Study of a large area avalanche photodiode as a fast photon and a soft X-ray detector

We have studied the fundamental properties of a 16 mm diameter (large area) avalanche photodiode (APD) to determine its suitability as a low-signal detector. The dependence of the gain on the bias-voltage and temperature is presented. In particular, the timing characteristics were investigated as a function of the detection area. The 16 mm ⊘ APD had a moderate timing performance. A 0.5 mm ⊘ APD provided a much smaller timing jitter, less than 200 ps, which is close to that of PMTs. The performance of an APD as a photon counter for scintillation light from a CsI(Tl) crystal was also studied. The RMS (root mean square) resolution obtained for 662 keV γ-rays is 4% for a small scintillator block and 12% for a large scintillator block, respectively. The performance of the APD was also found to be maintained with shaping times as short as 200 ns. Our study was extended to the capability of the APD to detect soft X-rays. X-rays of ∼600 eV were detected above the noise level of the 16 mm ⊘ APD by cooling it to ∼ −20°C.