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The Mu2e calorimeter is composed of 1400 un-doped CsI crystals, coupled tolarge area UV extended Silicon Photomultipliers (SiPMs), arranged in twoannular disks. This calorimeter has to provide precise information on energy,timing and position resolutions. It should also be fast enough to handle thehigh rate background and it must operate and survive in the high radiationenvironment. Simulation studies estimated that, in the highest irradiatedregions, each photo-sensor will absorb a dose of 20 krad and will be exposed toa neutron fluency of 5.5x10^11 n_(1MeV)/cm^2 in three years of running, with asafety factor of 3 included. At the end of 2015, we have concluded anirradiation campaign at the Frascati Neutron Generator (FNG, Frascati, Italy)measuring the response of two different 16 array models from Hamamatsu, whichdiffer for the protection windows and a SiPM from FBK. In 2016, we have carriedout two additional irradiation campaigns with neutrons and photons at theHelmholtz-Zentrum Dresden-Rossendorf (HZDR, Dresden, Germany) and at theCalliope gamma irradiation facility at ENEA-Casaccia, respectively. Anegligible increment of the leakage current and no gain change have beenobserved with the dose irradiation. On the other hand, at the end of theneutron irradiation, the gain does not show large changes whilst the leakagecurrent increases by around a factor of 2000. In these conditions, the too highleakage current makes problematic to bias the SiPMs, thus requiring to coolthem down to a running temperature of ~0 {\deg}C.

Irradiation study of UV Silicon Photomultipliers for the Mu2e Calorimeter