The Actinide Science Group conducts physical and chemical researches on materials containing actinide elements. In particular, the JAEA beamlines SPring-8 BL22XU and BL23SU are equipped with experimental stations in the RI Laboratory, which can handle radioactive materials. They are the unique facilities for synchrotron radiation experiments on actinide materials.

1.Fundamental research on physical properties of actinide compounds by synchrotron radiation X-ray spectroscopy

In terms of the fundamental condensed matter physics, actinide compounds are strongly-correlated electron systems that exhibit a variety of novel physical properties. In order to elucidate the mechanism of their physical properties, we study their electronic and magnetic states by synchrotron radiation X-ray spectroscopy. The group has the soft X-ray angle-resolved photoemission spectroscopy (ARPES) station, the soft X-ray absorption magnetic circular dichroism spectroscopy (XMCD) station, and the scanning transmission X-ray microscope (STXM) station at SPring-8 BL23SU.ARPES is an experimental technique that can experimentally derive the band structure and Fermi surfaces of materials. In particular, by using soft X-rays as incident light, the bulk electronic structures of strongly correlated electron systems can be studied. Meanwhile, we study the magnetic properties of materials by XMCD. Soft X-ray synchrotron radiation from BL23SU, can directly excite p-orbital absorption edges in transition metal elements and d-orbital absorption edges in rare-earth and actinide elements, and we can study the spin and orbital moments of the d and f orbitals responsible for magnetism by XMCD. STXM is an experimental technique for measuring spatially-resolved X-ray absorption spectra by focusing X-rays to a few tens of nanometers that enable the evaluation of electronic states in small areas. We conduct researches on spatially inhomogeneous samples such as environmental samples and decommissioning research samples.

2.Environmental restoration of Fukushima and decommissioning

We are conducting researches contributing to Fukushima environmental restoration and decommissioning research. The hard X-ray beamline BL22XU is equipped with a hard X-ray photoelectron spectroscopy (HAXPES) station, and its extremely high bulk sensitivity is used to study environmental samples such as Cs containing samples and simulated structural materials in nuclear reactors. We have discovered that cesium is strongly adsorbed on clay minerals in contaminated soil, especially weathered biotite, by chemical methods. We are exploring highly efficient desorption methods for cesium by physical pulverization using a ball mill, chemical treatment using an interlayer expander, and molten salt electrolysis. We also investigate the possibility of soil recycling, including the development of thermoelectric properties from treated soil and the discovery of strontium absorption effects.

3.Separation chemistry of difficult-to-separate elements and isotopes

For rare metal ions such as lanthanides and actinides, our group has developed mutual metal separation systems by solvent extraction and precipitation separation using novel ligands, and we have investigated the structural determination of metal complexes and the mechanism of metal separation. Besides, photochemical processes including the f-f transitions are investigated to apply them for the mutual separation of f-block elements. We are also engaged in establishing a new principle of isotope separation which is based on a quantum walk; a quantum counter part to the random walk, with developing techniques required to demonstrate the principle such as broadband terahertz wave generation.

4.Safety assessment for the geological disposal system

Groundwater is the transport media for the long-lived fission products relevant to the safety assessment of the geological disposal system. An electrochemical investigation of selenium species, using cyclic voltammetry, is carried out for the purpose of determining the standard redox potential, which is crucial for understanding the predominant Se species in groundwater.

5.Chemistry of transplutonium elements

The hydration structure of einsteinium was observed by XAFS measurements to determine its ionic radius. In order to investigate the electronic state of metal ions from ultra-trace tranplutonium solution samples, we utilized our two powerful tools; research laboratory for RI samples at the Nuclear Science Research Institute and the ultra-sensitive XAFS measuring environment at the Harima RI Laboratory. We will apply these methods to other RI nuclides in the future.

Electronic structure of strongly-correlated uranium compounds by soft x-ray angle-resolved photoemission spectroscopy

We experimentally study band structures and Fermi surfaces of various strongly-correlated uranium compounds by angle-resolved photoemission spectroscopy (ARPES) using soft x-ray synchrotron radiation. The experimental results are compared with relativistic band-structure calculations to elucidate their 5f electronic structures to understand the mechanism of their physical properties. [(Invited Review) S. Fujimori et al., J. Phys. Soc. Jpn. 85, 062001 (2016).]　Among the strongly-correlated uranium compounds, UTe₂ was found to be a spin triplet superconductor at the end of 2018. It has found potential applications as a superconducting qubit in quantum computers and is being studied extremely competitively. Our research group has succeeded in clarifying the electronic structure of UTe₂ by ARPES for the first time in the world [S. Fujimori et al., J. Phys. Soc. Jpn. 88, 103701 (2019).]