Structure of the neutron-halo nucleus 6He

The {sup 6}Li({sup 7}Li,{sup 7}Be){sup 6}He charge-exchange reaction leading to the neutron-halo nucleus {sup 6}He has been studied at {ital E}({sup 7}Li) = 350 MeV. Magnetic analysis was used to observe transitions to the known {ital J}{sup {pi}} = 0{sup +} ground state and the {ital J}{sup {pi}} = 2{sup +} state at {ital E}{sub {ital x}} = 1.8 MeV as well as pronounced resonances at {approximately}5.6 MeV, {approximately}14.6 MeV, and {approximately}23.3 MeV. Coincidences with 430-keV Doppler-shifted {gamma} rays from the deexcitation in flight of the {ital J}{sup {pi}} = 1/2{sup {minus}} first-excited state in {sup 7}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 {theta}{sub c.m.} = 0{degree} to 18{degree} 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 {sup 6}He carries significant Gamow-Teller strength {ital B}(GT). Contributions with higher {ital L} values frommore » the central spin flip and the tensor interactions {ital V}{sub {sigma}{tau}} and {ital V}{sub {ital T}{tau}} are responsible for the mostly structureless distributions observed, and the 0{degree} cross sections are not proportional to {ital B}(GT). The strong resonances at {approximately}5.6 MeV and {approximately}14.6 MeV are interpreted as 2{sup +} and (1,2){sup {minus}} resonances, respectively, with cross sections stronger than predicted presumably due to mixing with continuum states leading to quadrupole and dipole enhancements. (Abstract Truncated)« less