Instrument Response Functions of the Antarctic Demonstrator for the Advanced Particle-astrophysics Telescope (ADAPT)

The Antarctic Demonstrator for the Advanced Particle-astrophysics Telescope (ADAPT) gamma-ray/cosmic-ray instrument serves as a precursor to the proposed APT mission. The APT mission is designed to improve sensitivity in the MeV-TeV gamma-ray range by an order of magnitude compared to current missions and is optimized for dark-matter and multimessenger research. The ADAPT instrument uses scintillating fibers for particle tracking and sodium-doped cesium iodide (CsI:Na) tiles read out with wavelength shifting (WLS) fibers for imaging, with solid-state silicon photomultipliers (SiPMs) for calorimetry. It includes four layers of imaging calorimeter detectors and scintillating-fiber trackers, functioning both as a Compton and Pair telescope for gamma-ray measurements from several hundred keV to several GeV. The ADAPT balloon flight aims to advance the technical readiness for the APT gamma-ray mission concept and, during its long duration flight, will provide better sensitivity to 1-100 MeV transients than Fermi or other gamma-ray instruments. We have optimized event reconstruction models based on simulations incorporating the optical properties of CsI tiles, WLS signal attenuation measurements, and SiPM and preamplifier board characterizations. The realistic detector construction been simulated and the instrument's response has been evaluated as functions of both the incident gamma-ray direction and gamma-ray energy. We present the Instrument Response Functions (IRFs) for APT and ADAPT, including the detectors’ effective area, angular resolution as given by the point-spread function (PSF), and energy dispersion.