Fission fragments mass distributions of nuclei populated by the multinucleon
transfer channels of the 18O+232Th reactionK.Hirose1), K.Nishio1), R.Léguillon1), H.Makii1), I.Nishinaka1), R.Orlandi1), K.Tsukada1),
J.Smallcombe1,2), S.Chiba3), Y.Aritomo4), T.Ohtsuki5), R.Tatsuzawa6), N.Takaki6), N.Tamura7),
S.Goto7), I.Tsekhanovich8), C.M.Petrache9), A.N.Andreyev10,1)
1): ASRC, JAEA 2): TRIUMF, Canada 3): Tokyo Institute of Technology 4): Kindai Univ.
5): KUR, Kyoto Univ. 6): Tokyo City Univ. 7): Niigata Univ. 8): Univ. of Bordeaux, France9): Univ. Paris-Saclay, France 10):Univ. of York, United Kingdom

Nuclear fission process is usually described as an evolution ofa nuclear shape on a potential-energy surface resulting from theinterplay of macroscopic (collective) and microscopic (singleparticle) degrees of freedom in a nucleus. In particular, the mass
division in fission is directly influenced by nuclear shell effects.
Then fission-fragment mass distribution (FFMDs) and their
sensitivity to the excitation energy and isospin provide a deep
insight into the mechanism of the process of fission.
The scope of the present work is to explore the potential ofthe multi-nucleon transfer (MNT) reactions to measure FFMDs
and their excitation energy dependence for the neutron-richactinides including nuclides which cannot be accessed byparticle-capture and/or heavy-ion fusion reactions. Advantage ofthis method is that fission of many compound nuclides can bestudied in a single experiment. A key development for thismethod is a silicon ΔE-E detector to separate ejectiles toidentify transfer channels and determine the excitation energy ofcompound nuclei. Fission fragments were detected incoincidence using multi-wire proportional counters. We studied
the reaction 18O + 232Th using the 18O beam from the JAEA
tandem accelerator.
Figure 1 shows the FFMDs of 231-234Th*, 232-236Pa*, and 234238U*, for excitation energies with an interval of 10 MeV [1].
The experimental FFMDs for all nuclides studied havepredominantly asymmetric shape in the lowest excitation energy,
while the symmetric component grows with increasing
excitation energy. The data for 231,234Th*, 234,235,236Pa* wereobtained for the first time, which exhibits a larger advantage ofthe MNT reactions for fission studies. It is found that themeasured spectra reveal larger peak-to-valley ratio in theFFMDs for nuclei with large isospin values, as seen for the mostneutron rich isotopes of the same element, considered at thesame excitation energy, for instance at the E*=20-40MeV. Thismight be explained by the growing influence of the magic 132Sn
nucleus on the mass division toward neutron-rich nuclei.
The general behaviour of the FFMDs at low excitationenergies is explained by the calculation based on the fluctuationdissipation fission model [2] shown by the red curves in Fig.1.
The measured FFMDs however show the double-humped shape
more clearly at the highest excitation energy. By a further
analysis, it is found that the asymmetric FFMDs observed at
higher energies are due to the effects of multi-chance fission;
contribution of the fissions of several lower-excited lighter
nuclei produced after neutron evaporation (or emission).
From this study, the MNT reaction was found to be a
powerful tool to explore fission observables in a wide nuclide
region up to the high excitation energy of 60 MeV. In higher
excitation energies, an interesting observation was reported thatthe number of the prompt-fission neutrons only from the heavy
fragment increases when the excitation energy of a fissioning
nucleus increases [3]. Now we are planning to measure theprompt-fission neutrons as well as FFMDs by combining
neutron detector array with the present setup.
References[1] R. Léguillon et al., Phys. Lett. B761. 125 (2016).
[2] Y. Aritomo et al., Phys. Rev. C 88, 044614 (2013).
[3] A.A. Naqvi et al., Phys. Rev. C 34, 218 (1986).

Fig.1 FFMDs obtained in the MNT channels in 18O+232Th. The blue curves are the examples of original Langevin calculation, which are broadenedwith the mass resolution (red curves) to make a comparison to the measured spectra [2].