  We have investigated the effects of X-ray irradiation to doses of 0, 200 Gy,
20 kGy, 2 MGy, and 20 MGy on the Hamamatsu silicon-photomultiplier (SiPM)
S10362-11-050C. The SiPMs were irradiated without applied bias voltage. From
current-voltage, capacitance/conductance-voltage, -frequency, pulse-shape, and
pulse-area measurements, the SiPM characteristics below and above breakdown
voltage were determined. Significant changes of some SiPM parameters are
observed. Up to a dose of 20 kGy the performance of the SiPMs is hardly
affected by X-ray radiation damage. For doses of 2 and 20 MGy the SiPMs operate
with hardly any change in gain, but with a significant increase in dark-count
rate and cross-talk probability.
