Gamma-ray observations at stratospheric altitudes with the Gamma-Ray Polarimeter Experiment (GRAPE)

The Gamma-Ray Polarimeter Experiment (GRAPE) is a high-altitude balloon experiment designed for polarization studies of transient sources with a long-term goal of observing Gamma-Ray Bursts (GRBs) aboard a series of Long Duration Balloon flights (LDBs). Polarization studies of GRB prompt emission is necessary to constrain existing physics models describing the radiation processes and magnetic field structures of astrophysical jets. GRAPE is a wide field of view (FoV) Compton polarimeter measuring γ-ray polarization in the 50-500 keV energy range with a broad range (20 keV – 3 MeV) for spectroscopy. The novel 3-dimensional geometry of the current design provides GRAPE with Compton imaging in addition to polarization capabilities. The instrument consists of 245 optically isolated SiPM coupled to either high-Z (GAGG:Ce) or low-Z (para-Terphenyl) scintillator arranged in a 7x7x5 cuboid structure. At the center of the scintillator array two low-Z detectors contain a ~25 nCi Co-60 calibration source for onboard calibrations. A test flight of this design was flown on August 27, 2023, from Fort Sumner, NM to validate the scientific capability of the instrument design. We will report on res