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

RIKEN Accel. Prog. Rep. 54 (2021)

Properties of 187 Ta revealed through isomeric decay†
P. M. Walker,∗1 Y. Hirayama,∗2 G. J. Lane,∗3 H. Watanabe,∗4,∗5 G. D. Dracoulis,∗3 M. Ahmed,∗2,∗6 M. Brunet,∗1T. Hashimoto,∗7 S. Ishizawa,∗5,∗8,∗2 F. G. Kondev,∗9 Yu. A. Litvinov,∗10 H. Miyatake,∗2 J. Y. Moon,∗7M. Mukai,∗6,∗2,∗5 T. Niwase,∗2,∗5,∗11 J. H. Park,∗7 Zs. Podolyák,∗1 M. Rosenbusch,∗2 P. Schury,∗2 M. Wada,∗2,∗6X. Y. Watanabe,∗2 W. Y. Liang,∗12 and F. R. Xu∗12We report results from the production and separation of a low-energy beam of the neutron-rich tantalum
isotope, 187 Ta114 , together with its high-spin isomericstate. The production process exploits multi-nucleon
transfer (MNT) reactions which have been shown to beeﬀective for making neutron-rich nuclei.1) The large angular momentum transfer in MNT reactions is a key aspect for the formation of high-spin isomers.
The experiment was performed at the RIKEN Nishina
Center with the recently commissioned KEK Isotope
Separation System (KISS) facility.2) This is the ﬁrst facility of its kind, capable of stopping heavy-ion reaction
products in a high-pressure (80 kPa) argon gas cell, performing laser resonant ionization for element (Z) selectivity, and achieving mass (A) separation of the electrostatically extracted, singly charged, 20 keV ions in a
dipole magnet with a resolving power A/∆A = 900.
The 187 Ta ions were produced by MNT reactions ofa 50 particle-nA beam of 136 Xe at 7.2 MeV/nucleon,delivered by the RIKEN Ring Cyclotron. The beam
was incident on a 5 µm thick natural tungsten target atthe entrance to the argon gas cell.
The 20 keV secondary beam of laser-ionized tantalum was mass separated (1.5 ions/s of 187 Ta) and transported to a moving-tape collection point, surrounded bya low-background, 32-element gas proportional counter
with 80% of 4π solid angle for β particles and conversion
electrons3) and four Super Clover germanium γ-ray detectors with a total absolute full-energy-peak eﬃciencyof 15% at 150 keV. The tape transport was operated
with equal beam-on/beam-oﬀ periods, with the radioactivity moved to a shielded location at the end of eachcycle. The chosen beam-on periods were 30 s, 300 s and1800 s, with ﬁve days of data taking.
An isomer in 187 Ta at 1789(13) keV had been identiﬁed in the Experimental Storage Ring at GSI,4) but
without observation of the decay radiations. Details ofthe excited-state structure of 187 Ta have now been reCondensed from the article in Phys. Rev. Lett. 125, 192505(2020)∗1 Department of Physics, Surrey University∗2 Wako Nuclear Science Center (WNSC), IPNS, KEK∗3 Department of Nuclear Physics, ANU∗4 School of Physics, Beihang University∗5 RIKEN Nishina Center∗6 Graduate School of Science and Engineering, Tsukuba University∗7 Rare Isotope Science Project, Institute for Basic Science (IBS)∗8 Graduate School of Science and Engineering, Yamagata University∗9 Physics Division, ANL∗10 GSI Helmholtzzentrum für Schwerionenforschung GmbH
∗11 Department of Physics, Kyushu University∗12 School of Physics, Peking University†Fig. 1. Level scheme for 187 Ta based on γ-ray transitions
observed following the decay of a T1/2 = 7.3(9) s isomer.

vealed by the isomeric γ-ray emissions, as illustrated inFig. 1. The isomer is found to have an excitation energyof 1778(1) keV, with a half-life of 7.3(9) s.
Despite the proximity to N = 116, which is predicted to be the critical point for a ground-state prolateoblate shape transition, the reduced hindrance for the191.7 keV, E2 isomeric decay remains substantial, withfν = 27(1), indicating that K is approximately conserved, and therefore that axial symmetry is not stronglyviolated. Nevertheless, weak violation of axial symmetry
is indicated by the observed irregularity in the 9/2[514]rotational band that is populated through the isomer decay. Comparison with the rhenium isotone, 189 Re, supports calculations showing that axial symmetry is better
conserved for the lower-Z nuclei, through the N ≈ 116
shape transition region.
The new capability to produce low-energy beamsof neutron-rich tantalum isotopes and isomers demonstrates the power of the gas-stopping technique for nuclear structure studies of exotic neutron-rich nuclei, evenwith refractory elements. This marks a milestone on theway to the exploration of nuclei predicted to have welldeformed oblate ground states.
References

1) Y. X. Watanabe et al., Phys. Rev. Lett. 115, 172503 (2015).
2) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res. B
463, 425 (2020).
3) M. Mukai et al., Nucl. Instrum. Methods Phys. Res. B
463, 421 (2020).
4) M. W. Reed et al., Phys. Rev. Lett. 105, 172501 (2010).

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