The KM3NeT Multi-PMT digital optical module

Abstract

The KM3NeT collaboration is currently constructing the first phase of a cubic kilometer-scale neutrino detector in the Mediterranean Sea. The basic detection element, the Digital Optical Module (DOM), houses 31 three-inch photomultiplier tubes (PMTs) inside a 432mm glass sphere. This multi-PMT concept increases the photocathode area by a factor of three compared to a design with a single 10 inch PMT, leading to a significant cost reduction. Moreover, this concept allows for an accurate measurement of the light intensity (photon counting) and offers directional information with an almost isotropic field of view. We will discuss these aspects and the enabling technologies, which include 3D-printed PMT support structures, and custom low-powered PMT bases, which provide the HV and digitization of the analog signal. An FPGA-based readout system transfers all sub-nanosecond timestamped photon signals to shore via optical fibres. The DOM design has been validated and its physics potential has been proven in the operation of prototypes deployed in the French and Italian detector sites at 2500m and 3500m depth respectively.