Planar LAAPDs: temperature dependence, performance, and application in low energy X-ray spectroscopy

An experiment measuring the 2S Lamb shift in muonic hydrogen (μ^-p) is being performed at the Paul Scherrer Institute (PSI), Switzerland. It requires small detectors for 1.9 keV X-rays (2P-1S transition) with an energy resolution around 25%, a time resolution better than 100 ns, a large solid angle coverage, and insensitivity to a 5 T magnetic field. We used large area avalanche photodiodes (LAAPDs) from radiation monitoring devices (RMD) for the latest data taking period in 2003. For soft X-ray spectroscopy applications, they have to be cooled, hence, a series of tests were performed to choose the optimal operation temperature. 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 our experiment as well as their response to alpha particles are presented.