Linear polarization of high-energy gamma-rays ( 10 MeV-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 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 3.06 \sigma .
This attractive polarimeter will be applied to the GRAINE project , a balloon-borne experiment that observes cosmic gamma-rays with an emulsion-based pair conversion telescope .