  A novel photon detector, the Silicon Photomultiplier (SiPM), has been tested
in proximity focusing Ring Imaging Cherenkov (RICH) counters that were exposed
to cosmic-ray particles in Ljubljana, and a 2 GeV electron beam at the KEK
research facility. This type of RICH detector is a candidate for the particle
identification detector upgrade of the BELLE detector at the KEK B-factory, for
which the use of SiPMs, microchannel plate photomultiplier tubes or hybrid
avalanche photodetectors, rather than traditional Photomultiplier Tubes (PMTs)
is essential due to the presence of high magnetic fields. In both experiments,
SiPMs are found to compare favourably with PMTs, with higher photon detection
rates per unit area. Through the use of hemispherical and truncated pyramid
light guides to concentrate photons onto the active surface area, the light
yield increases significantly. An estimate of the contribution to dark noise
from false coincidences between SiPMs in an array is also presented.
