  We report on the design and estimated performance of a balloon-borne hard
X-ray polarimeter called HX-POL. The experiment uses a combination of Si and
Cadmium Zinc Telluride detectors to measure the polarization of 50 keV-400 keV
X-rays from cosmic sources through the dependence of the angular distribution
of Compton scattered photons on the polarization direction. On a one-day
balloon flight, HX-POL would allow us to measure the polarization of bright
Crab-like sources for polarization degrees well below 10%. On a longer (15-30
day) flight from Australia or Antarctica, HX-POL would be be able to measure
the polarization of bright galactic X-ray sources down to polarization degrees
of a few percent. Hard X-ray polarization measurements provide unique venues
for the study of particle acceleration processes by compact objects and
relativistic outflows. In this paper, we discuss the overall instrument design
and performance. Furthermore, we present results from laboratory tests of the
Si and CZT detectors.
