Ⅱ-1. Nuclear Physics
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

The study of the core-excited component in 11 Li
Y. Li,∗1,∗2 Z. Yang,∗3 Y. Kubota,∗1 and T. Uesaka∗1 for the SAMURAI18 Collaboration
11Li is one of the most well-known drip-line nuclei innuclear physics. The discovery of a spatially extended
structure of neutrons in 11 Li, which is now widely known
as “halo” structure, opened the very active ﬁeld of research with unstable nuclear beams.1) 11 Li has the Borromean nature, generally considered as a 3-body systemof 9 Li plus 2 well-decoupled valence neutrons. However,recent theoretical studies pointed out that contributionof the excited 9 Li core could also be signiﬁcant2,3) in theground state of 11 Li. But no experiment has hitherto
succeeded in providing a direct information about theexcited-core in 11 Li.
The SAMURAI18 experiment performed at the Radioactive Isotope Beam Factory (RIBF) in RIKEN employed the quasi-free (p, pn) reaction. Kubota et al.4) reported the dineutron correlation localized radially on the11Li surface with the data of the 11 Li(p, pn)9 Li(g.s) + nchannel. The core-excited component associated withbound excited states of 9 Li (J π = 1/2− , Ex = 2.69 MeV)
was also studied from coincident gamma-ray measurement and no signiﬁcant contribution of this component
was observed, which can be attributed to the spin-parityconstraints.5) Thus, it can be expected that the coreexcited component in 11 Li that we are interested inshould be associated with a 9 Li excited state with J π= 3/2− and with a higher excitation energy above the8Li + n breakup threshold. By looking into the 8 Li + 2nchannel of the SAMURAI18 experiment, we will be ableto probe the 9 Li*(unbound) + n component in 11 Li.
Secondary 11 Li beams (∼ 1 × 105 pps, ∼246 MeV/nucleon) were produced from the fragmentation of 48 Ca
beam at 345 MeV/nucleon and selected by the BigRIPS
fragment separator. They were then tracked onto the150-mm-thick MINOS target using two multiwire driftchambers. After a (p, pn) reaction, the target protonand one of the neutron of 11 Li were scattered to large
polar angles. The recoil particle detector (RPD) wascomposed of a multiwire drift chamber and a plastic scintillator hodoscope. The neutron detector array WINDS
was installed to measure the scattering angle and thetime of ﬂight of the knockout neutron. The beam-likeresidues—the charged fragment 8 Li and two neutrons—
were analyzed by the SAMURAI spectrometer and theneutron detector array NEBULA, respectively. Single
incoming neutron could induce signals in multiple detectors of NEBULA, a phenomenon commonly called crosstalk. The time-space-separation cuts and the causality
cuts of velocity were applied to eliminate these fake twoneutron events.6)
The relative energy spectrum of 10 Li is reconstructedfrom coincident 8 Li + n + n events using the invariant∗1∗2∗3Spin isospin Laboratory, RIKEN
Institute of Modern Physics, Chinese Academy of Sciences
School of Physics, Peking University- 23 -mass method and is presented in Fig. 1. Two resonancelike structures are observed at Erel ∼ 0.3 MeV and atErel ∼ 1.6 MeV. The spectrum can be well ﬁtted usinga sum of an Is-wave virtual state for the ﬁrst peak anda p-wave resonance for the second peak. Figure 2 shows
the Dalitz plot of the relative energy of the subsystem
8Li + n (Efn ) versus that of 8 Li + n + n (Erel ). The correlation pattern in the Erel range of 1.3–1.7 MeV is consistent with the sequential two-neutron emission via anintermediate 9 Li resonant state with Efn ∼ 0.3 MeV, andFig. 1. Relative energy spectrum for 8 Li + 2n. The blue
dashed line, red dashed line and the green line represent
the the s-wave virtual state ﬁt, p-wave resonance ﬁt andsum of these two components, respectively.

Fig. 2. Dalitz plot of the relative energy. The horizontal
axis represents the relative energy of 8 Li + n + n (Erel )and vertical axis represents the relative energy of 8 Li +n (Efn ).

Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 56 (2023)

this component should thus be assigned to the excitedcore conﬁguration in the ground state of 11 Li. For thenext step, we will analyze the momentum distributionof the knockout neutron to determine its single-particle
orbital occupation. By combining with the theoretical
calculation that is now in progress we will be able to pin
down the excited-core component in 11 Li.
References
1) I. Tanihata et al., Phys. Rev. Lett. 55, 2676 (1985).
2) G. Potel et al., Phys. Rev. Lett. 105, 172502 (2010).
3) Y. Kikuchi et al., Phys. Rev. C 87, 034606 (2013).
4) Y. Kubota et al., Phys. Rev. Lett. 125, 252501 (2020).
5) Y. Kubota, Ph.D Thesis, University of Tokyo (2016).
6) Y. Kondo et al., Nucl. Instrum. Methods Phys. Res. B
463, 173 (2020).

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