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It is known that thermal and/or mechanical pretreatment influences the luminescent behaviour of phosphors. It has been reported that mechanically pretreated NaCl : Tl specimens, exposed to ultraviolet radiation, exhibit a pronounced glow peak at 420 K. The present work concerns the study of the intensity of this peak as a function of impurity concentration. There is a remarkable growth in the intensity of the 420 K glow peak with decreasing concentration. This can be explained on the basis of the interaction between impurity ions and moving dislocations.

The phosphorescence decay rates of thallium-doped ammonium chloride (NH4Cl:Tl) phosphors, prepared by crystallization from aqueous solution, have been studied at room temperature for near-ultraviolet emission. The effects of impurity concentration as well as thermal and/or mechanical treatment on the decay rates have been examined. Phosphorescence centres consisting of a Tl+ion and a nearby negative ion vacancy are suggested to be responsible for the observed luminescence decay. The changes in the decay characteristics after pretreatments are explained on the basis of the location of the centres in normal and distorted regions of the host lattice.