Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

μs isomers of 158,160Nd†
E. Ideguchi,∗1 G.S. Simpson,∗2 R. Yokoyama,∗3 Mn. Tanaka,∗1 S. Nishimura,∗4 P. Doornenbal,∗4 G. Lorusso,∗4P. -A. Söderström,∗4 H. Sakurai,∗4 T. Sumikama,∗4∗5 J. Wu,∗4 Z. Y. Xu,∗4 andthe EURICA RIBF-86 collaboration
T1/2 = 339(20) ns–(1648.1)(6 )The doubly mid-shell nuclei of the mass numberA∼160 region are well known for undergoing a rapid
increase in the deformation of their ground states whengoing from the neutron number N =88 to 901) . FromN = 92 onwards their quadrupole deformation is close
to saturation and these nuclei possess well-deformed
prolate ground-state rotational bands. Several of theN = 98 and 100 isotopes here are known to possess
2-qp isomeric states,2–4) , one may therefore expect
similar long-lived states to exist in 158,160 Nd, whoselevel schemes are unknown. The presence of isomericstates oﬀers the opportunity to study the structureof nuclei via the use of delayed γ-ray spectroscopy of
in-ﬂight, mass-separated beams of ﬁssion fragments.
The observation of K-isomers allows the position ofsingle-particle Nilsson states to be mapped in regions
dominated by collective structures, which is useful for
testing the predictions of mean-ﬁeld models. In thepresent work we have studied excited states in the nuclei 158,160 Nd via delayed γ-ray spectroscopy.
In-ﬂight ﬁssion of a 345-MeV/nucleon 238 U beam
was performed at RIBF. Ions including 158,160 Nd wereselected using the BigRIPS spectrometer and implanted in the WAS3ABi stopper5) , or a copper plate,situated at the F11 focal plane. The WAS3ABi stopper
allowed detected β decays to be correlated with identiﬁed and implanted ions, whereas the use of a passive
stopper allowed a higher ion implantation rate for isomer studies. The EURICA Ge array5) was used todetect any γ rays emitted following ion implantation
at the BigRIPS focal plane. Use of ion–γ and ion–γ −γcoincidences has allowed the ﬁrst level schemes to beestablished for the nuclei 158,160 Nd. More details on
the experiment can be found in Ref.6) .
Delayed γ rays with energies of 151.7, 233.4 and
1197.1 keV were found to be in coincidence with 158 Ndions. A delayed cascade proceeding by transitions withenergies of 65.2, 149.9 and 892.8 keV was assigned
to 160 Nd nuclei. The level schemes of 158,160 Nd constructed in the present work are shown in Fig. 1. The
order of the decays was assigned from the level systematics in the neighboring nuclei Nd2) . The spins
and parities of the isomers were assigned from theirdecay patterns and half-lives, as explained in Ref.6) .
†∗1∗2∗3∗4∗51197.1

T1/2 = 1.63(21) μs–(4 )1107.9892.8(6 + )(451.0)233.4(217.6)(4 + )
151.7+(65.9)
(2 +)
00158

Nd215.1(4 + )
149.9+(2 +)
65.2
00
160

NdFig. 1. Level schemes of 158,160 Nd obtained in the presentwork.

The conﬁgurations of the isomeric states of 158,160 Nd
have been assigned with the aid of blocked-BCS(BBCS) calculations. These have allowed a ν5/2[523]⊗
ν7/2[633] conﬁguration to be assigned to the (6− ) isomer of 158 Nd and a ν1/2[521] ⊗ ν7/2[633] one to the(4− ) isomer of 160 Nd, the same as found in the stable 170 Yb4) . It is worth noting that one experimental
decay scheme is incompatible with the results of projected shell-model7) and Hartree-Fock calculations8) .
A peak appears in the r-process abundances aroundmass 160 and its origin remains unclear. Measurements of β-decay half-lives performed within the EURICA campaign at RIBF have shown variations inthese values in the same region, which have a nuclear structure origin9) . Identiﬁcation of further Nilsson states and analysis of the β-decay data gathered in
the our experiment will help resolve these anomalies.
References
1) R. Casten et al., Phys. Rev. C 33, 1819 (1986).
2) G. Simpson et al., Phys. Rev. C 80, 024304 (2009).
3) Z. Patel et al., Phys. Lett. B 753, 182 (2016).
4) P. Walker et al., Nucl. Phys. A 365, 61 (1981).
5) P.A. Söderström et al. Nucl. Instrum. Meth. Phys. Res.
B 317, 649 (2013).
6) E. Ideguchi et al., Phys. Rev. C. 94, 064322 (2016).
7) Y.-C. Yang et al., J. Phys. G 37, 085110 (2010).
8) S.K. Ghouri et al., Int. Mod. J. Phys. 21, 1250070(2012).
9) J. Wu et al., Phys. Rev. Lett. 118, 072701 (2017).

Condensed from the article in Phys. Rev. C. 94, 064322(2016)
RCNP, Osaka University
LPSC, Grenoble
CNS, University of Tokyo
RIKEN Nishina Center
Department of Physics, Tohoku University- 29 -