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Ionisation Process of Uranium and its Enhancement in Thermal Ionisation Mass Spectrometry

Monia Kraiem¹, Maria Wallenius, Evelyn Zuleger, Thomas Gouder, Alice Seibert, Jean-Pol Hiernaut, Joan Horta, Klaus Mayer, Klaus Luetzenkirchen

Correspondence address: Monia Kraiem, European Commission, Joint Research Centre, Institute for Transuranium Elements, Nuclear Chemistry, P.O.Box 2340, Karlsruhe, 76125 Germany.

Keywords: Ionization Efficiency; Ionization Process; Ionization, Thermal; Isotope Ratio MS.

Novel aspect: The results of the study contribute to a thorough understanding of the chemical reactions and interactions on a filament and the evaporation and ionisation processes occurring during a TIMS measurement of uranium.

 

Thermal ionisation mass spectrometry (TIMS) is a technique offering the potential for highly accurate isotope ratio measurements of elements having a relatively low ionisation potential, such as uranium and plutonium. The technique has profited from significant instrumental improvements on the detection side, e.g using multi-collector Faraday set-ups, and more recently also multiple ion counting devices. This has allowed down-scaling the sample size from few micrograms of uranium down to the low picogram range. This enables the measurement of the uranium isotopic composition, including the minor abundant isotopes ²³⁴U and ²³⁶U in micrometer sized particles as required by modern safeguards. The challenge in the measurement of such samples is in the availability of small quantities of analyte for the measurement. The major uncertainty component in the final result originates from counting statistics. Thus any increase in ionisation efficiency will result in a higher ion current and the measurement uncertainty will be reduced. A number of empirical procedures have been reported in the literature that help to increase the ionisation efficiency of uranium.

In this paper, the results of an experimental study of the ionisation process of uranium on a TIMS filament are presented. First, known amount of uranium were loaded on rhenium filaments using different additives and filament coatings (e.g. collodion, graphite, gold and platinum) in order to determine the overall ionisation efficiency of uranium. Then, further studies were performed on selected systems. X-ray photoelectron spectroscopy (XPS) was used to study the filament chemistry, i.e. to observe the different U-species forming on the filament during the heating and to see if any interactions between the filament material, the coating and the uranium take place. A modified Knudsen Cell combined with a mass spectrometric analyser was used to study the charged and uncharged species evaporating from the filament. Scanning electron microscopy was used to study the changes in sample morphology during the heating of the filament.

The results of the study contribute to a thorough understanding of the chemical reactions and interactions on a filament and the evaporation and ionisation processes occurring during a TIMS measurement of uranium.