  For the SPECTRAP experiment at GSI, Germany, detectors with Single-Photon
counting capability in the visible and near-infrared regime are required. For
the wavelength region up to 1100 nm we investigate the performance of 2x2 mm^2
avalanche photo diodes (APDs) of type S0223 manufactured by Radiation
Monitoring Devices. To minimize thermal noise, the APDs are cooled to
approximately -170 deg. C using liquid nitrogen. By operating the diodes close
to the breakdown voltage it is possible to achieve relative gains in excess of
2x10^4. Custom-made low noise preamplifiers are used to read out the devices.
The measurements presented in this paper have been obtained at a relative gain
of 2.2x10^4. At a discriminator threshold of 6 mV the resulting dark count rate
is in the region of 230/s. With these settings the studied APDs are able to
detect single photons at 628 nm wavelength with a photo detection efficiency of
(67+-7)%. Measurements at 1020 nm wavelength have been performed using the
attenuated output of a grating spectrograph with a light bulb as photon source.
With this setup the photo detection efficiency at 1020 nm has been determined
to be (13+-3)%, again at a threshold of 6 mV.
