Cosmic ray showers at large Zenith angles

Abstract

A large vertical array of muon detectors, of sensitive area 34 square metres, has been constructed and used to determine the frequency of observation of groups of coincident muons in E.A.S. as a function of the zenith angle and of the number of muons in the group. These observations are compared with the results of theoretical predictions, based on the E.A.S. model of De Beer et al. (1966), and a composite sea-level electron size spectrum based on the results of many workers. Two alternative predictions have been made, one based on a model of the primary spectrum in which the primary flux suffers rigidity modulation at ~10(^15) eV, the other on a model in which the heavy primaries fragment above a critical energy (~10(^15) eV) and the primary flux consists solely of protons. The experimental observations are sensitive to the mean transverse momentum and the results suggest that this remains nearly constant at O.4 GeV/c over the range of interaction energy 200 - 2000 GeV. The results also suggest that the primary spectrum model containing a pure proton flux above ~10(^15) eV is to be preferred, thus there is no evidence from this work for an increasing primary mass above ~10(^15) eV as would have been expected from the rigidity modulation hypothesis.