Performance of photomultiplier tubes for cryogenic applications

The use of noble liquid detectors draws a great interest in particle physics research, especially for direct searches of Dark Matter. This kind of experiments requires to go to several tons of noble liquid in order to explore deeper regions of the parameter space. The signal produced in large tons of liquid is usually read out by photomultipliers.In this paper we report on results of tests performed on 2 in. and 3 in. photomultiplier tubes developed by Electron Tubes Ltd. for operation at cryogenic temperatures in the WArP experiment. The measurements have been performed at the Photomultipliers Test Facility set-up in Naples laboratory. The Photomultipliers Test Facility cryogenic equipment uses fused silica fibers driven LED/LASER generated light in the 400 nm region.The photomultipliers have been tested in liquid nitrogen (77 K). The behaviour of photomultipliers at cryogenic temperature has been studied on more than 350 samples. All the photomultipliers show typical gain behaviour when immersed in liquid nitrogen. Generally, this gain exponentially decreases to asymptotic values, some 20–30% lower. Charge resolution and signal-to-noise ratio show a long-term stability in cryogenic environment.