Research Group for Heavy Element Nuclear ScienceGroup Leader : A. Andreyev
Members : K. Nishio, K. Tsukada, Y. Utsuno, M. Asai, H. Koura, A. Toyoshima, K. Hirose,T.K. Sato, H. Makii, R. Orlandi, M. Vermeulen, N.M. Chiera, T. Tomitsuka, R. Kean, F. Endo

Studies of the chemical properties of the heaviest elements,and determination of the limits of nuclear stability, are among
the most interesting, but also most challenging, topics in modern
chemistry and nuclear physics. The research conducted by ourgroup strongly focussed toward addressing 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 include
also nuclear structure and reaction studies 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 byrelativistic effects. Atomic and chemical properties of exoticatoms and molecules are studied via an atom-at-a-timechemistry approach.
Role of multi-chance fission in the description of fissionfragment mass distributions
For further public acceptance of nuclear power, it is essential
to reduce the already-existing and newly produced nuclear
waste. The use of accelerator-driven systems (ADS) is
considered as one of the viable options for the incineration
and/or transmutation of the long-lived minor actinides into
shorter-lived fission products. In the ADS approach, energetic
spallation neutrons could be used to irradiate the fissionable
minor actinides. This leads to fission with higher excitationenergies than the traditional light water reactors. In high-energy
fission the so-called “multi-chance fission”, i.e. fission after
neutron-evaporation from compound nucleus, plays a role. We
have investigated the effects of multi-chance fission on fissionfragment mass-distributions (FFMDs) in detail, from themeasurement of FFMDs for twelve nuclides by multinucleon
transfer channels in the reaction 18O +238U at the JAEA tandem
facility [1]. It was found that the mass distributions for all thestudied nuclides maintain a double-humped shape up to thehighest measured energy in contrast to expectations ofpredominantly symmetric fission due to the washing out of
nuclear shell effects, see Fig.1. From a comparison with the
dynamical calculation based on the fluctuation-dissipation
model (Langevin equations), it was found that the massasymmetric FFMDs observed at the high-energy region arecaused by the fissions of lower-excited lighter isotopes
generated by higher-order neutron-evaporation steps.
Systematic shell-model calculations of β-decay properties inneutron-rich nuclei
β-decay properties, including half-lives and delayed neutron
emission probabilities, are among the most fundamental
nuclear-structure data, which are also crucially needed for
application purposes, e.g. for calculations of criticality ofreactors. The primary aim of this study is to demonstrate thatlarge-scale shell-model calculations are very useful to describe
these data. We have calculated β-decay half-lives and β-delayed
neutron emission probabilities for 78 nuclei with 13 ≤ Z ≤ 18
and 22 ≤ N ≤ 34 [2]. The experimental values are wellreproduced by our calculations, as presented in Fig. 2 for the βdecay half-lives. The quality of our calculation is evaluated by
the standard deviation for the ratio of the calculated andexperimental data. This value is as small as 1.58 with ourcalculation, whereas remarkably higher value of 3.04 is given
with the standard QRPA calculation [3]. It is expected that thepresent results will provide more reliable input data for thecalculation of r-process nucleosynthesis.

Fig. 2 β-decay half-lives compared between experiment and theory. The
filled symbols are experimental data, and the open symbols with thesolid and the dashed lines stand for the calculated values using theexperimental and calculated Q values, respectively.

References[1] K. Hirose et al., Phys. Rev. Lett. 119, 222501 (2017).
[2] S. Yoshida, Y. Utsuno et al., Phys. Rev. C 97, 054321(2018).
[3] P. Möller et al., Phys. Rev. C 67, 055802 (2003).
Fig.1 Fission-fragment mass distributions of uranium isotopes. The
red and blue curves are the results of Langevin calculation with andwithout taking into account the multi-chance fission.