Year
2000
Abstract
The detection, characterization, and quantification of trace quantities of uranium and plutonium play a vital role in nuclear safeguards. Current methods of analysis, including thermal ionization mass spectrometry (TIMS), provide precise isotopic and quantitative measurements at the picogram and above level, but below a few picograms conventional instruments encounter detection limitations. The IAEA has expressed an urgent need for instrumentation which will operate well below the picogram level for desired elements. The objective of this research is to build and test a high efficiency cavity type ion source and to integrate it with a commercial thermal ionization mass spectrometer. This combination will provide a 10- 100 times enhancement in ionization efficiency for uranium and plutonium. Resulting from this will be lower detection limits, allowing analysis of samples previously too low in U and Pu for conventional TIMS analysis. Commercially available mass spectrometers based on thermal ionization from heated filaments operate at the limit of their ionization efficiency (about 0.2% for uranium under best conditions). The cavity ion source is not based on a hot rhenium ribbon, but on a tungsten tube with a 0.5 mm diameter cavity into which the sample is loaded. As atoms are vaporized within the cavity, they experience a greater number of interactions with ionizing surfaces than conventional filament TIMS due to the restricted volume and large surface area of the cavity; the result is a greater ionization efficiency. Preliminary data were obtained at LANL using a quadrupole mass analyzer. Although the inherent operating conditions of this device facilitate interfacing the cavity, quadrupoles can not match the isotope ratio precision obtainable with magnetic sector devices. Inherent to the design of most magnetic sector instruments is an operating voltage that exceeds 7000 V; this makes interfacing the cavity much more difficult. During this presentation, we will report our preliminary design considerations. Preliminary efficiency and isotope ratio data from several rare earth elements will show the power of this approach for safeguards applications