYNTHESIS by different percentages by volume of water to H 2 SO 4 for a 20 g batch is studied. Crystals of maximum TL sensitivity are obtained with the addition of 200% by volume of water to H 2 SO 4. Further addition of water reduces the TL sensitivity of the crystal slightly and at 400% by volume of water showed about 15% reduction. The variation in TL intensity with the quantity of HNO 3 acid added to water upto 30% by volume was studied. A minimum quantity of about 6% by volume of HNO 3 acid to water showed optimal TL sensitivity about 25% more than without HNO 3 acid in water. Further addition resulted only in a small increase in TL sensitivity. The action of HNO 3 in promoting the TL sensitivity could be related to its oxidising property. Earlier reports suggest that the addition of a small amount of an oxidising medium such as H 2 O 2 to concentrated H 2 SO 4 enhances the TL sensitivity of CaSO 4 :Dy made by recrystallisation process. The addition of H 2 O 2 was reported to hinder the graying of organic matter accidentally getting into the acid media (14). Effect of sintering temperature The change in the TL intensity of new phosphor with the sintering temperature in the range of C subsequent to co-precipitation and acid evaporation was studied. The variation in TL sensitivity of new phosphor was similar to phosphor prepared through recrystallisation method. Maximum TL sensitivity of the phosphor was obtained after the sintering treatment between 650 and 7508C for 1h which about 60% higher than un-sintered sample. Sintering at high temperatures is believed to stabilise the defect sites in the CaSO 4 lattice. Addition of acid to water The above method of co-precipitation of CaSO 4 :Dy crystal by the addition of water to hot acid is highly exothermic at least in the initial stages of preparation. As an alternate to this technique, concentrated H 2 SO 4 solution containing dissolved CaSO 4 and Dy 2 O 3 was added slowly to hot water (1008C) containing trace quantity of nitric acid under stirring conditions and the co-precipitated phosphor was tested. The reaction in this case was found to be less exothermic but the TL sensitivity was found to be relatively less, about 85% of the standard phosphor. Although the TL glow curve structure was found satisfactory, the crystals were found to be cylindrical in shape in this case unlike the cuboidal crystal morphology obtained with the addition of water to hot acid discussed above. Since the TL sensitivity was lower, this technique was not pursued further. 49 CONCLUSION For the first time, a simplified co-precipitation technique has been developed for the synthesis of CaSO 4 :Dy phosphor, which circumvent the cumbersome procedure, used so far, namely, evaporation of (highly corrosive) concentrated H 2 SO 4. High TL sensitivity, uniform microcrystalline morphology, lower grain size suitable for manufacturing dosemeters in solid form, better glow curve structure, lesser glow peak shift and better linearity and simplified preparation technique make the new phosphor a better alternative. The new recipe of CaSO 4 :Dy based on co-precipitation technique is not only economical but also compatible for largescale production. A novel method of synthesis of CaSO 4 :Dy TLD powder is suitable for manufacturing teflon discs used in personnel monitoring. REFERENCES 1. Yamashita, T., Nada, N., Onishi, H. and Kitamura, S. Calcium sulphate activated by thulium or dysprosium for thermoluminescent dosimetry. Health Phys. 21, (1971). 2. Danby, R. J., Boas, J. F., Calvert, R. L. and Pilbrow, J. R. ESR of thermoluminescent centres in CaSO 4 single crystals. J. Phys. C: Solid State Phys. 15, 2483 (1982). 3. Rao, G. S., Iyer, R. K., Gokhale, Y. W., Gupta, S. K, Deshpande, S. G. and Gupta, S. S. 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9 11. Lakshmanan, A. R., Bhuwan, C. and Bhatt, R. C. Gamma radiation induced sensitization in CaSO 4 :Dy TLD phosphor. Nucl. Instrum. Meth. 153, (1978). 12. Lakshmanan, A. R., Jose, M. T., Ponnusamy, V. and Vivek Kumar, P. R. Luminescence in CaSO 4 :Dy phosphor-dependence on grain agglomeration, sintering temperature, sieving and washing. J. Phys. D: Appl. Phys. 35, 386 (2002). A. R. LAKSHMANAN ET AL. 13. Lakshmanan, A. R. Controlled thermonuclear fusion of hydrogen nuclei during sodium metal dissolution in aqueous epsom solution at a critical salt concentration through cavitation induced metastable nano crystal nucleation can water serve as an infinite source of energy? Infinite Energy, September (2008) (in press). 14. Kasa, I. Dependence of TL response of CaSO 4 :Dy and CaSO 4 :Tm TL phosphor on grain size and activator concentration. Radiat. Prot. Dosim. 33(1/4), (1990). 50