The Distribution of Multiplicities of Neutrons Produced by Cosmic-Rayμ-Mesons Captured in Lead

Abstract

The nature of the $\mu$-meson capture process in lead has been investigated by studying the number of neutrons emitted by the excited nucleus. Working under 2000 g ${\mathrm{cm}}^{-2\mathrm{}}$ of clay and limestone and a 144 g ${\mathrm{cm}}^{-2\mathrm{}}$ lead filter, events were studied in which a single charged particle penetrated a triple coincidence telescope and stopped in an 86 g ${\mathrm{cm}}^{-2\mathrm{}}$ lead absorber, with one or more delayed coincident neutron counts from an array of ten ${\mathrm{B}}^{10}$ ${\mathrm{F}}_3$ counters in a paraffin moderator placed below the absorber. Events in which more than one G-M tube in any coincidence tray was discharged were rejected. A large paraffin barrier was interposed between the absorber and filter to reduce the sensitivity to neutrons originating above the absorber. The neutron detecting geometry is thought to have had an efficiency sensibly independent of neutron energy up to about 10 Mev. On the basis of 327 delayed neutron coincidences, the mean multiplicity of disintegration neutrons per stopped negative $\mu$-meson was found to be 2.16±0.15, with ±10 percent additional error due to the uncertainty in the strength of the standard neutron source used to determine neutron detecting efficiency. The mean squared multiplicity was found to be 5.2±1.9 on the basis of 3 double-neutron coincidences. On the evaporation model the expected mean multiplicity for 100-Mev excitation energy is about 6, while a calculation based on the ${\mu }^{-}+P\to N+\nu$ hypothesis, using the distribution of excitation energy calculated on the free-particle model, and Weisskopf's statistical theory of the nucleon evaporation process, leads to an expected mean multiplicity of 0.95.