Advanced SiPM readout for the AMEGO CsI Calorimeter

We propose a program to advance the silicon photomultiplier (SiPM) readout for the CsI(Tl) calorimeter subsystem on the All-sky Medium-Energy Gamma-ray Observatory (AMEGO). AMEGO is a Probe-class mission concept, formerly known as ComPair; NRL is the PI institution for the CsI calorimeter – one of three major AMEGO subsystems. The other subsystems include a stack of double-sided silicon strip detectors (DSSD) and the CZT calorimeter. The AMEGO team, comprising over 200 scientists and 80 different institutions, has recently submitted 15 science white papers for inclusion in the Astronomy and Astrophysics Decadal Survey (Astro2020). This proposed program is a follow on to a previously funded APRA (PI J. E. Grove & Co-I R. Woolf, NRL - CsI Calorimeter for the ComPair Balloon Prototype, FY17-18) and companion to the currently-funded APRA (PI: J. McEnery, GSFC & NRL PI: R. Woolf - ComPair: Steps to a Medium-Energy Gamma-ray Mission, FY16-20). During the program led by PI Grove, we developed and built a hodoscopic, CsI (with SiPM readout) prototype calorimeter for a balloon flight, in conjunction with the other subsystems developed during the McEnery APRA. Aside from the prototype, the main goals were to understand the performance of a commercial SiPM array and an ASIC readout for SiPMs. Our work revealed multiple topics that demand further investigation going forward in order to advance the CsI calorimeter towards a full flight instrument. The first component of this proposal will be to design, fabricate and test a custom SiPM readout array. The array will be comprised of a mixture of small (1mm^2 or 9mm^2) square and large (36mm^2) square SiPMs, with each SiPM containing an optimized microcell size. The commercial SiPM arrays used during Grove's APRA do not provide adequate dynamic range needed for a single crystal in the calorimeter (desired 30 keV – 300 MeV). The mixture of small and large SiPMs will expand the dynamic range; this is the same dual gain range technique used by the Fermi LAT calorimeter with PIN diodes. Additionally, we found that the commercial SiPM array coupled to CsI demonstrated non-linear behavior when exposed to high-energy (>10 MeV) gamma rays. We will mitigate the non-linearity by testing with varying-sized SiPM microcells, ranging from 10 µm to 50 µm. This work will allow us to understand how the SiPM microcell size affects the non-linearity and will enable us to assess the dynamic range using different-sized SiPMs. Next, we will develop a customized readout board designed to integrate multiple commercial SiPM ASICs. This is considerably more complex than using an off-the-shelf product, but a necessary step to advance the CsI calorimeter program moving forward. During the Grove's APRA, we used a single 16-channel SiPM ASIC that performed well. The next logical step is to transition from a single ASIC readout board (with limited I/Os and channel count) to a custom readout board, incorporating multiple ASICs per board, that can interface with the other AMEGO subsystems. These readout boards will be compatible with a CsI calorimeter the size of a full-scale flight unit. Lastly, we will construct a two-layer deep calorimeter, comparable in size to a flight-like unit. The hodoscopic calorimeter will consist of flight-length CsI bars, dual gain range custom SiPMs, and multiple custom readout boards, integrated into a working instrument.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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