Scintillation properties of CsI:Na thin films from viewpoint of nanoparticle formation

We have investigated scintillation properties of Na-doped CsI (CsI:Na) thin films grown by vacuum deposition. The Na-related luminescence, which is the dominant scintillation of CsI:Na, is activated by heat treatment and associated with formation of NaI nanoparticles. It is found that the Na-related luminescence, which is remarkably degraded in atmosphere, the so-called hygroscopic property, is reversibly recovered in vacuum, while the luminescence due to the self-trapped exciton of CsI hardly decreases in ambient conditions. This finding indicates that the hygroscopic property of CsI:Na results from degradation of NaI nanoparticles embedded in the CsI:Na thin film and that the recovery in vacuum is due to recrystallization. This is a novel proposal to explain the high hygroscopicity of CsI:Na. Furthermore, it is demonstrated from the temperature dependence of the luminescence properties that the energy transfer from the host material of CsI to the NaI nanoparticles is mediated through thermally activated hopping of the self-trapped exciton of CsI.