I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Oblate shapes and metastable states of 92, 94 Se†
P. -A. Söderström,∗1,∗2,∗3,∗4 C. Lizarazo,∗1,∗2 V. Werner,∗1 N. Pietralla,∗1 P. M. Walker,∗5 G. X. Dong,∗6F. R. Xu,∗7 T. R. Rodríguez,∗8 F. Browne,∗9 P. Doornenbal,∗3 S. Nishimura,∗3 C. R. Nit, ă,∗10 andA. Obertelli∗1,∗3,∗11 for the SEASTAR2015 collaboration
The main goal of the second SEASTAR campaign1)
performed at the RIBF was the exploration of the nuclear structure evolution in the region of a possible onset of deformation and shape transition between the twocorner-stone nuclei 78 Ni2) and 110 Zr.3) As this region isknown for the large abundance of nuclear isomers,4) theEURICA decay setup5,6) was installed at the end of theZeroDegree spectrometer in addition to the SEASTAR
instrumentation. Thus, complementary to the in-beamspectroscopy data of the Se chain,7) it was possible toobserve internal isomeric decay using high-resolution γray spectroscopy.8)
The type of metastable conﬁgurations observed in deformed atomic nuclei, whereby two quasiparticle states
are formed by breaking pairs of nucleons close to theFermi level, signiﬁcantly changing the angular momentum projection on the symmetry axis is known as
K isomers. These typically originate from deformed
Nilsson orbitals coming up from lower-lying shells inwell deformed prolate nuclei. Analogously, the typical
downsloping of these orbitals on the oblate side opens
up the possibility for a new region of K isomers at low
Z and oblate deformation for exotic nuclei, involving
the same orbitals from higher-lying shells as within theprolate deformed Z ∼ 72 region.
In Ref. 8), we report on the observation of suchstates for 92, 94 Se from EURICA during the SEASTAR
campaign, see Fig. 1, as well as the impact of theirdecay pattern on the discussion of shape evolution
at N = 60. Potential energy surface calculations
suggest oblate K π = 7− (ν11/2− [505] ⊗ ν3/2+ [402])
and K π = 9− (ν11/2− [505] ⊗ ν7/2+ [404]) conﬁgurations for the negative-parity states in 92 Se, and K π =7− (ν11/2− [505] ⊗ ν3/2+ [411]) for the isomer of 94 Se.
For 94 Se, this leads to a hindrance factor of FW =2.49 × 108 and a reduced hindrance fν ∼ 25. Characteristic values for E1 K-trap decays with ν = 6 arewithin 27 ≲ fν ≲ 42.9,10)
Also, the observed J π = 7− states have very diﬀerent
Condensed from the article in Phys. Rev. Lett. 124, 222501(2020)∗1 Institut für Kernphysik, Technische Universität Darmstadt∗2 GSI Helmholtzzentrum für Schwerionenforschung GmbH
∗3 RIKEN Nishina Center∗4 ELI-NP, IFIN-HH∗5 Department of Physics, University of Surrey∗6 School of Science, Huzhou University∗7 State Key Laboratory of Nuclear Physics and Technology,Peking University∗8 Departamento de Física Teórica, Universidad Autónoma deMadrid∗9 School of Computing Engineering and Mathematics, University of Brighton∗10 DFN, IFIN-HH∗11 IRFU, CEA, Université Paris-Saclay†(9- ) T1/2 =15.7(7)μs

3072
3005

(7- )

++2502 (6 , 5 )

2430

(7- )

T1/2=0.68(5) μs(6+1 ) 1935++1605 (42 , 31 )(4+1 ) 1252968(2+1 ) 539(4+1 ) 1113

(2+1 ) 470
92

0+gs

(2+2)

Se58

0+gs

94

Se60Fig. 1. Level schemes constructed for 92, 94 Se. The arrow
width of each transition indicates its intensity.

decay paths for the two isotopes. The decay properties
can partially be explained considering that the collective wave functions predicted by the SCCM method for92
Se7) are relatively soft, implying that K is not a goodquantum number due to the lack of well-deﬁned rigid
axial symmetry. For 94 Se, however, the SCCM method
proposes a rigid oblate ground state and a rigid prolate yrare band, increasing the wave-function overlap
between an oblate isomer and the oblate ground state
relative to the prolate yrare band.
This work presents the observation and interpretationof 92, 94 Se isomeric states. The diﬀerent isomeric decay
paths of each isotope contrast the relatively smooth systematics of the ground-state and K = 2 excited bands
observed from in-beam spectroscopy.7) Also, a similar
32 ns isomer was recently observed for 94 Kr at the ALTO
facility of the IPN Orsay with the ν-Ball array,11) allowing the systematic evaluation of the 7− and 9− statesalong N = 58.
References

1) P. Doornenbal et al., RIKEN Accel. Prog. Rep. 49, 35(2016).
2) R. Taniuchi et al., Nature 569, 53 (2019).
3) N. Paul et al., Phys. Rev. Lett. 118, 032501 (2017).
4) D. Kameda et al., Phys. Rev. C 86, 054319 (2012).
5) S. Nishimura, Prog. Theor. Exp. Phys. 2012, 03C006 (2012).
6) P. -A. Söderström et al., Nucl. Instrum. Methods Phys.
Res. B 317, 649 (2013).
7) S. Chen et al., Phys. Rev. C 95, 041302(R) (2017).
8) C. Lizarazo et al., Phys. Rev. Lett. 124, 222501 (2020).
9) G. D. Dracoulis et al., Rep. Prog. Phys. 79, 076301 (2016).
10) Z. Patel et al., Phys. Rev. C 96, 034305 (2017).
11) R. -B. Gerst et al., Phys. Rev. C 102, 064323 (2020).

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