Čerenkov Light in Extensive Air Showers and the Chemical Composition of Primary Cosmic Rays at1016eV

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 ${\mathrm{cm}}^2$ 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 ${10}^4$ photons/${\mathrm{m}}^2$. For showers in the size interval ${10}^6<N<\mathrm{}{10}^8$ particles, the CL intensities $\rho (r, N)$ in the radial region $r=25-300\mathrm{}$ m are fitted fairly well by the function $\rho (r, N)=k{N}^{0.7\pm 0.1}\exp (-\alpha r+\beta r^2)$, where $\alpha =1.8\mathrm{}\times {10}^{-2\mathrm{}}$ ${\mathrm{m}}^{-1\mathrm{}}$, $\beta =1.3\mathrm{}\times {10}^{-5\mathrm{}}$ ${\mathrm{m}}^{-2\mathrm{}}$, and $k=155\mathrm{}\pm 80$ (visible photons) ${\mathrm{m}}^{-2\mathrm{}}$. The distribution of CL intensity (normalized to constant size and radius) indicates that the primary cosmic-ray beam at ${10}^{16}$ eV is probably mixed in chemical composition, much as it is at ${10}^{12}$ eV.