A Cloud-Chamber Study of Neutron Production by Sea-Level Cosmic Rays with Particular Reference to μ-Mesons Stopped in Lead
A multiple-plate cloud chamber has been used at sea level to test the conclusion drawn from counter experiments that neutrons of several-Mev energy are liberated when negative μ-mesons are stopped in lead. The stopped mesons are recognized by comparison of visually estimated ionization and range. With the plate thicknesses used, mesons can be distinguished from electrons and protons. To discriminate against π-mesons, which would have to be produced locally, there is a minimum of condensed material above the chamber, and mesons accompanied by any other particle in the top compartment are excluded from the statistics. Alternate plates of Pb and C of equal stopping power are used, evaporation neutrons being expected from mesons stopped in the Pb but not from mesons stopped in the C. The chamber is surrounded by paraffin and BF3 counters, for the detection of neutrons originating in the plates; a G-M tube telescope above the chamber selects charged particles directed through the chamber. The chamber is expanded when a telescope coincidence is associated with at least one detected neutron. In 1207 accepted pictures, there are 14 definite cases of mesons stopping in the Pb, none of mesons stopping in the C. Including probable as well as definite identifications, the corresponding numbers are 19 and 1. These results confirm the production of evaporation neutrons in μ-meson capture in Pb. The large Pb: C ratio shows that π-meson contamination is negligible when our selection criteria are used. The neutron coincidence pictures reveal the various processes giving rise to these events. Electronic showers account for a large fraction; the "giant resonance" of 15-Mev photons in Pb is probably responsible. About 20 percent of the events are energetic nuclear interactions. There is a significant yield of single penetrating particles, some of which must be fast μ-mesons interacting in Pb with small energy loss. The rate is consistent with underground measurements of the neutron yield. A V0 decay has been found with both branches heavily ionizing; it appears to be a V10 with Q in the range 30-55 Mev.