Research Group for Heavy Element Nuclear ScienceGroup Leader : Andrei ANDREYEVMembers : NISHIO Katsuhisa, TSUKADA Kazuaki, ASAI Masato, SATO K. Tetsuya, HIROSEKentaro, MAKII Hiroyuki, Riccardo ORLANDI, ITO Yuta, SUZAKI Fumi, NAGAME Yuichiro,TOMITSUKA Tomohiro, Kun Ratha. KEAN, SHIRAI Kaori, TANAKA Shoya, TOKOI
Katsuyuki, SUZUKI Hayato

Effects of multichance fission on isotope dependence ofFFMD at high energies
In our recent study [3] on the measurement of FFMDs using
multinucleon transfer reactions, we reached a conclusion that themass-asymmetric shape of the FFMD remaining at highexcitation energies is due to the multichance fission (MCF), i.e.,fission after evaporation of neutrons. As excitation energy is
successively lowered by multiple neutron emission, fission fromhigher-order fission chances tends to have a pronounced doublehumped shape in FFMD due to revival of the shell effects
responsible for mass asymmetry.

Fig.1 Fission-fragment mass distribution for neptunium isotopes(black dot with error bars). Initial excitation energy of thecompound nucleus E* is shown at the right-hand side. Solid curves
are the Langevin calculations (see text).

We extended the measurement to obtain data for a wide set ofnuclides. For example, FFMDs of neptunium isotopes are shownin Fig.1. We also carried out a calculation using the Langevinapproaches [4]. The blue curves are the results which do not take

into account the MCF. Large deviation is evident at the excitationenergies 𝐸𝐸 ∗ ≥ 35 MeV. The results after invoking the MCF
nicely reproduce all the data, showing double-peak structure evenat the highest energies. Looking at exact detail, the experimental
FFMDs clearly show the dependence on the isotope for 𝐸𝐸 ∗ > 30
MeV, exhibiting deeper peak-to-valley ratio toward heavier
isotopes. A larger number of evaporated neutrons for heavier
isotopes accounts for these trends.

Effects of the nuclear structure of fission fragments on
the high-energy prompt fission 𝜸𝜸 -rayspectrum in𝟐𝟐𝟐𝟐𝟐𝟐
𝐔𝐔(𝒏𝒏𝐭𝐭𝐭𝐭 , 𝒇𝒇 )We have measured the PFGS in 235U(𝑛𝑛th , 𝑓𝑓 ) by significantly
extending the high-energy limit of 𝐸𝐸 ∗ to about 20 MeV, incontrast to ∼ 7 MeV in literature (see Fig.2) [5]. This wasachieved by the high-efficiency setup consisting of large volume
LaBr3 (Ce) scintillators and compact fission-fragment detectors.
We compared our data with a statistical Hauser-Feshbach
model calculation to find the origin of high-energy γ-rays. Here,
we adopted a fission model which can reproduce experimental
data on the average properties of neutron and 𝛾𝛾-ray emissions inorder to calculate the initial excitation-energy and spin
distributions of primary fission fragments (FFs) [3]. The solid
curve in Fig. 2 is the results of the calculation, where available
experimental data on low-lying nuclear levels of the FFs are used.
In this analysis we found
102
that dominant contributions
101
to the spectrum at 𝐸𝐸𝛾𝛾 > 10100
MeV are given only by
10-1
specific FFs, such as 86Br,
10-2102,103
Nb , and 109Tc ,10-3
known as having low-lying
10-4
high-spin states which open
10-5
the 𝛾𝛾 -ray transition fromhigh-energy and high-spin
10-6PeelleVerbinski
states of primary FFs.
10-7
Oberstedt
Significant underestimation-8
Present10Calculationof the spectrum at 𝐸𝐸𝛾𝛾 > 14-910 Prompt fission γ-ray spectrumFig.2
15200510MeV suggests the presence
Energy (MeV)in 235U(𝑛𝑛γ-rayth , 𝑓𝑓).
of neutron-rich nuclei
whose low-lying high-spinstates are not found yet.
Yield (γ's / fission MeV)

Determining the limits of stability of heaviest nuclei is the mostinteresting and challenging topics, and the relevant problem, i.e.,
nuclear fission is the core subject in nuclear physics program ofour group. Our experiments also include nuclear structure studiesof broad range of nuclides, in addition to super-heavy nuclei. The
heavy elements open a unique opportunity for studies ofrelativistic effects in chemistry and atomic physics. For thechemical properties of the heaviest elements, many experimental
approaches are promoted to determine the energy and orbitals of
the valence electrons.
In the chemistry program in FY2019, the formation of NbOCl3and TaOCl3 and their adsorption behaviour on quartz surfaces
were explored by applying an isothermal gas-chromatographic
method for the basic study of dubnium (Db, element 105) [1,2].
The future studies with DbOCl3 under the same experimental
conditions will quantify the volatility trend in Group-5 elements.
In this report, we present two results of fission studies, i.e., theeffects of neutron emission from compound nucleus on thefission-fragment mass distribution (FFMD) and the measurementof high-energy prompt fission γ-ray spectrum (PFGS) in thefission of 235U induced by a thermal neutron, 235U(𝑛𝑛th , 𝑓𝑓 ).

References[1] N.M. Chiera et al., Inorg. Chim. Acta 486, 361 (2019).
[2] N.M. Chiera et al., J. Radioanal. Nucl. Chem. 320, 633 (2019).
[3] K. Hirose et al., Phys. Rev. Lett. 119, 222501 (2017).
[4] S. Tanaka et al., Phys. Rev. C 100, 064605 (2019).
[5] H. Makii et al., Phys. Rev. C 100, 044610 (2019).