Performance of the most recent avalanche photodiodes for future X-ray and gamma-ray astronomy

We report on the performance of the most recent avalanche photodiodes produced by Hamamatsu Photonics, as low-energy X-rays and γ-rays detectors. APDs share good features of both photo diodes and PMTs, as they are very compact, produce an internal gain of 10-100, and have high quantum efficiency close to 100 % in the visible right. Until very recently, however, APDs were limited to very small surfaces, and were mainly used as a digital device for light communication. We have developed large area (up to 10×10 mm²) APDs which can be used in the physics experiments. The best energy resolution of 6.4 % (FWHM) was obtained in direct detection of 5.9 keV X-rays. The FWHM results of 9.4 % and 4.9 % were obtained for 59.5 keV and 662 keV γ-rays respectively, as measured with the CsI(Tl) crystal. The minimum detectable energy for the scintillation light was as low as 1 keV at lightly cooled environment (-20°C). Note that our results are the best records ever achieved with APDs. Various applications of APDs are presented for future space research and nuclear medicine. In particular 2-dimensional APD arrays will be a promising device for a wide-band X-ray and γ-ray imaging detector.