The intensity of cosmic ray muons deep underground

Abstract

The vertical intensity, I(_v), and exponent of the angular distribution, n, (where I(_o) = I(_v)cos(^n) 0), of muons underground have been measured using simple telescopes comprising plastic scintillators, geiger counters and neon flash tubes at depths of 816, 1812 and 4110 m.w.e. in the Kolar Gold Fields, India. The observed values of n are 1.92 ± 0.33, 3.26 ± 0.10 and 5.33 ± 0.50 respectively, and those of I(_v) are (2.29 ± 0.09)10(^-6), (1.98 ± 0.05)10(^-7) and (4.47 ± 0.34)10(^-9) cm(^-2)sr(^-1)sec(^-1). These results have been compared with the results of other workers and the intensity-depth relation of Osborne et al (1964). The agreement is quite good at 816 and 4110 m.w.e. but the observed intensity is higher by ~30% at 1812 m.w.e. The integral sea-level muon intensities have been inferred from the range-energy relation, allowing for fluctuations in energy losses. From comparison with ɤ - ray cascade data it is shown the K/π ratio is sensibly constant at ~30% over the range of primary energy 10(^3) – 10(^6_) GeV. No significant non-Poissonian contribution to the distribution of time intervals between the arrival of successive events underground was noted indicating that the phenomenon of bursts of particles at short time intervals reported by Cowan et al (1964) has not been observed in the present experiment. The probability of electromagnetic interactions of muons in rock and lead was found to Increase (with increasing depth) demonstrating the growing Importance of energy losses by bremsstrahlung and direct pair production. Analysis of multiple penetrating particle events shows that they cfm be attributed to the production o muons (via pions) in extensive air showers, and to the nuclear interaction of muons in rock. Several neutrino induced muons and atmospheric muons have been detected in a new experiment, still in operation, at 7600 m.w.e. The vertical intensity of muons has been derived from the latter and compared with that inferred from the South African neutrino experiment at 8500 m.w.e. Finally, a best estimate of the intensity depth relation is given.