Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Isomer study on neutron-rich Pm isotopes using EURICA at RIBF
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,∗11 L. Sinclair,∗17,∗4 H. Suzuki,∗4 H. Takeda, ∗4 J. Taprogge,∗18,∗19 Zs. Vajta,∗14H. Watanabe∗20 and A. Yagi∗10It has been known that large prolate deformation
develops in neutron-rich Z ∼ 60 nuclei. This can beseen from the systematics of excitation energies of theﬁrst 2+ states of even-even Z = 55 to 66 nuclei asshown in FIG.1 of ref1) . In this deformed region, many
K-isomers with micro second half-lives have been discovered. For example, K π = 4− isomers are systematically observed in Z = 62 to 68, N = 100 isotones2,3) .
It is interesting to investigate whether the same kindof isomers exist in lower Z isotones, as this information will be helpful in understanding the deformed shellstructure of such highly neutron-rich nuclei.
We performed isomer and β-γ spectroscopy on
neutron-rich Z = 56 to 61 isotopes at RIBF. Theneutron-rich isotopes were produced using in-ﬂight ﬁssion of a 345MeV/nucleon 238 U beam. Fission fragments were identiﬁed by measuring the time-of-ﬂight(TOF) and magnetic rigidity (Bρ) in the second stageof BigRIPS and by measuring the energy loss (∆E) by
the ion chamber at the ﬁnal focal plane, F11. The measurement was conducted in two diﬀerent setups. In one
setup, the beam was implanted into an active stopper,
WAS3ABi4) which consists of ﬁve layers of DoubleSided-Silicon-Strip Detectors (DSSSDs) with 40 × 60strips, in order to obtain β-γ and isomer data at thesame time. In this setup, the total implantation rate
was limited up to ∼100 cps. In the other setup, a copper stopper was introduced instead of the DSSSD toaccept a wide range of nuclides with a high total rate,

Fig. 1. Preliminary γ-ray energy spectra of 158 Pm, 159 Pm
and 161 Pm. The time windows of 158 Pm is up to 16µs
and that of 159 Pm and 161 Pm are up to 5µs. Events
close to the timing of the beam implantation are excluded. Half-lives of 159 Pm and 161 Pm are obtained
from the 330 and 728keV γ ray respectively.

in order to optimize the isomer search. The γ raysfrom the isomeric states were detected by EURICA5) ,which is an array of 12-cluster Ge detectors. Each cluster consists of seven crystals that enable adding backCompton-scattered events in the neighboring crystals.
After the analysis of the data, isomers were found in
Pm isotopes with A = 158, 159, and 161. Fig. 1 shows
the preliminary energy spectra of the delayed γ rays
for the Pm isotopes. Many new γ rays were observed
for them. Half-lives of 159 Pm and 161 Pm were obtainedby ﬁtting the timing spectra gated by the γ-ray energy.
158Pm was found to have a half-life much longer than
the 16µs time window. Further analysis of the γ-γ
coincidence and relative intensities are in progress toconstruct the level schemes. β-γ analysis will also beperformed to obtain more information on the low-lying
states of these nuclei.
References∗1

Center 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∗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 Física Teórica, Universidad Autónoma deMadrid∗20 Department of Physiscs, Beihang University∗21) R. F. Casten et al.: Phys. Rev. Lett. 47, 1433 (1981)
2) G. D. Dracoulis et al.: PRC 81, 054313 (2010)3) S. Go et al.: RIKEN Accel. Prog. Rep. 46, 21 (2013)4) S. Nishimura et al.: RIKEN Accel. Prog. Rep. 46, 182(2013)
5) S. Nishimura: Nucl. Phys. News 22, No. 3 (2012)- 11 -