Temperature dependences of LaBr3(Ce), LaCl3(Ce) and NaI(Tl) scintillators

The temperature dependence of light output, energy resolution and decay time constants of the light pulses of NaI(Tl), LaCl3(Ce) (LaCl3) and LaBr3(Ce) (LaBr3) crystals were measured over the temperature range of −30 to 60 °C. In the study of the light output, the number of photoelectrons produced by the scintillators in the XP2020 photomultiplier was measured and corrected for by the temperature dependence of the quantum efficiency determined for 360 and 420 nm, respectively. It showed a high stability of the light output of LaBr3 of about 0.01%/°C and a comparable uniformity of LaCl3 at a long peaking time of 12 μs. The well-known thermal instability of NaI(Tl) was confirmed at a short peaking time of 2 μs. However, a much better stability of NaI(Tl) at low temperatures was observed for a long peaking time. The study of the decay of light pulses from LaCl3 and LaBr3 crystals confirmed earlier measurements, while NaI(Tl) showed a complex behavior at different temperatures. At low temperatures a strong contribution of a slow component of up to 60% of the total light was observed, while at elevated temperatures a well-known initial slow decay was replaced by a delayed maximum and the slow component became insignificant. The results of the study of energy resolution seem to be correlated with the variation of both the light output and a dependence of the decay time constants of the light pulses at changing temperature. This is particularly interesting in the case of NaI(Tl), where different dependencies of the energy resolution as a function of temperature for different peaking times in the spectroscopy amplifier were found. Tests of the XP2020 PMT itself showed that the thermal instability of the gain of the dynode structure of about−0.4%/°C is a dominating effect. The opposite effect on an increasing quantum efficiency, partly compensating for the gain instability, was observed above 10 °C for the longer wavelength of 420 nm.