Research Group for Heavy Element Nuclear ScienceGroup Leader : Andrei N. ANDREYEVMembers : NISHIO Katsuhisa, TSUKADA Kazuaki, ASAI Masato, MAKII Hiroyuki, SATO
Tetsuya, HIROSE Kentaro, Riccardo ORLANDI, ITO Yuta, SUZAKI Fumi, TANAKA Shoya,
AOKI Ryota, UCHIBABA Yuta

The heavy elements open unique opportunities for studies ofrelativistic effects in chemistry and atomic physics, for which
single-atom chemistry is promoted. Determination of the limitsof existence of the heaviest nuclei is among the most interesting
and challenging topics in nuclear physics, and nuclear fission is
our core subject in the nuclear physics program. Our experiments
also include nuclear structure studies of a broad range of nuclides,
in addition to super-heavy nuclei. The experiments are carried out
at the JAEA tandem accelerator facility.
One of the highlights of our nuclear physics experiments is toquantify the angular momentum of excited compound nuclei(CN) populated in multinucleon transfer reaction 18O+237Np.
Here, angular momentum was derived by measuring angulardistributions of fission fragments relative to the rotational axis ofthe CN. The results are presented as a Research Highlight.
In this report, we describe a recent nuclear-structure study and ameasurement of neutron-capture cross sections relevant to stellar
nucleosynthesis achieved via the surrogate-reaction method.
Nuclear structure of 248Cf and shell gaps in heavy nuclei
The limits of existence of the heaviest nuclei are strongly
connected with the presence of shell gaps which enhance theirstability against fission. The shell gaps in the superheavy element
region are predicted by different models to occur in nuclei with
proton number Z = 114, 120 or 126 and neutron number N = 184.
Important benchmarks to improve theoretical models can beobtained from deformed actinides lying near smaller, “deformed”
shell gaps, that are believed to be pronounced near (Z, N) ~(100,152). Still, different models predict different locations.
Furthermore, in these heavy actinides, part of the nucleon orbitals
above the predicted spherical shell gaps at Z=114 or N=184 canappear as excited states due to deformation of the nuclear shape,
typical for such actinide nuclides.
248Cf (Z=98, N=150) has only two protons and two neutrons
below the deformed Z=100 and N=152 gaps, but the location ofthe gaps is debated and Z=98 is also suggested. To test thedifferent claims, we searched for excited states in 248Cf sensitive
to the location of the proton shell gap.
248Cf was produced at the JAEA Tandem accelerator via the18O+249Cf multi-nucleon transfer reaction, and γ rays emitted by

its excited states were detected. Several new transitions wereobserved and, for the first time, the mean lifetimes (τ) of excitedstates in the nanosecond range were measured. One of the isomers
found in this work is shown in Fig. 1. We also found evidence ofa long-lived (τ > 200 ns) excited state at (882) keV, to which wetentatively assigned a 5- spin parity. The low energy of this (5-)
state, which was predicted since the 1970s by calculations, but
until now not observed, corroborates that the deformed protonshell gap occurs at Z=100, not at Z=98.
Study of 59Fe(n,γ)60Fe cross sections via a surrogate reaction
The radioactive isotope 60Fe (t1/2=2.62 Myr) is a key nucleus inastrophysical processes. Since its half-life is much shorter than
the age of the galaxy, its detection permits us to study ongoingstellar processes in our galactic neighbourhood. 60Fe is produced
in stars with masses at least 8 times larger than the sun (M ≥ 8M⊙),
through capture reactions in the high neutron fluxes reachedduring the carbon (C-shell) burning stage and the core-collapse
supernova explosion.
To elucidate the production of 60Fe, accurate knowledge of the59Fe(n,γ)60Fe neutron-capture reaction is required. The short halflife of 59Fe (t1/2=44.5 days) prevents the production of a 59Fe
target. Thus, at the JAEA Tandem accelerator, the neutron
capture cross sections were determined using the surrogate ratio
method. In this method, the ratio of the γ-decay probability of thecompound nuclei 60Fe* and 58Fe*, R = Pγ(60Fe*)/Pγ(58Fe*),produced by the 58Fe(18O,16O)60Fe* and 56Fe(18O,16O)58Fe*
transfer reactions, respectively, were measured. The 59Fe(n,γ)60Fe
cross sections were then determined by multiplying the known
57Fe(n,γ)58Fe cross sections by R. Figure 2 shows the measured
cross sections as a function of equivalent neutron energy. Theimpact of our new results is that now the main uncertainties in the60Fe yields from massive stars are related to the models of theprogenitor star and of the supernova explosion, rather than to thevalue of the 59Fe(n,γ)60Fe rates.

Fig. 2 The cross sections of 59Fe(n,γ)60Fe as a function of equivalent
neutron energy. Our data are shown by solid circles. The results of theUNF and TALYS code, with their parameters constrained by the γdecay probability ratios measured in this work, are also shown.
Fig. 1 Decay data of the 551-keV γ ray that represent the lifetime of the592-keV state, shown in the level scheme. The red curve is the result ofthe decay-curve fit.

References[1] R. Orlandi et al, “K isomers in 248Cf and the Z=100 shell gap”
100 Years of Nuclear Isomers Workshop, Berlin, Germany, 2022.
[2] S.Q. Yan et al., The Astrophys. J. 919, 84 (2021).