RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Study of α-cluster structure in 22 Mg
K.Y. Chae,∗1 S.M. Cha,∗1 K. Abe,∗2 S.H. Bae,∗3 D.N. Binh,∗4 S.H. Choi,∗3 N.N. Duy,∗5 K.I. Hahn,∗6
S. Hayakawa,∗2 B. Hong,∗7 N. Iwasa,∗8,∗9 D. Kahl,∗10 L.H. Khiem,∗11,∗12 A. Kim,∗13 D.H. Kim,∗13 E.J. Kim,∗14G.W. Kim,∗13 M.J. Kim,∗1 K. Kwak,∗15 M.S. Kwag,∗1 E.J. Lee,∗1 S.I. Lim,∗13 B. Moon,∗7 J.Y. Moon,∗16S.Y. Park,∗13 V.H. Phong,∗9 H. Shimizu,∗2 H.Yamaguchi,∗2 L. Yang,∗2 and Z. Ge∗9The study of the α-cluster structure in a nucleus isone of the most important subjects in nuclear physics.
In the case of self-conjugate 4N nuclei, it is well known
that the energy levels with large α-reduced widths form
the rotational bands, which indicates the molecularlike cluster structures of the nuclei. The α-cluster
structure of rare isotopes is, however, still poorly understood especially for N < Z proton-rich nuclei.
As pointed out by Dufour and Descouvemont,1) theproton-rich radionuclide 22 Mg is expected to show anα-cluster structure. The 22 Mg nucleus (18 Ne + α system) was investigated using the generator coordinate
method (GCM), which obtained results indicating theexpected doubling cluster states with J π = 1− and3− at the energy range of 12 < Ex < 13 MeV. However, the energy levels were not clearly observed in thefollow-up experiment performed by Goldberg et al.2)
The elastic scattering of 18 Ne(α, α)18 Ne was measured in the inverse kinematics at the Center for Nuclear Study Radioactive Ion Beam Separator (CRIB)
of the RIKEN Nishina Center in September 2016 for
10 days. A thick target method was adopted so that awide energy range of 22 Mg nuclei could be scanned.
A beam of rare isotope 18 Ne was produced by the3He(16 O, 18 Ne)n reaction using the primary 16 O beamfrom the AVF cyclotron (Ebeam = 8.0 MeV/u). The3He gas target density was 1.54 mg/cm2 . The typicalsecondary beam intensity was about 3 × 105 particles
per second during the runs. The 18 Ne with a beam energy of ∼ 50 MeV impinged on the 470 Torr of the 4 He
gas target at the ﬁnal focal plane. The energy levelsof 22 Mg up to Ex ∼ 18 MeV were observed.

Recoiling α particles were measured by two ΔE-E
silicon detector telescopes located 430 mm downstream
from the target entrance window, which enabled αparticle identiﬁcation using the standard energy loss
technique. No signiﬁcant contamination from otherlight charged particles was found. A background run
with an 87-Torr Ar gas target was performed to see
if the observed α particles originated from the desired
reactions. Figure 1 shows a typical α energy spectrum.
The black and red lines indicate the α energy spectra
for the 4 He and Ar gas targets, respectively. Relative
counts are shown in the ﬁgure for comparison.
A detailed calibration and energy reconstruction will
be performed to determine the precise α energy spectrum. The experimental excitation function of the18
Ne + α system will be extracted and compared withthe theoretical R-matrix calculation.3) The propertiesof populated energy levels such as excitation energies,spins, parities, and the α-reduced widths will be constrained for the α cluster structure information.

Fig. 1. Typical α spectrum. The black (red) line indicatesthe 4 He (Ar) gas run. All data were normalized bybeam intensity. The strong peak near 13 MeV represents leaky α particles that originated at the production
target chamber.

∗1Department of Physics, Sungkyunkwan University∗2 Center for Nuclear Study, University of Tokyo∗3 Department of Physics and Astronomy, Seoul National University∗4 30 MeV Cyclotron Center, Tran Hung Dao Hospital∗5 Department of Physics, Dong Nai University∗6 Department of Science Education, Ewha Womans University∗7 Department of Physics, Korea University∗8 Department of Physics, Tohoku University∗9 RIKEN Nishina Center∗10 School of Physics and Astronomy, University of Edinburgh∗11 Institute of Physics, Vietnam Academy of Science and Technology∗12 Graduate University, Vietnam Academy of Science andTechnology∗13 Department of Physics, Ewha Womans University∗14 Division of Science Education, Chonbuk National University∗15 Department of Physics, School of Natural Science, Ulsan
National Institute of Science and Technology (UNIST)∗16 Institute for Basic Science

References
1) M. Dufour and P. Descouvemont, Nucl. Phys. A 726,53 (2003).
2) V. Z. Goldberg et al., Phys. Rev. C 69, 024602 (2004).
3) A. M. Lane and R. G. Thomas, Rev. Mod. Phys. 30,
257 (1958).

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