An Advanced Design for the Gamma-RAy Polarimeter Experiment (GRAPE)

Understanding the underlying physics governing astrophysical jets associated with Gamma-Ray Bursts (GRBs) is necessary to advance the field of gamma-ray astronomy. Existing physics models can be constrained through GRB polarization studies. The Gamma-Ray Polarimeter Experiment (GRAPE) is a high-altitude balloon experiment designed to measure GRB polarization over the energy range of 50-500 keV at flight altitudes. A flight of the newest GRAPE design is scheduled to fly from Fort Sumner, NM in August 2023. The new design is based on an arrangement of small scintillation detector elements designed to measure photon interactions in three dimensions and provide modest imaging capabilities. The flight instrument consists of a 3-dimensional (7x7x5) array of high-Z (GAGG) and low-Z (P-terphenyl) scintillators each read out by individual Hamamtsu Multi-Pixel Photon Counters (MPPCs). Previous GRAPE missions have been sensitive to M-class solar flares and observations of the Crab Nebula with low signal to background ratios. The new design improves performance relative to the previous GRAPE design through the use of advanced scintillator materials, the ability to perform modest Compton imaging for source localization and background rejection, and by completely eliminating optical cross-talk. Background reduction is achieved using the imaging capabilities allowing for some level of event rejection for events inconsistent with the source direction. This paper will present the new module design and simulated response parameters to provide an estimate of the balloon payload sensitivity.