Fast Meson Interactions in Nuclear Emulsions. I.π−Mesons

${\pi }^{-}$ mesons produced in the Berkeley cyclotron are collimated and monochromatized to 35±2 Mev by a toothed channel in the magnetic field of the cyclotron. Tracks of these mesons obtained in Ilford G5 emulsions are followed and studied for scatter, nuclear stars, and disappearances in flight. The energy distribution of the accepted mesons is critically examined and a low energy cutoff for the initial energy is established. 65 percent of the meson track is above 30 Mev in the region of observation, while the remaining 35 percent may drop to 20 Mev. The selection of the high energy particles and their identification by small angle scatter and grain density is discussed. Proton contamination is eliminated largely by small angle scatter. Electrons are ruled out by grain count.