Description of the payload
X-Calibur is a sensitive hard X-ray polarimeter built to be installed in the focal plane of a focussing hard X-ray telescope. X-Calibur measures polarization based on detecting the scattering angle of Compton scattered X-ray photons. The photons scatter preferentially into a direction perpendicular to the orientation of the electric field vector. The experiment uses a low-atomic-number (plastic) slab as scatterer and high-atomic-number solid state detector ("CZT" detectors) to absorb the scattered photons. It operates in the 20-80 keV range. The instrument combines a >80% detection efficiency, with a Full Width Half Maximum (FWHM) energy resolution of 5 keV, and a very low background level. X-Calibur is the only hard X-ray polarimeter which achieves a sensitivity close to the limiting sensitivity dictated by the physics of Compton interactions.
X-Calibur is mounted in the focal plane of the InFOCuS X-ray telescope, a stabilized balloon-borne platform developed during the 2000's decade at the Goddard Space Flight Center.
Details of the balloon flight and scientific outcome
External references and bibliographical sources