Compton Polarimetry

Photons preferentially Compton scatter perpendicular to the plane of polarization. This property can be exploited to design instruments to measure the linear polarization of hard X-rays (∼10–100 keV). Photons may undergo two interactions in the sensitive volume of the instrument, i.e., a scattering followed by an absorption. Depending on the materials used to detect these two interactions, the Compton polarimeter can be classified as single-phase (same material for scattering and absorption detectors) or dual-phase (different materials). Different designs have been studied and adopted, and current instruments are predominantly with sensors based on scintillation or solid-state detectors. X-ray polarimetry requires much higher statistics than, e.g., spectrometry or timing; thus systematic effects must be accurately measured and accounted for. In this chapter we introduce the basic formalism of the Compton effect; we describe the design schemes developed so far for scattering polarimeters, including both the single-phase and dual-phase approaches; we overview the calibration methods to reduce the systematic effects; and we describe sources of background which affect the measurements.