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 degrees C, 220 degrees C and 375 degrees C, in contrast to the two glow peaks observed in the CaSO4: Dy phosphor at 130 degrees C and 220 degrees 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 degrees C correlates with a Ag2+ centre formed owing to gamma-irradiation and observable below -170 degrees C. The Ag2+ centre is characterized by an axial g-tensor with principal values g(parallel to) = 2.38 and g(perpendicular to) = 2.41. ESR studies further indicate that the precursor to a centre observable at low temperature (-170 degrees C) appears to act as the recombination centre for the TL peak at 375 degrees C; this radical is characterized by the g-values g(parallel to) = 2.0023 and g(perpendicular to) = 2.0038 and is assigned to the SO3- 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 x 10(4) Gy, but the intensity of the SO3- radical continues to increase up to the studied dose of 1.2 x 10(5) Gy. The intensity of the SO3- radical decreases with Ag concentration in CaSO4: Dy,Ag, in agreement with the TL model proposed in this work