Evaluation of a prototype detector for the LargE Area burst Polarimeter (LEAP)

The LargE Area burst Polarimeter (LEAP) is one of two NASA Missions of Opportunity proposals that are currently in a Phase A Concept Study, with a final selection due later this year. It is a wide Field of View (FoV) Compton polarimeter designed to study Gamma-Ray Burst (GRB) polarization over the energy range from 50-500 keV and to measure GRB spectra in the range from 20 keV - 5 MeV. It will be deployed as an external payload on the International Space Station in 2025 for a three-year mission. The science goal is to measure the polarization from at least 65 GRBs with a sensitivity level defined by a Minimum Detectable Polarization (MDP) of < 30%. LEAP measures polarization using seven independent polarimeter modules, each with a 12x12 array of high-Z and low-Z scintillation detectors readout by individual photomultipliers (PMTs). During the Phase A Concept Study, lab measurements were conducted with a small-scale (5x5) prototype polarimeter module. This included both spectral and polarization measurements with laboratory calibration sources. Here the prototype measurements and the comparisons made with simulations of the prototype detector are described. These results demonstrate the basic functionality of the LEAP design.