Development of a CsI Calorimeter for the Compton-Pair (ComPair) Balloon-Borne Gamma-Ray Telescope

There is a growing interest in astrophysics to fill in the observational gamma-ray MeV gap. We, therefore, developed a CsI:Tl calorimeter prototype as a subsystem to a balloon-based Compton and Pair-production telescope known as ComPair. ComPair is a technology demonstrator for a gamma-ray telescope in the MeV range that is comprised of four subsystems: the double-sided silicon detector, virtual Frisch grid CdZnTe, CsI calorimeter, and a plastic-based anti-coincidence detector (ACD). The prototype CsI calorimeter is composed of 30 CsI logs, each with a geometry of 1.67\,\, \times \,\, 1.67 \,\, \times \,\,10 cm3. The logs are arranged in a hodoscopic fashion with six in a row that alternate directions in each layer. Each log has a resolution of around 8% full-width-at-half-maximum (FWHM) at 662 keV with a dynamic energy range of around 250 keV-30 MeV. A 2\,\, \times \,\, 2 array of SensL J-series SiPMs read out each end of the log to estimate the depth of interaction (DOI) and energy deposition with signals read out with an IDEAS ROSSPAD. We also utilize an Arduino to synchronize with the other ComPair subsystems that comprise the full telescope. This work presents the development and performance of the calorimeter, its testing in thermal and vacuum conditions, and results from irradiation by 2-25 MeV monoenergetic gamma-ray beams. The CsI calorimeter will fly onboard ComPair as a balloon experiment in the summer of 2023.