We report on the optimization of the hard X-ray polarimeter X-Calibur for a
high-altitude balloon-flight in the focal plane of the InFOC{\mu}S X-ray
telescope from Fort Sumner (NM) in Fall 2013. X-Calibur combines a low-Z
scintillator slab to Compton-scatter photons with a high-Z Cadmium Zinc
Telluride (CZT) detector assembly to photo-absorb the scattered photons. The
detector makes use of the fact that polarized photons Compton scatter
preferentially perpendicular to the electric field orientation. X-Calibur
achieves a high detection efficiency of order unity and reaches a sensitivity
close to the best theoretically possible. In this paper, we discuss the
optimization of the design of the instrument based on Monte Carlo simulations
of polarized and unpolarized X-ray beams and of the most important background
components. We calculate the sensitivity of the polarimeter for the upcoming
balloon flight from Fort Sumner and for additional longer balloon flights with
higher throughput mirrors. We conclude by emphasizing that Compton polarimeters
on satellite borne missions can be used down to energies of a few keV.
