  We report on Monte Carlo studies of the hard X-ray polarimeter X-Calibur. The
polarimeter will be used in the focal plane of a grazing incidence hard X-ray
telescope. It combines a low-Z Compton scatterer with a high-Z Cadmium Zinc
Telluride (CZT) detector assembly to measure the polarization of 10 keV - 80
keV X-rays. X-Calibur makes use of the fact that polarized photons Compton
scatter preferentially perpendicular to the electric field orientation. In
contrast of competing designs, which use only a small fraction of the incoming
X-rays, X-Calibur achieves a high detection efficiency of order unity. In this
contributions, we discuss a Monte Carlo study which compares X-Calibur's
polarimeteric performance achieved using different scattering materials
(Scintillator, Be, LiH, Li), and calculate the sensitivity of X-Calibur when
used with different balloon-borne and space-borne mirror assemblies.
