Compton Polarimetry

TL;DR

This paper reviews the principles, designs, calibration methods, and background considerations of Compton polarimeters used for measuring the linear polarisation of hard X-rays in the 10-100 keV range.

Contribution

It provides a comprehensive overview of Compton polarimetry, including formalism, design schemes, calibration techniques, and background sources, highlighting recent developments and challenges.

Findings

Different design schemes have been developed for Compton polarimeters.

Calibration methods are crucial for reducing systematic effects.

Background sources significantly impact measurement accuracy.

Abstract

Photons preferentially Compton scatter perpendicular to the plane of polarisation. This property can be exploited to design instruments to measure the linear polarisation of hard X-rays ($\sim$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.