A hard X-ray imaging polarimeter for balloon-borne astronomy

A polarimeter based upon an optical avalanche chamber is being developed at the Marshall Space Flight Center. The polarimeter will be carried aloft by balloon for imaged polarization measurements of celestial sources at energies between 40 keV and 100 keV. In the lab, using a small prototype polarimeter, we have measured a modulation factor of 0.3 at 54 keV. The polarimeter works by using a CCD camera to image the track of each photoelectron liberated when an X-ray is absorbed in the optical avalanche chamber. By virtue of this technique, we are able to measure the ejection direction of the photoelectrons (i.e. polarization of the incident X-ray beam) as well as locate the position of the absorbed X-ray to within a region smaller than the size of the charge cloud. As a result, we can obtain a map detailing the two dimensional distribution of polarization amplitude and polarization direction. We present details of this work as well as a description of the balloon experiment we are currently building.