) Chinese Journal ofAstronomy and Astrophysics GRAPE- A Balloon-Borne Gamma-Ray Polarimeter Experiment

Abstract This paper reviews the development status of GRAPE (the 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), although GRAPE could also be employed in the study of other astrophysical sources. Accurately mea-suring the polarization of the emitted radiation will lead to a better understating of both emis-sion mechanisms and source geometries. The GRAPE design consists of an array of plastic scintillators surrounding a central high-z crystal scintillator. The azimuthal distribution of photon scatters from the plastic array into the central calorimeter provides a measure of the polarization fraction and polarization angle of the incident radiation. The design of the detec-tor provides sensitivity over a large field-of-view (> steradian). The design facilitates the fabrication of large area arrays with minimal deadspace. This paper presents the latest design concept and the most recent results from laboratory tests of a GRAPE science model