Research Group for Exotic
Heavy Element
Heavy-Element
NuclearNuclear
ScienceScience

Group Leader : NISHIO
Andrei N.Katsuhisa
ANDREYEVMembers : NISHIOTSUKADA
Katsuhisa,
Kazuaki,
TSUKADAASAI Masato,
Kazuaki,
MAKII
ASAI
Hiroyuki,
Masato, SATOMAKIITetsuya,
Hiroyuki,
HIROSE
SATO
Kentaro,Tetsuya, Riccardo
HIROSE ORLANDI,
Kentaro, Riccardo
ITO Yuta,
ORLANDI,SUZAKI ITO
Fumi,
Yuta,
James
SUZAKISMALLCOMBE,
Fumi, TANAKA
PHAM Shoya,
Thanh
Tung,
AOKI AOKIRyota,Ryota,
UCHIBABAUCHIBABA
Yuta Yuta, Gyeongmin GONG, NATORI Hina, MIYACHI Yuta

Determination of the limits of existence of the heaviest nuclei
is among the most interesting and challenging topics in nuclearphysics. We investigate nuclear structure studies of a broad rangeof nuclides, in addition to super-heavy nuclei. The heavy
elements open unique opportunities for studies of relativistic
effects in chemistry and atomic physics.
One of the highlights in FY2022 is establishing a method tostudy the nuclear structure of neutron-rich heavy nuclei using
multinucleon transfer reaction, see Research Highlight. Here, weshow two experimental results, i.e. nuclear structure study ofselenium isotopes and fission of actinide nuclides.

preliminary analysis supports a description of mixing between
three competing shapes, leading to an interesting but complex
system which requires further study.

Mixing of different nuclear shapes in selenium isotopes
The study of nuclear shapes is a key tool for understanding how
the Strong Force governs these many-body systems. Of particularinterest is the phenomenon of Shape Coexistence, in which
multiple different shapes are possible almost simultaneously
within a nucleus. Which shape is dominant can switch from one
isotope to the next, with the addition of a single neutron. Such a
dramatic shape change occurs in the unstable selenium isotopes.
Through experimental studies of their shapes, and the quantum
mixing between the shapes, we aim to understand how the change
is driven, and what determines the point of the shape flip. Nuclear
shape affects how the electromagnetic field of a nucleus interacts
with the electrons of its own atom. In a process called internalconversion, the energy release in the transition between nuclear
shapes can be given to an electron, causing it to break away fromthe atom. The probability of such transitions is sensitive to shapemixing within the nucleus.

Role of heavy-fragments in the fission of actinide nuclides
Nuclear fission is a unique decay process of heavy nuclei andwidely used for atomic energy applications. Also, it is believedthat nuclear fission is important to explain the abundance ofelements in the universe, as it would happen in the last stage of
r-process nucleosynthesis in stars.
We are studying nuclear fission experimentally by populating
an excited compound nucleus in multinucleon transfer reactions.
This reaction is unique as it can produce many nuclides in one
reaction by exchanging different numbers of neutrons andprotons between target and projectile nuclei. We aresystematically measuring fission fragment mass distributions,known to exhibit mass-asymmetric fission for typical actinide
nuclides, as a signature of nuclear shell structure characterizing
fission process.
Figure 2 shows the average heavy (AH) and light fragment mass
numbers (AL) obtained in our experiments. In general, the heavyfragments maintain AH~140, whereas the light fragment linearly
increases with the mass number of fissioning nucleus (Ac).
Looking in detail, it is noticed that not only AL but also AH
increases with Ac for each elemental isotopes. Assuming that
proton to neutron ratio of fissioning nucleus is conserved infragments, we found that heavy fragments have an averagenuclear charge ZH=54 for all the studied nuclei, indicating adominant role in fission.

Fig. 1 Electron spectra showing the electron peak at 900 keV for theinteraction between two shapes in 72Se and the measured lifetime of theassociated process.

Fig. 2 Average heavy (AH) and light (AL) fragments as a function of themass number of fissioning nucleus.

Using a dedicated electron spectrometer, called SPICE [1], we
measured electron spectra (Fig. 1), and were able to clearly
identify the peak corresponding to mixing between shapecoexisting states in the isotope 72Se [2]. Our measurement
showed that there is strong mixing between the competing shapes
in this nucleus, and while two distinct shapes can be observed inselenium isotope, there isn't a sharp switch of dominant shapes.
Furthermore, our experimental work extended to 70Se, for which

References[1] M. Moukaddam, J. Smallcombe et al., Nuc. Inst. Meth. A, 905,180-187, (2018).
[2] J. Smallcombe et al., Phys. Rev. C, 106, 014312, (2022).
[3] K. Hirose et al., TASCA 23, 20th Workshop on Recoil
Separator for Superheavy Element Chemistry & Physics, 25-27
April 2023 Darmstadt, Germany.