The liquid rare gases have excellent scintillation properties and there are increasingly good prospects for their use in radiation detectors having a wide range of applications in different fields, such as medical radionuclide imaging (Chepel, 1993; Chepel et al., 1994), high and medium energy particle physics (Akimov et al.,1993), astrophysics (Aprile et al.,1989). Despite the new photosensitive devices that have been developed in recent years, photomultipliers are still the photodetectors most widely used for the detection of the liquid rare gases scintillation light. To increase the efficiency of light detection it is very often mandatory to place the photomultiplier very near, or even inside, the detector, which means operating the PM at low temperature (the boiling point of xenon and krypton is -108C and -153C respectively). The manufacturers usually give -30°C as the lowest working temperature and very little information on the behaviour of photomultipliers at lower temperatures is available in literature (Ichige et al.,1993). Thus, it is necessary to study the performance of photomultipliers in the range of 7r: temperatures of interest in view of the liquid rare gases detectors.
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