Experimental Studies For The Halo Structure In The Second Excited State Of ～6Li

Halo nuclei have been the hot point in the studies of nuclear physics since the discovery of the neutron halo (or neutron skin) in the light nuclei,such as "Li,near the neutron drip line in the middle of 1980's. A lot of experimental and theoretical efforts have been devoted to study this phenomenon,and not only the neutron halo structure in a series of light neutron-rich nuclei was discovered,but also the evidence of proton halo in some light proton-rich nuclei was observed. Those investigations have improved one's understanding of the structure,formation conditions,reaction mechanisms,and excitation modes of halo nuclei.Inl990's it was predicted that besides the ground state of the light nuclei near the drip line,the high isospin excited states may also have halo structure. The neutron halo structure of the first excited state of I3C and the neutron-proton halo structure of the second excited state of 6Li,which is the isobaric analog state of the ground state of 6He,were considered the most probable candidates to search for. The former has been observed,while the latter has not been found so far.In an attempt to investigate the halo structure of the second excited state of 6Li by comparing the branching ratio of the Fermi transition to the second excited state (0+) and the GT transition to the ground state (1+) of 6Li from the ground state of 6He,scientists at GANIL and MSU measured the differential cross sections near the zero degree for the 'HHeLQn reaction using the 6He secondary beams of 41.6 AMeV and 93 A MeV,respectively. The expected neutron -proton halo effect of the second excited state of 6Li was not observed in their experiments. Some microscopic theoretical analysis indicates the branching ratio of the transitions near zero degree is not sensitive to the halo structure. The complete angular distribution of the 'H(6He,6Li)n reaction has to be measured to reveal the halo structure of the second exited state of 6Li.We have measured the angular distributions and the cross sections of the 1H(6He,6Li)n reactions populating the ground state and the second excited state of 6Li,with a 4.17AMeV 6He secondary beam on (CHz)n target at the secondary beam line of the HI-13 tandem accelerator. The cross sections of 35+11 mb and 117+10 mb are obtained for the reactions populating the ground state and the second excited state,respectively.Using different combinations of the nucleon density distributions of the ground state of 6He and the second excited state of 6Li with and without the halo,we have calculated the microscopic optical potentials of the input and output channels,and,with these potentials,calculated the angular distribution of the 'H( He,Li)n reaction populating the second excited states of 6Li by using a DWBA code.Comparing the results calculated with different nucleon density combinations,one can see that the existence of the halo structure has obvious effect on the angular distribution. Only assuming that both the ground state of 6He and the second excited state of 6Li have the halo structure as the three-body model predicted,can the calculated angular distribution conform to the experimental results. The root mean square radius of the corresponding nucleon densities are 2.53fm and 2.73fm for the ground state of 6He and the second excited state of 6Li,respectively.The present work provides the first experimental evidence for the theoretical prediction that the second excited state of 6Li is a neutron-proton halo state with an a cluster at its center.