A new tool for ultra-trace analysis of radionuclides : setup, optimization and characterization of the resonant laser-SNMS system for IRS Hannover

Abstract

Geochemical behavior and transport mechanisms of anthropogenic radionuclides in natural systems are of major importance to evaluate their distribution at contaminated areas and storage sites and for reliable estimation and reduction of radiation hazards for affected, inhabited areas. The research project SIRIUS focuses on the influence of the formation of radionuclide containing nanoparticles and sorption processes of radionuclides in different geological formations. The analysis of composition, spatial distribution and localization of hot particles, sorption and migration processes of trace amounts of radionuclides requires an excellent suppression of organic matrix material and isobaric contamination in combination with high spatial resolution while maintaining the natural structure of the sample. The new resonant Laser-SNMS system at the Institute for Radioecology and Radiation Protection (IRS) in Hannover was developed to meet those demands by an extension of the non-destructive spatially resolved analysis of a static TOF-SIMS with the high element selectivity of resonant laser ionization. This system was planned based on a test installation at the Institute for Nuclear Chemistry of the University Mainz. The herein presented PhD project comprises the development and installation of a dedicated Ti:sapphire laser system and the adaption, optimization and full characterization of the Laser-SNMS system as well as the first analytical measurements. The Ti:sapphire laser system was modified in collaboration with the LARISSA group at the Institute of Physics in Mainz to fulfill all requirements for laser post-ionization of a sputtered neutral particle cloud. Besides mechanical modifications of the conventional TOF.SIMS 5 by IONTOF to provide access for the focused laser beams, the operational parameters of the TOF mass analyzer required an optimization for Laser-SNMS application. The complexity of the strongly correlated operational parameters necessitated a simulation-based approach, that included the simulation of the sputtered neutral particle cloud, the complete mass analyzer geometry and the resulting ion trajectories of the laser ions through the mass spectrometer. The results of this optimization lead to a gain of Laser-SNMS ion signal and improve the measurement conditions as demonstrated during several tests with uranium, plutonium and technetium samples. The characterization measurements of synthetic samples create a solid basis for future radioanalytical tasks and enabled determination of the influence of laser ionization and of the sensitivity achievable for ultra-trace analysis. The Laser-SNMS mass spectra of synthetic uranium, plutonium and technetium samples demonstrated the expected gain for the signal-to-background ratio in comparison to conventional SIMS. First analytical measurements verify the sufficient suppression of interfering elements and molecular constituents in the sample and prove the applicability of resonant Laser-SNMS on environmental samples. Furthermore, with the ability to record the starting location of every detected resonant laser ion, it was possible to create isotope selective images of hot particles in environmental sample material.