Spatial polarization distribution measurements of MeV gamma rays produced by inverse Compton scattering

Yang, Y.*; Taira, Yoshitaka*; Shizuma, Toshiyuki*; Omer, M.   

Highly polarized MeV gamma rays produced via inverse Compton scattering (ICS) between a polarized laser and electron beam provide a unique probe for fundamental and applied physics. ICS gamma rays are characterized by energy tunability, quasi-monochromaticity, high polarization, and low divergence angle (1 mrad), and are used for electron beam diagnostics, nuclear physics experiments, and evaluation of detectors for polarized gamma rays. A polarimeter that can measure the spatial polarization distribution of gamma rays is being developed at the UVSOR synchrotron facility. The measurement principle exploits the azimuthal asymmetry in Compton scattering cross-sections. Incident linearly polarized gamma rays interact with an iron target, and seven NaI detectors arranged in the scattering plane record the angular intensity distribution of scattered gamma rays. The determination of the polarization axis is achieved through systematic analysis of the azimuthal modulations. The spatial polarization distribution is measured by scanning a 1-mm diameter collimator in the beam cross-section in two dimensions. In this work, we discuss the measurement results of the spatial polarization distribution of horizontally and vertically linearly polarized gamma rays.