S.N. Menon

Identification of defect centres using TSL, PL, OSL and ESR studies in LiAlO2 based phosphors

LiAlO2 : Mn/Ce/Tb phosphors were prepared by combustion synthesis. LiAlO2 : Mn exhibits two thermally stimulated luminescence (TSL) peaks around 150 and 320 °C, while LiAlO2 : Ce/Tb exhibits TSL peaks around 170 and 280 °C. Photoluminescence emission of LiAlO2 : Mn consists of a peak around 500 nm when excited with 450 nm, while LiAlO2 : Ce shows a broad band around 400 nm when excited by 320 nm. The TSL emission spectra of LiAlO2 : Mn/Tb indicate that Mn2+/Tb3+ is the luminescence centre. LiAlO2 : Ce and LiAlO2 : Tb exhibit optically stimulated luminescence when stimulated with 470 nm light.Electron spin resonance (ESR) studies were carried out to identify defect centres responsible for TSL peaks observed in LiAlO2 : Ce. Room temperature ESR spectra of irradiated LiAlO2 : Ce/Tb phosphor appear to be in a superposition of at least four distinct centres. Two defect centres (centre I and centre II) are identified in irradiated LiAlO2 : Ce/Tb phosphor and these centres are assigned to O− and F+ centres, respectively. O− centre appears to correlate with the 170 °C TSL peak and F+ centre appears to correlate with both 170 and 280 °C TSL peaks in LiAlO2 : Ce.

Beta response of LiMgPO4:Tb,B based OSL discs for personnel monitoring applications

Properties such as high optically stimulated luminescent (OSL) sensitivity, ease of preparation and dose linearity over nine decades (µGy-kGy) make LiMgPO4:Tb, B (LMP) a unique phosphor for dosimetry applications. This led to the investigation of the beta response of highly sensitive LMP-based Teflon-embedded OSL discs for personnel monitoring applications. A PTB beta secondary standard calibration setup (BSS2), which contains three beta sources viz. (147)Pm, (85)Kr and (90)Sr/(90)Y, was used. The relative response with respect to (137)Cs photons for 0.4-mm thick LMP discs was found to be ∼7.32, ∼53.5 and 100 % for (147)Pm, (85)Kr and (90)Sr/(90)Y beta energies, respectively. The response of LMP discs under various filter combinations viz. 0.18-mm thick mylar (25 mg cm(-2)), 0.625-mm thick Poly-allyl-diglycol carbonate (PADC, 81 mg cm(-2)), 1-mm thick polythene (95 mg cm(-2)), 1-mm thick Perspex (118 mg cm(-2)), 1.25-mm thick PADC (162 mg cm(-2)) and 1.6-mm thick (189 mg cm(-2)) Perspex filters was also studied and the ratio of the response of open disc to the response under filters (DOpen/DFilter) of different thicknesses (mg cm(-2)) was evaluated. Studies were also performed for the mixed field of low- ((85)Kr) and high-energy ((90)Sr/(90)Y) beta particles and the DOpen/DFilter ratio was evaluated. The angular dependence of the response of OSL discs to (85)Kr and (90)Sr/(90)Y beta sources was also studied. Studies were also carried out for (204)Tl, (32)P, natural uranium and (106)Ru/(106)Rh beta sources and the ratios of the response of open disc to that of under 1.6-mm thick Perspex (DOpen/DFilter) filter were measured. A study with various beta sources for the evaluation of the DOpen/DFilter ratio was necessary as these ratios are used to estimate the energy of beta particles and to apply the correction factor while evaluating the beta dose/design of dose estimation algorithms.

Electron spin resonance and thermoluminescence studies in CaSO4: Dy,Ag phosphor

The defect centres formed in the thermoluminescence (TL) phosphor CaSO4 : Dy,Ag are studied using the technique of electron spin resonance (ESR). The Ag co-doped phosphor exhibits three glow peaks around 130°C, 220°C and 375°C, in contrast to the two glow peaks observed in the CaSO4 : Dy phosphor at 130°C and 220°C, at gamma ray dose of 1 Gy when the TL measurements were carried out in the spectral region 300–650 nm. ESR studies show that the additional peak at 375°C correlates with a Ag2+ centre formed owing to γ-irradiation and observable below −170°C. The Ag2+ centre is characterized by an axial g-tensor with principal values g|| = 2.38 and g⊥ = 2.41. ESR studies further indicate that the precursor to a centre observable at low temperature (−170°C) appears to act as the recombination centre for the TL peak at 375°C; this radical is characterized by the g-values g|| = 2.0023 and g⊥ = 2.0038 and is assigned to the radical. It is observed that there is more incorporation of Ag in the CaSO4 : Dy system as compared with that in the pure CaSO4 system. The variation of Ag2 + ESR intensity with Ag concentration as well as with dose in the range 0.3–120 kGy is studied. The Ag2 + ESR signal and TL saturate at 2 × 104 Gy, but the intensity of the radical continues to increase up to the studied dose of 1.2 × 105 Gy. The intensity of the radical decreases with Ag concentration in CaSO4 : Dy,Ag, in agreement with the TL model proposed in this work.

ESR and TL mechanism in CaSO4?:?Ag co-doped phosphors

Thermoluminescence (TL) studies have been carried out on CaSO4?:?Mn, CaSO4?:?Ag,Mn, CaSO4?:?Ag,Nd, CaSO4?:?Ag,Mn,Nd, CaSO4?:?Ag,Zr and CaSO4?:?Ag,Mn,Zr phosphors. It is found that TL at 360??C in CaSO4?:?Ag,Mn,Nd is about 40 times that of CaSO4?:?Ag,Mn and about 170 times that of CaSO4?:?Ag,Nd. This shows that the presence of an efficient luminescent centre (Mn2+) as well as a trivalent impurity (such as Nd3+ or Y3+) is important for enhancement of TL at 360??C in the CaSO4?:?Ag,Mn,Nd phosphor system. Electron spin resonance (ESR) studies show that the peak at 360??C correlates with an Ag2+ centre formed due to ?-irradiation and observable only below ?170??C. The Ag2+ centre is characterized by an axial g-tensor with principal values g|| = 2.38 and g? = 2.41. ESR studies further indicate that the precursor to a centre observable at low temperature (?170??C) appears to act as the recombination centre for the TL peak at 360??C. The low temperature centre is characterized by the g-values g|| = 2.0023 and g? = 2.0038 and is assigned to the radical.

Environmental monitoring using LiMgPO4:Tb, B based optically stimulated luminescence dosimeter

A new optically stimulated luminescence based environmental dosimeter (EnOSLD) was developed using LiMgPO 4 :Tb, B phosphor (LMP). The dosimeters were deployed along with the conventional thermoluminescent based environmental thermoluminescent detectors (EnTLD) for a period of six quarters in the environs of a nuclear power plant in India. The dose estimated by the EnOSLDs was compared with that of the dose estimated by EnTLDs. The mean ratio of the doses measured by thermoluminescent detector to that measured by optically stimulated luminescence dosimeters was found to be 1.04 ± 0.11. The results show that LMP based OSLDs can be used as environmental dosimeter.