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 (>pi
steradian), at the same time offering the ability to be close-packed with
multiple modules in order to reduce deadspace. We plan to present in this paper
the latest laboratory results obtained from GRAPE using partially polarized
radiation sources.
