The Proton Component of the Cosmic Radiation at Sea Level

Abstract

The range spectra of mesons and protons in the sea-level cosmic radiation have been investigated with a cloud chamber that had within its volume a total of 10 cm, of lead equivalent absorbing material. Measurements were made with absorbers equivalent to 2, 7, 17, and 48 cm of lead above the apparatus. The masses of stopping particles were determined by observing the momentum and range of each particle. One hundred and sixty-one mesons ($\mathrm{mass}=205\mathrm{}\pm 2m_e$) and 72 protons ($\mathrm{mass}=1900\mathrm{}\pm 44m_e$) were observed. No evidence was found for the existence of any particles with intermediate mass. Data obtained under each of the four thicknesses of moderating absorber were used to give four points on the differential range spectra for measons and protons. The meson spectrum has a peak occurring in the range between 17 cm and 30 cm of lead. The observed proton component can be interpreted either as the residual of a flux of protons which is incident on top of the moderator and degraded in energy with passage through the moderator, or as a flux of protons produced in the moderator by an incident neutral radiation, probably neutrons. The proton range spectrum fits an exponential absorption law with a mean free path of 12 cm of lead. This is just the order of attenuation to be expected from nuclear interactions of protons or neutrons. Thus, if the protons are incident on the moderator, their differential range spectrum would have to have been flat in the region between 8 and 54 cm range in lead. This implication, together with the behavior of the proton component in the lead absorber plates observed in the cloud chamber, points to the incident neutron hypothesis as the more probable explanation of the observed protons.