Scintillation Response of Activated Inorganic Crystals to Various Charged Particles

Abstract

Experimental studies of the response of activated ionic crystals such as NaI(Tl) and CsI(Tl) to heavy charged particles indicate decreasing scintillation efficiency with increasing particle mass, and a nonlinearity in pulse height versus energy for heavier particles. Recent experiments indicate that the scintillation efficiency to electrons, however, is less than that to protons. In an attempt to account for these effects, this paper presents a calculation based on a model of the process of energy transfer from the incoming particle to the activator sites. In this model, the energy carriers are taken to be excitons resulting from recombination of electronhole pairs in the wake of the particle. The migration of carriers to activator sites is described by a one-velocity diffusion equation in which the density of unoccupied activator sites, $N_a$, is a function of space and time. The diffusion equation is coupled with a second differential equation describing the time dependence of $N_a$. The solution to these equations indicates that the depletion of available activator sites by a particle with high $\frac{\mathrm{dE}}{\mathrm{dx}}$ can account for observed saturation effects. This model further contains the activator concentration as a parameter, and permits a prediction of scintillation efficiency as a function of both $\frac{\mathrm{dE}}{\mathrm{dx}}$ and concentration. The low scintillation efficiency to electrons is predicted as a consequence of the smaller recombination probability for particles of very low $\frac{\mathrm{dE}}{\mathrm{dx}}$. Finally, for a low-$\frac{\mathrm{dE}}{\mathrm{dx}}$ particle in a crystal of 0.1-mole-percent activator concentration the diffusion length of energy carriers is found to be of order 20 A.