Halo Structure of the Neutron-Dripline Nucleus 19B

The heaviest bound isotope of boron B-19 has been investigated using exclusive measurements of its Coulomb dissociation, into B-17 and two neutrons, in collisions with Pb at 220 MeV/nucleon. Enhanced electric dipole (E1) strength is observed just above the two-neutron decay threshold with an integrated E1 strength of B(E1) = 1.64 +/- 0.06(stat) +/- 0.12(sys) e(2) fm(2) for relative energies below 6 MeV. This feature, known as a soft E1 excitation, provides the first firm evidence that B-19 has a prominent two-neutron halo. Three-body calculations that reproduce the energy spectrum indicate that the valence neutrons have a significant s-wave configuration and exhibit a dineutronlike correlation.