Determination of minor isotope ratios of individual uranium particles by accelerator mass spectrometry

Abstract

The determination of uranium isotope ratios in uranium particle is an essential technique in the analysis of environmental samples for nuclear safeguards. At present, mass spectrometry has been widely used to measure isotope ratios of uranium particles. We developed a new analytical method for measuring minor isotope ratios of individual uranium particles directly. The technique includes single particle identification by energy dispersive X‐ray spectrometer (EDX), transfer by scanning electron microscope (SEM) combined with micromanipulator and isotope ratios analysis with accelerator mass spectrometry (AMS). The minor isotope ratios of individual uranium particles with four isotopic abundances and different sizes from certified reference materials were measured by AMS. The results show that the relative error (RE, i.e., the deviation of measured value from the certified value) of 234U/235U and 234U/236U isotope ratios was 9.0% and 19.3%, respectively, and the relative standard deviation (RSD) was within 9.9% and 16%, respectively. The minimum particle determined was about 2 μm. The method was found to be an alternative analytical means for nuclear safeguards.

Comparison of AMS, TIMS, and SIMS techniques for determining uranium isotope ratios in individual particles

“…A high‐purity silver (99.99%) target cone was designed to satisfy the demand of sample preparation 28,29 . The high‐vacuum grease was diluted with n‐heptane to 0.01 g/ml.…”

“…The high‐purity silver target cones with diluted vacuum grease (0.01 g/ml, 2 μl) and a clean carbon planchet containing uranium particles of U200 were introduced into the chamber of SEM (JSM‐6360 V) simultaneously. The individual uranium particles with diameters of 3 to 5 μm on the carbon planchet were identified with energy‐dispersive X‐ray spectrometer (EDX) and picked up with a tungsten needle attached to a micromanipulator (MM3A) and transferred onto the center of each high‐purity silver target cone 28,29 . The preparation of uranium particle samples (2–7 μm) from U850 were the same as above.…”

“…The isotope ratios were determined with an AMS (300 kV) which was developed by CIAE 28,29,31 . 234 U, 235 U, 236 U and 238 U were measured in a peak‐jumping mode, respectively.…”

“…The experimental condition of SEM‐TIMS was optimized by using a new kind of thermal ion emitter to enhance the ionization efficiency of uranium 27 . In addition, a new method to directly measure individual uranium particles by a miniaturized AMS with atmospheric insulation system developed by China Institute of Atomic Energy (CIAE) has been studied 28,29 . IAEA has been pursuing improvements or new techniques for isotope ratios analysis of uranium particles to verify the absence of undeclared nuclear activities.…”

“…China Institute of Atomic Energy (CIAE) has been studied. 28,29 IAEA has been pursuing improvements or new techniques for isotope ratios analysis of uranium particles to verify the absence of undeclared nuclear activities. Almost all spectral interferences that would be problematic in smaller geometry SIMS instruments can be eliminated when large-geometry SIMS instruments (LG-SIMS) are used for measurements of uranium isotope ratios in individual particles.…”