The Scattering of 45-Mev Positive Pions by Hydrogen

Abstract

Standard Ilford G.5 plates were exposed in an improvised 46-Mev external 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 scattered pion. Energy-momentum conservation permits separation of similar looking nonhydrogen events from the pion-proton scatterings. The average pion energy in the plates was determined to be 45±2 Mev from 15 events where the recoil proton stops in the emulsion. A total cross section of (12±3) millibarns has been obtained from 37 pion-proton scatterings. No scatterings were observed less than 79°. Only four are less than 90°. A phase-shift analysis of the data gives ${\alpha }_3=-(5.7\mathrm{}\pm 1.2)°$, ${\alpha }_{33}=(4.4\mathrm{}\pm 1.1)°$, and ${\alpha }_{31}=(2.4\mathrm{}\pm 1.8)°$. Since these particular phase shifts happen to give very little Coulomb interference, these data do not determine the absolute signs of the phase shifts. Upon comparison of these results with scattering data at other energies, ${\alpha }_3$ appears to vary as the first power of the pion momentum and ${\alpha }_{33}$ with the third power at pion energies less than 80 Mev.