Defect chemistry of Eu dopants in NaI scintillators studied by atomically resolved force microscopy

Activator impurities and their distribution in the host lattice play a key role in scintillation phenomena. Here a combination of cross-sectional noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) was used to study the distribution of Eu2+ dopants in a NaI scintillator activated by 3% of EuI2. Two types of precipitate structures were found. First, needle-shaped EuI2 precipitates with a layered structure are likely responsible for scattering the scintillation light. In transparent crystals with good scintillation properties, precipitates with a cubic crystal structure and a size below 4 nm were found. A surprisingly low concentration of point defects was detected in all of the investigated samples. Upon annealing, Eu segregates towards the surface, which results in the formation of an ordered hexagonal overlayer with the EuI2 composition and a pronounced, unidirectional moire pattern.