Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Investigation of octupole correlations of neutron-rich Z ∼ 56 isotopesby β-γ spectroscopy

Counts/keV

R. Yokoyama,∗1 E. Ideguchi,∗2 G. Simpson,∗3 Mn. Tanaka,∗2 S. Nishimura,∗4 P. Doornenbal,∗4P.-A. Söderström,∗4 G. Lorusso,∗4 Z. Y. Xu,∗5 J. Wu,∗4,∗6 T. Sumikama,∗7 N. Aoi,∗2 H. Baba,∗4 F. Bello,∗8F. Browne,∗9,∗4 R. Daido,∗10 Y. Fang,∗10 N. Fukuda,∗4 G. Gey,∗3,∗4,∗11 S. Go,∗1,∗4 N. Inabe,∗4 T. Isobe,∗4D. Kameda,∗4 K. Kobayashi,∗12 M. Kobayashi,∗1 T. Komatsubara,∗13 T. Kubo,∗4 I. Kuti,∗14 Z. Li,∗6M. Matsushita,∗1 S. Michimasa,∗1 C.-B. Moon,∗15 H. Nishibata,∗10 I. Nishizuka,∗7 A. Odahara,∗10 Z. Patel,∗16,∗4
S. Rice,∗16,∗4 E. Sahin,∗8 L. Sinclair,∗17,∗4 H. Suzuki,∗4 H. Takeda, ∗4 J. Taprogge,∗18,∗19 Zs. Vajta,∗14H. Watanabe,∗20 and A. Yagi∗10Neutron-rich Ba isotopes (Z = 56, N ∼ 88) are expected to have a signiﬁcant octupole collectivity due
to the interactions between orbitals with ∆j = ∆l = 3
around the Fermi surface. Recently, the reﬂectionasymmetric shape, octupole deformation, has beenreported in 144 Ba by Bucher et al.1) The theoretical calculations exhibit diﬀerent predictions for octupole correlations in this region. For example, themicroscopic-macroscopic method2) predicts some β3
values, whereas the Hartree-Fock calculation3) argues
that there is no state with a dipole moment relevantto the octupole collectivity. Therefore, experimental
studies of more neutron-rich Ba isotopes are required.
We performed the β-γ spectroscopy of the 150 Cs
decay at RIBF using the in-ﬂight ﬁssion of a345 MeV/nucleon 238 U beam bombarding a 3-mm
thick Be target. Fission fragments were identiﬁed by
the TOF-Bρ-∆E method using BigRIPS.4) The secondary beam was implanted into an active stopper
WAS3ABi,5) which consists of ﬁve layers of doublesided-silicon-strip detectors for ion-β correlation. Theγ rays from the implanted nuclei were detected usingEURICA,6) an array of 12-cluster Ge detectors.
The γ-ray energy spectrum of the 150 Cs decay isshown in Fig. 1. The ion-β time window was set to0.2 s considering the previously reported half-life of150Cs, 0.84(8) ms.7) Since the peak count was small,5(a) 100150Cs to Ba < 0.2 s (without addback)

4200

3597

no addback
background2significance105(b)

Preliminary

432100200

400

600

800
1000
Energy (keV)

Fig. 1. (a) The preliminary γ-ray energy spectrum of theβ decay from 150 Cs to 150 Ba. The red curve shows theestimated continuum background. (b) The significancespectrum by a likelihood ratio test.

a log-likelihood ratio test was performed for the energy spectrum. The signiﬁcance spectrum is shown inFig. 1(b). The background was estimated by smoothing and scaling the energy spectrum with a time window of 2-to-10 s after ion implantation. We requested
4σ as a conﬁdence level to identify signiﬁcant peaks inthe spectrum. There are three signiﬁcant peaks at energies of 100, 200, and 597 keV. The peak at 200 keV
is not assigned to an excited state of 150 Ba because itbecame more pronounced in a spectrum with a longer
time window up to 2 s than 0.2 s. Analysis on the 100and 597-keV γ-ray is in progress.

∗1Center for Nuclear Study, The University of Tokyo
Research Center for Nuclear Physics, Osaka University∗3 LPSC, Université Grenoble-Alpes, CNRS/IN2P3∗4 RIKEN Nishina Center∗5 Department of Physics, The University of Tokyo∗6 Department of Physics, Peking University∗7 Department of Physics, Tohoku University∗8 Department of Physics, University of Oslo
∗9 School of Computing Engineering and Mathematics, University of Brighton∗10 Department of Physics, Osaka University∗11 ILL, Grenoble∗12 Department of Physics, Rikkyo University∗13 Department of Physics, University of Tsukuba∗14 MTA Atomki, Hungarian Academy of Science, Hungary∗15 Department of Display Engineering, Hoseo University∗16 Department of Physics, University of Surrey∗17 Department of Physics, University of York∗18 Instituto de Estructura de la Materia, CSIC
∗19 Departamento de Fı́sica Teórica, Universidad Autónoma deMadrid∗20 Department of Physiscs, Beihang University∗2References
1) B. Bucher et al., Phys. Rev. Lett. 116, 112503 (2016).
2) P. A. Butler et al., Nucl. Phys. 533, 249 (1991).
3) W. Nazarewicz et al., Nucl. Phys. 429, 269 (1984).
4) N. Fukuda et al., Nucl. Instrum Methods B 317, 323(2013).
5) S. Nishimura et al., RIKEN APR 46, 182 (2013).
6) S. Nishimura, Nucl. Phys. News 22, No.3, 38 (2012).
7) J. Wu et al., Phys. Rev. Lett. 118, 072701 (2017).

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