Isospin and spin-isospin modes in charge-exchange reactions

The microscopic structure of the Gamow-Teller resonance (GTR) and spin-dipole resonance (SDR) in Bi-208 has been investigated in the Pb-208(He-3,tp)Pb-207 reaction at E(He-3) = 450 MeV and very forward scattering angles. The partial and total branching ratios and the escape widths for GTR and SDR decay to the residual neutron-hole states in Pb-207 were deduced. These are found to be in good agreement with recent theoretical estimates. The (He-3,tp) reaction on Pb at E(He-3) = 177 MeV was also studied in order to locate isovector monopole strength corresponding to 2 (h) over bar w transitions. Monopole strength at excitation energies above 25 MeV was discovered and compared to calculated strength due to the isovector giant monopole resonance and the spin-flip isovector monopole resonance. Calculations in a normal-mode framework show that all isovector monopole strength can be accounted for if the branching ratio for decay by proton emission is 20%.