RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

β-delayed γ-ray spectroscopy of 168 Dy∗1,∗2∗1,∗2,∗3∗4,∗7∗1,∗2∗1,∗2,∗3
∗4 ∗4
∗5 ∗5
∗6 ∗6
∗4,∗7
G.G.
X.X.
Zhang,
H.H.
Watanabe,
P. P.
M.M.
Walker,J. J. J.
Liu,
J. J.
Wu,
P. P.
H. H.
Regan,P-A.
Zhang,
Watanabe,
Walker,Liu,
Wu,
Regan,
∗3
∗10
∗8∗3
∗3 ∗8
∗8 ∗9
∗9
∗10 ∗3∗8Söderström,
H. Kanaoka,H. Z.
Korkulu, Z. P.
S. Lee, P.
S.S.
Nishimura,A. Yagi, andEURICA 2014
P-A. Söderström,Kanaoka,
Korkulu,Lee,
S. Nishimura,A. Yagi,collaborationand EURICA
2014 collaboration

Atomic nuclei tend to be deformed when moving
away from shell closures, giving rise to rotational motion. In well-deformed nuclei, characteristic rotational
bands can be built on intrinsic states, including not
only the ground state, but also any kinds of vibrational
states and other quasiparticle conﬁgurations. These
intrinsic excitations play an important role in stabilizing the nuclear shape. Recent spectroscopic studies on170Dy1) and 172 Dy2) using the EUROBALL-RIKEN
Cluster Array (EURICA) setup3) revealed that theγ-vibrational levels emerge at unusually low excitation energy compared to the adjacent even-even nuclei. This result indicates an enhancement of the γvibrational mode in this doubly mid-shell region, leading presumably to an excursion from axial symmetry.
This report presents a preliminary result of γ-ray
spectroscopy of 168 Dy following the β decay from
168Tb. The same decay channel has been studied before in Ref. [4], where only three γ rays at 75, 173,
and 227 keV were assigned for the excited states in168
Dy. The former two γ rays have been conﬁrmed as
the 2+ → 0+ and 4+ → 2+ transitions in the groundstate rotational band, respectively, by means of multinucleon transfer reactions5) . In the present work, we
explore the level structure of 168 Dy with higher statistics than Ref. [4] with a particular focus on the nonyrast states being characteristic of γ- and octupolevibrational modes.
The decay spectroscopy experiment for neutronrich Dy isotopes has been performed at RIBF at
the RIKEN Nishina Center. The nuclei of interest
were produced by the in-ﬂight ﬁssion of a 238 U primary beam at 345 MeV/u incident on a Be target
with an average intensity of 12 pnA. About 1.2 × 104
168
Tb65+ ions were transported through the BigRIPSZeroDegree spectrometers and ﬁnally implanted into
the WAS3ABi6) active stopper system comprised of
two double-sided silicon-strip detectors (DSSSD). EachDSSSD has a thickness of 1 mm and an active area of
60×40 mm2 with 1 mm pitch. The DSSSDs recorded
events of heavy-ion implantation and subsequent decayelectrons (both β rays and internal conversion electrons). Gamma rays were detected by EURICA witha full energy-peak eﬃciency of about 10 % for 1 MeV
γ rays. Figure 1 shows the β-delayed γ-ray spectrum
measured within 20 s after the implantation of 168 Tb.
In addition to the transitions reported previously,4,5)
several new γ rays are clearly visible at 216, 322, 915
and 1131 keV. A ﬁt of the electron time distribution
measured in coincidence with these γ rays yields a halflife of 10.4(5) s (see Fig. 2), which is consistent withthe value obtained independently from the analysis ofβ-decay half lives7) . The detailed level scheme of 168 Dy
including these new transitions is still under consideration.

∗1References

Fig. 1. Gamma-ray spectrum following implantation of168
Tb within 20 s.

Fig. 2. Electron time distribution after implantation of168
Tb measured with a sum of gates on the γ-ray peaks
labeled in Fig. 1 except for the 75-keV line and X rays.
An exponential function with a constant backgroundwas used for ﬁtting.

IRCNPC, Beihang UniversitySchool of Physics and Nuclear Energy Engineering, Beihang
University∗3 RIKEN Nishina Center∗4 Department of Physics, University of Surrey∗5 Department of physics, Hong Kong University∗6 School of Physics, Peking University∗7 National Physical Laboratory, Teddington∗8 Department of Physics, Osaka University∗9 INR, Hungarian Academy of Sciences∗10 Department of Physics, Chung-Ang University

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