Development of 2 cm-square Hamamatsu avalanche photodiodes for high-resolution X-rays and γ-rays detection

The avalanche photodiodes (APDs) have attracted considerable attention in various field of experimental physics, but their uses are still limited in only a few experiments, possibly due to their small surface areas. Here, we report the development of the large-area (∼20mm square) APDs, for future applications to high-resolution X-rays and γ-rays detection. We have made two prototypes of reverse-type APDs based on different concepts, one consists of a 2×2 array of 10×10mm2 pixels (APD1) and the other is a monolithic pixel of 19×19mm2 size (APD2) to achieve a large effective area. By comparing the dark current and gain characteristics at room temperature (+20°C) and lightly cooled environment (-20°C), we quantitatively discussed the origin of predominant noise source at different temperatures. As a performance demonstration of newly developed APDs, we made a scintillation γ-ray detector consisting of a 20×20×5mm3 CsI(Tl) crystal and a 2 cm-square APD. The best FWHM energy resolution of 5.5±0.2% were obtained for 662keVγ-rays at room temperature. Similarly, the best FWHM energy resolution of 8.5±0.2% were obtained for 122 keV γ-rays at lightly cooled environment. We showed that the minimum detectable energy for scintillation light was 15 keV at 20°C and less than 5 keV at -20°C.