II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Isomer spectroscopy using multi-nucleon transfer reaction on 248 Cm

∗1
∗2
∗3
∗4
∗5
∗6
∗7

10000

Counts / keV

8000
6000
4000

Gated by 0p transfer channels

265.7(249Cm)

The Island of Stability, which is linked to the predicted doubly closed proton and neutron shells in superheavy nuclei, is still an unexplored region in nuclear physics. Owing to low expected cross sections
with current experimental techniques, the region has
not been accessible for detailed decay studies. Excitedstates in actinide nuclei can have an important rolein characterizing the stability of the island, as partof degenerate orbitals above predicted energy gaps at
Z = 114 and N = 184 can appear as excited states owing to the systematic development of quadrupole deformation. Pioneering studies1,2) identiﬁed a large component of high-j quasiparticle states originating fromthe νk17/2 orbital above the gap in 249 Cm. Thus, dataaround this region can provide benchmarks for theoretical calculations of the location and properties ofthe Island of Stability.
One method to study the region of heavy actinides
is to produce them via the transfer reactions of heavy
ions onto radioactive actinide targets. The diﬃcultyof using actinide targets lies in their strong radioactivity. An experimental method, called the isomerscope method,3) is one of measurements to avoid thisproblem with the space-time separation. This method
was originally developed at JAEA to investigate isomeric states via deep inelastic collisions. The combination of γ-ray detectors and absorbers signiﬁcantly
enhanced the peak-to-background ratio of the delayed
γ-ray component. We applied this method to perform isomer spectroscopy of nuclei produced via multinucleon transfer reactions on actinide targets.
The experiment was performed at the JAEA tandem accelerator facility in May 2021. An actinide target of 248 Cm was prepared via electrodeposition onto a
1.8 μm aluminium backing. The target was irradiatedusing a 18 O beam with an energy of 96.5 MeV with anintensity of approximately 2.0 particle nA. The beamenergy was set to detect the light ejectiles identiﬁedat backward angles. The produced nuclei were identiﬁed via speciﬁc transfer channels using an array ofΔE-E telescopes. The selection of the beam energy

177.5
247
192.5 ( Cm)(249Cm)S. Go,∗1,∗2,∗3 R. Orlandi,∗4 T. Niwase,∗5,∗3 K. Sugiyama,∗1 T. Tomimatsu,∗1 T. Kai,∗1 K. Hirose,∗4E. Ideguchi,∗6,∗3 Y. Ishibashi,∗1 Y. Ito,∗4 H. Makii,∗4 S. Matsunaga,∗1 K. Morita,∗1,∗3 D. Nagae,∗2,∗3Y. Nagata,∗1 H. Nishibata,∗1,∗3 K. Nishio,∗4 T. T. Pham,∗6 S. Sakaguchi,∗1,∗3 T. K. Sato,∗4 T. Shizuma,∗7F. Suzaki,∗4 K. Tsukada,∗4 K. Washiyama,∗2 and M. Asai∗4,∗22000

100

150200250300350

400450500

Energy (keV)

Fig. 1. Delayed γ-ray spectrum gated using 0p transfer
channels by the ΔE-E silicon telescope. The 177.5and 265.7-keV peaks resulted from the isomeric state of247
Cm with a half-life of 100.6 ns.4) The 192.5- and265.7-keV peaks resulted from the isomeric state of249
Cm with 19-ns half-life.2)

enabled us to place the telescope at the backward angles, which eﬀectively reduced the radiation damage on
the silicon detectors. An ejectile passing through oneof the four ΔE detectors (20 μm thick) was stopped in
the E detector (300 μm thick) to measure the residualenergy. Heavier nuclei were scattered in the forward
direction and stopped in a catcher foil. Four germanium and four LaBr3 detectors were placed close to
the foil to detect the delayed γ rays. A tungsten shield
was placed between the target and γ-ray detectors toabsorb the strong γ-radiation around the target.
Several known isomers belonging to nuclei in the region of 248 Cm were successfully observed in this study.
An example of the delayed γ-ray spectrum is shownin Fig. 1. The 178-keV peak correspond to the γ-ray
transition from the isomeric state of 247 Cm.4) The halflife value deduced by the preliminary analysis agreed
well with the literature value of 100.6 ns.4) The 192.5and 265.7-keV peaks corresponded to transitions froman isomeric state in 249 Cm.2) This short-lived 19-ns
isomer was successfully identiﬁed in this experimental
setup. Further analysis to identify new isomeric statesis in progress.

Department of Physics, Kyushu University
Research Center for Superheavy Elements, Kyushu University
RIKEN Nishina Center
Advanced Science Research Center, Japan Atomic Energy
AgencyWako Nuclear Science Center (WNSC), IPNS, KEK
Research Center for Nuclear Physics, Osaka University
Tokai Quantum Beam Science Center, National Institute forQuantum Science and Technology

References
1) I. Ahmad et al., Nucl. Phys. A 646, 175 (1999).
2) T. Ishii et al., Phys. Rev. C 78, 054309 (2008).
3) T. Ishii et al., Nucl. Instrum. Methods Phys. Res. A395, 210 (1997).
4) I. Ahmad et al., Phys. Rev. C 68, 044306 (2003).

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