Neutron production by cosmic-ray particles at sea level and underground

Abstract

Further measurements are reported of disintegration neutrons produced in Pb by charged cosmic-ray particles that stop in the Pb ((AB−C:N) events) or that penetrate the Pb ((ABC:N) events). In order to disentangle the different mechanisms producing these events, the same apparatus was used at two different sites, one near sea-level and the other in the sub-basement of a skyscraper, at an equivalent depth below sea-level of 1200 g cm−2 air. At this depth the star-producing radiation should be attenuated at least 60-fold, the penetrating shower producing radiation at least 9-fold. Both the (AB−C:N) and the (ABC:N) rates, are however, only reduced by a factor of about 6. It is concluded that with the 158 g cm−2 Pb filter in place above the telescope all of the underground (AB−C:N) rate and about half of the sea-level (AB−C:N) rate is due to the capture of negative μ-mesons that stop in the Pb absorber below the telescope. The average neutron multiplicity per stopped negative meson comes out to be about 2. The persistence of the (ABC:N) rate underground speaks for neutron production by fast μ-mesons though the depth of the site is not sufficient to make this conclusion certain. Comparison withW. B. Fowler's counter data shows that practically none of the sea-level (ABC:N) rate can be ascribed to fast μ-mesons; underground, the mesonic contribution is appreciable. At both sites, removal of the Pb filter from above the telescope increases the (AB−C:N) rate; it is probable that part of this soft neutron-producing radiation consists of electron-photon showers secondary to the fast mesons.