Structure Of Neutron-rich Boron Isotope

Investigations of exotic nuclei have made rapid progress by means of the development of radioactive ion-beam facilities and detection techniques during the last few decades. Many interesting properties have been observed and of which halo phenomenon is the most interesting one. If the neutron-proton ratio is large and the radius of neutron is a bit little larger than that of protons, there should be existed a novel structure called "neutron skin". The skin is generally 0.2fm or so in thickness. The nuclei with very large ratio could have "neutron halo" based on the small neutron separation energy. Halo nucleus provides a new way of studying the nuclear matter of low density, as well as the structure and dynamics properties of large density fluctuations system. The research on halo nuclei has very important significance to the celestial nuclei synthesis and nuclear physics itself.Research shows that there is a clear dependency for the change of electromagnetic moment with neutrons of neutron-rich boron isotopes. This may be caused by some changes of structure, such as the cluster and halo. Physicists predict that 17B is of a double neutron halo structure, while 19B is of a loose neutron halo structure.The interaction cross section of 19B have been measured at around 800 A MeV by Suzuki et al and a valence radius analysis suggests a "15B+4n" neutron skin structure in 19B. At the same time, Hiroki et al have investigated the fragmentation mechanism reflecting the cluster structure of 19B. The clustering structure of 19B is reflected in its fragmentation as the dynamical cluster breakup into He and Li isotopes,depending on the incident energy.In this paper, we systematically calculate the ground state properties of neutron-rich boron isotopes, such as binding energy, radius and density distribution. The coupling equations are solved in a spherical box whose radius is 20fm and calculation step length is 0.1fm. The parameter sets NLSH, PK1, TM2 are used which are good to describe light nuclei. The contribution from continua is restricted within a cut-off energy Ecut～120MeV, center density is nuclear material densityρ0= 0.152 fm-3. The odd nuclei adopts single particle blocking. The neutron pairing strength can be obtained by comparing the theoretical and experimental binding energy. The calculations can well meet the experimental value of binding energy and radius. We confirmed the predictions about the exotic structure of 17B and 19B by analyzing the radial distribution of neutron and proton density, the single particle level, the occupy probability of level, the contribution from continua, the neutron separation energy and so on. The calculations show that 17B is of a double neutron halo structure. The neutron halo is formed mainly by the occupied valence neutron level 2s1/2 with small centrifugal barrier and small binding energy. The four-neutron separation energy and two-neutron separation energy of 19B is 0.43MeV and 1.73MeV respectively. This is very similar to 8He, not so as to 11Li. The level 1d5/2 of 19B has large centrifugal barrier. So 19B is unlikely to be of a neutron halo structure. The outer shell of 19B is occupied by four neutrons and these four nucleons should be treated together. We consider 19B has a neutron skin structure consisted of 15B plus four neutrons.