The neutron distribution in the surface of heavy nuclei

Abstract

A recent study of the capture of K − mesons in light and heavy nuclei present in photographic emulsion has led to the conclusion that primary K − interactions with neutrons are, relative to interactions with protons, approximately five times more probable in heavy than in light nuclei. The ratio M o/ M 1 of the K −-nucleon transition amplitudes in the T=0 and T=1 channels for hydrogen, deuterium, helium and light and heavy nuclei are discussed, and it is shown that the increased number of interactions on neutrons in heavy nuclei is unlikely to be attributable to changes in the transition amplitudes with nuclear binding energy but has rather to be explained in terms of an increased neutron to proton ratio in the nuclear periphery where K − capture mainly occurs. Various proton and neutron distributions ae discussed and it is shown that the magnitude of the effect could be understood if, for instance, in a heavy nucleus, the protons and neutrons had a distribution of Fermi shape with the same half-density radius but with the neutron surface thickness 50% greater than that of the proton distribution. Other experimental evidence for a neutron rich atmosphere in heavy nuclei, as suggested by Johnson and Teller, is briefly reviewed and the value of K − capture studies in different materials as a sensitive means of investigating the neutron distribution in the nuclear surface is discussed.