II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

β-decay spectroscopy of 187 Ta
M. Mukai,∗1 Y. Hirayama,∗2 Y. X. Watanabe,∗2 S. C. Jeong,∗2 H. Miyatake,∗2 S. Ishizawa,∗1,∗2,∗3T. Niwase,∗1,∗2,∗4 M. Rosenbusch,∗2 P. Schury,∗2 M. Wada,∗2 H. Watanabe,∗1,∗5 M. Brunet,∗6 F. G. Kondev,∗7G. J. Lane,∗8 Yu. A. Litvinov,∗9 Zs. Podolyák,∗6 and P. M. Walker∗6The nuclear data of neutron-rich nuclei around N =126, such as the nuclear mass, decay half-life, and decay scheme, are key for understanding the formation of
the third peak around A = 195 through the r-process
in explosive stellar environments, and the data are required to be investigated experimentally. Furthermore,
the neutron-rich region around Z = 75 is not only a transition region of nuclear deformation, but also expected tocontain high-energy long-lived isomers. Owing to the astrophysical and nuclear physics interests in this nuclear
region, nuclear spectroscopy has been performed at theKEK Isotope Separation System (KISS) 1,2) installed in
the Radioactive Isotope Beam Factory (RIBF), RIKEN.
The isomeric states of 187 Ta (Z = 73, N = 114) wereﬁrst observed in the Experimental Storage Ring (ESR)at GSI.3) In that study, the half-lives and excited energies were determined to be 2.3(6) min for the groundstate (gs), 22(9) s for the ﬁrst isomeric state (m1) at theexcitation energy Ex = 1789(13) keV, and >5 min forthe second isomeric state (m2) at Ex = 2935(14) keV.3)
We performed the β- and γ-decay spectroscopy of 187 Tato investigate the nuclear structure of the ground andtwo isomeric states at KISS. They are predicted to havenuclear spins of 25/2− and 41/2+ for the m1 and m2states, respectively. The results of successful experiments on the m1 isomer were reported in Ref. 4). The
present report describes the progress of the analysis investigating the unknown β-decays of 187gs, m1, m2 Ta.
187gs, m1, m2
Ta were produced using multi-nucleon
transfer reactions of a natural tungsten target (5 µmthick) and 136 Xe beam (7.2 MeV/nucleon, 50 particlenA). The target-like fragments were thermalized andneutralized in a gas cell ﬁlled with puriﬁed Ar gas of∼1 atm2) and re-ionized element-selectively by using alaser resonance ionization technique5) at the exit of thegas cell. Subsequently, the mass number was chosen byusing a dipole magnet with a mass resolving power ofA/∆A ∼ 900. The mass-analyzed ions were transported
to the decay station, which consists of a tape transport
device, a multi-segmented proportional gas counter,6)
∗1
∗2

∗3
∗4
∗5
∗6

∗7
∗8

∗9RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), Institute of Particleand Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK)
Graduate School of Science and Engineering, Yamagata University
Department of Physics, Kyushu University
International Research Center for Nuclei and Particles in theCosmos, Beihang University
Department of Physics, University of Surrey
Physics Division, Argonne National Laboratory
Department of Nuclear Physics, Australian National University
GSI Helmholtzzentrum für Schwerionenforschung GmbHFig. 1. Energy spectrum of γ-rays associated with the βdecay of 187 Ta and 187 W, as well as some of the internaltransitions of 187m1 Ta. The labels indicate the energy(keV) and possible origin of the observed γ-rays. The
inset shows a part of the low-energy level scheme in 187 W.
The values in parentheses are the relative intensities oftransitions.7)

and four clover-type germanium detectors.
Figure 1 shows the γ-ray energy spectrum in coincidence with events of the gas counter telescope, which
are sensitive to electron energies of >100 keV.6) The
measurement was performed with the time sequence ofbeam-on/oﬀ = 1800/1800 sec. We observed γ-rays originating from β-decays of 187 Ta as well as 187 W (T1/2 =24.0 h), which is the daughter nucleus of 187 Ta, and fromthe internal decay of 187m1 Ta (T1/2 = 7.3(9) sec)4) emitting high-energy (>100 keV) conversion electrons. The
γ-ray transitions were assigned not only through comparison with previously reported transition energies,7)
but also by checking the half-lives from the energy-gatedtime spectra for each gamma peak. Consequently, all of
the observed γ-rays emitted from 187 W were identiﬁed(upper right scheme in Fig. 1). The observed γ-raysemitted from 187 Re agree with those measured in previous decay studies.
Further analysis to determine the decay branch fromthe three states of 187 Ta is ongoing.
References

1) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res. B
353, 4 (2015).
2) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res. B
412, 11 (2017).
3) M. W. Reed et al., Phys. Rev. C 86, 054321 (2012).
4) P. M. Walker et al., Phys. Rev. Lett. 125, 192505 (2020).
5) Y. Hirayama et al., Rev. Sci. Instrum. 90, 115104 (2019).
6) M. Mukai et al., Nucl. Instrum. Methods Phys. Res. A884, 1 (2018).
7) V. Bondarenko et al., Nucl. Phys. A 811, 28 (2008).

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