  Silicon Photo-Multipliers (SiPM) are becoming the photo-detector of choice
for increasingly more particle detection applications, from fundamental physics
to medical and societal applications. One major consideration for their use at
high-luminosity colliders is the radiation damage induced by hadrons, which
leads to a dramatic increase of the dark count rate. KETEK SiPMs have been
exposed to various fluences of reactor neutrons up to $\Phi_{neq}$ =
5$\times$10$^{14}$ cm$^{-2}$ (1 MeV equivalent neutrons). Results from the I-V,
and C-V measurements for temperatures between $-$30$^\circ$C and $+$30$^\circ$C
are presented. We propose a new method to quantify the effect of radiation
damage on the SiPM performance. Using the measured dark current the single
pixel occupation probability as a function of temperature and excess voltage is
determined. From the pixel occupation probability the operating conditions for
given requirements can be optimized. The method is qualitatively verified using
current measurements with the SiPM illuminated by blue LED light.
