Gamma-Ray Observations from a Balloon Test Flight of an Improved Gamma-Ray Polarimeter Experiment (GRAPE)

The Gamma-Ray Polarimeter Experiment (GRAPE) is a high-altitude balloon experiment designed for polarization studies of Gamma-Ray Bursts (GRBs) with a long-term goal of making observations at stratospheric altitudes aboard a series of Long Duration Balloon flights (LDBs). Polarization studies of prompt emission from GRB jets is the next step in unfurling the mysteries of the central engine that governs astrophysical jets. Understanding the underlying physics of these jets is necessary to constrain existing physics models describing the internal radiation mechanisms and magnetic field structures of GRB jets. GRAPE is a wide field of view (FoV) Compton polarimeter measuring γ-ray polarization from transient sources in the 50-500 keV energy range with a broad range (20 keV – 3 MeV) for spectroscopy. GRAPE’s novel 3-dimensional geometry enables improved background rejection capabilities through source localization with Compton imaging. The latest design incorporates advanced scintillator materials and eliminates optical crosstalk for improved performance. The instrument consists of 245 optically isolated SiPMs each coupled to either a high-Z (GAGG:Ce) or low-Z (para-Terphenyl) scintillator arranged in a 7x7x5 cuboid structure. Co-60 calibration sources (~25 nCi) imbedded within two low-Z detectors located in the center of the array allow for onboard calibrations. We will report on results from a 2023 test flight, conducted on August 27, 2023, from Fort Sumner, NM, as well as plans for an upcoming beam campaign to further characterize the instrument performance.