Isobaric States in Nonmirror Nuclei

The 14.8-Mev protons from the Livermore variable-energy cyclotron have been used to study ($p, n$) spectra from 13 selected target nuclei with mass numbers from 48 to 93 for a laboratory angle of 23°. Neutron energies were determined using standard time-of-flight techniques. All spectra show 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 is the usual Coulomb energy displacement. The strong forward peaking of the angular distribution of the neutrons corresponding to the analog state in ${\mathrm{V}}^{51}(p, n){\mathrm{Cr}}^{51}$ confirms the direct nature of the reaction. Although it is tempting, on the basis of the narrow width of the isobaric states (<300 kev), to imply some validity to isotopic spin as a quantum number for masses up to $A=93\mathrm{}$, Lane and Soper have shown that isobaric states in adjoining nuclei can occur even in mass regions where isotopic spin is not a good quantum number.