ZDependence of Positive Photopion Production

The $Z$ dependence of positive photopion production has been measured with 550-Mev bremsstrahlung. The data are compared with the optical model using the real and imaginary potentials given by Frank, Gammel, and Watson. If variations in the production cross section due to the real potential, including the Coulomb potential, are taken into account, the agreement for pion energies in the range 33 to 152 Mev is surprisingly good considering both the approximations in the model and the experimental errors. The observed absorption mean free path varies from ∼9×${10}^{-13\mathrm{}}$ cm for 33-Mev pions to ∼1×${10}^{-13\mathrm{}}$ cm at the highest energies.