Direct observation of individual cosmic ray showers

Abstract

THE detection of the cores and directions of large cosmic ray showers became possible in the late 1950s as a result of the combination of the techniques of density sampling, using an array of particle detectors to find the shower centre of symmetry¹ and fast timing using spaced scintillation counters to fix the shower arrival direction². Previous Cerenkov light measurements in extensive air showers have concentrated on the gross properties of the light flux on the ground using a similar approach to the particle measurements. It has been suggested by Fomin and Khristiansen³ and Efimov et al.⁴. that the full-width half maximum (FWHM) of pulses of Cerenkov light may depend on the longitudinal development of the shower, and so on the atomic mass number of the primary. We have gone further and used precise measurements of the detailed shape of the Cerenkov light pulses recorded by several well spread simple detectors to reconstruct accurately an image of the shower in Cerenkov light. This has been done without the usual prerequisite of first determining the shower's axis of symmetry and arrival direction. The data so obtained may be of use in studies of the average shower characteristics, the search for extreme fluctuations and, we hope, for the labelling of individual showers with an indication of the mass of their primary.