Scintillation Process in CsI(Tl). II. Emission Spectra and the Possible Role of Self-Trapped Holes

The luminescence emission spectra of CsI(Tl) crystals containing varying amounts of Tl have been measured upon excitation by gamma rays, protons, and alphas. The principal features include a broad Tl emission band centered near 5500 Å and a narrower band centered near 3300 Å which is characteristic of the host crystal. This 3300-Å band is analogous to near-ultraviolet emission bands observed in NaI and KI. It is found for CsI(Tl) that the intensity of the 3300-Å band relative to the Tl band is greatest upon excitation by radiation of high $\frac{\mathrm{dE}}{\mathrm{dx}}$. It is suggested that the 3300-Å band arises from the recombination of free electrons with self-trapped holes. Evidence from other investigations supporting this hypothesis is summarized.