We describe a novel photon detector which operates under an intense flux of
neutrons. It is composed of lead-aerogel sandwich counter modules. Its salient
features are high photon detection efficiency and blindness to neutrons. As a
result of Monte Carlo (MC) simulations, the efficiency for photons with the
energy larger than 1 GeV is expected to be higher than 99.5% and that for 2
GeV/c neutrons less than 1%. The performance on the photon detection under
such a large flux of neutrons was measured for a part of the detector. It was
confirmed that the efficiency to photons with the energy >1 GeV was
consistent with the MC expectation within 8.2% uncertainty.Comment: 16 pages, 16 figures, submitted to Prog. Theor. Exp. Phy
