D.S. Simons, A.J. Fahey, J.G. Gillen, S.A. Wight, and C.J. Zeissler

Objective: To develop procedures that improve the detectability and reliability of screening measurements for uranium by secondary ion mass spectrometry (SIMS).

Problem: For several years, the Analytical Microscopy Group has been developing and improving procedures to screen environmental swipe samples for the presence of enriched uranium. The driving force for this project is the mandate of the International Atomic Energy Agency (IAEA) to verify that signatories of the Treaty on the Non-Proliferation of Nuclear Weapons do not permit civilian nuclear materials to be diverted for military purposes. The IAEA conducts inspections of uranium enrichment plants and collects cloth swipe samples from them. These samples are analyzed to determine the isotopic composition of the uranium that is collected. We have developed a procedure based on SIMS to survey particulate matter extracted from the cloth swipe material and to measure the uranium isotopic composition from individual particles by a quantitative imaging method. In the course of analyzing samples from the IAEA, we have noted some analytical challenges related to the sparseness of uranium-bearing particles in some samples, and to molecular ions that produce spectral interferences with uranium in others. We have recently developed some new approaches to address these issues.

Approach: It would be useful to have a method to prescreen samples to decide whether enough uranium is present to make the SIMS analysis worthwhile. We have investigated the use of multi-day gamma spectral acquisitions and exposures to radiation-sensitive phosphor imaging plates as possible nondestructive screening methods applied to the cloth swipes. After particles are extracted from the cloth and deposited on a suitable substrate, the SIMS measurements are made. At this point, different procedures can be used depending on the density of uranium particles that are present. If the density is high, isotopic measurements can be made from each analytical area. This approach is time-consuming and usually covers only a few square millimeters of the substrate on which the particles are deposited. However, if the uranium density is low, it is more time-efficient to survey for only the major isotope of uranium (238U), and return to those locations where uranium was detected for an isotopic analysis. In this way an area greater than 1 cm2 can be surveyed in one day. We have noted that molecular ion interferences can perturb the apparent isotopic composition of uranium. In particular, 208Pb27Al and 207Pb28Si signals can be confused with 235U and lead to a false detection of enriched uranium. We have found that the ratio of signals at m/z 234 to m/z 235 can be used as a diagnostic to indicate the presence of these spectral interferences.

Results and Future Plans: A comparison of low background gamma spectral acquisition and phosphor image plate exposure of test swipes for similar exposure times revealed that the phosphor imaging plate was able to detect activity from the cloth in several cases even though the gamma measurement was negative. The samples with positive detection were also found to have enough uranium to be suitable for SIMS measurements, indicating that the phosphor imaging plate can be a viable screening tool. The large area SIMS scanning method was applied to a sample of particles extracted from a cloth and dispersed on a graphite disk. A survey was made of an array of 6400 circular areas, each 150 mm diameter. Figure 1 shows a bubble plot in which the log of the 238U intensity is represented by the diameter of the bubble. The plot clearly shows the locations where high uranium signals were detected. Isotopic ratios were measured by SIMS from particles in these high-signal areas. In future work, this large area scanning method will be explored further to determine the minimum time in which a full sample can be surveyed.

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