Spatially resolved analysis of actinides using resonant ToF laser SNMS

The BMBF-funded research project Secondary Ionisation of Radioactive Isotopes for Ultra trace analysis with Spatial resolution (SIRIUS) on the migration of radionuclides from a future nuclear waste repository focuses on the migration and dissolution behavior of micro- and nanoparticles. A non-destructive characterization of synthetically generated or environmental particles is indispensable as a starting point for the following speciation and leaching experiments on individual particles with regard to their elementary and isotopic composition, as it allows to determine the origin and history of considered particles. Radiometric methods like $\alpha$- and $\gamma$-spectrometry offer first information regarding the isotope composition besides scanning electron microscopy, but can either only detect the short-lived radionuclides with correspondingly high decay energies or are limited to the composition of the main elements. An alternative detection method for a range of stable as well as radioactive isotopes is the almost non-destructive resonant laser secondary neutral time-of-flight mass spectrometry, which provides an isotope-selective surface analysis with a lateral resolution of down to 100 nm. The research project is focussed on the geochemical migration behavior of uranium, plutonium, americium and technetium, with the present work dealing with the ionization behavior of sputtered uranium, plutonium and americium from nuclear fuel particle surfaces. Existing and newly developed two- and three-step resonance ionization schemes have been investigated and optimized for the suppression behavior of isobaric interferences and an equal response of different isotopes of one element. The influence of laser beam polarizations on the isotope signal was investigated and the total angular momentum quantum number of an autoionizing state of plutonium was determined. In addition, the first unambiguous non-destructive detection of ${}^{238}$Pu in the presence of an excess of ${}^{238}$U by at least five orders of magnitude from a fuel particle sample originating in the Chernobyl Exclusion Zone was performed. In addition, a two-step excitation scheme for americium developed by the Institute of Physics was successfully applied for detection of ${}^{242m}$Am in the same sample with an estimated total mass of about one femtogram.