Demonstration of polarization sensitivity of emulsion-based pair conversion telescope for cosmic gamma-ray polarimetry

Linear polarization of high-energy gamma-rays (10MeV–100 GeV) can be detected by measuring the azimuthal angle of electron–positron pairs and observing the modulation of the azimuthal distribution. To demonstrate the gamma-ray polarization sensitivity of emulsion, we conducted a test using a polarized gamma-ray beam (0.8–2.4 GeV) at SPring-8/LEPS. Emulsion tracks were reconstructed using scanning data, and gamma-ray events were selected automatically. Using an optical microscope, out of the 2381 gamma-ray conversions that were observed, 1372 remained after event selection, on the azimuthal angle distribution of which we measured the modulation. From the distribution of the azimuthal angles of the selected events, a modulation factor of 0.21+0.11−0.09 was measured, from which the detection of a non-zero modulation was established with a significance of 3.06σ. This attractive polarimeter will be applied to the GRAINE project, a balloon-borne experiment that observes 10–100 GeV cosmic gamma-rays with an emulsion-based pair conversion telescope.