Response of parylene-coated NaI(Tl) scintillators at low temperature

Despite that it is widely used as a scintillator at room temperature, the hygroscopicity of NaI com- plicates its handling and limits its application for many purposes, for example as a cryogenic detector. To overcome this problem we study coating materials that can act as humidity barriers, in particular parylene, a polymer that can be deposited in very radiopure, thin and conformal layers. In this work, several NaI(Tl) samples coated with 2-5 µm parylene-C were tested at low temperature. Luminescence spectra under X-ray excitation are presented at several temperatures as well as the light output vs temperature at 1.5-300 K. Several thermoluminescence peaks were observed at around 60, 95 and 150 K during warm up to room temperature. The mechanical resistance of the coating under thermal cycles was also investigated, and we observed a degra- dation of the optical appearance and the light output after cooling down to about 100 mK, which compromises the reusability of the samples.