  In this work we study the performance of silicon photomultiplier (SiPM) light
sensors after exposure to the JULIC cyclotron proton beam, of energy $\sim$ 39
MeV, relative to their performance before exposure. The SiPM devices used in
this study show a significant change in their behavior and downward shift of
their breakdown voltage by as much as $\sim$ 0.4$\pm$0.1 V. Single photon
measurements appear to be no longer possible for the SiPMs under study after
exposure to a dose of $\sim$ 0.2 Gy (corresponding to an integrated proton flux
of $\sim$$\phi_{p}$=1.06x10$^{8}$ p/cm$^{2}$). No visible damage to the surface
of the devices was caused by the exposure.
