The CsI calorimeter for AMEGO-X

Astronomical observations with gamma rays in the range of several hundred keV to hundreds of MeV represent an under-explored region of the electromagnetic spectrum. To help increase the sensitivity for measurements in this so-called "MeV gap", a mission concept known as the All-sky Medium-Energy Gamma-ray Observatory eXplorer (AMEGO-X) is in development, led by NASA Goddard Space Flight Center (GSFC). One of the main instrument subsystems for AMEGO-X is a calorimeter based on hodoscopically arranged CsI:Tl bars readout on each end by dual-gain SiPMs, led by the U.S. Naval Research Laboratory (NRL). NRL designed and built a prototype CsI:Tl calorimeter instrument using a commercial-off-the-shelf (COTS) silicon photomultipliers (SiPMs) and front-end to assess the performance and dynamic energy range. Per log, we observed significant non-linearities in the energy response and limited dynamic range using COTS SiPMs, well below the four orders of magnitude in energy required for the instrument. To mitigate this issue, we developed dual-gain SiPM boards with two SiPM geometries, read out separately, to increase the dynamic energy range of the readout and improve non-linearity across that range. SiPMs that utilize 40 μm microcell pitch for both SiPM geometries demonstrate the desired response when tested with high-energy beams. The approximate energy range of the calorimeter is 0.25 − 400 MeV with overlap in the different SiPM groups of around 2.5−30 MeV. NRL is currently working with FBK to develop custom SiPM readout and constructing a prototype calorimeter for the AMEGO-X instrument.