STRUCTURE OF THE NEUTRON-HALO NUCLEUS HE-6

The Li-6(Li-7,Be-7)He-6 charge-exchange reaction leading to the neutro n-halo nucleus He-6 has been studied at E(Li-7) = 350 MeV. Magnetic an alysis was used to observe transitions to the known J(pi) = 0(+) groun d state and the J(pi) = 2(+) state at E(x) = 1.8 MeV as well as pronou nced resonances at similar to 5.6 MeV, similar to 14.6 MeV, and simila r to 23.3 MeV. Coincidences with 430-keV Doppler-shifted gamma rays fr om the deexcitation in flight of the J(pi) = 1/2(-) first-excited stat e in Be-7 were measured to permit the identification of spin-flip tran sitions. All observed transitions appear to have spin-flip characteris tics. The shapes of the experimental angular distributions from theta( c.m.) = 0 degrees to 18 degrees are well described by microscopic one- step finite-range distorted-wave calculations with theoretical shell-m odel transition amplitudes. For the two low-lying shell-model states t he absolute cross sections are also well described. The internal struc tures of the projectile and ejectile are taken into consideration. A l arge number of contributions is permitted by the angular momentum coup lings. Only the ground state of He-6 carries significant Gamow-Teller strength B (GT). Contributions with higher L values from the central s pin flip and the censor interactions V-sigma tau and V-T tau are respo nsible for the mostly structureless distributions observed, and the 0 degrees cross sections are not proportional to B(GT). The strong reson ances at similar to 5.6 MeV and similar to 14.6 MeV are interpreted as 2(+) and (1,2)(-) resonances, respectively, with cross sections stron ger than predicted presumably due to mixing with continuum states lend ing to quadrupoIe and dipole enhancements. It appears that the resonan ce at similar to 5.6 MeV does not represent a soft dipole mode origina lly predicted at E(x)=4-7 MeV.