Two-Neutron Halo is Unveiled in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">F</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>29</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>
S. Bagchi, R. Kanungo, Y. Tanaka, H. Geißel, P. Doornenbal, W. Horiuchi, G. Hagen, Toshio Suzuki, Naofumi Tsunoda, D. S. Ahn, H. Baba, J. K. Behr, F. Browne, S. Chen, M. L. Cortés, A. Estradé, N. Fukuda, M. Holl, K. Itahashi, N. Iwasa, G. R. Jansen, W. G. Jiang, S. Kaur, A. O. Macchiavelli, S. Y. Matsumoto, S. Momiyama, I. Murray, T. Nakamura, S. J. Novario, H. J. Ong, T. Otsuka, T. Papenbrock, S. Paschalis, A. Procházka, C. Scheidenberger, P. Schrock, Y. Shimizu, D. Steppenbeck, H. Sakuraï, D. Suzuki, Hiroaki Suzuki, M. Takechi, H. Takeda, Satoshi Takeuchi, R. Taniuchi, K. Wimmer, K. Yoshida
Abstract
We report the measurement of reaction cross sections (σ_{R}^{ex}) of ^{27,29}F with a carbon target at RIKEN. The unexpectedly large σ_{R}^{ex} and derived matter radius identify ^{29}F as the heaviest two-neutron Borromean halo to date. The halo is attributed to neutrons occupying the 2p_{3/2} orbital, thereby vanishing the shell closure associated with the neutron number N=20. The results are explained by state-of-the-art shell model calculations. Coupled-cluster computations based on effective field theories of the strong nuclear force describe the matter radius of ^{27}F but are challenged for ^{29}F.