Circular polarimetry of gamma-rays with tracking detectors

S. Tashenov1,2 1GSI, Darmstadt, Germany; 2Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, SE-10691 Stockholm, Sweden Photon helicity a projection of the photon spin on its momentum direction, also called circular polarization, can provide information about processes in which angular momentum and spin interactions play an important role. In particular it can be used in atomic, nuclear and particle physics to study effects of parity violation for testing the Standard Model of elementary particles [1]. In gamma-ray astronomy it might be able to distinguish stars and galaxies composed of matter and antimatter [2]. First measurements of circular polarization of gamma rays were performed in 1950’s by exploitingCompton scattering off polarized electrons [3]. This technique, however, has a substantial limitation associated with the mandatory use of passive magnetized scattering targets. In particular it does not allow for gamma-ray imaging. Modern segmented imaging detectors which exploit the principles of gamma-ray tracking have a number of advantages over their older non-segmented counterparts. Gamma-ray tracking algorithms, which are developed for such detectors, reconstruct the sequence of the photon scatterings for the purposes of imaging, linear polarimetry and background reduction. Recently it was demonstrated that it is also possible to reconstruct the secondary gamma-ray tracks produced by the bremsstrahlung photons emitted by the Compton recoiled electrons [4]. This possibility opens up new perspectives for gamma-ray circular polarimetry. Spin transfer in Compton scattering of a circularly polarized gamma-ray produces a polarized energetic recoiled electron from an initially unpolarized electron. While slowing down in the solid detector volume such an electron may emit bremsstrahlung. A transversally polarized electron emits photons asymmetrically to the left and to the right of the plane defined by its momentum and spin [5], see Fig.1. Therefore measurement of the left-right asymmetry of the emitted bremsstrahlung photons will yield the circular polarization measurement of the incoming photons. For Compton scattering of circularly polarized photons off initially unpolarized electrons, the recoiled electrons obtain polarization [6]