Nuclear split in fission
K. Hirose1), K. Nishio1), S. Tanaka2), R. Lèguillon1), H. Makii1), I. Nishinaka3), R. Orlandi1),
K. Tsukada1), J. Smallcombe4), M. J. Vermeulen1), S. Chiba5), Y. Aritomo2), T. Ohtsuki6),
K. Nakano7) S. Araki7), Y. Watanabe7), R. Tatsuzawa8), N. Takaki8), N. Tamura9), S. Goto9),
I. Tsekhanovich10), and A. N. Andreyev11,1)
1) : ASRC, JAEA 2): Kindai Univ. 3): QST 4): TRIUMF 5): Tokyo Tech. 6): Kyoto Univ.
7): Kyushu Univ. 8): Tokyo City Univ. 9): Niigata Univ. 10): Univ. of Bordeaux11): Univ. of York

Treatment of radioactive wastes of long-lived minor actinides,
produced in a light-water reactor, is one of urgent issues to besolved in the usage of nuclear power. One of the options is totransmute these nuclides into shorter-lived fission products by
irradiating them with higher-energy neutrons in a so-called
accelerator-driven system (ADS). Thus, in such case, fissioning
nuclei have higher excitation energies. Design of the ADS
requires high-energy neutron-induced fission data.
Nuclear fission is a phenomenon where a nucleus deforms
and finally splits into two lighter nuclei. Since a probability
distribution of masses of both fission fragments, i.e., fission
fragment mass distribution (FFMD), sensitively reflects thedeformation path, it is of essential importance to observe thefragment masses for the elucidation of fission mechanism.
However, the available FFMDs in terms of nuclides andexcitation-energy range are quite limited.
A highly excited nucleus decays either via fission or via
neutron emission which leads to a different nucleus with alighter mass and lower excitation energy. The latter then decays
again either via fission or via neutron emission. This
competition between fission and neutron emission continues
until the excitation energy of the descendant nucleus drops
below its fission barrier. As shown in Fig. 1, a 240U nucleus
with a high excitation energy, for example, can evaporate
several neutrons. Therefore, in a fission measurement of theinitially highly excited state, several lighter isotopes withdifferent excited states are included. The so-called multi-chancefission concept is well known to explain the structure of fission
cross sections, but its impact on the FFMD have not beenstudied in detail.
In this work, we investigated the effects of multi-chancefission on the FFMDs [1]. For this purpose, we have measured alarge set of FFMD (237-240U, 239-242Np and 241-244Pu) with a wide
coverage in the excitation energy (10–60 MeV) using multinucleon transfer channels in the 18O+238U reaction (see also [2]).
The experimental data were interpreted by combining a
dynamical fission model based on the fluctuation-dissipation
theorem and a concept of the multi-chance fission. From thepresent analysis, it is clarified that the experimental FFMD
consists of fission of six nuclides as shown by the dashed lines
in Fig. 2, and it is evident that the observed mass-asymmetric
distribution, i.e. a double-humped shape, remaining at such ahigh-excitation energy is caused by fission of the lower-excitedlighter isotopes. Furthermore, the FFMDs for fission of highly
excited nucleus were obtained like the black dashed line shownin Fig. 2, which is an important observable to understand thefission mechanism. The result of this work opens the highenergy fission research.
This work is supported by “Comprehensive study of delayedneutron yields for accurate evaluation of kinetics of high burnup reactors” and “Development of prompt-neutron measurement

in fission by surrogate reaction method and evaluation ofneutron-energy spectra” by the Ministry of Education, Culture,Sports, Science and Technology of Japan (MEXT). This work
was partly done with in an IAEA CRP on beta-delayed neutrons(F41030). The Langevin calculations have been done using thecluster computer system (Kindai-VOSTOK) which is supportedby “Research funds for External Fund Introduction” by Kindai
University. This work was also supported partly by STFC of theUnited Kingdom.
References[1] K. Hirose et al., Phys. Rev. Lett 119, 222501 (2017).
[2] R. Lèguillon et al., Phys. Lett. B 761, 125 (2016).

Fig. 1 An example illustrating the process for fission and neutron
evaporation from an initially excited nucleus, 240U.

Fig. 2 Open circles are the observed FFMD for the initial 240U
nucleus populated at 45 MeV. This FFMD is a mixture ofcontributions from 235-240U as shown by dashed curves, populated bymultiple neutron emission. Each curve is calculated by a Lagevinequations with a fraction determined by statistical model. The
obtained theoretical curve is broadened with the experimental
resolution to make a direct comparison with the measured data.