Development of Dual-Gain SiPM Boards for Extending the Energy Dynamic Range

Astronomical observations with gamma rays in the range of several hundred keV to hundreds of MeV currently represent the least explored energy range. To address this so-called MeV gap, we designed and built a prototype thallium-doped CsI (CsI:Tl) calorimeter instrument using the commercial off-the-shelf (COTS) silicon photomultipliers (SiPMs) and front-ends, which may serve as a subsystem for a larger gamma-ray mission concept. During development, we observed significant nonlinearity in the energy response. In addition, using the COTS readout, the calorimeter could not cover the four orders of magnitude in energy range required for the telescope. We, therefore, developed dual-gain SiPM boards that make use of two SiPM species that are read out separately to increase the dynamic energy range of the readout. In this work, we investigate the SiPM's response with regards to active area ( 3\,\times \, 3 and 1 \,\times \, 1 \mathrm {mm}^{2} ) and various microcell sizes (10, 20, and 35 ~\mu \mathrm {m} ). We read out 3\times 3\times 6\,\,\mathrm {cm}^{3} CsI:Tl chunks using dual-gain SiPMs that utilize 35- \mu \mathrm {m} microcells for both SiPM species and demonstrate the concept when tested with high-energy gamma-ray and proton beams. We also studied the response of 17\times 17\times 100\,\,\mathrm {mm}^{3} CsI bars to CsI bars to high-energy protons. With the COTS readout, we demonstrate a sensitivity to 60-MeV protons with the two SiPM species overlapping at a range of around 2.5-30 MeV. This development aims to demonstrate the concept for future scintillator-based high-energy calorimeters with applications in gamma-ray astrophysics.