Elastic Scattering of 118-Mev Negative Pions by Hydrogen

Standard Ilford G5 plates were exposed in the 122-Mev external negative pion beam of the Chicago cyclotron. Scatterings of the pions by the hydrogen in the emulsion were observed by scanning for recoil protons and examining the proton beginnings for an incoming and a scattered pion. Energy-momentum conservation permits separation of similar looking nonhydrogen events from the pion-proton scatterings. Both 400- and 600-micron plates exposed to about ${10}^6$ mesons/${\mathrm{cm}}^2$ were area scanned under 830× magnification. A scanning efficiency test composed of the events most difficult to see gave a lower limit of 90 percent for scanning efficiency. An average pion energy of (118±2) Mev was determined from 23 events where the recoil proton stops in the emulsion. A total cross section for nonexchange scattering of (9.6±2.0) millibarns has been obtained based on 59 events. The differential cross section obtained can be fitted with the formula $\frac{d\sigma }{d\omega }=(0.49\mathrm{}\pm 0.15)+(0.16\pm 0.21)cos\chi +(0.85\mathrm{}\pm 0.45){cos}^2\chi$ millibarns per steradian. $\chi$ is the scattering angle in the center-of-mass system. This is consistent in shape and magnitude with the results of the Chicago scintillation counter experiments.