Halo Structure of the Neutron-Dripline Nucleus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">B</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>19</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>
K. J. Cook, T. Nakamura, Y. Kondo, K. Hagino, Kazuyuki Ogata, A. Saito, N. L. Achouri, T. Aumann, H. Baba, F. Delaunay, Q. Deshayes, P. Doornenbal, N. Fukuda, J. Gibelin, Jongwon Hwang, N. Inabe, T. Isobe, D. Kameda, D. Kanno, S. Kim, N. Kobayashi, T. Kobayashi, T. Kubo, Sylvain Leblond, J. Lee, F. M. Marqués, R. Minakata, T. Motobayashi, Kotomi Muto, T. Murakami, D. Murai, T. Nakashima, N. Nakatsuka, A. Navin, Seiji Nishi, S. Ogoshi, N. A. Orr, H. Otsu, Y. Satou, Y. Satou, Y. Shimizu, Hiroaki Suzuki, Kazuaki Takahashi, H. Takeda, Satoshi Takeuchi, R. Tanaka, Y. Togano, J. Tsubota, A. G. Tuff, M. Vandebrouck, K. Yoneda
Abstract
The heaviest bound isotope of boron ^{19}B has been investigated using exclusive measurements of its Coulomb dissociation, into ^{17}B 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 ^{19}B 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.