Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Study of resonance states in 26Si by elastic scattering of 22Mg+
N.N. Duy,＊1 L.H. Khiem,＊1 S. Kubono,＊2 H. Yamaguchi,＊2 D. Kahl,＊2 T. Hashimoto,＊2 S. Ota,＊2 Y. Wakabayashi,＊3T. Komatsubara,＊4 T. Teranishi,＊5 S. Kato,＊6 N. Iwasa,＊7 T. Yamada,＊7 Y.K. Kwon,＊8 A. Kim,＊9 Y.H. Kim,＊9J. Song,＊9 J. Hu,＊10 and Y. Ito＊11The data of 26Si are important not only in the astrophysics
but also in the nuclear structure. The resonance states of 26Si
are valuable for investigating the reaction rate of22Mg(α,p)25Al and 25Al(p,)26Si reactions in the Supernovae
II and X-ray bursts, which are important in understanding
the astrophysical nomalies. In addition, the alpha-clusterstructure, above the alpha decay threshold (Ethr = 9.164
MeV), is expected to be obtained in the 26Si nucleus
because of the alpha-cluster threshold rule. Futhermore,
although the energy levels of the mirror nucleus, 26Mg, arewell-known, there are many unknown spin-parities. Thesequantum parameters can be assigned using the 26Si data.
There were some experimental efforts for the levels nearand above the alpha decay threshold, but the data is still
limited. Several resonance states in the energy region
considered were obtained by the works in ref.1, 2), but suchstates had a large uncertainty because of unknown
spin-parities. Therefore, we performed the elastic scatteringof the 22Mg(α,α)22Mg reaction using a radioactive ion (RI)
beam of 22Mg to obtain the experimental data corresponding
to the mentioned aims for 26Si at the CRIB facility ofRIKEN in October 2011. We performed the experiment of22Mg+α by applying the thick-target method using theactive-target detector GEM-MSTPC in inverse kinematics.3)
The RI beam of 22Mg, which was produced via the3He(20Ne,22Mg)n reaction, with the intensity of 1200
particles/s satisfied the energy condition of Ecm = 0.5 – 3.0
MeV in the center-of-mass of the elastic scattering
22Mg(,)22Mg.4) Therefore, the energy corresponded to theregion of Ex = 9.5 – 12.5 MeV in 26Si. The RI beam wasincident on the gas target, which was a gas mixture of
4He+CO2 (10%) at 140 Torr at room temperature. The gas
was filled in a chamber in which the electrode structures ofGEM-MSTPC and arrays of silicon detector telescopes
were installed. The trajectory and timing information of theincoming 22Mg were obtained using the beam monitors
PPACs for particle identification. The ejectiles coming fromthe elastic scattering 22Mg(α,α)22Mg were distinguishedfrom those due to the 22Mg(α,p)25Al reaction based on theE-E method by using the information from the silicon
telescopes. The events due to the elastic scattering and theproduction of the beam contaminants were identified basedon the Bragg curves of the outgoing 22Mg and thecontaminants determined by GEM-MSTPC.

The data were analyzed event by event. The calculationof the kinematics was carried out by considering the energy
loss of the projectile measured by the active-target detector.
The energy of alphas was measured by the active-target
detector and the silicon telescopes. The excitation functionof the cross section of the elastic scattering 22Mg(α,α)22Mg
could be determined in the forward angles, which related to
0-5 degrees and 5-10 degrees in the laboratory frame. The
resonances in the excitation function were applied to theR-matrix analysis performed by the AZURE code5) todeduce the quantum quantities of the resonance states of26
Si above the alpha threshold. We could obtain six states, inwhich three new states (11.245, 11.493, and 11.807 MeV)
and three lower states matched well with the results
obtained by previous works.1, 2) In addition, the spin-parity
assignment for the first and sixth states is satisfied with twovalues, as shown in Table 1.

Institute of Physics, Vietnam Academy of Science and Technology＊2Center for Nuclear Study, Graduate School of Science, Univ. of Tokyo＊3
Japan Atomic Energy Agency＊4
Department of Physics, University of Tsukuba＊5
Department of Physics, University of Kyushu＊6
Department of Physics, University of Yamagata＊7
Department of Physics, University of Tohoku＊8
Department of Physics, ChungAng University＊9
Department of Physics, Seoul University＊10
Institute of Modern Physics, Chinese Academy of Science＊11
Low Energy Experimental Nuclear Physics Gr., Univ. of Tsukuba

1) A. Matic et al., Phys. Rev. C 84 025801 (2011) 1 - 7.
2) J. C. Thomas et al., Eur. Phys. Jour. A 21, (2004) 419-435.
3) N. N. Duy et al., CNS Ann. Rep. 2011 (2013) 9 - 10.
4) N. N. Duy et al., Nucl.Instrum.Meth A 723 (2013) 99-101.
5) R. E. Azuma et al., Phys. Rev. C 81 045805 (2010) 1-17.
6) F. Ajzenberg-Selove et al., Nuclear Physics A114 (1968) 13-16.
7) L. R. Hafstad et al., Physical Review Vol.54 (1938) 681 – 692.

Table 1. The resonance states above the alpha threshold of26
Si obtained by the elastic scattering of 22Mg(α,α)22Mg.
LevelsEr (MeV)(MeV)
J
+
110.325  0.071
0.218  0.011(2 , 1-)210.678  0.016
0.194  0.006
0+310.831  0.113
0.186  0.013
1411.245  0.028
0.208 0.027
4+
511.493  0.216
0.292  0.010
3611.807  0.117
0.156  0.032(0+, 2+)
According to the study in ref.6), the first, third, and sixth
level in the 26Si nucleus are very close to the threeresonances located at 10.300 MeV (0+), 10.844 MeV (1-),
and 11.828 MeV (2-), respectively, in 12C. In addition, suchstates of 12C might have a structure of 3. 7) Therefore, the26
Si may exist under alpha cluster as a structure of(p+3+3+p).
The direct measurement of alpha elastic scattering was
performed for the first time. The data of 26Si is very limited
so far. The cluster structure of the isotopes with a number ofnucleon nearby the value of 4N (N = 2, 3, 4…) is very
uncertain. Hence, we need such type of data significantly
more to investigate the structure of 26Si in future.
References＊1- 43 -