Sea-Level Mesons Stopped in Thin Pb and Al Foils. I. Possible Emission of Charged Nuclear Particles and Related Events

Further cloud-chamber experiments are described on the stopping of sea-level mesons in thin foils of Pb (5 of 0.009 and 6 of 0.018 inch) and Al (8 of 0.004 and 3 of 0.032 inch). Eighty-one negative $\mu$ mesons are seen to stop in the Pb foils (50 in the thick foils and 31 in the thin ones). Only two cases show some evidence of slow proton or alpha emission, as indicated by a heavy dot at the end of the stopped negative meson. In the case of Al, there are 36 negative mesons stopped, 12 and 24 in the thin and thick foils, respectively. None of these cases gives any observable evidence of a nuclear event at the place where the negative meson stops. From these statistics, upper limits have been estimated for the probability of nuclear ejection of protons and alpha particles of different energies. It is thus found that, to be ejected from a Pb nucleus with reasonable probability, a proton must have an energy between 3 and 12 Mev, in agreement with the emulsion results of other workers. For Al, since no nuclear event has been observed, it is concluded that the maximum probability of nuclear ejection must be less than 10 percent for a proton of an energy in the above range. An alpha particle in this energy range is improbably emitted from either an Al or a Pb nucleus. As in the previous experiments, no long-track and narrow-angle electron pairs have been observed in either Pb or Al which can be attributed to high-energy photons over 15 Mev emitted from the stopped negative $\mu$ meson (or decaying meson). Thus we find no high-energy gamma rays and evidence for the rarity of even low-energy nuclear events. Consequently we draw the same conclusions as in earlier less extensive experiments: that following the nuclear interaction only a small part of the $\mu$ meson's rest energy is imparted to the nucleus, and that the remaining part must go off as neutral, nonelectromagnetic radiation, to escape observation in the cloud chamber.