The short-period time variation of the luminescence of a zinc sulphide phosphor under ultra-violet excitation

A photographic method, in some respects new, is described for measuring in detail the luminescent-intensity time relationships of a powdered phosphor. A zinc sulphide with silver and copper impurities was irradiated at 20, 40, 60 and 80° c. with various intensities of λ3650: the results are mainly on the blue phosphorescent intensity over some 300 milliseconds from the end of irradiation. The observations are found to correspond quantitatively to a bimolecular law with two types of activating centre. On this assumption four independent constants are deduced mathematically for each curve, two per centre. They are interpreted in terms of the initial concentrations of ionized centres, and the recombination coefficient for each kind of centre with free electrons. Using these constants, the ratio of the contributions from the two kinds of centre to the luminescent intensity at the beginning of the decay was deduced. From this ratio and the spectral distribution of the fluorescent emission, the centre with the faster decay was identified as silver and the other as interstitial zinc. The very slowly decaying green band present in the luminescent emission is ascribed to the presence of copper centres. The constants are found to change with temperature and with exciting intensity in a way which cannot be explained on the simple theory. These variations can, however, be accounted for by assuming that there are trapping centres closely associated with each "excited" activating centre. It is shown that the traps associated with the copper centres are of major importance, although the concentration of copper impurity is relatively small. As functions of exciting intensity the constants also have periodic terms superimposed on the main changes. An attempt is made to relate this qualitatively to the equations for the build-up of luminescence when more than one type of activating centre is present. The effect of the thickness of the layer of powder on the simplest type of two-centre bimolecular decay law and the relation, in outline, of our interpretation to the work of other observers are also considered.