Research Group for Reactions Involving Heavy Nuclei
Group Leader : A. Andreyev
Members : K.Nishio, I. Nishinaka, H. Koura, Y. Utsuno, H. Makii, K. Hirose, R. Orlandi,
R. Léguillon, J. Smallcombe, E. Maeda

Shell structure of nuclei has substantial effects on theirmasses, decays and nuclear reactions. The research objective ofthe group is to study the evolution of shell structure in exotic
heavy nuclei. For this purpose, nuclear decay properties of theexotic nuclear system and reaction process are investigatedexperimentally and theoretically. Nuclear fission process in the
new region of chart of nuclei is investigated to find new effectsof shell structure and dynamics dominating fission process.
Nuclear structure study for neutron- and proton-rich nuclei arealso our scope.
Study for fission of neutron-rich actinide nuclei using multinucleon transfer reaction
Nuclear fission is a fundamental process for nuclear energy
applications. Finding new phenomena in fission andunderstanding of the fission process would bring new idea tosolve issues related to reactor safety and transmutation of long
lived nuclear waste. We are measuring fission properties forneutron-rich actinide nuclei for this purpose.
In order to access neutron-rich nuclei, multi-nucleon transfer
reactions with actinide target nuclei are used. Here, we report on
the results obtained in the reaction 18O + 232Th. The experiment
was carried out at the JAEA tandem facility. A 232Th target wasirradiated by 18O beams. Projectile-like nuclei produced bymulti-nucleon transfer reaction were identified using silicon ΔEE detectors, and the fissioning nuclei and their excitation energy
were assigned event by event. Both fission fragments weredetected in coincidence with position-sensitive multi-wire
proportional counters, and the fragment masses are determinedby kinematic consideration.
In the reaction of 18O + 232Th, we obtained fission data for atleast 14 nuclei ranging from actinium to uranium isotopes [1].
Figure 1 shows the fission fragment mass distributions forseveral protactinium isotopes, where data for three neutron-richnuclei 234,235,236Pa were obtained for the first time.

The figure also shows the excitation energy dependence ofthe mass distributions. Transition from mass-asymmetric fission
to symmetric fission toward high excitation energy is obtained.
The data are also useful to determine the distribution of reaction
products generated in collisions between heavy nuclei.
Finding a New Magic Number 34 from the Level Structureof 54Ca
Searching for the appearance of new magic numbers and thedisappearance of conventional magic nubers is one of the central
issues in the physics of exotic nuclei. Since we predicted theexistence of a new magic number (N) 34 in a very neutron-richnucleus 54Ca about a decade ago [2], a number of experiments
have been carried out to verify the prediction. Experimental datafor its surrounding nuclei do not show any signs of the existenceof the N=34 magic number, and therefore the direct experimenton 54Ca has been strongly awaited. In this study, the 2+1 energy
level in 54Ca is measured for the first time via the in-beam -ray
spectroscopy at RIBF in RIKEN [3]. The measured energy, as
well as the ones for other calcium isotopes, is presented in Fig. 2.
The 2+1 level in 54Ca is much higher than the ones for non-magicnulclei with N=22, 24, 26, 30, and is almost equivalent to that ofa semi-magic nucleus 52Ca. Our shell-model calculation showsthat the 2+1 level in 54Ca indicates a large sub-shell gap between
the p1/2 and f5/2 orbits which amounts to approximately 2.5 MeV.
This is much larger than sub-shell gaps in nickel isotopes which
is typically less than 1 MeV. This experiment has thus
confirmed our prediction and has also validated the underlying
mechanism of the evolution of shell structure due to the nuclearforce.

Fig.2 The 2+1 levels in calcium isotopes compared between experiment(Expt.) and shell-model calculations (Calc.).

Fig.1 Fission fragment mass distribution for protactinium isotopes
produced by multi-nucleon transfer reaction 18O + 232Th. For eachnuclei, excitation energy dependence is shown.

References[1] R. Léguillon et al., “75-years in nuclear fission: Present
status and Future perspectives”, Munbai, India.
[2] T. Otsuka, R. Fujimoto, Y. Utsuno et al., Phys. Rev. Lett. 87,082502 (2001).
[3] D. Steppenbeck et al., Nature 502, 207 (2013).