Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 52 (2019)

First observation of 20 B and 21 B†
S. Leblond,∗1 F. M. Marqués,∗1 J. Gibelin,∗1 N. A. Orr,∗1 Y. Kondo,∗2 T. Nakamura,∗2 J. Bonnard,∗3
N. Michel,∗4,∗5 N. L. Achouri,∗1 T. Aumann,∗6,∗7 H. Baba,∗8 F. Delaunay,∗1 Q. Deshayes,∗1 P. Doornenbal,∗8N. Fukuda,∗8 J. W. Hwang,∗9 N. Inabe,∗8 T. Isobe,∗8 D. Kameda,∗8 D. Kanno,∗2 S. Kim,∗9 N. Kobayashi,∗2T. Kobayashi,∗10 T. Kubo,∗8 J. Lee,∗8 R. Minakata,∗2 T. Motobayashi,∗8 D. Murai,∗11 T. Murakami,∗12K. Muto,∗10 T. Nakashima,∗2 N. Nakatsuka,∗12 A. Navin,∗13 S. Nishi,∗2 S. Ogoshi,∗2 H. Otsu,∗8 H. Sato,∗8Y. Satou,∗9 Y. Shimizu,∗8 H. Suzuki,∗8 K. Takahashi,∗10 H. Takeda,∗8 S. Takeuchi,∗8 R. Tanaka,∗2Y. Togano,∗2,∗7 A. G. Tuﬀ,∗14 M. Vandebrouck,∗3 and K. Yoneda∗835

It is well established that the shell structure of thenucleus, that leads to an enhanced stability for systems
with “magic” numbers of protons (Z) and/or neutrons(N ) of 2, 8, 20... is modiﬁed as the limits of particle stability, or driplines, are approached. Neutron numbers
between 8 and 20 correspond to the ﬁlling of the sdshell neutron single-particle orbitals. Approaching thedriplines, the energies of these orbitals evolve, leadingfor example to the disappearance of the N = 20 magic
number for Z = 10–12 and to the appearance of newshell closures at N = 14, 16 in the oxygen isotopes.
In this respect, the most neutron-rich boron isotopes,
which lie below doubly-magic 22, 24 O and straddle theneutron dripline, are of considerable interest.
After removing one or two nucleons from secondary
beams of 22 N and 22 C, produced at the RIBF of theRIKEN Nishina Center, with a carbon reaction target, beam-velocity 19 B fragments and neutrons weredetected in the forward direction using the SAMURAI setup including the NEBULA neutron array.
The relative energy between the 19 B fragment andthe ﬁrst detected neutron is shown in Fig. 1. A
prominent resonance-like structure was observed atabout 2.5 MeV above the one-neutron decay threshold (Fig. 1) that, guided by theoretical considerations, has been identiﬁed as the 1− , 2− ground-state
doublet of 20 B, with energies Er = 1.56 ± 0.15 and2.50 ± 0.09 MeV. A weaker higher-lying peak was also
observed at 4.86 ± 0.25 MeV.
The data acquired for 21 B in the 19 B plus one(Fig. 1) and two-neutron channels were consistent withthe population of a resonance 2.47 ± 0.19 MeV above

25
20

22

O

23

O

24

21

N

22

N

2320

C

19BO
N

22C15N = 16N [counts / 200 keV]

30

1050012319456789Erel( B+n) [MeV]Fig. 1. Relative energy spectrum of 19 B+n events following proton-removal from 22 N (gray) and 22 C (hatchedhistogram). The gray dotted line in the inset delineates
the neutron dripline.

the two-neutron emission threshold, and thus tentatively assigned to be the expected 3/2− ground state.
These results allowed the ﬁrst determinations to bemade of the ground-state masses of 20, 21 B, which arein agreement with the extrapolations of the most recent atomic-mass evaluations taking into account the19B, 22 C and 23 N mass measurements. In this spirit,
the present 20, 21 B masses will permit mass-surface extrapolations in this region to be made with improved
precision and further from stability. In addition, 21 B
was found to exhibit direct two-neutron decay.
The identiﬁcation and ﬁrst spectroscopy of 20, 21 B
opens the way to the exploration of structure and correlations beyond the dripline below 24 O. In particular,
improvements in secondary-beam intensities and neutron detection should permit n-n correlations in thedecay of 21 B to be investigated and its ﬁrst excitedstate to be located. This, coupled with work underway
to investigate the excited states of 22 C, will provide direct insights into the N = 16 shell closure beyond theneutron dripline as well as stringent tests of a new generation of ab initio and related theoretical models.

†Condensed from the article in Phys. Rev. Lett. 121, 262502(2018), see also references therein.
∗1 LPC-Caen∗2 Department of Physics, Tokyo Institute of Technology∗3 Institut de Physique Nucléaire, Orsay∗4 NSCL/FRIB Laboratory, Michigan State University∗5 School of Physics, Peking University∗6 Institut für Kernphysik, Technische Universität Darmstadt∗7 ExtreMe Matter Institute EMMI, GSI∗8 RIKEN Nishina Center∗9 Department of Physics and Astronomy, Seoul National University∗10 Department of Physics, Tohoku University∗11 Department of Physics, Rikkyo University∗12 Department of Physics, Kyoto University∗13 GANIL, CEA/DRF-CNRS/IN2P3∗14 Department of Physics, University of York-1-