Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Breakup of proton halo nucleus 8 B at near-barrier energies†
L. Yang,∗1,∗2 C. J. Lin,∗1,∗3 H. Yamaguchi,∗2,∗4 A. M. Moro,∗5,∗6 N. R. Ma,∗1 D. X. Wang,∗1 K. J. Cook,∗7,∗8M. Mazzocco,∗9,∗10 P. W. Wen,∗1 S. Hayakawa,∗2 J. S. Wang,∗11 Y. Y. Yang,∗12 G. L. Zhang,∗13 Z. Huang,∗13A. Inoue,∗14 H. M. Jia,∗1 D. Kahl,∗15 A. Kim,∗16 M. S. Kwag,∗17 M. La Commara,∗18 G. M. Gu,∗17S. Okamoto,∗19 C. Parascandolo,∗20 D. Pierroutsakou,∗20 H. Shimizu,∗2 H. H. Sun,∗1 M. L. Wang,∗13 F. Yang,∗1and F. P. Zhong∗1,∗3
As recognized since the very ﬁrst measurements inthe mid-1980s,1) the most neutron-rich unstable isotopeof lithium, 11 Li, was reported to present an unexpected
halo-type structure. After this experimental discovery
in 11 Li, several other nuclear systems with neutron halo
structures were reported. On the proton-rich side, 8 B
is one of the few cases whose ground state presents theproton halo.2) The large extent of the nuclear matter
distribution and the very low threshold of the breakup
channel strongly impact the reaction mechanism, particularly at energies around the Coulomb barrier, wherecouplings to the breakup continuums are expected tobe particularly signiﬁcant. Consequently, the detailed
knowledge of the breakup mechanism is critical for anunderstanding of the reaction dynamics of halo nuclearsystems.
The breakup mechanism of the proton halo nuclei israther complicated. The dynamic Coulomb polarization eﬀect3) may produce a hindrance to both the proton transfer and breakup processes,4) which results in
the rather elusive character of proton halo systems. To
obtain a comprehensive understanding of the breakup
process, the coincident measurement of the breakup
fragments is the inevitable course. However, it is asigniﬁcant challenge for incident energies close to theCoulomb barrier, as it is not easy to carry out a coincidence measurement among the breakup fragments as

was done at higher energies owing to the much reduced
kinematic focusing. It becomes worse owing to the very
low beam intensities of proton halo nuclei.
To address this long-standing challenge, we performed the complete kinematics measurement of8B + 120 Sn at two energies around the barrier.5) Owing
to the highly eﬃcient silicon detector array, the correlation of the breakup fragments, 7 Be and proton, isderived for the ﬁrst time. As an example, the relative
energy spectra (Erel ) is presented in Fig. 1. The correlations can be explained by the state-of-the-art continuum discretized coupled channel calculations. The results indicate that 8 B presents distinctive reaction dynamics: the dominance of the elastic breakup. Thisbreakup occurs primarily through short-lived continuum states and almost exhausts the 7 Be yield. The
correlation reveals that the prompt breakup mechanism
dominates, occurring predominantly on the outgoing
trajectory. We also report that, as a large environment,
the continuum of 8 B breakup may not signiﬁcantly impact elastic scattering and complete fusion.

†Condensed from the article in Nat. Commun. 13, 7193 (2022)
Department of Nuclear Physics, China Institute of AtomicEnergy∗2 Center for Nuclear Study, University of Tokyo∗3 College of Physics and Technology & Guangxi Key Laboratory of Nuclear Physics and Technology, Guangxi NormalUniversity∗4 National Astronomical Observatory of Japan∗5 Departamento de FAMN, Universidad de Sevilla∗6 Instituto Interuniversitario Carlos I de Fı́sica Teórica y Computacional (iC1)∗7 Department of Physics, Tokyo Institute of Technology∗8 Facility for Rare Isotope Beams, Michigan State University∗9 Dipartimento di Fisica e Astronomia, Universita di Padova∗10 Istituto Nazionale di Fisica Nucleare-Sezione di Padova∗11 School of Science, Huzhou University∗12 Institute of Modern Physics, Chinese Academy of Sciences∗13 School of Physics, Beihang University∗14 Research Center for Nuclear Physics, Osaka University∗15 Extreme Light Infrastructure—Nuclear Physics, Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH)∗16 Center for Extreme Nuclear Matters, Korea University∗17 Department of Physics, Sungkyunkwan University∗18 Department of Pharmacy, University Federico II∗19 Department of Physics, Kyoto University∗20 Istituto Nazionale di Fisica Nucleare-Sezione di Napoli∗1Fig. 1. Measured Erel distribution (circles) for the breakup
fragment at 38.7 MeV. The vertical line indicates theexpected location of the peak from the ﬁrst 1+ resonanceof 8 B. The solid and dashed curves denote the simulated
distributions of Erel with the orbital angular momentum
up to l = 3 and the l = 1 (1+ ) state, respectively.

References
1) I. Tanihata et al., Phys. Rev. Lett. 55, 2676 (1985).
2) G. A. Korolev et al., Phys. Lett. B 780, 200 (2018).
3) M. Ito et al., Nucl. Phys. A 787, 267 (2007).
4) Y. Kucuk et al., Phys. Rev. C 86, 034601 (2012).
5) L. Yang et al., Nat. Commun. 13, 7193 (2022).

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