Gamma Polari-Calorimeter: Performing simultaneous polarization and energy measurements of gamma rays using the pair production process

This paper describes the Gamma PolariCalorimeter, an instrument that is being developed for measuring both energy and polarization degree of gamma beams in the energy range of 100 MeV to 2 GeV at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility. The instrument is being designed for performing these measurements using the polarization dependent cross-section of the pair production process. The electron-positron pair's kinematic properties are measured after being tracked using a magnetic field and thin pixelated silicon sensors, thus enabling the reconstruction of the incoming gamma photon that generated the pair. The degree of polarization of the beam can then be inferred from the amplitude of the modulation of the azimuthal distribution of the electron-positron pairs. Monte Carlo simulations show that for single incoming photons within the targeted energy range the energy resolution is below 10 % and the analyzing power is greater than 0.5. Performance degrades when considering multiple photons arriving at the detector simultaneously but the energy resolution is still kept below 10 % while simultaneously reconstructing as many as 12 photons per event.