Observations of the atmospheric Cerenkov light associated with EAS at 5 200 m altitude

Abstract

An array of four atmospheric Cerenkov light (CL) detectors has been installed on Mt. Chacaltaya (altitude 5 200 m) and operated in conjunction with the Bolivian Air Shower Joint Experiment (BASJE) for a period of several months. The CL detectors are located at radii of 15, 75, 100, and 300 m from the center of the BASJE array. Each detector consists of nine 5-in.-diameter photomultiplier tubes with a total sensitive area of 880 cm2. The angular response for incident photons is 65° FWHM about the vertical. The detector integration time is about 10−7 s, and the associated electronics has a logarithmic response of more than three decades. In the presence of background starlight, the minimum detectable photon pulse is about 104 visible photons/m2. The data exhibit the qualitative features expected for atmospheric CL emission. The average CL intensity varies approximately as N0.8 where the exact exponent depends upon the distance from the shower core. The fluctuations in CL intensity at a fixed distance from the core of EAS of fixed size are found to be greater than the experimental uncertainties. Therefore the observed fluctuations must reflect, in part, the inherent fluctuations in the development of EAS together with the effects of any mixture in the chemical composition of the primary cosmic-ray beam.