RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Long-lived K isomer and enhanced γ vibration in 172 Dy†∗1,∗2∗1H.H.Watanabe
forRIBF88
RIBF88collaboration
collaboration
WatanabeforThe excitation spectrum in deformed nuclei is characterized by rotational and vibrational motion, thelatter corresponding to oscillations around the equilibrium shape with a ﬁxed orientation of the nucleus.
For axially symmetric nuclei, the lowest order shapevibration is of quadrupole type (i.e., a phonon carries
two units of angular momentum), which can be classiﬁed into two modes, β and γ vibrations, in terms of thecomponent of (vibrational) angular momentum along
the symmetry axis, denoted by K. The β-vibrationalmode with K π = 0+ maintains axial symmetry, while
the K π = 2+ γ vibration represents a dynamical distortion from axial symmetry, which may prelude theemergence of γ instability or rigid triaxial deformation
approaching the transitional region where the nuclei
have less-deformed quadrupole shapes.
In the present work, we have explored neutron-rich
Dy isotopes (Z = 66) with a particular focus on thesystematic behavior of the ground-state (g.s.) and γvibrational bands. Based on a simple assumption thatthe axial quadrupole deformation increases as the number of valence nucleons increases, it is conjectured thatthe maximum ground-state deformation occurs in the
doubly mid-shell nucleus 170 Dy (N = 104). In actual
nuclei, however, the stability of shape is likely to besensitive to characteristic single-particle (Nilsson) orbitals near the Fermi surface. Experimentally, the existence of γ-vibrational levels at low excitation energycan be a signature of softening deformed shape with respect to the axially-asymmetric (γ) degree of freedom.
In this report, the ﬁrst spectroscopic results of the g.s.
and γ bands in 172 Dy (N = 106) are presented. Its
excited states have been populated through the decay
from a long-lived isomeric state, which has the same
conﬁguration as the K π = 8− isomers that had been
identiﬁed in the N = 106 isotones from Z = 68 to821) . Thus, the present work has extended a sequenceof the N = 106 isomers to the previously inaccessible
nucleus of Z = 66. It is notable that high-K isomers
can serve as a useful probe for the underlying nuclear
structure since their nature is sensitive to intrinsic orbits near the Fermi surface, pairing and other residualinteractions, and the degree of axial symmetry2) .
The level structure of 172 Dy has been investigatedby means of decay spectroscopy with the EURICA
setup following in-ﬂight ﬁssion of a 238 U beam. Prior
to the present work, no spectroscopic information had
been reported for this neutron-rich nucleus. The level
scheme of 172 Dy established in the present work isdisplayed in Fig. 1. A long-lived isomeric state with†∗1∗2Fig. 1. Partial level scheme of 172 Dy constructed in thepresent work.

T1/2 = 0.71(5) s and K π = 8− has been identiﬁed
at 1278 keV, which decays to the ground-state andγ-vibrational bands through hindered electromagnetictransitions, as well as to the daughter nucleus 172 Ho via
allowed β decays. The robust nature of the K π = 8−
isomer and the ground-state rotational band reveals
an axially-symmetric structure for this nucleus. Meanwhile, the γ-vibrational levels have been identiﬁed atunusually low excitation energy compared to the neighboring well-deformed nuclei, indicating the signiﬁcanceof the microscopic eﬀect on the non-axial collectivity
in this doubly mid-shell region.
References
1) G.D. Dracoulis et al.: Phys. Lett. B 635, 200 (2006).
2) G.D. Dracoulis: Phys. Scr. 2013, 014015 (2013).

Condensed from the article in Phys. Lett. B 760, 641 (2016)
School of Physics, Beihang University
RIKEN Nishina Center- 30 -