Circular polarimetry with gamma-ray tracking detectors

Measurement of circular polarization of gamma-rays with position sensitive solid state detectors is proposed. It is based on the transfer of the photon spin to the recoiled electron in the process of Compton scattering and subsequent detection of the electron spin polarization. Bremsstrahlung polarization correlations, in particular the left–right asymmetry of the photon emission from transversally polarized electrons, are used for this purpose. Compton scattering and electron bremsstrahlung events as well as the scattered and the bremsstrahlung photon absorptions take place inside the active volume of the position sensitive detector. These events are identified and their complete kinematics is reconstructed by means of gamma-ray tracking. No spin polarized targets and no inactive materials are needed for this technique. The proposed method is naturally integrated into the concept of a Compton telescope, allowing for the first time to build an imaging gamma-ray circular polarimeter. It should work in the energy region of several 100 keV up to several 10 MeV.