THERMOLUMINESCENCE OF PURE AND IMPURITY-DOPED KBR AND NACI CRYSTALS

Two thermoluminescence peaks are observed at 90 and 150°C in highly pure KBr crystals (background divalent cation impurity not greater than 1 p.p.m.). From the growth of F centres with the time of x-irradiation these two peaks have been interpreted in terms of the two types of F centres postulated previously. The presence of Cd impurity in KBr suppresses the formation of both the types of F centres while the presence of Zn impurity enhances the first-stage F centres but suppresses the second-stage F centres. Optical absorption studies of the KBr crystals doped with Cd and Zn are correlated with the thermoluminescence data. Plastic deformation of highly pure KBr crystals considerably increases the area under both the thermoluminescence peaks. No additional glow peak is observed either in plastically deformed or Cd- and Zn-doped KBr crystals. Thermoluminesence experiments performed with highly pure NaCl crystals (background divalent cation impurity not greater than 1 p.p.m.) also show two peaks at 170 and 210°C. However, in analar purity NaCl crystals (background divalent cation impurity about 20 p.p.m.) or in NaCl crystals containing Ca and Cd, an additional thermoluminescence peak at 80°C is also observed. The activation energies of both the types of F centres in KBr and NaCl and of the impurity centres in NaCl are evaluated from the thermoluminescence results using the 'initial rise method'.