Research Group for Heavy Element Nuclear ScienceGroup Leader: A. Andreyev
Members : K. Nishio, K. Tsukada, Y. Utsuno, M. Asai, H. Koura, I. Nishinaka, A. Toyoshima,K. Hirose, T.K. Sato, H. Makii, R. Orlandi, R. Léguillon, J. Smallcombe, Y. Kaneya, A. Mitsukai

Studies of chemical properties of the heaviest elements anddetermination of the limits of nuclear stability are among themost interesting, but also most challenging topics in modern
chemistry and nuclear physics. The research of the groupstrongly addresses these fundamental questions.
In nuclear physics, our core program is on fission, which is aunique physical process observed only in nuclear matter, andwhich provides the basis for numerous atomic energy
applications. Our experimental and theoretical programs
includes structure and reaction study of exotic neutron- andproton-rich nuclei as well as super-heavy nuclei. For nuclear
chemistry, our work is concentrated on the heaviest elements,
whose chemical properties are crucially determined by therelativistic effects. Atomic and chemical properties of the exotic
atoms and molecules will be studied on an atom-at-a-timechemistry approach.
Investigation of prompt fission measurement using multinucleon transfer reactions
Nuclear fission is a unique decay process which may bedescribed by the interplay of macroscopic (collective) andmicroscopic (single particle) degrees of freedom in a nucleus.
In particular, fission-fragment mass distribution (FFMDs) andtheir sensitivity to the excitation energy and isospin provide adeep insight into the mechanism of the process of fission.
The scope of the present work is to explore the potential of themulti-nucleon transfer (MNT) reactions to measure FFMDs andtheir excitation energy dependence for the neutron-rich actinide
nuclei, which cannot be accessed by particle-capture and/or
heavy-ion fusion reactions. Advantage of this method is thatfission of many compound nuclei can be studied in one
experiment. A key development for this method is a silicon
ΔE-E detector to separate ejectile isotopes to identify transfer
channels and determine the excitation energy of compound
nuclei. Fission fragments were detected in coincidence using
multi-wire proportional counters. We studied the reaction 18O +232
Th using an 18O beam from the JAEA tandem accelerator.

Figure 1 shows the FFMDs of 231-234Th*, 232-236Pa*, and 234U*, for excitation bins of 10 MeV [1]. Data of 231,234Th*,
234,235,236
Pa* nuclei were obtained for the first time. It is foundthat the measured spectra reveals larger peak-to-valley ratio in
the FFMDs for nuclei with large isospin values, as seen for themost neutron rich isotopes of the same element, considered at
the same excitation energy, for instance at the E*=20-40MeV.
This might be explained by the growing influence of the magic132
Sn nucleus on the mass division toward neutron-rich nuclei.
238

First on-line vacuum chromatography experiments withsingle atoms of Tl as the lighter homolog of E113
Our international collaboration with the Paul Scherrer Institute(PSI) demonstrated the feasibility of on-line vacuum
chromatography experiments with the heaviest elements. The
experiment at the JAEA Tandem accelerator facility acts as a
proof of principle aiming at a first unambiguous chemical
characterization of E113.
A first-ever on-line isothermal vacuum chromatography
experiment (IVAC) has been developed and successfully used
for determining the adsorption of thallium on quartz at oneatom-at-a-time level [3]. The radioactive nucleus as a tracer
184Tl (T1/2=10.1 s) was produced in the 152Gd(35Cl, 3n) reaction
and transported to the JAEA-ISOL setup with a CdI2/He aerosol
gas-jet system. 184Tl was ionized and the separated 184Tl+-ion
beam was guided to a newly developed IVAC experiment setup(Pexp <10-2 Pa). By changing isothermal temperature of theinnermost quartz column, retention time of the Tl atom on thequartz surface was measured. The retention time is correlatedwith a probability of the atom passing through the column. The
escaped atom from the column is attached on a gold surface of adiamond-based detector, where -decay of 184Tl was detected todetermine the fraction. The experimental data were comparedwith a microscopic kinetic model (Monte Carlo approach) [4]which can describe the adsorption process. The measured
adsorption enthalpy of 158+3 kJ/mol is in excellent agreementwith the calculation. The set up was found to have a fairly highthroughput efficiency of 37% at the highest temperature [3]. The
result demonstrates the applicability of the newly developed
IVAC system for the E113 chemistry and for even higherelements.
References[1] R. Léguillon et al., Phys. Lett. B, ***. *** (2016).
[2] Y. Aritomo et al., Phys. Rev. C. 88, 044614 (2013).
[3] P. Steinegger et al., J. Phys. Chem. C, 120 (2016) 7122-7132.
[4] I. Zvara, The Inorganic Radiochemistry of Heavy Elements,
1st edition, Springer, Netherlands (2008).

Fig.1 FFMD obtained in the MNT channels in 18O+232Th. The curves arethe Langein model calculation [2].