Research Group for Reactions Involving Heavy Nuclei
Group Leader : Satoshi Chiba
Members : Katsuhisa Nishio, Shinichi Mitsuoka, Hroyuki Koura, Ichiro Nishinaka, Yutaka Utsuno, Hiroyuki Makii,Yasuo Wakabayashi, Yoshihiro Aritomo, Shuya Ota, Tatsuro Nagayama

Neutron-induced fission cross section of short-lived nucleus
239
U determined by surrogate reaction method
Neutron-induced ﬁssion cross sections for 239U (half-life 23.5min) were determined using the surrogate reaction technique.
The compound nucleus 240U* was populated by the transferreaction 238U(18O,16O)240U*, and the fission probability of 240U*
was determined experimentally. The ﬁssion cross sections wereobtained by refering the know ﬁssion cross sections of 235U(n,f)
and measuring the ﬁssion probability of 236U* populated by thereaction 235U(18O,17O)236U*. The experiment was carried out at
the JAEA tandem accelerator facility. A thin uranium target layer
was bombarded by oxygen beams with 162 MeV. The transfer
channel was identiﬁed by detecting the projectile-like nucleus
using silicon ΔE-E detectors. Fission events were identifeidby detecting fission fragments with multi-wire proportionalcounters. The results are shown in Fig. 1.

Fig. 1 Neutron-induced fission cross sections (in barn)for 239U(n,f) plotted as a function of neutron energy(preliminary). The step structure associated with the ﬁrst
chance ﬁssion is seen at En = 7 MeV.
Reaction dynamics in collisions between heavy nuclei in theframework of fluctuation-dissipation model
Exotic nuclei far from stable isotopes are produced bycollisions between heavy nuclei. An example is super-heavy
nuclei (SHN), which can be produced by fusion-evaporation
reactions. Understanding the fusion process is essential topredict the cross section for SHN and to make an experimental
strategy. We have developed a model to calculate the evolutionof nuclear shape from the initial impact between a projectile
nucleus and an actinide target nucleus in the framework offluctuation-dissipation model [1]. The actinide nucleus isprolately deformed, so that we have effectively taken into
account the orientation effects on the reaction. Figure 1 shows
the time evolutions of nuclear shape for systems produced bythe 30Si + 238U and 36S + 238U reactions. Fusion is deﬁned as thecase that the nuclear shape attains the one of ground state ofSHN (compound nucleus). The calculation shows the difference
between fusion-ﬁssion and quasiﬁssion in the mass asymmetryof ﬁssion fragments and in the time scale for ﬁssion. The fusion
probability is obtained from the fusion-fission yield relative
to the total fission yield. The probability agreed with thosedetermined by the evaporation residue cross sections [2,3]. The
same model successfully described the nucleon-transfer inducedﬁssion [4].

Group Activities

The research objective of our group is to investigate heavyion induced reactions such as nucleon-transfer and fusion
reactions. One subject is to develop experimental and theoreticalmethods to determine neutron-induced reaction cross sections
with heavy-ion reactions. For nuclei with short half-lives,
it is practically impossible to measure the cross sections inexperiments using neutron source. The idea of the so-calledsurrogate reaction method is to populate the same compound
nucleus as the neutron-capture reaction by nucleon-transfer
reactions using available target nucleus and to measure thedecay probabilities for ﬁssion and gamma-ray emission to derivefission and capture cross sections. In addition, nuclear fission
and nuclear structure of exotic nuclei and reaction mechanism toproduce exotic nuclei are in our scope.

Fig. 2 Time evolutions of the probability distribution on thenuclear shape projected on on the mass asymmetry (massunit) and charge-center distance for the systems producedin the reactions of 30Si + 238U (left) and 36S + 238U (right).
Quasifission finishes at the time less than 30×10-21 s,
whereas fusion-ﬁssion appears later than 30×10-21 s.
References[1] Y. Aritomo et al., Phy. Rev. C 85, 044614 (2012).
[2] K. Nishio et al., Phys. Rev. C 82, 024611 (2010).
[3] K. Nishio et al., Phys. Rev. C 82, 044604 (2010).
[4] Y. Aritomo et al., Phys. Rev. C 84, 024602 (2011).

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