Thermoluminescence and optical absorption studies of Z1-centres in NaCl crystals doped with terbium

The formation of Z1-centres in NaCl:Tb single crystals as well as crystals quenched from 550 0C is studied with the help of optical absorption and thermoluminescence (TL) measurements. The glow curve of NaCl:Tb shows four glow peaks around 60, 90, 140, and 190 0C (T1, T2, T3, and T4, respectively). Bleaching of X-irradiated crystals with F-light shows that the T3-peak is enhanced whereas the T1- and T4-peaks get suppressed. Optical absorption studies of the crystal before and after F-bleaching show that the F-band decreases and the decrease is accompanied by the broadening towards the longer wavelengths. Annealing studies of X-irradiated crystals at room temperature show that the T1- and T2-peaks correspond to shallow traps. The T3-peak is attributed to Z1-centres and the T4-peak to F-centres. It is concluded that Kleefstra's model for the Z1-centre is a more probable model. (author)