ZDependence of Positive Pion Production by Photons

The relative yield of positive pions from H, C, Al, Cu, Ag, and Pb produced by 335-Mev bremsstrahlung has been measured with a counter telescope at 5 angles to the beam line ranging from 45° to 150° and at 9 meson energies extending from 12 Mev to 125 Mev. The pion production cross sections at all angles and at all energies of 65 Mev or lower follow approximately a $\frac{Z}{A^{\frac{1}{3}}}$ dependence. Fair agreement with the optical model is obtained at the lower energies, provided account is taken of effects of the Coulomb potential and the potential arising from the pion-nucleon interaction. In the intermediate region from 22.5 to 65 Mev, the yields from the heavy nuclei are somewhat lower than predicted. At pion energies of 95 and 125 Mev, where the production cross sections follow more nearly a $Z$ dependence, it is possible to get moderately good agreement with theory by considering the effects of the Coulomb potential on the production near the cutoff energy.