This is a repository copy of Two-Neutron Halo is Unveiled in ^{29}F. White Rose Research Online URL for this paper: https://eprints.whiterose.ac.uk/163580/ Version: Published Version Article: Bagchi, S., Kanungo, R., Tanaka, Y. K. et al. (44 more authors) (2020) Two-Neutron Halo is Unveiled in ^{29}F. Physical Review Letters. 222504. ISSN 1079-7114 https://doi.org/10.1103/PhysRevLett.124.222504 Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ PHYSICAL REVIEW LETTERS 124, 222504 (2020) Two-Neutron Halo is Unveiled in 29F S. Bagchi,1,2,3 R. Kanungo,1,4,* Y. K. Tanaka,1,2,3 H. Geissel,2,3 P. Doornenbal,5 W. Horiuchi,6 G. Hagen,7,8 T. Suzuki,9 N. Tsunoda,10 D. S. Ahn,5 H. Baba,5 K. Behr,2 F. Browne,5 S. Chen,5 M. L. Cort´es,5 A. Estrad´e,11 N. Fukuda,5 M. Holl,1,4 K. Itahashi,5 N. Iwasa,12 G. R. Jansen,7,13 W. G. Jiang,8,7 S. Kaur,1,14 A. O. Macchiavelli,15 S. Y. Matsumoto,16 S. Momiyama,17 I. Murray,5,18 T. Nakamura,19 S. J. Novario,8,7 H. J. Ong,20,† T. Otsuka,5,17 T. Papenbrock,8,7 S. Paschalis,21 A. Prochazka,2 C. Scheidenberger,2,3 P. Schrock,22 Y. Shimizu,5 D. Steppenbeck,5,22 H. Sakurai,5,17 D. Suzuki,5 H. Suzuki,5 M. Takechi,23 H. Takeda,5 S. Takeuchi,19 R. Taniuchi,17,21 K. Wimmer,17 and K. Yoshida5 1Astronomy and Physics Department, Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada 2GSI Helmholtzzentrum für Schwerionenforschung GmbH, D-64291 Darmstadt, Germany 3Justus-Liebig University, 35392 Giessen, Germany 4TRIUMF, Vancouver, British Columbia V6T 2A3, Canada 5RIKEN Nishina Center, Wako, Saitama 351-0198, Japan 6Department of Physics, Hokkaido University, Sapporo 060-0810, Japan 7Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 8Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA 9Department of Physics, Nihon University, Setagaya-ku, Tokyo 156-8550, Japan 10Center for Nuclear Study, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 11Department of Physics, Central Michigan University, Mount Pleasant, Michigan 48859, USA 12Department of Physics, Tohoku University, Miyagi 980-8577, Japan 13National Center for Computational Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 14Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada 15Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 16Department of Physics, Kyoto University, Kyoto 606-8502, Japan 17Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 18Institut de Physique Nucleaire, IN2P3, CNRS, Universit´eParis-Sud, Universit´eParis-Saclay, 91406 Orsay Cedex, France 19Department of Physics, Tokyo Institute of Technology, 2-12-1 O-Okayama, Meguro, Tokyo 152-8551, Japan 20RCNP, Osaka University, Mihogaoka, Ibaraki, Osaka 567 0047, Japan 21Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom 22Center for Nuclear Study, University of Tokyo, RIKEN Campus, Wako, Saitama 351-0198, Japan 23Graduate School of Science and Technology, Niigata University, Niigata 950-2102, Japan (Received 14 February 2020; revised manuscript received 17 April 2020; accepted 8 May 2020; published 5 June 2020) ex 27;29 We report the measurement of reaction cross sections (σR )of F with a carbon target at RIKEN. The ex 29 unexpectedly large σR and derived matter radius identify F as the heaviest two-neutron Borromean halo to date. The halo is attributed to neutrons occupying the 2p3=2 orbital, thereby vanishing the shell closure associated with the neutron number N ¼ 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 27F but are challenged for 29F. DOI: 10.1103/PhysRevLett.124.222504 ex In atomic nuclei the strong force binds protons and enhanced root-mean-square matter radius (Rm ) that can be neutrons into complex systems. Long-lived isotopes and β- extracted from the (unusually large) reaction cross section ex ex 1=3 stable nuclei exhibit a well-known shell structure [1,2]. σR , which deviates from the known trend Rm ∝ A with However, in some nuclei with a large neutron excess an mass number A. Some general conditions for halos are unusual type of structure emerges. In neutron-halo nuclei a summarized in Ref. [6]. These exotic nuclei are intricately large nuclear surface is formed that is almost entirely related to changes in the nuclear shell