Research Group for Heavy Element Nuclear ScienceGroup Leader : A.N. Andreyev
Members : Katsuhisa Nishio, Kazuaki Tsukada, Yutaka Utsuno, Masato Asai, Hiroyuki Koura,Atsushi Toyoshima, Kentaro Hirose, Tetsuya K. Sato, Hiroyuki Makii, Riccardo Orlandi, Yuta
Ito, Nadine M. Chiera, Kazuki Yoshida, Tomohiro. Tomitsuka, Kun Ratha Kean,Shoya
Tanaka, Katsuyuki Tokoi, Hayato Suzuki

Determination of the limits of stability of heaviest nuclei isone of the topics not only the most interesting but also the mostchallenging in modern nuclear physics. The produced heaviest
elements also open a unique opportunity in chemistry, wherechemical/atomic properties are crucially determined by therelativistic effects. The research conducted by our groupstrongly focusses toward addressing these fundamental
questions.
Nuclear fission, a unique physical process observed only innuclear matter, is our core subject, and understanding of thisphenomenon provides the basis for numerous atomic energy
applications. Our experimental and theoretical programs include
nuclear-structure studies of neutron- and proton-rich nuclei, inaddition to super-heavy nuclei. For identifying the chemical
properties of the heaviest elements, measurement of the firstionization potential IP1, as well as the investigation of complex
formation and its adsorption behaviour, is promoted todetermine the energy and orbitals of the valence electrons.
The highlight of our results in 2018 is the measurement of theIP1 of Fm (atomic number Z=100), Md (Z=101), and No(Z=102). Combined with our previous report for Lr (Z=103),
these results clearly demonstrate that the 5f orbital is fully
occupied by electrons at No and that Lr has a weakly boundelectron outside the No core [1]. More details are described as a
Research Highlight in this Annual Report.
Existence of two deformation paths in the fission of protonrich nucleus 178Pt (Z=78)
Fission of a proton-rich nucleus has nowadays become a hot
topic after the discovery of the mass-asymmetric fission of
180Hg, (see our recent review [2]). Several experimental andtheoretical results are revealing that there is a new island which
is dominated by the mass-asymmetry in fission in addition to thewell-known actinide region in the chart of nuclides. Fission datain the unexplored region were obtained by forming excitedcompound nuclei in heavy-ion fusion reactions at the JAEA
tandem facility. Advantage of this approach is that fission ofmany nuclides can be investigated by selecting available targets
and projectile nuclei, in contrast to the conventional β-delayed
fission and/or fission after Coulomb excitation in inverse
kinematics.

The reaction of 36Ar+142Nd was used to study fission of protonrich nucleus 178Pt [3]. We found two independent fission modes,i.e., different deformation paths in fission. One is the massasymmetric fission leading to average light (L) and heavy (H)fission-fragment masses, AL/AH ~79 u /99 u, close to ~80 u /100
u observed in low-energy fission of 180Hg, and the other is themass symmetric fission, which shows the relatively low total
kinetic energy (TKE), (see Fig.1). These two fission modes arewell reproduced by nuclear density functional theory.
Origin of the dramatic change of fission mode in fermium
isotopes investigated using Langevin framework
About 40 years ago, a very exciting result was found in thespontaneous fissions of fermium isotopes. In contrast to the
mass-asymmetric fission of 256Fm and lighter isotopes, 258Fm
shows sharp mass-symmetric fission. Sudden appearance of the
sharp symmetric fission, only by adding just two extra neutrons,
has not been explained quantitatively so far.
The mechanism becomes unveiled clearly using a Langevinframework [4]. The time evolution of the nuclear shape on thepotential surface reveals that the lighter fermium isotope 254Fm
showing the asymmetric fission-fragment mass distribution istrapped in the local minimum for a substantial length of time
before overcoming the saddle point B, (see Fig.2). This behavior
changes dramatically in 258Fm that disintegrate immediately
after overcoming the 1st saddle point, without surmounting thesaddle point B. The fission path is determined by a subtle
balance between the saddle point A and B. The second saddle
point A, which prevents the fission of 254Fm in this direction, islowered for 258Fm, opening the compact and sharp symmetricfission.

Fig.2 Potential energy landscape for 254Fm and 258Fm, plotted on thedeformation and elongation parameters of nuclear shape. Selectedsample trajectory typical for each nucleus is shown. 254Fm undergoes
fission by overcoming the saddle point B, whereas 258Fm disintegrates
through the saddle point A.

Fig.1 (a) TKE distribution and (b) mass-TKE distribution of thefission of 178Pt, produced in fusion 36Ar+142Nd (Ebeam=168.8 MeV).
(c) and (d) show the fission-fragment mass distributions, generatedusing high- and low-TKE gates, respectively.

References[1] T.K. Sato et al., J. Am. Chem. Soc. 140, 14609 (2018).
[2] A.N. Andreyev et al., Rep. Prog. Phys., 81, 016301 (2018).
[3] I. Tsekhanovich et al., Phys. Lett. B, 790, 583 (2019).
[4] Y. Miyamoto et al., Phys. Rev. C, 99, 051601(R) (2019)