Prospects for GRB Polarimetry with GRAPE

This paper discusses the latest progress in the development of GRAPE (Gamma‐Ray Polarimeter Experiment), a hard X‐ray Compton Polarimeter. The purpose of GRAPE is to measure the polarization of hard X‐rays in the 50–300 keV energy range. We are particularly interested in X‐rays that are emitted from solar flares and gamma‐ray bursts (GRBs). Accurately measuring the polarization of the emitted radiation from these sources will lead to a better understating of both the emission mechanisms and source geometries. The GRAPE design consists of an array of plastic scintillators surrounding a central high‐Z crystal scintillator. We can monitor individual Compton scatters that occur in the plastics and determine whether the photon is photo absorbed by the high‐Z crystal or not. A Compton scattered photon that is immediately photo absorbed by the high‐Z crystal constitutes a valid event. These valid events provide us with the interaction locations of each incident photon and ultimately produces a modulation pattern for the Compton scattering of the polarized radiation. Comparing with Monte Carlo simulations of a 100% polarized beam, the level of polarization of the measured beam can then be determined. The complete array is mounted on a flat‐panel multi‐anode photomultiplier tube (MAPMT) that can measure the deposited energies resulting from the photon interactions. The design of the detector allows for a large field‐of‐view (\u3e π steradian), at the same time offering the ability to be close‐packed with multiple modules in order to reduce deadspace. We present in this paper the latest laboratory results obtained from GRAPE using partially polarized radiation sources along with a brief description of our future plans for the GRAPE design