structure. In 11Li, for composed of neutrons [3,4]. Particularly interesting are so- example, the N ¼ 8 shell gap vanishes with the intruder called Borromean two-neutron halos [5]. These intriguing 2s1=2 orbital (35%–55%) that forms a Borromean halo in quantum systems consist of a bound state between a core the last bound isotone [7,8]. nucleus and two neutrons, where any of the two-body Do all traditional neutron shell closures vanish into subsystems are unbound. Examples known so far are 6He, Borromean two-neutron halos? We address this question 11Li, 14Be, 17B, and 22C. A neutron-halo nucleus exhibits an here for N ¼ 20 by reporting the discovery of the heaviest 0031-9007=20=124(22)=222504(7) 222504-1 © 2020 American Physical Society PHYSICAL REVIEW LETTERS 124, 222504 (2020) (a) Plastic scintillator NaI(Tl) array 10 m PPAC Borromean halo to date, and the first of its kind in the 48Ca beam Production target proton sd shell. The measured total reaction cross section F10 ex 29 F0 Wedge-shaped F11 σ of the N ¼ 20 nucleus F is much larger than that of degrader R Dipole F8 27 F5 magnet F1 F4 F6 F9 F. This observation implies a two-neutron halo structure F2 F3 F7 29 Quadrupole in F, and the corresponding melting of the traditional N ¼ magnet 20 2 Secondary shell gap is due to the intrusion of the p3=2 orbital from Ionization chamber target a higher shell. Therefore, the two weakly bound neutrons (b) (c) 103 103 experience only a small centrifugal barrier and have 11 11 Z extended wave function to form the halo. Z 102 The weakening of the N 20 shell gap was first hinted 10 102 10 ¼ 29F at from systematics of the two-neutron separation energies 9 9 (S2n) of sodium isotopes [9] and subsequently observed 10 10 Proton number through the low excitation energy [10] and enhancement of Proton number reduced electric quadrupole transition probability [11] of 8 29F 8 1 1 32Mg. Since then a large number of investigations in neon to 2.9 3 3.1 3.2 3.3 2.9 3 3.1 3.2 3.3 Mass-to-charge ratio A/Q Mass-to-charge ratio A/Q aluminum isotopes found intruder pf-shell components in level schemes [12,13], orbital configurations [14–17], and FIG. 1. (a) Schematic view of the experimental setup. The magnetic moment [18]. nuclei 27;29F are transported from the focal plane F0 to F8, where Monte Carlo shell model calculations [19] align well the reaction target is located. Unreacted 29F is identified using the with these findings. It suggests that the monopole tensor ZDS from F8 to F11. Particle identification (b) before the carbon 29 interaction contributes to the shell quenching [20,21]. The reaction target at F8 and (c) after the target at F11 with F events high atomic number (Z) boundary of the quenched shell is selected before the target. drawn at the aluminum isotopes. The low-Z shore of this quenched shell remains undetermined. The observed lowest beam with an average intensity of 570 pnA and an energy 28 resonance state of F can be explained by the USDB shell of 345A MeV interacting with a 10 mm thick rotating Be model interaction without appreciable need for any intruder target. The isotopes of interest were separated from the orbitals from the pf shell [22] thereby concluding 28Fto various contaminant fragments using the BigRIPS frag- follow normal shell ordering. Large-scale shell model ment separator and identified [Fig. 1(b)] using the tech- calculations, however, predict the Borromean nucleus 29F nique of in-flight energy deposit (ΔE), time of flight (TOF), to be at the boundary of normal to quenched shells [23]. and magnetic rigidity (Bρ). Achromatic wedge-shaped The boundary of bound nuclear landscape, the drip lines, aluminium degraders of thicknesses 15 mm and 5 mm are defined by the last bound isotopes or isotones [24].We were used at the dispersive foci F1 and F5 [black inverted have few data on nuclei close to the neutron-drip line of the triangle in Fig. 1(a)], respectively, to spatially separate the N ¼ 20 isotones. In 29F, the two-neutron separation energy beam contaminants. The Bρ was determined from a S2n ¼ 1.4ð6Þ MeV is only known with a low precision position measurement with parallel plate avalanche coun- [25].