Isospin and spin-isospin modes in charge-exchange reactions

The microscopic structure of the Gamow-Teller resonance (GTR) and spin-dipole resonance (SDR) in 208Bi has been investigated in the 208Pb(3He,tp)207Pb reaction at E(3He)=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 207Pb were deduced. These are found to be in good agreement with recent theoretical estimates. The (3He,tp) reaction on Pb at E(3He)=177 MeV was also studied in order to locate isovector monopole strength corresponding to 2ℏω 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%