  X-ray polarimetry promises to give qualitatively new information about
high-energy astrophysical sources, such as binary black hole systems,
micro-quasars, active galactic nuclei, neutron stars, and gamma-ray bursts. We
designed, built and tested a X-ray polarimeter, X-Calibur, to be used in the
focal plane of the balloon-borne InFOCuS grazing incidence X-ray telescope.
X-Calibur combines a low-Z scatterer with a CZT detector assembly to measure
the polarization of 20-80keV X-rays making use of the fact that polarized
photons scatter preferentially perpendicular to the electric field orientation.
X-Calibur achieves a high detection efficiency of ~80%. The X-Calibur detector
assembly is completed, tested, and fully calibrated. The response to a
polarized X-ray beam was measured successfully at the Cornell High Energy
Synchrotron Source. This paper describes the design, calibration and
performance of the X-Calibur polarimeter. In principle, a similar space-borne
scattering polarimeter could operate over the broader 2-100keV energy band.

Instrumentation and Methods for Astrophysics