Interactions of FastμMesons in Lead with Small-Energy Transfer

With the Pb target material divided into 1.27-cm slabs between hodoscoped Geiger tubes and surmounted by a magnet cloud chamber, a study has been made of the interactions of fast $\mu$ mesons in which at least one evaporation neutron is produced and no additional charged particles emerge from the slab. The cross section-average neutron multiplicity, $\sigma \overline{m}$, is (15.2±2.1)×${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^2$ per nucleon. From hodoscope observations without neutron coincidence it is found that for an electron-initiated shower to stay concealed in a 1.27-cm Pb plate its energy must be less than about 100 Mev. With the help of "approximation $B$" track length theory and experimental photonuclear neutron yields, we calculate that hidden knock-on showers contribute (5.8±1.2)×${10}^{-29\mathrm{}}$ to the total yield $\sigma \overline{m}$ (cross section times average multiplicity), leaving (9.4±2.4)×${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^2$ per nucleon as the result for the direct $\mu$-meson nuclear interaction. This analysis is supported by the agreement between the number of visible showers observed and calculated. The neutron yield in the direct interaction is found to increase with $\mu$-meson momentum. The Weizsäcker-Williams approximation is used to calculate the effect expected from the interaction between the electric charge of the $\mu$ meson and the nucleons. Within the rather large uncertainties involved in the use of this approximation, there is excellent agreement with the experimental results.