PREPARATION, DOSIMETRIC CHARACTERISATION AND ...

Radiation Protection Dosimetry Vol. 100, Nos 1–4, pp. 251–253 (2002) Nuclear Technology Publishing

PREPARATION, DOSIMETRIC CHARACTERISATION AND INVESTIGATION OF RELATED TSL DEFECT CENTRES IN Li3PO4:Mg,Cu PHOSPHOR B. C. Bhatt†, B. S. Dhabekar†, S. S. Sanaye†, S. S. Shinde†, S. V. Moharil‡ and T. K. Gundu Rao§ †Radiological Physics and Advisory Division Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India §Regional Sophisticated Instrumentation Centre, IIT Powai, Mumbai 400 076, India ‡Department of Physics, Nagpur University, Nagpur 440 010, India Abstract — Li3PO4:Mg,Cu, a low effective atomic number and high sensitivity TSL phosphor, has been prepared. Its TSL glow curve shows a major peak around 360°C with minor peaks around 110°C and 230°C. The optimum concentrations of the dopants are found to be 200 ppm each. Its gamma sensitivity is 1.2 times as compared to CaSO4:Dy (0.1 mol%). The optimum preirradiation annealing treatment is found to be 650°C, 15 min. Its PL emission shows a band at 370 nm with excitation band at 250 nm. Dose to TSL response shows that its response is linear up to the gamma dose of 100 Gy for irradiations carried out at RT. An irradiated sample shows a distinct new ESR signal, which is tentatively assigned to an electron/hole localised on one of the oxygen(s) of the phosphate group. Step annealing experiments show decay of the defect centre around 340°C. This correlates well with the TSL peak around 360°C.

Thermally stimulated luminescent (TSL) materials are usually divided into two main groups: tissue-equivalent materials with relatively low sensitivity to ionising radiations and highly sensitive materials but with high effective atomic number and consequently poor equivalence to tissue (1). In recent years, many tissue-equivalent, highly sensitive phosphors have been developed, LiF:Mg,Cu,P being one of them. But this phosphor suffers from severe anneal–readout temperature restrictions. More recently, Li3PO4:Mg,Cu TSL phosphor has been developed to study the ‘micro-phase’ in LiF:Mg.Cu,P (2). This phosphor has a low effective atomic number (Zeff = 10.6), comparable TSL sensitivity with that of CaSO4:Dy, and its TSL sensitivity is not affected by normal annealing or readout procedures (3). Investigations using the electron spin resonance (ESR) technique were carried out in order to study correlation between the defect centres and TSL glow peaks. MATERIALS AND METHODS The phosphor was prepared by precipitation from an aqueous solution of lithium hydroxide (LiOH) by H3PO4 acid. The desired impurities were added to the aqueous solution of LiOH before adding H3PO4 acid. The precipitate was thoroughly washed and dried. It was then heated with 2 wt.% NH4Cl flux at 400°C for 1 h. This technique was used previously to dope Cu+ in alkali fluorides (4). The sample was then heated at 860°C for 1 h in air and then quenched to room temperature. The Li3PO4 phase thus formed was confirmed by XRD.

For gamma irradiation, a gamma chamber containing a 60Co source was used. A locally made TSL reader with an EMI 6255S PM tube having S11 response was used for all TSL measurements. The heating rate was 300°C.min−1. The PL spectrum was recorded on a FP-750 spectrofluorometer (Jasco). ESR measurements were made using a Varian E-112 E-line Century series ESR spectrometer that utilises 100 kHz field modulation. Step heat treatments were performed to follow the decay and evolution of defect centres. These were carried out in situ in the ESR cavity using the Varian variable temperature accessory. RESULTS AND DISCUSSION Figure 1 shows a typical TSL glow curve for 2 TSL intensity (arb. units)

INTRODUCTION

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Figure 1. TSL glow curve of Li3PO4:Mg,Cu (solid line) and background signal for an unirradiated phosphor (dotted line). Test gamme dose was 1 Gy.

Contact author E-mail: bcbhatt1얀vsnl.net 251

B. C. BHATT, B. S. DHABEKAR, S. S. SANAYE, S. S. SHINDE, S. V. MOHARIL and T. K. GUNDU RAO

Li3PO4:Mg,Cu phosphor at a test gamma dose of 1 Gy. The main TSL glow peak occurs at about 360°C with minor peaks at 110°C and 230°C. The sensitivity of the phosphor is 1.2 times that of CaSO4:Dy (0.10 mol%) when heights of the main peaks of the phosphors are compared. The effect of pre-irradiation annealing on sensitivity of the phosphor was studied by annealing the samples at various temperatures and then exposing them to gamma radiation. The study shows the optimum treatment to be 650°C, 15 min (Figure 2). The dose against TSL response shows that its response is linear up to the gamma dose of 100 Gy for irradiations carried out at RT (Figure 3). The dose–TSL response studied at 200°C shows no loss in the sensitivity of the phosphor and is identical to the corresponding dose response curve studied for irradiations carried out at RT; the response is linear up to the studied gamma dose of 10 Gy. The minimum detectable dose was calculated from the standard deviation of background reading of the

