Preliminary Results of Uranium and Plutonium Efficiency Measurements Using a High Efficiency Cavity Ion Source Interfaced with a Finnigan MAT 262 Mass Spectrometer

Evolving goals of safeguards environmental sampling of uranium and plutonium demand decreased sample sizes. Thermal ionization mass spectrometry (TIMS), with samples loaded on metallic filaments for vaporization and ionization before mass analysis, has been the mainstay analytical technique for precise isotope ratio measurement and, using isotope dilution, abundance determination. However, the ionization efficiency (ratio of element ionized to element available) of TIMS is poor for uranium and plutonium. Thus, improved ionization efficiency would increase analytical sensitivity and reduce sample size requirements, enhancing environmental sampling safeguards methods. We have developed a High Efficiency Cavity Source (HECS) that is interfaced with a commercial TIMS instrument (MAT 262) at the IAEA Safeguards Analytical Laboratory. The sample is loaded in a cavity bored into a metal (tungsten, W or rhenium, Re) rod, which is heated by electron impact. The source’s confined geometry and ability to operate at much higher temperatures provide the potential for enhanced ionization efficiency compared to traditional TIMS sources. Preliminary tests yielded total efficiencies of 0.4% to 1.2 % for plutonium and 0.1 to 0.6% for uranium. Following enhancements in the optics of the HECS source the total efficiencies have increased to an average of more than 2 % total efficiency for both uranium and plutonium.