Quasielastic (p, n) Angular Distributions

Previously reported neutron spectra resulting from the ($p, n$) reaction all showed one strong neutron group at an energy corresponding in excitation in the final nucleus to the isobaric counterpart (analog state) of the target ground state, i.e., the $Q$ value for the reaction is the usual Coulomb displacement energy. Using time-of-flight techniques and the higher proton energies available from the 90-in. cyclotron after recent modifications, we have measured the angular distributions of the neutrons from the isobaric ($p, n$) reaction for 18.5-MeV protons on 19 targets between ${\mathrm{Be}}^9$ and ${\mathrm{Nb}}^{93}$. The optical model, with the inclusion of the isospin term as suggested by Lane, is capable of explaining the diffractive nature of the ($p, n$) angular distributions and the absolute magnitude of the cross sections. The strength of the isospin potential ($V_1$) obtained from the quasielastic ($p, n$) reaction ($V_1\approx 70-90$ MeV) is in agreement with the magnitude calculated from the proton potential anomaly ($V_1\approx 100$ MeV).