CERENKOV LIGHT IN EXTENSIVE AIR SHOWERS AND CHEMICAL COMPOSITION OF PRIMARY COSMIC RAYS AT 1O16-EV

An array of nine atmospheric Čerenkov light (CL) detectors has been operated on Mt. Chacaltaya, Bolivia (altitude 5200 m) in conjunction with the Bolivian Air Shower Joint Experiment (BASJE). Data were obtained during the periods July to November 1966 and June to November 1967. CL detectors were located at radii of 15, 150, and 300 m from the center of the BASJE particle-detector array. Each CL detector consisted of nine 5-in.-diam photomultipliers with a total sensitive area of 880 cm2 and an acceptance cone of 65°(full width at half-maximum) about the vertical. The minimum detectable CL pulse, in the presence of background starlight, was less than 104 photons/m2. For showers in the size interval 106<N<108 particles, the CL intensities ρ (r,N) in the radial region r=25-300 m are fitted fairly well by the function ρ (r,N)=kN0.7±0.1exp(-αr+βr2), where α=1.8×10-2 m-1, β=1.3×10-5 m-2, and k=155±80 (visible photons) m-2. The distribution of CL intensity (normalized to constant size and radius) indicates that the primary cosmic-ray beam at 1016 eV is probably mixed in chemical composition, much as it is at 1012 eV. © 1969 The American Physical Society.