A Cloud-Chamber Study of Nuclear Interactions in Lead and Aluminum. Part II

The results of the cloud-chamber Experiment B described in Part I are used to discuss some properties of the particles produced in nuclear interactions. Evidence is presented showing that the electronic showers produced in nuclear interactions are all photon-initiated. Upper limits on the proportion of directly produced electrons are found to be 20 percent in interactions in aluminum and 41 percent in lead. The possibility that the photons are decay products of neutral $\pi$-mesons is strongly indicated, although no definite proof can be given. It is shown that the mean free paths for nuclear interaction of secondary penetrating particles are 172±30 g/${\mathrm{cm}}^2$ in lead and 164±50 g/${\mathrm{cm}}^2$ in aluminum. Assuming that the penetrating particles consist of equal numbers of $\pi$-mesons and protons, one obtains an upper limit of 250 g/${\mathrm{cm}}^2$ for the mean free path for interaction of $\pi$-mesons in lead.