Low-energy X-ray spectroscopy with RGB-HD SiPMs coupled to CsI(Tl) scintillator

The continuous improvement of Silicon Photomultiplier (SiPM) characteristics opens new perspectives for the application of these detectors in middle to low energy X-ray scintillation light readout. Compared to traditional PET detectors this application poses an additional challenge because of the much lower number of photons generated in the scintillator. Many SiPM characteristics influence the energy resolution of the detected radiation, including Photon Detection Efficiency (PDE), primary noise and Excess Noise Factor (ENF). The most recent development of the RGB-HD SiPM technology at Fondazione Bruno Kessler (FBK, Italy) provides a PDE up to 40% at 550 nm while keeping the Dark Count Rate (DCR) below 500 kc.p.s./mm2. A low DCR value is fundamental in improving the energy resolution for soft X-rays. In this work we coupled a 1×1 mm2 RGB-HD SiPM to a 0.9×0.9×2 mm3 CsI(Tl) crystal allowing the detection of 55Fe 5.9 keV photons with an energy resolution of 39% FWHM. A 4×4 mm2 RGB-HD SiPM with a 3×3×5 mm3 CsI(Tl) was used for the readout of a 57Co source (6.4 keV, 14.4 keV and 122 keV) in order to prove the dynamic range of the system. The energy resolution of these three peaks shows different trends with respect to the SiPM bias and to the signal integration time. This is related to the fact that SiPM parameters such as PDE, DCR and ENF, and readout parameters such as integration time, become more or less relevant depending on the energy of the primary photon.