Elastic Scattering of 26-Mev Negative Pions by Hydrogen in Emulsion

Standard Ilford G5 plates were exposed to a fairly monoenergetic 30-Mev ${\pi }^{-}$-meson beam obtained by slowing down the Chicago 90-Mev beam with absorber and then using magnetic analysis. Range curves were taken indicating a 35 percent contamination from muons and electrons in this geometry. This figure was independently determined from the grain density distribution of tracks in the emulsion. A flux of 2.2×${10}^5$ tracks per ${\mathrm{cm}}^2$ was used for area scanning. The average beam energy in the plates is (26±2) Mev as determined from measuring the recoil proton energies in the 5 pion-proton scatterings which were found. An equivalent pion path length of 13×${10}^4$ cm was scanned giving a cross section of (1.15±0.6) mb for the 5 events. This cross section is for scatterings greater than 50°. Smaller angle scatterings were not considered because of the possibility that such short recoil protons might be missed by area scanning. Various scanning efficiency checks indicate a scanning efficiency above 90 percent. The data are analyzed to give (4.7°±2.7°) for the phase shift combination ($2{\alpha }_1+{\alpha }_3$).