Calibration Facility for Detector Strings for the KM3NeT/ARCA Neutrino Telescope at the Capacity Laboratory

KM3NeT is a network of submarine Cherenkov neutrino telescopes under construction at two different sites in the Mediterranean Sea. Astroparticle Research with Cosmics in the Abyss (ARCA), near Sicily, Italy, is optimized for the detection of cosmic neutrinos, while Oscillation Research with Cosmics in the Abyss (ORCA), near Toulon, France, is optimized for atmospheric neutrinos. ARCA and ORCA are both arrays of thousands of optical sensors [digital optical modules (DOMs)], each consisting of 31 small photomultipliers (PMTs) housed inside a glass sphere, which detect the Cherenkov light produced by the secondary particles generated in the neutrino interactions. Eighteen DOMs are arranged on flexible strings, referred to as vertical detection units (DUs), anchored to the sea floor. Once completed, ARCA and ORCA will consist of 230 and 115 DUs, respectively. Each DOM of a string communicates at a dedicated wavelength to the shore station via a network of optical fibers transmitting optical and acoustic signal information as well as orientation information. Before the deployment, each DU is tested and calibrated in a dark room. The test bench is equipped with a full data acquisition (DAQ) system for communication, data processing, and time synchronization. Several steps are needed to accomplish the DU calibration, including the high-voltage (HV) tuning of the PMTs, checking of the acoustic receivers and calibration light sources in the DOM, and time calibration using laser signals distributed to all DOMs. Here, we describe the DU test and calibration facility at the Campania AstroPArtiCle InfrastrucTure facilitY (CAPACITY) Laboratory, Caserta, focusing on the functional tests and calibrations performed at the end of the DU integration.