  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.
