Analog Giant Resonance Excitation via the (N,p) Reaction

The direct (n,p) reaction is a selective tool for studying isovector transitions which are induced by an isovector interaction and in which there is a transfer of one unit of isospin to the target. Furthermore the (n,p) selection rule △T₃ = △T = +1 implies that only T=Tₒ (tgt) + 1 states (the analogs of Tₒ + 1 states of the target) are excited. Our measurements of (n,p) spectra on selected targets ranging from ⁴He to ²⁰9Bi reveal that the excitations of the analogs of giant resonance states particularly the "E1" GDR and "M1" are strong for light nuclei. Essentially 100% of the GDR sum rule, as given by the Goldhaber-Teller (GT) model, is exhausted. For M1 transitions the use of matrix elements from ß-decay allows the central S=1 part of the isovector interaction to be determined. A larger-than-expected N-Z dependence is found for the Ni(n,p) cross sections of the even Ni isotopes. For heavy nuclei the filling of neutron ahead of proton orbitals produces the expected blocking of (n,p) to the analog GDR excitations, and low-lying resonances in ²⁰9Bi(n,p) appear to be isovector E2 analogs.