DarkSide-20k Veto Photon-Detector Units: construction and characterization

Abstract

DarkSide-20k is a global direct dark matter search experiment situated underground at LNGS (Italy), designed to reach a total exposure of 200 tonne-years nearly free from instrumental backgrounds. The core of the detector is a dual-phase Time Projection Chamber (TPC) filled with 50 tonne of low-radioactivity liquid argon.The entire TPC wall is surrounded by a gadolinium-loaded polymethylmethacrylate (Gd-PMMA), which acts as a neutron veto, immersed in a second low-radioactivity liquid argon bath enclosed in a stainless steel vessel. The neutron veto is equipped with large-area Silicon PhotoMultiplier (SiPM) array detectors, placed on the outside of the TPC wall. SiPMs are arranged in a compact design meant to minimize the material used for PCBs, cables and connectors: the so-called Veto Photon-Detector Units (vPDUs).A vPDU comprises 16 vTiles, each containing 24 SIPMs, together with front-end electronics, and amotherboard, which distributes voltage and control signals, sums vTiles channels, and drives theelectrical signal transmission. The neutron veto will be equipped with 120 vPDUs. The paper willfocus on the production of the first vPDUs, describing the assembly chain in the U.K. institutes, inorder to underline the rigorous QA/QC procedures, up to the final characterization of the firstcompleted prototypes. Tests will be extensively performed in liquid nitrogen baths either for thesingle vTiles and for the assembled vPDUs, with the purpose of assigning a “quality passport” toeach component.