Research Group for Superheavy Elements
Group Leader : Matthias Schädel
Members : Kazuaki Tsukada, Masato Asai, Atsushi Toyoshima, Tetsuya Sato
Yusuke Kaneya, 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. Examples for the progress
in FY2013 are reported in the following.
Measurement of the Md3+/Md2+ reduction potential studied
with the flow electrolytic chromatography
Relativistic effects are strongly affecting ground-state electron
configurations of the heaviest elements. For the actinides,
theoretical calculations predict that the relativistic expansion of
the inner 5f orbital contributes to the actinide contraction. Thesechanges of relativistic origin influence binding energies ofvalence orbitals and the stabilities of electron configurations. It
is, however, difficult to directly measure energetic levels of theheaviest actinides with atomic number > 100 by any
spectroscopic method as these elements are not available inmacro amount. Redox reactions between the divalent andtrivalent oxidation states on the actinides are related to theenergies of the electron transitions between the 5fn7s2 and 5fn16d7s2 states. This indicates that redox potentials of actinides
help providing information on the relative stabilities of the 5f
orbital and other electron configurations.
Recently, we have developed a novel method of electrolytic
column chromatography available for single ions. Carbon
fibers modified with Nafion perfluorinated ion-exchange resin
were employed as a working electrode as well as a cationexchanger. This technique was successfully applied to theoxidation of nobelium (No) [1]. In this present report, thereduction potential of Md was determined with similarelectrolytic column chromatography [2].
The isotope 255Md (half-life, t1/2 = 27 min) was produced in
the irradiation of a 248Cm target with 11B beams at the JAEA
tandem accelerator. Attached to tiny KCl particles in a He gas
stream, nuclear reaction products, recoiling out of the target,
were transported to a chemistry laboratory. After collection,
trivalent actinide ions including Md were chemically separated
from KCl with a chromatographic HDEHP column in 0.1 M
HCl. Subsequently, a dried sample was dissolved with 0.10 M
HCl. This solution was then fed with 0.10 M HCl into a Nafion
electrode column. Potentials between -0.3 and -0.9 V vs. aAg/AgCl reference electrode (SSE) in 1.0 M LiCl were applied.
To determine the elution position of Md, the effluent from theNafion electrode was collected on eight Ta discs and evaporated
to dryness. Remaining products were then stripped with 3.0 M
HCl and were also collected and dried. Alpha particle

measurements started ~12 min with passivated implanted planarsilicon detectors.
In Figure 1, reduction probabilities of Md and Eu are shown
as a function of the applied potential. From the applied potential
at 50% reduction probability, formal reduction potentials of the
Md3+ + e- → Md2+ and Eu3+ + e- → Eu2+ couples were evaluatedto be -0.40 ± 0.03 V and -0.70 V vs. a SSE, respectively.
Applying the SSE versus a normal hydrogen electrode (NHE)
relationship as well as the correction with a linear relationshipbetween our measured formal potentials with standard potentials,
we obtain -0.16 ± 0.05 V vs. a NHE for the reduction potentialof Md. The present result helps to validate and improve
empirical models and it provides an opportunity to test model
calculations based on modern relativistic quantum-chemicaltreatments.

Md

Eu

Fig.1 The reduction probabilities of Md(III-II) and Eu(III-II) studied
with a flow electrolytic chromatography apparatus.

Spectroscopy of element 115 decay chains
An international team created a few short-lived atoms ofelement 115 at the GSI Helmholtz Center for Heavy Ion
Research in Germany by bombarding a 243Am target with 48Cabeams [3]. Thirty correlated -decay chains were detected and ahigh-resolution , x-ray, and -ray coincidence spectroscopy
experiment was conducted. The events are consistent withprevious assignments of similar decay chains to originate fromelement Z=115 [4]. This includes first candidates for Z
fingerprinting the decay of Mt by means of characteristic K-xray detection along -decay chains anticipated to start fromelement Z=115. There is potential for direct determination of theatomic number of the decay products of superheavy elements.
This paper was selected among “the Top Ten Physics
Newsmakers of 2013” by APS News.
References[1] A. Toyoshima et al., J. Am. Chem. Soc. 131, 9180 (2009).
[2] A. Toyoshima et al., Inorg. Chem. 52, 12311 (2013).
[3] D. Rudolph et al., Phys.Rev. Lett. 111, 112502 (2013).
[4] Yu. Ts. Oganessian et al., Phys. Rev. C 69, 021601 (2004).