Ortsaufgelöste Ultraspurenanalyse mittels Sekundärionen-Massenspektrometrie und Laser- Sekundärneutralteilchen-Massenspektrometrie
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Abstract
Abstract:
The applicability of clay rock and cement as geological and geotechnical barriers for long-term nuclear waste storage can be assessed by diffusion and sorption studies. Such experiments are conducted using radionuclide concentrations in the trace regime. Information about spatial distributions of the radionuclide in those samples is of great interest to monitor their migration through the sample or to find correlations between different minerals and the radionuclide.
In this work, a setup for laser postionization in a time-of-flight secondary ion mass spectrometer
(ToF-SIMS) was established and optimized to enable laser secondary neutral mass spectrometry (Laser-SNMS). The key characteristics of the realized approach lies within a suppression of secondary ion background with a delayed extraction and an extraction bias voltage along with a resonant laser ionization with a three step excitation scheme. This way, the method should combine a high sensitivity and selectivity from the resonant ionization with the lateral resolution of the SIMS instrument.
The setup consists of a commercial ToF-SIMS instrument and a laser system based on three tunable titanium-sapphire lasers that are pumped by an internally frequency doubled neodymium-YAG laser. The ToF-SIMS and the laser system were optically coupled via a glass fibre. A synchronisation of the pulsed processes of both systems was achieved by using the SIMS trigger signal as a masterclock after enhancing it with a pulse generator.
In order to perform Laser-SNMS on a ToF-SIMS instrument, the settings and applied voltages had to be changed and optimized to comply with the new ionization process. These optimizations were performed separately for conducting and non-conducting sample types. They included the coordination and exact timing of the SIMS duty cycle and the laser pulses, changes in the settings of the primary ion gun and the mass analyzer as well as the charge compensation for non-conducting samples.
After the optimizations, also some application examples were investigated. First SIMS studies on clay minerals and a neptunium diffusion sample in Opalinus clay rock showed promising results but also the limits of this method. A combined approach of SIMS and Laser-SNMS measurements was then used to examine a plutonium diffusion sample in clay rock and a plutonium sorption sample on a cement thin section. It was possible to reproduce findings from previous, synchrotron based measurements and to demonstrate the applicability of the newly established Laser-SNMS method for the analysis of migration and sorption experiments. Another method, the resonance ionization mass spectrometry (RIMS), was used to determine plutonium and technetium in environmental samples at an ultratrace level.