Direct (p,n) Reaction in MediumANuclei: A Configuration-Selective Process

Time-of-flight techniques have been used on the Livermore variable energy cyclotron to study ($p,n$) spectra from selected target nuclei. Previously reported neutron spectra for 14.8-MeV protons all showed one strong neutron group at an energy corresponding in excitation in the final nucleus to the isobaric counter-part (analog state) of the target ground state, i.e., the $Q$ value is the usual Coulomb displacement energy. With the increase in cyclotron energy to 18.5 MeV, measurements of isobaric ground states have been extended to include ${\mathrm{Rh}}^{103}$, Ag, ${\mathrm{In}}^{115}$, Sn, and ${\mathrm{Pr}}^{141}$. The measured width of the isobaric state is less than 100 keV even for elements as heavy as Ag and Sn where the Coulomb displacement energy is ≃13.5 MeV. Excited isobaric states are found in bombarding even-even nuclei from ${\mathrm{Ar}}^{40}$ to Se, in correspondence with the excited states of the target nucleus. No such prominent excited states are observed in odd-even target nuclei ${\mathrm{V}}^{51}$ and ${\mathrm{Co}}^{59}$. For the "deformable" odd-even nuclei ${\mathrm{Rh}}^{103}$ and Ag, excited isobaric states are observed corresponding to states in the target nucleus with large Coulomb excitation cross sections. Prominent high-energy neutron groups are observed corresponding to the configuration states predicted by Lane and Soper.