(Peter F. Bloser (UNH), Lorraine Hanlon (UCD), Jason S. Legere (UNH), Sheila McBreen (UCD), James M. Ryan (UNH), and Alexey Uliyanov (UCD))
The Gamma Ray Polarimeter Experiment (GRAPE) is designed to investigate one of the most exotic phenomena in the universe – gamma-ray bursts (GRB). There has been intense observational and theoretical research in recent years, but research in this area has been largely focused on studies of time histories, spectra, and spatial distributions. Theoretical models show that a more complete understanding of the inner structure of GRBs, including the geometry and physical processes close to the central engine, requires the exploitation of gamma-ray polarimetry. Over the past several years, we have developed the GRAPE instrument to measure the polarization of gamma-rays from GRBs over the energy range of 50 to 500 keV. 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. The new concept employs a large number of small (1 cm), optically-isolated scintillator cubes, each of which is read out by its own silicon photomultiplier (SiPM). These cubes are stacked in a three-dimensional arrangement that allows the determination of event interaction locations in three dimensions within the sensitive volume. The resulting three-dimensional location data provides a moderate level of Compton imaging capability (1 sigma angular resolution of 10-15°). Imaging can be used to significantly reduce the instrumental background by limiting the influence of the cosmic diffuse flux, resulting in an improved polarization sensitivity. Here we shall describe this concept and the expected performance for GRB polarization measurements. We will also describe our efforts to develop a working prototype, which is initially focused on the testing of various scintillators (both organic and inorganic) with SiPM readouts.