A Polarimeter for Studying Hard X-Rays from Solar Flares

We present a modular design for a Compton scatter polarimeter that will be used for studying the polarization of hard X-rays (50-300 keV) from solar flares. A complete polarimeter module fits on the front end of a 5-inch position-sensitive photomultiplier tube (PSPMT). The PSPMT is used to determine the Compton interaction location within an annular array of small plastic scintillator elements. Some of the photons that scatter within the plastic scintillator are subsequently absorbed by a small centrally-located array of CsI crystals that is read out by an independent multi-anode PMT. The independence of the two PMT readout schemes provides appropriate timing information. Monte Carlo simulations indicate that one such module, with a scintillator thickness of 12.7 cm, has a peak effective area of almost 3.5 cm2 at 200 keV and a polarization modulation factor in excess of 50% from 50 keV up to 250 keV. A small array of such detectors would be capable of measuring polarization levels of less than 1% in X-class solar flares. We are currently testing a fully-functional science model based on this design Incident Photons concept. These tests are designed to evaluate the performance characteristics of the design and to more fully validate our Monte Carlo simulation code. Here we shall review the characteristics of this modular design and report on the status of the laboratory testing. We will also outline the potential of this design for performing polarization measurements of solar CsI Array f l a r e s , including the possibility of incorporating such detectors into an imaging polarimeter. Plastic Scintillator Rods