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