Luminescence of Silver Bromoiodide Crystals

Abstract

Crystals of simple and mixed silver halides are known to show a strong fluorescence but little afterglow at low temperatures. However, the slow buildup of luminescence under weak excitation indicates the presence of effective trapping processes. These processes have been studied in more detail on silver bromoiodide crystals. The lifetimes of the trapping states were estimated by observing the dependence of buildup rates on the dark interval between two exposures. The trapping states are also revealed by the effects of infrared radiation. If applied during excitation, it reduces the stationary brightness; if it is applied in the interval between exposures, it reduces the rate of re-excitation. The pattern of behavior of these phosphors is interpreted by a model of the type generally used in the description of zinc sulfide and similar photoconducting phosphors. If the absence of afterglow is taken into account, the model requires that the efficiency of the phosphor should increase with the square root of the exciting intensity. This has been confirmed experimentally over a range of 3 powers of 10. An afterglow can be produced in the silver bromoiodide by incorporating certain divalent cations (such as ${\mathrm{Cd}}^{++}$ or ${\mathrm{Sr}}^{++}$) into the crystal. From an extension of the model to this more complex system, a number of emission and stimulation effects can be deduced which have been found in a series of qualitative observations.