Excitonic and activator recombination channels in binary halide scintillation crystals

The aim of this paper is to study the efficiency of excitonic and activator recombination channels in activated alkali halides depending on the activator concentration. Both theoretical considerations and experimental verification of competition of these channels were carried out. For a model system a CsI-based scintillator was chosen. It is shown that at low temperature (20K) the excitonic luminescence significantly drops in a hyperbolic law with increase of activator (Tl or In) emission. A corresponding increase of the activator-related recombination channel at 20K was found to be only 20% of the excitonic yield of pure crystals, reaching a saturation at about 0.04mol% of the activator. The energy loss observed is attributed to trapping of electron excitations at different centers. This is connected with the competition of electron-hole recombination and trapping of electrons on activators. Estimation of recombination efficiency was done based on the generalized Onsager model. Activator-induced screening, broad spatial distribution of thermalized electrons (mean thermalization length in CsI is about 180nm) and strongly inhomogeneous electron track structure were taken into account in the model. Energy transfer from the excitons to the activator was suggested to be the main cause of the activator emission observed in doped CsI crystals at 20K.