Phosphorescence and conduction current relaxation in ZnSe crystals

Abstract

In scintillation crystals, at X-ray or gamma quanta registration, a noticeable time delay of the pulse is observed. This is due to the thermal delocalization of charge carriers from traps. The study of phosphorescence and relaxation of the conduction current can give information on these processes. However, their investigation requires knowing theoretical dependences of phosphorescence intensity attenuation and conduction current relaxation. A multicenter model of a crystal is proposed in which at least three types of traps (shallow, phosphorescent and deep) take part in the delocalization of charge carriers at the excitation temperature. For such a model, theoretical dependences for the phosphorescence intensity and the conduction current relaxation are obtained. These dependences take into account the processes of re-trapping of free charge carriers on more shallow traps. They show that the level of filling of different traps depends not only on the total radiation dose, but also on the intensity of excitation. An explanation of an experimental fact of intersection of phosphorescence and conduction current relaxation curves at the same irradiation dose, but at different excitation intensities, contradicting classical kinetic theory of luminescence, is obtained. All experimental dependences of phosphorescence intensity and conduction current relaxation for ZnSe crystals at different temperatures are well described by the obtained theoretical dependences. It is also established that the time of emptying of each type of traps is determined not only by the probability of thermal delocalization, but also by the ratio of the concentration of these traps to the total concentration of deeper traps.