Positive pion-nucleus elastic scattering at 40 MeV

Abstract

Differential cross sections for elastic scattering of 40 MeV positive pions from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ were measured at 18 angles in the angular range 25°-160°. Relative uncertainties of 2.5% to 7% and overall normalization uncertainties of 4% to 7% were obtained. An optical potential fit of the data indicates that while the magnitudes of the strength parameters, except for $\mathrm{Re}{b}_1$, are different from values obtained from free $\pi N$ scattering amplitudes, the variation of these magnitudes with target mass is roughly as predicted by the impulse approximation. Coulomb effects, especially in the cases of ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, are extremely important, but impossible to unambiguously separate from the strong interaction in the optical model.