Development of wideband X-ray and gamma-ray spectrometer using transmission-type, large-area APD

The avalanche photodiode (APD) is a high-performance and compact light sensor recently applied in various fields of experimental physics. Among several types of APDs, the reach-through APD offers an advantage in direct X-ray detection, thanks to its thick depletion layer (≥ 100 μ m) in front of the amplification region. This type of APD is also sensitive to weak scintillation light from gamma-ray scintillators with a high quantum efficiency of ∼ 80 % (at λ ≃ 500 nm). In this paper, we propose a novel design of a compact X-ray-to-gamma-ray detector widely applicable between 1 keV and several hundreds of keV. The prototype consists of a reach-through APD (transmission type) optically coupled with a cubic CsI(Tl) crystal 4 × 4 × 4 mm³ in size. By applying the pulse shape discrimination technique to the APD output, we successfully discriminated the X-ray signals directly detected within the APD (1-40 keV), and gamma-ray signals absorbed in a CsI(Tl) scintillator (10-800 keV) located immediately behind the APD. Optimum FWHM energy resolutions of 15.1 ± 0.2 %, 6.6 ± 0.4 %, and 7.6 ± 0.1 % were obtained for 5.9 keV X-rays, 32 keV X-rays, and 662 keV gamma rays, respectively, measured at + 20 ^{{ring operator}} C. This stacked configuration is viable for various future applications in space science and nuclear medicine.