SPIN-ISOSPIN MULTIPOLE EXCITATIONS BY MEANS OF THE (LI-6, HE-6) REACTION AT 100 A.MEV

The charge exchange reaction (Li-6, He-6) has been investigated around 0-degrees at 100 A.MeV on C-12, C-13, Ni-58, Zr-90 and Pb-208 targets . Strong selectivity for L = 0, S = 1 states in N-12 and N-13 nuclei i s observed. Due to the finite angular opening of the spectrometer (+/- 1.3-degrees) the known Gamow-Teller resonances (L = 0) in Cu-58, Nb-9 0 and Bi-208 appear in the residual energy spectra superimposed on muc h broader structures, located at higher excitation energies, most prob ably related to a strong excitation of the Spin Dipole (1homegaBAR, L = 1), Spin Quadrupole (2homegaBAR, L = 2) and Spin Isovector Monopole (2homegaBAR, L = 0) resonances. An attempt has been made to reproduce the excitation energy spectra up to 40 MeV, assuming that only L = 0, 1 and 2 spin-multipole resonances contribute to the (Li-6, He-6) cross section on Ni-58, Zr-90 and Pb-208 targets. A very good overall fit o f the spectra is obtained with the excitation energy and width paramet ers very close to the original values. A general trend of the (Li-6, H e-6) reaction at 100 A.MeV is that the GT state (L = 0) contains about 6% of the cross section observed in the investigated angular range (a lmost-equal-to 5-10 mb/sr), whereas the remaining 94% of the cross sec tion is accounted for by the strong population of the (1homegaBAR, L = 1) and (2homegaBAR, L = 0,2) resonances.