Dependence of the NaI(Tl) Pulse Shape on Thallium Concentration and Temperature

The pulse shape of γ-induced photopeak scintillations in NaI(Tl) has been measured as a function of temperature for several standard commercial crystals and for crystals containing known thallium concentrations. A detailed analysis of the data indicates that the first two microseconds of the pulse is characterized by at least nine separate decay processes. Four of these are attributed to one radiative and three metastable states associated with a thallous ion. Two fast processes are attributed to emission characteristic of pure NaI while two others are associated with high thallium concentration. Energy transport is suggested as the ninth process.