Fast Protons from the Capture ofπ−Mesons in Photographic Emulsions

Abstract

The spectrum of fast protons from 1631 ${\pi }^{-}$ meson absorptions in Ilford G-5 and C-2 emulsions has been studied. It has been found that (16.1±1.8) percent of all ${\pi }^{-}$ meson absorptions are accompanied by protons of energy greater than 20 Mev, and that (9.5±1.3) percent of all absorptions are accompanied by protons of energy greater than 30 Mev. The proton energies were estimated by grain counting in comparison with ${\pi }^{-}$ mesons of known residual range. Allowance was made for the variation of grain density with position and depth in the emulsion. The energy spectrum is fairly flat from 30-70 Mev, but only three events were found above 70 Mev. Examination of stars from which the fast protons were emitted shows more prongs ejected opposite to the direction of the fast proton than are accounted for by evaporation from a recoiling nucleus. The energy spectrum favors the capture of the meson by a proton, with the recoil momentum taken by a small, variable number of nucleons. Furthermore, the evidence suggests that the low energy star prongs result from knock-on collisions of the primary nucleons with other nucleons inside the nucleus.