  Characteristic properties of a Geiger Mode APD (G-APD) in a THGEM-based
cryogenic two-phase Ar avalanche detector were studied in view of potential
applications in rare-event experiments. G-APD signal amplitude and noise
characteristics at cryogenic temperatures turned out to be superior to those at
room temperature. The effective detection of avalanche scintillations from
THGEM-multiplier holes in two-phase Ar has been demonstrated using a G-APD
without wavelength shifter. At an avalanche gain of 60, the avalanche
scintillation yield measured by the G-APD was as high as 0.9 photoelectrons per
avalanche electron, extrapolated to 4pi acceptance.
