Study of pure NaI at room and liquid nitrogen temperatures

Three pure NaI crystals grown in different batches by Scionix (The Netherlands) were studied at room and liquid nitrogen temperatures. The room temperature studies performed with photomultiplier readout covered measurements of emission spectra, photoelectrons yield and energy resolution. Moreover, the scintillation light pulses were measured by single photon method. Two crystals showed the photoelectron yield of about 1000 phe/MeV measured at room temperature with an XP2020Q photomultiplier. The energy resolution of the 662 keV photopeak was measured to be about 16% and was limited mainly by a low number of photoelectrons and a large nonproportionality of the light yield. The fast light pulse had the main decay time components of 1 and 5 ns. The high initial photon intensity of the tested NaI crystals revealed a good timing resolution of about 140ps obtained for ⁶⁰Co γ-rays. At liquid nitrogen temperatures the measurements of light yield and energy resolution were performed using an Advanced Photonix, Inc. avalanche photodiode readout. A high light output above 80000 ph/MeV was measured for the best of the tested crystals. The energy resolutions of the 662 keV photopeak of the three crystals differ a lot, varying from an excellent value of 3.8% to 6.2%. This suggests that the measured energy resolutions depend on the purity of the scintillator samples reflected in the emission spectra. The studies that were performed demonstrate the attractive properties of pure NaI at both room and LN₂ temperature.