Development of 2 cm-square Hamamatsu avalanche photodiodes for high-resolution X-rays and -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 ( ∼ 20 mm 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 × 10 mm 2 pixels (APD1) and the other is a monolithic pixel of 19 × 19 mm 2 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 × 5 mm 3 CsI(Tl) crystal and a 2 cm-square APD. The best FWHM energy resolution of 5.5 ± 0.2 % were obtained for 662 keV γ -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 .