The continued development of a low energy Compton imager for GRB polarization studies

The Gamma Ray Polarimeter Experiment (GRAPE) is designed to investigate gamma-ray bursts (GRB) in the important energy range of 50-500 keV. Our eventual goal is to fly GRAPE on a long duration balloon (LDB) platform to collect data on a significant sample of GRBs. Our experience with two balloon flights (in 2011 and 2014), coupled with further design efforts focused on orbital payloads, has led to an improved polarimeter concept that represents a natural evolution of the current design. The new concept employs a large number of small (2 cm3 ), optically-isolated scintillator cubes, each of which is read out by its own silicon photomultiplier (SiPM). These cubes are stacked in an arrangement that allows the determination of event interaction locations in three dimensions. The resulting three-dimensional location data provides a moderate level of Compton imaging capability (1σ angular resolution of 10-15). This level of imaging can be used to significantly reduce the instrumental background by limiting the impact of the cosmic diffuse flux, dramatically improving the polarization sensitivity. Here we shall describe this concept, some results from initial laboratory studies, and the expected performance parameters. We are currently working to optimize this design in preparation for a prototype balloon flight in the summer of 2020. Our long-term goal (pending acquisition of continued funding) is to fly a prototype balloon payload in the summer of 2020 and to be prepared for a first long duration balloon (LDB) flight at the end of 2021.