Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

SAMURAI18: comprehensive study of 11 Li
Y. Kubota,∗1,∗2 A. Corsi,∗3 Z. H. Yang,∗4 P. André,∗3 Y. Li,∗2,∗5 M. Miwa,∗6 Y. L. Sun,∗7 and T. Uesaka∗1,∗2for the SAMURAI18 Collaboration
Since the discovery of the neutron halo, 11 Li has been
one of the most attractive neutron drip-line nuclei. It
is known as a Borromean nucleus, described as a threebody system with two weakly bound valance neutrons
around a 9 Li core nucleus, with none of the two-bodysubsystems (10 Li or 2 n) having bound states. A hypothetical bound state of two neutrons, known as thedineutron,1) is thought to be closely related to its existence.
The quasi-free knockout reaction in inverse kinematics was employed in this study to reveal the two-neutron
correlation in 11 Li without breaking quantum coherence.2) By introducing the liquid hydrogen target system MINOS3) to the RIKEN RIBF, an unprecedentedly high luminosity has been realized. The dedicated(p, pn) setup combined with the SAMURAI spectrometer4) allowed kinematically “too” complete measurement resulting in high reliability.
By knocking out a valence neutron, the unbound 10 Li
was produced, and the decay channel of 9 Li + n wasinvestigated. The known low-lying structures of s-wave
virtual state and p-wave resonances were identiﬁed, andthe resonance parameters were signiﬁcantly improved.
Furthermore, an exceptionally narrow resonance witha width of 0.7 MeV at a relative energy of 5.5 MeV
was newly found and identiﬁed as a d-wave resonanceby the multipole decomposition analysis. Recently, theanalysis of the decay channel of 8 Li + 2n has revealed
an interesting structure that provides insight into the9Li core excitation in the ground state of the 11 Li.5)
The in-nucleus momentum k of the knockout neutronis reconstructed from the measured momentum of therecoil proton and the knockout neutron. This value isused as a measure of the radial position of the dineutron as well as to deﬁne the correlation angle θnf , whichis the measure of the dineutron strength. The relationship between θnf and k reveals that the dineutron is
localized at the surface of 11 Li and changes its shape inother region, such as near the center of the nucleus andin the tail of the halo.6) The same analysis has been
carried out for 14 Be and 17 B, and the universality ofthe dineutron is becoming apparent.7)
Only dineutron in the ground state can be studied
with one neutron knockout. In order to overcome thislimitation, the proton knockout from 12 Be was used
∗1
∗2
∗3
∗4
∗5
∗6
∗7

RIKEN Cluster for Pioneering ResearchRIKEN Nishina CenterDépartement de Physique Nucléaire, IRFU, CEA, Université
Paris-Saclay
Department of Physics, Peking University
Institute of Modern Physics, Chinese Academy of Sciences
Department of Physics, Saitama University
Institut für Kernphysik, Technische Universität Darmstadt- 26 -Multi-wiredrift chambers

WINDS

Dipole magnet
NEBULAnn

11Li

p

Plastic
scintillators
MINOS

RPD
1m

re r
wie
lti- mb
Mu t chaf
dritorllanti

sci
9Li
sticPla aysarr

Fig. 1. Schematic of the setup.

to produce the excited 11 Li. The decay mode of the9Li + 2n channel was investigated through the energy
sharing between the two-body subsystems and it was
found that in most cases 11 Li decays through 10 Li, i.e.,emitting the two neutrons sequentially.8)
From the interest of the tensor force, the formation of deuteron clusters in 11 Li was investigated using the deuteron knockout reaction. Since deuterons
are weakly bound systems, approximately half dissociates during the knockout process. However, choosing
the appropriate kinematics can distinguish deuteronoriginated protons and protons from proton knockout.
The deuteron spectroscopic factor could be obtained bycomparing it with the distorted-wave impulse approximation (DWIA) calculation, though it contains large
systematic uncertainties.9)
The proton knockout channel was also investigated
for the 10 He spectroscopy. A three-body correlation
analysis showed convincing result on the long-debatedground state of 10 He.10)
In summary, 11 Li was comprehensively studied using
the quasi-free knockout reaction. In addition to twoneutron correlation in the ground and excited states,
the structure of unbound subsystems and the formationof deuteron clusters were investigated.
References
1) A. B. Migdal, Soviet J. Nucl. Phys. 16, 238 (1973).
2) Y. Kikuchi et al., Prog. Theor. Exp. Phys. 2016,
103D03 (2016).
3) A. Obertelli et al., Eur. Phys. J. A 50, 8 (2014).
4) T. Kobayashi et al., Nucl. Instrum. Methods Phys. Res.
B 317, 294 (2013).
5) Y. Li, SAMURAI International Workshop (2022).
6) Y. Kubota et al., Phys. Rev. Lett. 125, 252501 (2020).
7) A. Corsi et al., Phys. Lett. B 840, 137875 (2023).
8) P. André, Direct Reactions with Exotic Beams (2022).
9) M. Miwa, Direct Reactions with Exotic Beams (2018).
10) Y. L. Sun, Direct Reactions with Exotic Beams (2022).