RHESSI as a Hard X-Ray Polarimeter

Polarization measurements of hard X-rays from solar flares can provide a direct handle on the extent to which the accelerated electrons are beamed, which, in turn, has important implications for particle acceleration models. Past efforts to measured X-ray polarization have met with only limited success. Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is also capable of measuring the polarization of hard X-rays (20-100 keV) from solar flares. This capability arises from the inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low energy photons (below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low energy photons from the Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux. Sensitivity estimates, based on Monte Carlo simulations and in-flight background measurements, indicate that a 20-100 keV polarization sensitivity of less than a few percent can be achieved for X-class flares.