Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Search for isomers in neutron-rich Cs isotopes
A. Yagi,∗1 A. Odahara,∗1 R. Daido,∗1 Y. Fang,∗1 H. Nishibata,∗1 R. Lozeva,∗2 C.-B. Moon,∗3S. Nishimura,∗4 P. Doornenbal,∗4 G. Lorusso,∗4 P.-A. Söderström,∗4 T. Sumikama,∗5 H. Watanabe,∗6T. Isobe,∗4 H. Baba,∗4 H. Sakurai,∗7,∗4 F. Browne,∗8,∗4 Z. Patel,∗9,∗4 S. Rice,∗9,∗4 L. Sinclair,∗10,∗4J. Wu,∗11,∗4 Z.Y. Xu,∗7 R. Yokoyama,∗12 T. Kubo,∗4 N. Inabe,∗4 H. Suzuki,∗4 N. Fukuda,∗4D. Kameda,∗4 H. Takeda,∗4 D.S. Ahn,∗4 D. Murai,∗13 F.L. Bello Garrote,∗14 J.M. Daugas,∗15
F. Didierjean,∗2 E. Ideguchi,∗16 T. Ishigaki,∗1 H.S. Jung,∗17 T. Komatsubara,∗18 Y.K. Kwon,∗18
S. Morimoto,∗1 M. Niikura,∗7,∗4 I. Nishizuka,∗5 T. Shimoda,∗1 and K. Tshoo∗18(TOF), magnetic rigidity (Bρ) and energy loss of thefragments to deduce mass-to-charge ratio (A/Q) andatomic number. The particle identiﬁcation for our Bρ
setting is shown in ref. 3. Figure 1 shows the A/Q
spectrum of the Cs isotopes deduced using the information of Bρ at F5 and F7 as well as TOF between
F3 and F7. The isomers were searched for on the basisof the timing information between the γ ray detectedby EURICA and ion passage in the plastic scintillator just upstream of WAS3ABi. The long ﬂight time
(approximately 650 ns) limited the half lives of longer
than hundreds of nanoseconds.
In this time range, we found new isomers in the nuclei 145 Cs, 146 Cs, 147 Cs, and 148 Cs. As an example, adecay curve of the isomer in 146 Cs is shown in Fig. 2.
The half life and decay scheme obtained in this workfor 144 Cs were consistent with the results reported inref. 4. The decay schemes of newly found isomers in
145−148
Cs have been established. The isomers in oddodd Cs isotopes are caused by the direct low-energy
deexcitation from the isomer, which is eﬀected by theproton and neutron interaction. In contrast, the decay
pattern of the isomers in the odd-Cs isotopes suggeststhat these isomers are supposed to be K-isomer candidates.
Systematic studies of the isomers in the Cs isotopes
are expected to provide new insights on shape evolution as well as proton-neutron interaction in various
deformed systems. Detailed analysis is in progress.

Neutron-rich nuclei in the northeast region of the
doubly-magic 132 Sn are attracting much attention for
the investigation of shape evolution from spherical(single-particle like) shapes to deformed (collectivelike) prolate shapes as a function of the neutron number. Additionally, a variety of collective modes, suchas the octupole collective mode and so on, is expectedto appear in this mass region. As isomers are eﬃcient
probes of nuclear structure, we performed an isomersearch experiment for the neutron-rich Cs isotopes inthe framework of the EURICA project1) .
The isomers were produced through in-ﬂight ﬁssion of a 345 MeV/nucleon 238 U beam. The ﬁssion
fragment separator system of BigRIPS and Zero Degree Spectrometer2) was tuned for neutron-rich Sb, Te,I, Xe, and Cs isotopes with A=140−150. The isotopes with a rate of approximatelly 50 pps were implanted into a stack of 5 double-sided Si strip detectors (WAS3ABi)1) . The β rays and γ rays emitted
from the stopped isotopes were detected by WAS3ABiand EURICA, which consists of 12 cluster-type Ge detectors, respectively. Particle identiﬁcation was performed on the basis of the information of time-of-ﬂight
104

Counts103

148 55+Cs
146 55+
Cs 147Cs55+
149 55+Cs
145 55+Cs

145 54+ 146 54+
Cs

Cs

102

150 55+Cs

147 54+

Cs

151 55+Cs
Cs

148 54+

101
100
2.64

2.66

2.68

A/Q

2.70

2.72

2.74

CountsFig. 1. A/Q spectrum of neutron-rich Cs isotopes.
∗1Department of Physics, Osaka University
IPHC/CNRS and University of Strasbourg∗3 Department of Display Engineering, Hoseo University∗4 RIKEN Nishina Center∗5 Department of Physics, Tohoku University∗6 Department of Physics, Beihang University∗7 Department of Physics, University of Tokyo∗8 CEM, University of Brighton∗9 Department of Physics, University of Surry∗10 Department of Physics, University of York∗11 Department of Physics, Peking University∗12 CNS, University of Tokyo∗13 Department of Physics, Rikkyo University∗14 Department of Physics, University of Oslo
∗15 CEA/DAM∗16 RCNP, Osaka University∗17 Department of Physics, University of Notre Dame
∗18 IBS∗2

Time difference between EURICA andplastic scintillator [25ns/channel]

Fig. 2. Decay curve of the newly found isomer in 146 Cs.

References
1) S. Nishimura et al., Prog. Theor. Exp. Phys. 2012(2012) 03C006.
2) T. Kubo et al., Prog. Theor. Exp. Phys. 2012 (2012)
03C003.
3) R. Lozeva et al., in this report.
4) T. Rzaca-Urban et al., Phys. Rev. C 80 (2009) 064317.

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