Observations of atmospheric Cerenkov light produced in high energy cosmic ray showers

Abstract

An experiment to observe to atmospheric Cerenkov light produced in extensive air showers, has been under development at the British Universities' joint air shower array at Haverah Park, since 1972. This thesis is concerned with the Cerenkov light measurments made at Haverah Park during the U.K. winter of 1975/76, when the experimental equipment was first improved to its present specification. An introduction to extensive air showers and the relevance of Cerenkov light studies, is followed by an account of the theoretical aspects of Cerenkov radiation. The most recent computer simulations of the features of Cerenkov light in extensive air showers are briefly described. A detailed account of the design and performance of the current Cerenkov light detection equipment is given, and the measurements made are reported. Very detailed measurements of the Cerenkov light in air showers can be made with the equipment. The average characteristics of the Cerenkov light in showers of energies around 5 x 10(^17) eV are established. Measurements of both the photon density and the details of the light pulse shape are made at core distances up to about 600 m. The curvature of the Cerenkov light front in air showers is also determined, from measurements of the arrival times of the light. A Cerenkov light measure of primary particle energy is identified. The dependences of the Cerenkov light parameters on shower energy and zenith angle are quantified, and are used to estimate the sensitivities of the parameters to depth of shower cascade maximum. The data measured here agree well with earlier measurements, where such comparison may be made. The measured average characteristics of the Cerenkov light in air showers are found to be in good agreement with predictions from the recent computer simulations. Comparison of the experimental and theoretical data, leads to an estimate of the average atmospheric depth at which showers reach cascade maxima. It is not possible, however, to interpret this estimate in terms of primary particle mass. Finally, the fluctuations in Cerenkov light signals are examined, by studying the light measurements made in a small sample of individual showers. It is shown that fluctuation values of Cerenkov light parameters, contain information about shower development, which is additional to that derived from the average characteristics of the light parameters. An upper limit for the range of atmospheric depths over which showers reach cascade development maxima, as a result of variations in the nucleon interactions in their cascades, is derived. It is concluded that future Cerenkov light measurements in a large sample of showers, may contribute significantly towards the identification of the masses of the primary cosmic ray particles.