Effect of the Activator Impurity on the Scintillation Yield in Alkali-Halide Crystals

The influence of different activator impurities on the scintillation yield of alkali halides has been investigated as a function of temperature. Luminescence spectra of pure and activated CsI and NaI scintillation crystals were measured under X-ray and VUV excitation at temperatures from 10 to 300 K. In indium- and thallium-doped crystals activator centers can capture electrons. Along with self-trapping of holes at low temperatures, electron capture by the dopant results in energy storage. This leads to a significant decrease of luminescence yield. In Eu-doped NaI and CsI crystals activator centers capture a hole first. In this way, at low temperatures electrons recombine either with self-trapped holes (yielding STE emission), or with holes trapped by the activator (giving rise to Eu emission band). No energy loss at low temperature is evident in CsI:Eu and NaI:Eu crystals