The luminescence of thallium-activated potassium chloride phosphors

Abstract

This paper describes and discusses new experimental investigations of the excitation, absorption and emission spectra, phosphorescent decay curve characteristics, thermoluminescence and photoconductivity of potassium chloride phosphors activated by thallium and of the thermoluminescence of potassium chloride phosphors activated by barium, by strontium or by thermal or mechanical strains. It is concluded that for concentrations of more than about 0·001 mol. Tl the KCl:TlCl phosphors contain a number of different centres. Reasons are given for suggesting that some of these may be: (1) Tl + ions at substitutional sites and not near to other Tl + ions at neighbouring positive ion sites, (2) TlCl n complexes at special sites near defects or along dislocations, (3) lattice distortions produced by the incorporation of Tl + or other foreign ions. Since the main emission peaks appearing in the thermoluminescence of KCl: Tl at 200 and 300 °K also occur in the KCl phosphors activated by barium, by strontium or by thermal or mechanical strain, it would appear that the electron traps responsible for the phosphorescence of KCl:Tl cannot be, as Williams suggests, metastable levels in Tl ions. It is suggested that they are at lattice defects created either by Tl or other foreign ions or by thermal or mechanical strains. The complexity of the absorption, emission and excitation spectra suggest that the existing theories (such as those of Williams and his co-workers and of Knox & Dexter) of the luminescence of KCl: Tl require further development.