Neutron Production byμMesons Which Penetrate Lead

The mean product of cross section and multiplicity ($〈\sigma m〉$) for evaporation neutrons produced when fast cosmic-ray $\mu$ mesons traverse a Pb producer has been measured. A magnet cloud chamber above the producer and a hodoscope-absorber array below it serve to identify $\mu$ mesons; neutron-producing events are selected by the detection of a delayed coincidence of a thermalized neutron with a penetrating particle. $〈\sigma m〉=(22\mathrm{}\pm 8)\times {10}^{-29\mathrm{}}$ ${\mathrm{cm}}^2$/nucleon. Theoretical calculations based on experimental photoneutron yields give $〈\sigma m〉=15\mathrm{}\times {10}^{-29\mathrm{}}$ ${\mathrm{cm}}^2$/nucleon for the yield due to knock-on showers, and $〈\sigma m〉=9\mathrm{}\times {10}^{-29\mathrm{}}$ ${\mathrm{cm}}^2$/nucleon for the direct yield from equivalent photons of the $\mu$-meson field. This theory also agrees with the results of underground measurements at higher average momenta. There is no evidence of $\mu$-meson scattering in excess of Coulomb scattering. Protons which suffer nuclear interactions produce more than 50 neutrons, but seldom produce penetrating secondaries.