Determination of the Number of Neutrons Associated with the Stoppage of Negative μ -Mesons in Lead

Negative mesons (at sea level and therefore assumed to be μ-mesons) were focused onto a lead absorber by means of a magnetic cosmic-ray spectrometer. The number of μ-mesons which stopped in the absorber was determined with an anticoincidence arrangement. Neutrons issuing from the lead absorber were registered during intervals of from 5 to 135 {μ}sec after each such stoppage by a counting system consisting of enriched-BF₃ counters surrounded by 8 cm of paraffin, whose efficiency was measured with the help of a calibrated Na-Be source of {}1-Mev photo-neutrons.For 5500 stopped μ-mesons, the average number of neutrons emitted per {μ}-meson stoppage was found to be 1.96±{}0.72. This result is consistent with the assumption that a neutrino (or light neutral meson) is emitted in the interaction between a μ-meson and a nucleon.