An experiment to search for discrete sources of cosmic gamma rays in the 1011 to 1012 eV region

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Abstract
A large optical reflector, designed to detect the Cerenkov radiation produced in the night sky by cosmic gamma rays, is being built for operation in the spring of 1968. The f/0.7 reflector will have an effective aperture of 34 ft, consisting of a mosaic of 252 2-ft hexagonal mirrors supported on a fully steerable frame. The mirrors will be front-aluminized so that by use of S-13 photocathodes the ultraviolet component of the Cerenkov radiation down to 3 000 Å can be detected. The optical efficiency of the reflector will be such that 75% of the light between 3 000 and 5 000 Å from a point source on the optical axis at infinity will be concentrated into a 2.5-in.-diameter disk in the focal plane. The instrument will be located on the 7 600-ft level of Mount Hopkins, Arizona. The mount for the reflector is so constructed that an additional 34-ft reflector can be added at a later date. With the large-aperture, high-speed electronics (100 Mc s−1 bandwidth), and S-13 optical wave band, the present energy threshold for the detection of gamma rays by the atmospheric Cerenkov light technique will be lowered by at least an order of magnitude. Theoretical models for the Crab nebula based on the inverse Compton effect predict a gamma-ray flux 10 times greater than the minimum flux sensitivity of the instrument. Several other possible sources will also be surveyed.