Production and Quality Assurance of the Mu2e Calorimeter Silicon Photomultipliers

The Mu2e calorimeter consists of 1348 undoped CsI crystals coupled to two large area UV-extended Silicon Photomultipliers (SiPMs). A modular and custom SiPM layout, a 3x2 matrix of 6x6 mm(2) monolithic SiPMs, has been developed to satisfy the Mu2e requirements. As well as ensuring the performances needed for the muon-to-electron conversion search, these photosensors have to guarantee a good reliability while operating maintenance-free in the Mu2e hostile environment: any failure can only be replaced during a long technical shut-down scheduled once a year. After testing prototypes from different vendors, we selected Hamamatsu and the final production of about 4000 pieces is now ongoing. A detailed Quality Assurance (QA) program is then mandatory to minimize the risk of an unexpected further degradation in the performances. The QA process for each photosensor includes a first visual inspection and the subsequent characterization of each of its monolithic cells by means of an automatized test station, able to measure the breakdown voltage, the gain and the dark current. For each production batch (similar to 300 pieces), 5 devices are exposed to a neutron fluency up to similar to 1.4x10(11) 1 MeV (Si) eq. n/cm(2); others 15 devices are undergone an accelerated aging in order to verify the Mean Time To Failure (MTTF) of the batch. A summary of the QA and the results for the firsts 4 production batches are presented in the paper.