Structure of the neutron-halo nucleusHe6

The ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ ${(}^7$Li${,}^7$Be${)}^6$He charge-exchange reaction leading to the neutron-halo nucleus ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ has been studied at E${(}^7$Li) = 350 MeV. Magnetic analysis was used to observe transitions to the known ${\mathit{J}}^{\mathrm{\pi }}$ = $0^{+}$ ground state and the ${\mathit{J}}^{\mathrm{\pi }}$ = $2^{+}$ state at ${\mathit{E}}_{\mathit{x}}$ = 1.8 MeV as well as pronounced resonances at ∼5.6 MeV, ∼14.6 MeV, and ∼23.3 MeV. Coincidences with 430-keV Doppler-shifted γ rays from the deexcitation in flight of the ${\mathit{J}}^{\mathrm{\pi }}$ = 1/$2^{\mathrm{-}}$ first-excited state in ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ were measured to permit the identification of spin-flip transitions. All observed transitions appear to have spin-flip characteristics. The shapes of the experimental angular distributions from ${\mathrm{\theta }}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$ = 0° to 18° are well described by microscopic one-step finite-range distorted-wave calculations with theoretical shell-model transition amplitudes. For the two low-lying shell-model states the absolute cross sections are also well described. The internal structures of the projectile and ejectile are taken into consideration. A large number of contributions is permitted by the angular momentum couplings. Only the ground state of ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ carries significant Gamow-Teller strength B(GT). Contributions with higher L values from the central spin flip and the tensor interactions ${\mathit{V}}_{\mathrm{\sigma }\mathrm{\tau }}$ and ${\mathit{V}}_{\mathit{T}\mathrm{\tau }}$ are responsible for the mostly structureless distributions observed, and the 0° cross sections are not proportional to B(GT). The strong resonances at ∼5.6 MeV and ∼14.6 MeV are interpreted as $2^{+}$ and (1,2${)}^{\mathrm{-}}$ resonances, respectively, with cross sections stronger than predicted presumably due to mixing with continuum states leading to quadrupole and dipole enhancements. It appears that the resonance at ∼5.6 MeV does not represent a soft dipole mode originally predicted at ${\mathit{E}}_{\mathit{x}}$=4–7 MeV. © 1996 The American Physical Society.