The nature of luminescence centers in NaI:Eu single crystals

The nature of luminescence centers in NaI:Eu scintillation crystals was studied in a wide excitation energy and temperature ranges. Radio- and photoluminescence spectra and TSL curves were measured for crystals with varied activator concentrations. Two main narrow luminescence bands at 2.8 eV (center I) and 2.6 eV (center II) were found at 10 К. Center I dominates for low Eu concentration (<0.07%). Increase of activator content leads to suppression of center I emission and enhancement of center II. Presence of oxygen containing impurities in heavily doped crystals is due to high hygroscopicity of the NaI matrix and EuI2 dopant. Analysis of emission and thermoluminescence features allows justifying the role of anion impurities (oxygen/hydroxyl ions) in formation of activator center II, which is responsible for scintillation efficiency of NaI:Eu crystals at room temperature.