Neutron Halo Structure in exiotic nuclei (^<17>B,^<22>N,^<23>O etc.)

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The two--neutron removal cross section (σ_<-2n>) and the longitudinal momentum distribution of ^<15>B fragments from the breakup of ^<17>B on ^9Be were measured at 70 A MeV. The distribution in the projectile rest frame is characterized by a FWHM of 80±10 MeV/c for ^<15>B. The σ_<-2n> is found to be 0.22±0.05 b. A Glauber--type analysis of the data provides clear evidence of a two-neutron halo structure in ^<17>B.The reaction cross section (σ_R) for the neutron-rich nucleus ^<17>B on a carbon target has been measured at an energy of 77 AMeV by a transmission method. A large enhancement of σ_R was observed (1400mb). The density distribution of ^<17>B was deduced through the energy dependence of σ_R together with high-energy data using a finite-range Glauber model calculation under an optical limit approximation. It is confirmed that the neutron-tail in the density distribution is necessary for ^<17>B in order to reproduce the measured σ_R. Based on the assumption of a core (^<15>B) plus valence two-neutron picture, the mixture of v (2s_<1/2>) and v (1d_<5/2>) was studied without treating the correlation between two neutrons.