A Study on 3-in- and 10-in-Diameter Photomultiplier Tubes for the KM3NeT Project

Photomultiplier tubes (PMTs) are widely used in neutrino and astroparticle detectors, inserted into transparent pressure-resistant glass vessels called optical modules (OMs). They compose of an array of photo sensors located in the underwater or under the ice environment to measure the Cherenkov light emitted by charged particles produced by neutrino interactions. Currently, there are two main designs for the OMs used in the operating neutrino telescopes: a design with a single large photomultiplier, typically with a photocathode diameter of 10 in, contained in a pressure resistant glass vessel of 17-in or 13-in diameter, and an innovative design of a pressure-resistant 17-in-diameter glass vessels that hosts 31 PMTs of 3-in photocathode diameter. The KM3NeT collaboration is building a cubic-kilometer-sized neutrino telescope in the depths of the Mediterranean Sea, and OMs of both types are produced to compose a string-like underwater detection units, which use small-area PMTs, and tower-like detection units, which use the single large area photomultiplier design. The main characteristics of two batches of 3-in and 10-in photomultipliers were investigated and the main results are reported in this paper.