Fine structure in the charge ratio of cosmic ray muons at large zenith angles

Abstract

A horizontal spectrograph having a maximum detectable momentum of 200 GeV/c was constructed and used to measure the charge ratio of cosmic muons as a function of energy and zenith angle. The effect of the earth's magnetic field on muon propagation in the atmosphere was evaluated and the observed ratios were corrected for this effect. The corrected ratios together with the results from the Japanese spectrograph, Kamiya et al. (l969), were examined in an investigation of the peaking in the charge ratio at muon energies ~20GeV as observed in the results from the previous Durham spectrographs, Kelly et al. (1968). The results of this investigation do not lend support to the existence of a fine structure at a unique energy of 20 GeV but high values of the charge ratio in the results from the present spectrograph at 40 GeV do indicate the possibility of a fine structure at an energy dependent oh the azimuthal arrival direction of the muons. If this peaking is not of statistical origin then it would point to some further propagation effect. From the various possibilities considered, if such a fine structure is subsequently verified, it would appear that a further, detailed examination of the effect of Coulomb scattering on muon propagation in the atmosphere might profitably be pursued. It was concluded, however, that although there exists the possibility of an azimuthal effect, the results point to those muons arriving at sea level with energies < 150 GeV having been produced as a result of high energy nucleon interactions in the upper atmosphere with interaction characteristics conforming to the models discussed by MacKeown and Wolfendale (1966). The spectrograph was also used in a subsidiary experiment to investigate the anomalous increase in the energy loss of relativistic charged particles in a plastic scintillation material as indicated by the results of Crispin and Hayman (l964), Smith and Stewart (1966) and Jones et al. (1968),The results from this experiment point to normal behaviour of muons in the context of the Sternheimer energy loss theory.