Dual phase Compton polarimeter working principle

Animation showing the working principle of a Compton polarimeter. A high-energy astrophysical source is emitting X/gamma-rays that enter the segmented polarimeter. Each photon is first Compton scattered in a scatterer segment (made of low-density scintillator) and is then absorbed in an absorber segment (made of a high-Z scintillator). The relative position of the two segments gives the azimuthal scattering direction, which is directly correlated with the polarization vector direction of the incoming photon. The incoming photon is indeed scattering preferentially orthogonally to its polarization vector. By performing this measurement for many photons from a source, one gets the azimuthal scattering angle distribution, also known as 'modulation curve'. This histogram is expected to be cos^2 modulated for a polarized incoming flux, with the relative amplitude of the modulation being linked to the polarization degree (PD) of the source, and the phase of the modulation to its polarization angle (PA). This modulation is ruled by the Klein-Nishina cross-section, but has a more complex structure than a simple theoretical cos^2 due to instrumental/geometrical effects.