An improved method for determination of isotope ratios in individual uranium particles by thermal ionization mass spectrometry

Abstract

The determination of isotope ratios of individual uranium particles in environmental swipe samples is very important for nuclear safeguards. A new technique to measure isotope ratios for individual uranium particle was developed in this work, which was a combination of particle transfer by scanning electron microscope (SEM) and isotope ratios analysis by thermal ionization mass spectrometry (TIMS). The technique was verified by measuring isotope ratios in individual micrometer-size uranium particles of known isotopic abundance. The particles were identified by SEM combined with energy dispersive X-ray spectrometer (EDX) and transferred by micromanipulator, which could improve the efficiency of sample preparation and simplify the procedure. The experimental condition was optimized by using a new kind of thermal ion emitter to enhance the ionization efficiency of uranium. Several individual uranium particles from certified reference materials were successfully measured by SEM-TIMS. The results show that the measured isotope ratios were in good agreement with the reference values. The relative errors of 13 particles were within 2.7% for 234U/238U, 1.1% for 235U/238U and 4.5% for 236U/238U isotope ratios, respectively, and the relative standard deviation (RSD) were within 1.6% for 234U/238U, 0.5% for 235U/238U and 3.3% for 236U/238U, respectively. It is expected that this method will become a promising alternative technique for determining uranium isotope ratios in particle analysis. The particles used in this study had sizes between 1.3 and 4.7 μm.