  An experiment measuring the 2S Lamb shift in muonic hydrogen mup is being
performed at the Paul Scherrer Institute, Switzerland. It requires small and
compact detectors for 1.9 keV x rays (2P-1S transition) with an energy
resolution around 25% at 2 keV, a time resolution better than 100 ns, a large
solid angle coverage, and insensitivity to a 5 T magnetic field. We have chosen
Large Area Avalanche Photodiodes (LAAPDs) from Radiation Monitoring Devices as
x-ray detectors, and they were used during the last data taking period in 2003.
For x-ray spectroscopy applications, these LAAPDs have to be cooled in order to
suppress the dark current noise, hence, a series of tests were performed to
choose the optimal operation temperature. Specifically, the temperature
dependence of gain, energy resolution, dark current, excess noise factor, and
detector response linearity was studied. Finally, details of the LAAPDs
application in the muonic hydrogen experiment as well as their response to
alpha particles are presented.
