Determination of the isotopic composition of trace amounts of actinides in environmental micro-particles by Resonance Ionisation Mass Spectroscopy (RIMS) of sputtered neutrals (SN)

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Resonance ionisation mass spectroscopy (RIMS) should be developed and applied for the investigation of the isotopic composition of trace amounts of actinides, most important uranium and plutonium, in micron-size particles. Such particles are of high interest in the context of a risk assessment of contaminated areas, nuclear forensic analyses as well as IAEA and Euratom safeguards programs. They usually consist of a uranium oxide matrix, which contains plutonium and americium traces. The determination of the actinide isotopic composition is used to deduce the origin, age and history of the particles. The standard technique for the analysis of small particles, secondary ion mass spectrometry (SIMS), uses ion gun sputtering in combination with mass-selective detection of the secondary ions. It suffers from isobaric interferences (238U/238Pu, 241Am/241Pu). The use of RIMS in combination with ion gun sputtering, post-ionising the sputtered neutrals rather than using the secondary ions, promises to be an elegant tool to overcome the problem of isobaric interference. In addition to extremely high element selectivity when using multi-step resonant ionisation, an increase in efficiency can be expected as ion gun sputtering produces predominantly neutral particles. The proposed set-up requires upgrading the RIMS system the Institut für Kernchemie at the University of Mainz with an ion gun that has a small spot size for sputtering off material from micron-size particles and a new sample stage that allows the handling of particulate samples. A laser system for resonant ionisation and a time-of-flight mass spectrometer for the registration of the created ions already exist. A feasibility study was performed in a previous project.