Title: Experimental Investigation of the Ionisation Mechanism of Uranium in Thermal Ionisation Mass Spectrometry in the Presence of Carbon
Citation: INTERNATIONAL JOURNAL OF MASS SPECTROMETRY vol. 289 no. 2-3 p. 108-118
Publication Year: 2010
JRC N°: JRC52628
ISSN: 1387-3806
URI: http://www.elsevier.com/locate/ijms
DOI: 10.1016/j.ijms.2009.10.001
Type: Articles in periodicals and books
Abstract: Thermal ionization mass spectrometry (TIMS) is a well established instrumental technique for providing accurate and precise isotope ratio measurements of elements with reasonably low first ionization potential. In nuclear safeguards and in environmental research, it is often required to measure the isotope ratios in small samples of uranium. Empirical studies had shown that the ionization yield of uranium and plutonium in a TIMS ion source can be significantly increased in the presence of a carbon source. But, even though carbon appeared crucial in providing high ionization yields, processes taking place on the ionization surface were still not well understood. This paper describes the experimental results obtained from an extended study on the evaporation and ionization mechanisms of uranium occurring on a rhenium mass spectrometry filament in the presence of carbon. Solid state reactions were investigated using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Additionally, vaporization measurements were performed with a modified-Knudsen cell mass spectrometer for providing information on the neutral uranium species in the vapor phase. Upon heating, under vacuum, the uranyl nitrate sample was found to turn into a uranium carbide compound, independent of the type of carbon used as ionization enhancer. With further heating, uranium carbide leads to formation of single charged uranium metal ions and a small amount of uranium carbide ions. The results are relevant for a thorough understanding of the ion source chemistry of a uranyl nitrate sample under reducing conditions. The significant increase in ionization yield described by many authors on the basis of empirical results can be now fully explained and understood.
JRC Directorate:Nuclear Safety and Security

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