Research Group for Heavy Element Nuclear ScienceGroup Leader : A. Andreyev
Members : K. Nishio, K. Tsukada, Y. Utsuno, M. Asai, H. Koura, A. Toyoshima, K. Hirose,T.K. Sato, H. Makii, R. Orlandi, M. Vermeulen, Y. Kaneya, A. Mitsukai, T. Tomitsuka

Studies of the chemical properties of the heaviest elements,and determination of the limits of nuclear stability, are the mostinteresting, but also most challenging, topics in modern
chemistry and nuclear physics. The research conducted in ourgroup strongly focussed toward addressing these fundamentalquestions.
In nuclear physics, our core program is on fission, which is aunique physical process observed only in nuclear matter, andwhich provides the basis for numerous atomic energy
applications. Our experimental and theoretical programs include
nuclear structure and reaction studies of exotic neutron- andproton-rich nuclei, as well as super-heavy nuclei. For nuclear
chemistry, our work is concentrated on the heaviest elements,
whose chemical properties are crucially determined byrelativistic effects. Atomic and chemical properties of exotic
atoms and molecules are studied via an atom-at-a-timechemistry approach.

γ / (fission MeV)

Measurement of high-energy prompt γ-rays in thermalneutron induced fission of 235U
Prompt fission γ-ray spectra (PFGS) are important as they
allow us to study the structure and de-excitation process ofneutron-rich nuclei. Also, it is required as a nuclear data for adesign of new types of reactors, such as the Generation-IVreactors. For spontaneous fission of 252Cf, the PFGS show ahump structure associated with a giant dipole resonance (GDR)
observed in fission fragments around 15 MeV [1]. For thermal
neutron-induced fission, the PFGS data are limited up to about 7
MeV, even in recent experiments for 235U(nth,f). This prompted
us to extend the measurement up to energies as high as 20 MeV,
enough to observe the GDR in 235U(nth,f).
The measurement was carried out using a cold-neutron beamfrom the high-flux reactor (PF1B line) of the Institut LaueLangevin (ILL), Grenoble, France. In the present measurement,
we used two multi-wire proportional counters (MWPCs) todetect both fission fragments in coincidence, and two LaBr3(Ce)scintillators to measure the γ-rays in fission. The improved
sensitivity of our detector setup revealed the PFGS up to ~20
MeV as shown in Fig. 1 [2]. The hump structure associated withthe GDR is seen.

We also observed a local structure around 4 MeV and 6 MeV,
which were also reported in 252Cf(SF) [3]. From the correlation
between the fission fragment mass and the γ-ray spectrum, it isfound that they are emitted from the fragments in the vicinity of
doubly-magic nucleus 132Sn.

Improvement of the gross theory for the description of βdecay half-lives
β− decay is the dominant decay mode of neutron-rich nuclei
and plays an important role in an astrophysical nucleothynthesis
and/or nucler reactor technologies. The gross theory [4], basedon the macroscopic concept, can predict a global trend for the βdecay half-lives and delayed-neutron yields over the chart ofnuclides, which have been a guide for β- -decay measurement
and associated structure studies. Due to its statistical treatment
the model does not take into account the microscopic properties,
characterized by the spin and parity of a nuclear level, and theireffects on β--decay. This results in a local disagreement from theexperimental data, particularly for nuclei in the vicinity ofclosed-shell nuclei.
To give a higher accuracy in our model, the microscopic
effects are included, where a possible change in the parities
between the parent and daughter nuclei significantly suppresses
the nuclear matrix elements. The calculated results are shown inFig. 2. A better agreement is found in the half-lives of theneutron-rich tin, indium and cadmium isotopes beyond theneutron magic number of N = 82.

Fig. 2. β−-decay half-lives for tin, indium and cadmium isotopes in theneutron-rich region. Thick solid red lines are from the improved grosstheory [5], which is compared with a previous model [4] (red dottedcurve) and FRDM + Quasi-RPA calculation [6] (blue dotted curve).
Location of the neutron magic number N=82 is shown by the vertical
bar.

1010
1010-1
10-2
10-3
10-4
10-5
10-6
10-7
0 2 4 6 8 10 12 14 16 18 20
γ-ray Energy (MeV)

Fig. 1. The PFGS for 235U(nth,f) obtained in the present measurement.

References[1] H. van der Ploeg et al., Phys. Rev. C 52, 1915 (1995).
[2] H. Makii et al., EPJ Web of Conference, in print.
[3] A. Hotzel et al., Z. Phys. A 356, 299 (1996).
[4] T. Tachibana et al., ENAM95 (Gif-sur-Yvette), p.763 (1995).
[5] H. Koura et al., Phys. Rev. C 95, 064304 (2017).
[6] P. Möller et al., Atom. Dat. Nucl. Dat. Tab. 66, 131 (1997).