The lateral distribution of Cerenkov light in large cosmic ray showers as a measure of longitudinal development

Abstract

This thesis reports measurements made on the longitudinal cascade of high energy cosmic ray showers using observations of atmospheric Cerenkov light. The lateral distribution of the Cerenkov light shower has been shown to be one of the measurable ground parameters which is sensitive to cascade development. The interpretation of such measurements together with other depth sensitive parameters in recorded showers has allowed inferences to be made about the mass distribution of the primary particles and about the nature of the high energy interactions which govern the generation of the cascade through the atmosphere. Measurements of atmospheric Cerenkov light from showers between 10(^15) – 10(^18) eV were made at Dugway Proving Grounds, Utah, U.S.A. between October 1977 and March 1980, using an array of fast photo multipliers to record both the lateral distribution of light density and the time structure of the light pulses. This thesis concerns the detailed analysis and interpretation of the lateral distribution data. These results were subsequently combined with those from pulse timing measurements to present composite results on the gross feature of shower development, the depth of cascade maximum. Analysis techniques have been developed to determine both the average characteristics of the lateral distribution and the fluctuations in the data which could be attributed to intrinsic fluctuations in cascade development. The results of these analyses were then related, through the results of computer simulations of shower development, to the depth of cascade maximum. A change in the elongation rate and the magnitude of fluctuations between 10(^16) and 10(^17) eV has been observed and this is interpreted as a change in primary mass composition from predominantly heavy particles ~ 10(^16) eV to include a greater proportion of light nuclei ~ 2 x 10(^17) eV. The combined measurement of the mean and fluctuations in the depth of maximum has allowed certain interaction models to be rejected. These were those involving scaling in the central region or using an interaction cross section which remains constant with energy. Results from other observations of cosmic ray showers show further indication of the change in primary mass composition between ~ 10(^16) eV and ~ 2 x 10(^17) eV. At higher energies these other results indicate a mass composition changing little with energy.