unexposed sample using the indigenously developed research reader. The dose value equivalent to 3 times the standard deviation of the background variation from an unirradiated sample was taken as a minimum measurable dose. It was found to be about 0.13 mGy for a readout cycle going up to 425°C. The minimum measurable dose of CaSO4:Dy (0.1mol%) was found to be 0.014 mGy for a readout cycle going up to 280°C. The relatively higher value of minimum detectable dose for Li3PO4:Mg,Cu may be due to a higher contribution of infrared signal during these measurements. The PL emission spectrum of Li3PO4:Mg,Cu shows an emission band at 370 nm when excited by 250 nm excitation wavelength (Figure 4). Variation of intensity of this band with Cu concentration shows concentration quenching of the 370 nm band beyond 200 ppm of Cu concentration. Variation of the 370 nm band intensity correlates well with the TSL intensity (Figure 5). The ESR spectrum of unexposed Li3PO4:Mg,Cu with optimum concentration of the dopants at room temperature displays a spectrum originating from a Cu2+ ion. The line width of the ESR line is about 800 gauss and 600

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Figure 2. Variation of TSL intensity with pre-irradiation annealing temperature.

Figure 4. PL spectra of Li3PO4:Mg,Cu TSL phosphor.

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Figure 5. Variation of gamma sensitivity and fluorescence intensity with Cu concentration. 252

PREPARATION AND CHARACTERISATION OF Li3PO4:Mg,Cu PHOSPHOR

the principal g-value is found to be 2.085. The intensity of the Cu2+ ESR signal does not show any variation after gamma irradiation: thus, Cu may not have any role in the trapping of charge carriers. The ESR spectrum of defect centres after gamma irradiation (1 kGy) in Li3PO4:Mg,Cu recorded at room temperature is shown in Figure 6. The scan range has been selected to record only the lines in the vicinity of free-electron resonance. A single, relatively broad ESR line is observed at room temperature with a principal gvalue of 2.0028 and a line width of 10 gauss. The centre has been tentatively assigned to a species wherein the electron/hole is trapped at the phosphate group. The absence of phosphorus hyperfine splitting (31P with 100% abundance, spin = 1/2) leads to speculation that the trapped electron/hole is localised on the oxygen(s) of the phosphate group. The stability of the centre was studied using the step annealing technique. The thermal annealing behaviour shows that the centre decays around 340°C. This decay correlates with the TSL peak at about 360°C (Figure 7). 2mT H

CONCLUSIONS (1) Li3PO4:Mg,Cu is a low effective atomic number and high sensitivity TSL phosphor. Its dose against TSL response is linear up to a dose of 100 Gy. (2) Irradiation carried out at 200°C did not result in any change in TSL sensitivity of the phosphor when compared with the corresponding samples irradiated at RT. Thus, this phosphor can be used in dosimetry at elevated temperatures, e.g. in a reactor environment. (3) The relatively higher value of minimum detectable dose (0.13 mGy) may be due to a higher contribution of IR noise during TSL measurements as the main TSL peak occurs at about 360°C. (4) The variation of 370 nm band intensity correlates well with the TSL intensity. (5) The Cu ion in the Li3PO4:Mg,Cu system does not appear to play any role in trapping of charge carriers. (6) An irradiated sample shows a distinct ESR signal which is assigned to an electron/hole localised on the oxygen(s) of the phosphate group in the Li3PO4:Mg,Cu system. This centre correlates with the TSL peak at about 360°C.

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Figure 7. Variation of intensity of ESR signal of defect centre in Li3PO4:Mg,Cu with step annealing temperature.

Figure 6. Powder ESR spectrum of gamma-irradiated Li3PO4:Mg,Cu phosphor recorded at room temperature.

REFERENCES 1. Azorin, J. Luminescence Dosimetry, Theory and Applications (Mexico, D.F.: Technico-Cientificians) (1990). 2. Naranje, S. M. and Moharil, S. V. Thermoluminescence in Lithium Phosphates, Phys. Status Solidi a 165, 489–494 (1998). 3. Sanaye, S. S., Shirva, V. K., Bhatt, B. C., Shinde, S. S., Naranje, S. M. and Moharil, S. V. Characterisation of New Li3PO4:Mg,Cu TL Phosphor. Proc. National Conf. on Luminescence and its Applications, Raipur. pp. 219–222 (1997). 4. Patil, R. R. and Moharil, S. V. Photoluminescence of Cu+ in Alkali Halides. J. Lumin. 63, 339–344 (1995).

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