A Comparative Study of Undoped NaI Scintillators With Different Purity

The performance of undoped NaI crystal of a high purity of 7 N grade, developed by Saint-Gobain Crystals for a dark matter search, was studied at liquid nitrogen temperature using an avalanche photodiode. The measured quantities covered light output expressed in the electron-hole (e-h) pair number, non-proportionality characteristics, energy resolution and finally intrinsic resolution of the studied crystals. In contrast to the previous study, the new crystals showed comparable non-proportionality curves to that known for NaI(Tl) at room temperature and a poor energy resolution of about 8% for 662 keV gamma rays from a ¹³⁷Cs source. The performed study highlights a role of undoped NaI crystals in a better understanding of limitations of energy resolution in scintillation detectors. A high sensitivity to traces of doping may help to find the way of a modification of non-proportionality of other scintillators by a selective doping or co-doping. A further study of slow components of NaI light pulses and their influence on the energy resolution may clear up its possible deterioration caused by defects in the crystal structures.