Effect of anion interstitials on the thermoluminescent properties of CaSO4:Dy

Abstract

Efforts were made to synthesise CaSO4 based thermoluminescent (TL) phosphors by solid state reaction route. ZnSO4 flux was found to be quite efficient in the incorporation of Dy into CaSO4 lattice as witnessed from the high TL intensity peak at about 100°C following sintering at 750°C in air. Its TL intensity is even higher than that of the 260°C peak appearing in recrystallised CaSO4:Dy. Similar low temperature TL peak was found in recrystallised CaSO4:Dy samples sintered at 1000°C in air. The results were explained on the basis of incorporation of interstitial oxygen anions which act as hole traps. Firing in sulphur or ammonium sulphate atmosphere did not shift the low temperature TL glow peak. But firing in carbon atmosphere at 750°C with ZnSO4 flux enhanced the intensity of TL glow peaks at 250°C and at 400°C due to the partial re-conversion of oxygen ions to sulphate ions. Firing at 850°C in reduced atmosphere, however, quenched the intensity of all TL peaks due to the removal of oxygen i.e. partial reduction of CaSO4 to CaS. The slow decline in the intensity of 250° TL peak on prolonged annealing at 400°C in recrystallised CaSO4:Dy indicates the thermal migration of defects causing the TL peak. Redox mechanism involving such interstitial ions and anion vacancies in the presence or absence of cation vacancies could lead to the emission of high (~250°C) or low (~100°C) temperature TL peaks, respectively. Certain other co-dopants tried such as Al3+ and SiO44− simply quenched the TL efficiencies of CaSO4:Dy and CaSO4:Mn, respectively. Among the monovalent sulphates tried, Na2SO4:Dy gave a TL peak around 100°C which is 20% of the 250°C TL intensity of recrystallised CaSO4:Dy.