Noise Pulses in Large Area Optical Modules

Large area photomultipliers are widely used in neutrino and astro-particle detectors to measure Cherenkov light in media like water or ice. The key element of these detectors are the so-called "optical module", which consists of a photodetector enclosed in a transparent pressure-resistant container to protect it and ensure good light transmission. Noise pulses at the anode of each photomultiplier strongly affect the performance of the detector. A large study was conducted on noise pulses of large area photomultipliers, considering time and charge distributions of dark pulses, pre-pulses, delayed pulses, and after pulses. The contribution to noise pulses due to the presence of the external glass vessels was also studied, even comparing two vessels of different brands.