The Advanced Particle-astrophysics Telescope: Simulation of the Instrument Performance for Gamma-Ray Detection

We present simulations of the instrument performance of the Advanced Particle-astrophysics Telescope (APT), a mission concept of a γ-ray and cosmic-ray observatory in a sun-Earth Lagrange orbit. The key components of the APT detector include a multiple-layer tracker composed of scintillating fibers and an imaging calorimeter composed of thin layers of CsI:Na scintillators. The design is aimed at maximizing effective area and field of view for γ-ray and cosmic-ray measurements, subject to constraints on instrument cost and total payload mass. We simulate a detector design based on 3-meter scintillating fibers and develop reconstruction algorithms for γ-rays from a few hundreds of keV up to a few TeV energies. At the photon energy above 30 MeV, pair-production/shower reconstruction is applied; the results show that APT could provide an order of magnitude improvement in effective area and sensitivity for γ-ray detection compared with the Fermi Large Area Telescope (LAT). A multiple-Compton-scattering reconstruction at photon energies below 10 MeV achieves sensitive detection of faint γ-ray bursts (GRBs) and other γ-ray transients down to ∼ 0.01 MeV/cm² with degree-level to sub-degree-level localization accuracy. The Compton analysis also provides a measurement of polarization where the minimum detectable degree of polarization for ∼ 1 MeV/cm² GRBs is below 20%. In addition to the APT simulations, we present the simulated performance of the Antarctic Demonstrator for APT, a 0.5m-square cross section balloon experiment that includes all of the key elements of the full APT detector.