A fast optical Cerenkov system for directional studies of possible gamma-ray sources

Abstract

The system observes pulses of light from the night sky, produced by small showers in the atmosphere. It uses two 90-cm f/0.5 mirrors in coincidence and with effective full field of view about 1°; but unlike previous systems of this kind it has a reduced integration time of about 3 ns and coincidence resolving time of 5 ns, the object being the selection of the fast collimated light cone originating from the high-energy early generations of the shower above 10 km rather than the slower, less directed, but more intense component from the vicinity of the maximum. The threshold energy for γ-rays showers is difficult to determine from the observed counting rate for proton showers, since the early development of the showers is different, but is believed to be in the vicinity of 10¹² eV. Preliminary drift scans have been carried out in the winter of 1966–67 at a site 300 m above sea level near Dublin. Several drift scans have been carried out on the Crab nebula, 3C196, and 3C286; and individual runs on some other objects including a possible source of neutral primaries at about 1100 + 20 reported by Hesse et al. The Crab nebula results show an apparent positive effect which, if it is real, would correspond to a flux of about 10⁻¹⁰ photons/cm² s, but the results are difficult to assess because of poor atmospheric conditions. Some other apparent positive effects have been observed, but none are significant statistically.