  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.
