Design and tests of the hard X-ray polarimeter X-Calibur

X-ray polarimetry will give qualitatively new information about high-energy astrophysical sources. We designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOCμS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 10–80keV X-rays making 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.