  The effects of radiation damage in silicon photomultipliers (SiPMs) from
gamma rays have been measured and compared with the damage produced by
neutrons. Several types of MPPCs from Hamamatsu were exposed to gamma rays and
neutrons at the Solid State Gamma Ray Irradiation Facility (SSGRIF) at
Brookhaven National Lab and the Institute for Nuclear Research (Atomki) in
Debrecen, Hungary. The gamma ray exposures ranged from 1 krad to 1 Mrad and the
neutron exposures ranged from 10$^8$ n/cm$^2$ to 10$^{12}$ n/cm$^2$. The main
effect of gamma ray damage is an increase in the noise and leakage current in
the irradiated devices, similar to what is seen from neutron damage, but the
level of damage is considerably less at comparable high levels of exposure. In
addition, the damage from gamma rays saturates after a few hundred krad, while
the damage from neutrons shows no sign of saturation, suggestive of different
damage mechanisms in the two cases. The change in optical absorption in the
window material of the SiPMs due to radiation was also measured. This study was
carried out in order to evaluate the use of SiPMs for particle physics
applications with moderate levels of radiation exposures.
