Development of a prototype detector for the gamma-ray polarimeter experiment (GRAPE)

The Gamma-RAy Polarimeter Experiment (GRAPE) is a NASA-funded high-altitude scientific balloon experiment. For over a decade, GRAPE has been developing wide field of view (FoV) scintillator-based Compton polarimeters designed to measure gamma-ray polarization from GRBs over the energy range of 50-500 keV. A variant of a GRAPE design was tested in flight in 2011 and 2014. A new design is scheduled to fly in Fort Sumner, New Mexico in August of 2023. The new design of GRAPE incorporates an assemblage of 245 optically isolated high-Z and low-Z scintillators each read out by individual silicon photomultipliers (SiPMs) and arranged in a 3-dimensional 7x7x5 cuboid layout which provides moderate imaging capabilities. The cube design of GRAPE is expected to improve performance with respect to previous GRAPE missions by eliminating optical cross-talk and using the instrument's imaging capability to reduce instrument background. To standardize the development process for the flight instrument, select optimal technology to optimize the design features, and build a framework for the analysis and simulation of the 245 detector instrument, a small-scale prototype instrument of the cube design was developed and studied in the lab using simulations and lab measurements of unpolarized sources. The results of these studies are presented here along with an overview of the 7x7x5 flight instrument and mission.