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

Negative mesons (at sea level and therefore assumed to be $\mu$-mesons) were focused onto a lead absorber by means of a magnetic cosmic-ray spectrometer. The number of $\mu$-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 μsec after each such stoppage by a counting system consisting of enriched-B${\mathrm{F}}_3$ 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.