Ⅱ-2. Nuclear Physics (Theory)

RIKEN Accel. Prog. Rep. 52 (2019)

Dineutron correlation and large quadrupole collectivity in deformed Mg
isotopes near neutron drip line
M. Yamagami∗1
Dineutron correlation is one of the exotic features innuclei near the neutron drip line. The pair excitation
into continuum states plays a key role in creating thestrong spatial correlation between two neutrons. Dineutron correlation is considered to be a universal phenomenon around the drip line but the experimental evidence is still under intense debate except for light mass
nuclei. In this study, we discuss low-lying quadruple excitations in deformed Mg isotopes to clarify the continuum eﬀects in pairing correlation, which could suggest
the presence of dineutron correlation.
First, we solve the Hartree-Fock-Bogoliubov (HFB)
equation with Skyrme energy density functional (EDF).
We demonstrate the result using the SkM* EDF but theUNEDF0 EDF draws the same conclusion. The pairingcorrelation is active for single-particle states whose en(+)
ergy ε satisﬁes ε < λ + Epair . Here, λ is the chemical

Fig. 1. K π = 0+ isoscalar quadrupole transition strengths(+)
with Epair = 10 MeV in 40, 42, 44 Mg. QRPA strengths
ignoring the dynamical pairing eﬀect are compared.

(+)

potential and Epair = 10 MeV is used. The predicted
neutron drip line nucleus is 44 Mg with neutron chemical
potential λn = −0.15 MeV. The quadrupole deformations of 34, 36, 38, 40, 42, 44 Mg are β2 = 0.35, 0.30, 0.28,0.28, 0.21, and 0.15, and the neutron pairing gaps are∆n = 1.21, 1.20, 1.17, 0.98, 1.05, and 1.00 MeV respectively.
On top of the HFB states, we solve the quasiparticle random phase approximation (QRPA) equationFig. 2. Summation of strength SIS2 (6 MeV) in neutron-richin the matrix form.1) Figure 1 shows the K π = 0+Mg isotopes are shown. Eﬀects of continuum states andIS
isoscalar quadrupole transition strength B(Q 2; Eν ) =pairing correlation are investigated.
| ⟨ν| r2 Y20 |0⟩ |2 to the excited state |ν⟩ at excitation energy Eν in 40, 42, 44 Mg. The QRPA excitation is gen(+)
Epair = 10 MeV. It should be noted that the singleerated by the coherent superposition of excitations ofparticle state with ε ≈ 10 MeV has a wave number
both particle-hole and particle-particle types. The trank ≈ 0.27 fm−1 and the spatial size ∆x ≈ 1/k ≈ 3.7 fm
sition strengths without the dynamical pairing eﬀects,
corresponds to the diameter of a dineutron predicted
i.e., QRPA calculation ignoring the residual pairing inaround 40 Mg.2)
teractions, are also shown.
Figure 2 also shows the SIS2 (6 MeV) of RPAIt should be noted that the transition strength of lowand QRPA ignoring the dynamical pairing correlation.
lying states signiﬁcantly reduces when the dynamical
These two results coincide with each other except for
pairing eﬀect is ignored. This excitation mode is in44
Mg. SIS2 (6 MeV) increases by 38.9% when static pairduced by the ﬂuctuation of neutron-pair occupation ining correlation is added in 44 Mg. This is due to an addiNilsson orbits with diﬀerent spatial shapes. For examtional particle-hole conﬁguration from the resonant stateple, the prolate-type orbits [310]1/2 and [301]1/2, and[321]1/2 to resonant state [301]1/2.
the oblate-type orbit [303]7/2 are the main contributors
In conclusion, the coupling to continuum states inaround 40 Mg.
pairing
correlation enhances the low-lying transition(6MeV)=Figure2
shows
thesummation
ofstrength
SIS2
∑
IS
34, 36
strengthsof K π = 0+ isoscalar quadruple excitations
B(Q2;E).
InMg,
thetypicalν
0<Eν <6 MeV(+)
in Mg isotopes near the neutron drip line. This suggests
model size for stable nuclei, Epair = 4 MeV, gives reathe presence of dineutron correlations.
sonable results. The eﬀect of continuum states above(+)

Epair = 4 MeV becomes gradually sizable as it approaches the drip line. SIS2 (6 MeV) converges with∗1

Department of Computer Science and Engineering, Universityof Aizu

- 71 -References

1) M. Yamagami et al., RIKEN Accel. Prog. Rep. 50, 92(2017).
2) M. Yamagami et al., Phy. Rev. C 77, 064319 (2008).