Nuclear capture of negative μ mesons in photographic emulsions

Abstract

Ilford C-2 emulsions have been exposed to negative μ mesons from the University of Chicago cyclotron. The μ mesons were separated from the π mesons of the same momentum by utilizing the difference in the ranges of the μ and π mesons in Cu. About 24 000 meson tracks which stopped in the emulsion have been studied. In 591 cases (2.4 percent) the stopped meson is accompanied by the emission of one or more charged particles. A large percentage of these «stars» are one prong stars. The energy distributions of protons and alpha-particles from the one prong stars have been obtained. The general shapes of the energy distribution curves indicate that the protons and alpha-particles from these stars are mostly evaporation products from a moderately excited nucleus such as silver or bromine. The average energy of the charged particles from these stars is lower than from negative π meson stars. The negative π meson contamination is found to be less than 0.3 percent. In several cases the characteristics of the star make it possible to identify the nucleus in which the meson was captured and to evaluate the energy given to the nucleus. In these cases the energy given to the nucleus is found sometimes to be as high as 45 MeV.