Front-End Computational Modeling and Design for the Antarctic Demonstrator for the Advanced Particle-astrophysics Telescope

The Advanced Particle-astrophysics Telescope (APT) is a planned space-based observatory designed to localize MeV to TeV transients such as gamma-ray bursts in real time using onboard computational hardware. The Antarctic Demonstrator for APT (ADAPT) is a prototype high-altitude balloon mission scheduled to fly during the 2025–26 season. Gamma-ray-induced scintillations in CsI tiles will be captured by perpendicular arrays of optical fibers running across both tile surfaces, as well as SiPM-based edge detectors to improve light collection and calorimetry. Signal samples are captured by analog waveform digitizer ASICs then sent to the front end of the computational pipeline, which is designed to be deployed on a set of FPGAs. This paper presents a model for uncertainty in the measured positions and deposited energies of Compton scatters in ADAPT, informed by simulations of the scintillation response and optical propagation properties of the CsI tiles, as well as existing characterizations of the SiPM and preamplifier boards. Anisotropic background radiation and event pileup are also considered. We describe our current implementation of event processing and data reduction for individual gamma rays, including both pedestal subtraction and signal integration. Preliminary work shows that high-level synthesis (HLS) enables the logic for pedestal subtraction and signal integration across 96 ASIC channels to run in 302 clock cycles on a single Kintex-7 FPGA. This demonstrates the feasibility of using FPGA hardware to accelerate the front-end event-building stage prior to back-end reconstruction and localization.