Research Group for Superheavy Elements
Group Leader : Matthias Schädel
Members : Kazuaki Tsukada, Masato Asai, Tetsuya Sato, Atsushi Toyoshima, Kazuhiro Ooe,
Sunao Miyashita, Yuichiro Nagame

The research objectives of this group are to understandchemical and nuclear properties of superheavy elements (SHEs)
placed at the uppermost end of the Periodic Table and on theheaviest frontier of the nuclear chart. To clarify the chemical
properties of SHEs, we investigate valence electronic structuresof SHEs through experimental determinations of ionization
potentials, redox potentials, and compound formations of SHEs.
To elucidate the limits of stability of the heaviest nuclei, weinvestigate the shell structure of superheavy nuclei throughexperimental assignments of proton and neutron single-particle
orbitals and through the evolution of nuclear deformation at thehighest proton and neutron numbers. To shed light on aspects
related to the accident at the Fukushima Daiichi NPP, we havemade contributions exploiting our experience in radioactivity
measurements. Typical progresses in FY2012 are reported in thefollowing.
First successful ionization and mass-separation of Lr(Z=103)
The first ionization potential (IP) is a fundamental physicaland chemical property of an element. Information on the IP of
the heaviest elements can provide a test and better
understanding of relativistic effects which are significantly
noticeable for heavy elements. Recently, we successfully
ionized and mass-separated Lr ions for the first time toward theIP measurement using a newly developed surface ionization ionsource as part of the JAEA-ISOL [1]. The details of the results
are described in the research highlights in this issue.
Ground-state configuration of the N=157 nucleus 259No
Although many theoretical studies have predicted the stability
and shell structure of superheavy nuclei, the location of protonand neutron shells in the superheavy region have not finallybeen established [2]. Energy spacings and the order of singleparticle orbitals are the most sensitive probes to clarify the shellstructure. Thus, experimental assignments of spin-parities andsingle-particle configurations of the ground states as well as
excited states in odd-mass superheavy nuclei provide valuable
information on the shell structure of superheavy nuclei. For oddmass N > 157 nuclei, nothing is known about spin-parities andsingle-particle configurations experimentally. In the presentwork, the ground-state configuration of the N=157 nucleus

Fig. 1 γ-ray spectrum observed in coincidence with 7260–7810 keV αparticles which corresponds to the α-energy range of 259No.

259

No
producedintheCm(18O,α3n) reaction using
the JAEA tandem accelerator
and the RIKEN AVF cyclotron
has been identified through α-γcoincidence
andα-singles
measurements [3]. Three γtransitions were observed forthe first time in the α decay of259
No, see Fig. 1, and its decay
scheme was established as259
shown in Fig. 2. The neutron Fig. 2 α-decay scheme of No
proposed in the present work.
9/2+[615] configuration wasassigned to the ground state of259
No as well as to the 231.4 keV level in 255Fm. Ground-state
deformations and neutron single-particle energies in Z=102
isotopes were calculated with a macroscopic-microscopic model[4]. The 9/2+[615] orbital was calculated to be the highest
among the five orbitals between the N=152 and 162 deformed
shell gaps. This is consistent with the experimental onequasiparticle energies in N=153 and 155 isotones [5,6], but isinconsistent with the present experimental result of the
9/2+[615] ground state configuration at N=157 [7]. To get the
9/2+[615] ground state at N=157, the order of the neutron
orbitals should be different between the N=153 and 157 isotones.
248

Efficiency calibration of Ge detector in soil samples
We applied our experience in radioactivity measurements, tomeasure 131I, 134Cs, and 137Cs in soil contaminated by theaccident of the Fukushima Daiichi NPP. In addition, efficiency
calibrations of Ge detectors and cascade summing correctionsfor 131I and 134Cs in soil samples which has a large volume anddensity were performed [8]. Gamma-ray detection efficiencies
were determined precisely using standard soil samples spiked
with 134Cs, 137Cs, 175Hf, and 88Zr in the laboratory. Cascadesumming corrections for γ rays of 134Cs in soil samples wereevaluated experimentally. To easily calculate the cascadesumming corrections for volume sources, we examined asimplified method using averaged efficiencies, and evaluated its
validity through a comparison of the calculated correction
factors with the experimental ones. The calculated correction
factors agreed very well with the experimental ones,
demonstrating the validity of this method.
References[1] T. Sato et al., Rev. Sci. Instrum. 84, 023304 (2013).
[2] A. Sobiczewski and K. Pomorski, Prog. Part. Nucl. Phys. 58,
292 (2007).
[3] M. Asai et al., Phys. Rev. C 87, 014332 (2013).
[4] P. Möller et al., At. Data Nucl. Data Tables 59, 185 (1995).
[5] M. Asai et al., Phys. Rev. C 83, 014315 (2011).
[6] M. Asai et al., Phys. Rev. Lett. 95, 102502 (2005).
[7] I. Ahmad and R. R. Chasman, Phys. Rev. C 80, 064315(2009).
[8] M. Asai et al., J. Nucl. Radiochem. Sci. 12, 5-10 (2012).