X-ray absorption fine structure (XAFS)-based radionuclide research at the KIT Light Source
Authors: Rothe Jörg 
Published: September 2023

Abstract. In the past two decades, X-ray absorption spectroscopy (XAS) and related
synchrotron-based radionuclide speciation techniques have become
indispensable, supporting open issues in fundamental radiochemistry and
nuclear waste disposal alike. Physicochemical structure information –
encoded in the element-specific X-ray absorption fine structure (XAFS) –
is obtained by the precise determination of the linear mass absorption
coefficient μ(E) of a specimen in an X-ray energy window ΔE encompassing an electronic bonding level of a selected atom type. The characteristic modulations of μ(E) in the vicinity of an “absorption
edge” (i.e., the steeply rising absorption probability of photons tuned to
the binding energy of inner shell “core” electrons) are attributed to
electronic transitions into unoccupied states due to the photoelectric effect. While absorption features close to the edge (arising from excitations into molecular orbitals or empty band structure levels close to
the Fermi level) are generally denoted as an X-ray absorption near-edge
structure (XANES), the scattering states at higher excitation energies are
causing the well-known extended X-ray absorption fine structure (EXAFS). EXAFS oscillations of the post-edge μ(E) are caused by the periodicity of the constructive and destructive interference of outbound and backscattered inbound photoelectron waves (superimposing at the absorbing
atom site) when continuously increasing the excitation energy. Quantitative
analysis of EXAFS data based on electron scattering theory is a well-established short-range structural probe, providing precise values of
interatomic distances (between the absorber atom and its neighbors) and
additional information on the type, number, and ordering of neighbor atoms
within a local cluster. Recent significant advancements in the quantum theoretical description of core-excited final states have been successful in overcoming persisting ambiguities in XANES data interpretation. Meanwhile, although there are more than 50 synchrotron radiation centers
operational in the world, facilities with specialized beamline infrastructure and safety protocols to allow experiments on radioactive samples with activities beyond the exemption limits are still scarce. At the KIT Light Source (at the Karlsruhe Institute of Technology, Karlsruhe, Germany), the Institute for Nuclear Waste Disposal (Institut für Nukleare Entsorgung, INE) operates two beamline end-stations dedicated to the investigation of radionuclide materials by XAFS spectroscopy and related techniques, i.e., the INE-Beamline (operational since 2005) and the ACT station of the CAT-ACT (CATalysis and ACTinide) wiggler beamline (operational since 2016). Samples with total activities amounting to 1.0×106 times the isotope specific exemption limits and 200 mg of fissile isotopes 235U and 239Pu are feasible at both beamlines. Highly versatile detection systems and sample holders – generally avoiding extensive shielding or sample handling in a glove box – have been conceived since the initial commissioning of the beamlines and have enabled unique XAFS measurements such as the first ever investigation of aqueous technetium species at the Tc L3 edge. Investigations of highly radioactive specimens (e.g., spent nuclear fuel or nuclear waste glass fragments) are feasible, regardless of the actual contact dose rate if the radiation field is limited to a few microsieverts per hour at the edge of the experimental table.