RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Shape coexistence in the N =19 neutron-rich nucleus 31 Mg
explored by β-γ spectroscopy of spin-polarized 31 Na†

Asymmetry paramter (A)H. Nishibata,∗1,∗2 T. Shimoda,∗1 A. Odahara,∗1 S. Morimoto,∗1 S. Kanaya,∗1 A. Yagi,∗1 H. Kanaoka,∗1
M. R. Pearson,∗3 C. D. P. Levy,∗3 and M. Kimura∗4
One of the highlights of the recent nuclear physics
is the shape coexistence of nuclei located far from theβ-stability line. In particular, neutron-rich nuclei withneutron numbers close to the neutron magic number N= 20, so-called the “island of inversion”1) , have been
intensively studied. However, the structures are still
not clear because there exists few information on excited states, such as spin-parity, for most of the islandof-inversion nuclei.
We study the neutron-rich nucleus 31 Mg which islocated around the “island of inversion” region. The
level structure of odd-mass 31 Mg is one of the most sensitive probes of shape coexistence because the last neutron orbit, which governs the spin-parity of the level,is strongly aﬀected by the nuclear structure. In thepresent work, detailed data are presented for the excited states of 31 Mg, obtained through a unique andextremely promising method2) to measure spin-parity
based on the β-decay spectroscopy of spin-polarized
31Na.
Our method uses an asymmetric β decay of the polarized nucleus in the allowed transition. The angular
distribution is expressed as W (θ)  1+ AP cosθ, whereA, P , and θ are the asymmetry parameter for each βtransition, the spin polarization, and the emission angle of the β rays with respect to the polarization direction, respectively. The essential point is that theasymmetry parameter takes three diﬀerent values depending on the spins of the parent and daughter states.
In the case of the β decay of 31 Na (I π = 3/2+ ), theasymmetry parameter is either −1.0, −0.4, or +0.6 forthe possible daughter state with a spin-parity of 1/2+ ,3/2+ , or 5/2+ , respectively. Therefore, from the experimental asymmetry value, the spin-parity can beunambiguously assigned. Further information on thismethod can be found in Ref.2)
The experiment was performed at the Isotope Separator and Accelerator (ISAC) facility in TRIUMF,
where a highly polarized Na beam was achieved by simultaneously pumping both atomic ground-state levels
with two laser frequencies. The β-rays and γ-rays associated with 31 Na β decay were measured at the end
†
∗1
∗2
∗3
∗4

1.0

5/2+
(A = +0.6)

0.5
0.944
0−0.5

2.015
0.050

3/2+
(A = −0.4)

0.673

−1.0
2.244
−1.5

00.5
1.0
1.5
Excitation Energy (MeV)

1/2+(A = −1.0)

2.0

Fig. 1. Experimental asymmetry parameters for the levels
at 0.050, 0.673, 0.944, 2.015, and 2.244 MeV.

of the Osaka beam line (for detailed information on theexperimental setup, see the original Letter† ).
Figure 1 shows the experimental asymmetry parameters. It is clearly seen that the experimental values
are consistent with one of the expected values. Hencethe spins and parities are ﬁrmly assigned as 1/2+ forthe 2.244-MeV level, 3/2+ for the 0.050- and 0.673MeV levels, and 5/2+ for the 0.944- and 2.015-MeVlevels. The spins of negative-parity levels are also limited based on transition intensities from the levels withspin-parity assigned in the present work. Furthermore,
11 γ rays and 2 levels are newly found by analyzing theγ-γ coincidence data.
From the obtained spins and energies of the levels in 31 Mg, we propose that the positive-parity levels [1/2+ 0 MeV, 3/2+ 0.050, 5/2+ 0.944, (7/2+ )1.155] and negative-parity levels [(3/2− ) 0.221, (7/2− )0.461, (1/2− ) 1.029, (11/2− ) 1.390] are members of
the K π = 1/2+ and 1/2− rotational bands, respectively. Through a level-by-level comparison between
the experimental results and theoretical calculationsof the AMD+GCM,3) it is found that the levels at0.673 [3/2+ ] and 2.015 MeV [5/2+ ] show good correspondence with spherical levels predicted at 0.81
and 1.85 MeV. Furthermore, the experimental level
at 0.942 MeV [(1/2− or 3/2− )] is probably the 3/2−
level of the band-head of K π = 3/2− rotational bands
in the AMD+GCM calculations. However, the 2.244MeV 1/2+ level cannot be explained by any theories
at present. These results show clear evidence for shapecoexistence in a low excitation energy region of 31 Mg.

Condensed from the article in H. Nishibata et al., Phys.
Lett. B 767, (2017).
Department of Physics, Osaka University, Osaka 560-0043,
Japan
RIKEN, Wako, Saitama 351-0198, Japan
TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T 2A3,
CanadaDepartment of Physics, Hokkaido University, Sapporo 0600810, Japan

References
1) E. K. Warburton et al., Phys. Rev. C 41, 1147 (1990).
2) K. Kura et al., Phys. Rev. C 85, 034310 (2012).
3) M. Kimura, Phys. Rev. C 75, 041302(R) (2007).

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