Determination of Minor Isotope Ratios in Uranium by Multiple Collector Inductively Coupled Plasma Mass Spectrometry

The isotopic fingerprints of uranium are of particular interest for nuclear forensics and nuclear safeguards. Especially the need for lower detection uncertainties of minor isotopes，including ²³⁴U and ²³⁶U, is considered to be of major importance for establishing relations between materials. This work described the utilization of multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the determination of uranium isotope ratios of uranium materials. The EC nuclear reference material 199 (EC-NRM199) were diluted to about 40.0 ng/g by 2% HNO₃ (V/V). The digested liquid samples of two kinds of yellow cakes were diluted to about 21 000 ng/g and 180 ng/g, respectively. Mass fractionation was corrected by external standardization and standard sample bracketing (SSB) method. The analytical results of the 180 ng/g uranium sample solution showed that the precision for the major isotope ratio ²³⁵U/²³⁸U could be smaller than 0.014%. A faraday-ground-connect method was developed to measure the minor isotope ratios, such as ²³⁴U/²³⁵U, ²³³U/²³⁵U and ²³³U/²³⁴U by faraday cups. The electric current produced by ²³⁸U⁺ in the faraday cup was conducted to the ground potential before arriving the preamplifier. The relative uncertainty for ²³⁴U/²³⁵U could be smaller than 0.020% for the 21 000 ng/g uranium sample solution under magnetscan mode. Variation of the measured isotope signatures would help in identification and source attribution of different uranium materials. An alternative method for analyzing ²³⁶U/²³⁵U was to set the magnet value of the center faraday detector named Ax at mass 234. Through the method, the relative uncertainty of ²³⁶U/²³⁵U for both GBW04234 and GBW04238 was less than 0.11%, and the results agreed with the certificated values within uncertainty range. The results demonstrated that this technique is a precise and accurate method for the determination of uranium isotopic fingerprints in nuclear forensics and nuclear safeguards. Furthermore, the faraday-ground-connect method shows great potential in measuring minor isotope ratios of other elements for nuclear and geological applications.