A Gamma Ring-Imaging Telescope for High-Energy Photon Detection

We describe experimental results obtained using a new type of detector for high-energy gamma astrophysics. The detector will measure gamma rays in the GeV range, with an angular resolution of a few mrad and a field of view of 50°. The telescope is based on the imaging of the Cherenkov light produced in a dense medium (liquid or solid) limited by a parabolic or spherical reflecting surface. The high-energy electrons produced at the start of the electromagnetic shower retain enough information about their initial direction to generate a ring-shaped photon image. This may produce hundreds or thousands of photoelectrons on an appropriate photon-imaging detector placed at the focal plane of the system, which is at the entrance surface of the converter. A prototype consisting of a liquid radiator (C₆F₁₄), a multistep parallel-plate avalanche chamber and an optical readout system was tested in a 9 GeV/c charged-particle beam. Preliminary results show that an angular resolution of a few mrad can be obtained. Furthermore, the capability of such a detector to identify e/π in the GeV range seems promising.