STUDIES OF THE SCINTILLATION PROCESS IN CsI(Tl)

Abstract

The scintillation response of CsI(Tl) crystals, having various thallium contents, was measured for excitation of the crystals by monoenergetic gamma rays, protons, and alpha particles. The investi gation was made to provide a test of some of the features of a theoretical model of the scintillation process in thallium-activated alkali iodides proposed by Murray and Meyer. In order to insure that the results obtained in this program would provide a critical test of the scintillation model, special attention was paid to technical effects which could influence the interpretation of the experimental data. For example, the effect of the pulseanalysis time on the relative scintillation response of CsI(Tl) to various charged parti cles was investigated. In addition, the emission spectra of the CsI(Tl) crystals were measured for excitation by x rays, protons, and alpha particles. The results of the investigation showed that the scintillation efficiency of CsI(Tl) is a continuous function of dE/dx, as assumed in the scintillation model, within the accuracy of the experiments when the effect of delta ravs are considered. This led to the conclusion that the light output of CsI(Tl) crystals is, in general, a nonlinear function of the energy of the particle. The shape of the dL/dE versus dE/dx curve for CsI(Tl) crystals is shown to be nearly independent of the thallium content of the crystal. This is in distinct contrast to the predictions of the scinti llation model. An examination of the emission spectra of CsI(Tl) crystals showed that light is emitted in a band, centered at about 3300A, which is characteristic of the emission of the pure crystal, as well as the characteriatic thallium luminescence. The relative intensity of this luminescence with respect to the thallium- activator luminescence increases as the ionization density in the crystal increases and increases as the thallium content decreases. It is suggested that the emission band characteristic of pure CsI crystals is associated with the decay of an iodine molecule ion of the type I/sub 2/ which has trapped an electron. On this basis it is suggested that thallium-activator luminescence results from the successive capture of an electron and a hole at a thallium center. The 3300A emission band of CsI overlaps the optical-excitation spectrum of CsI(Tl) for thallium-activator luminescence. Some thallium centers in CsI(Tl) are therefore excited by optical emission in the 3300A band. (auth)