  Measured response functions and low photon yield spectra of silicon
photomultipliers (SiPM) were compared to multi-photoelectron pulse-height
distributions generated by a Monte Carlo model. Characteristic parameters for
SiPM were derived. The devices were irradiated with 14 MeV electrons at the
Mainz microtron MAMI. It is shown that the first noticeable damage consists of
an increase in the rate of dark pulses and the loss of uniformity in the pixel
gains. Higher radiation doses reduced also the photon detection efficiency. The
results are especially relevant for applications of SiPM in fibre detectors at
high luminosity experiments.
