Results for the "REIMEP 18" Inter-Laboratory Comparison Campaign for the Measurement of Uranium Isotope Ratios

Inter-laboratory comparisons are organized for a variety of sample types and elements in order to assess the performance of isotopic measurements on a worldwide level. The REIMEP 18 (Regular International Measurement Evaluation Program) campaign for the measurement of uranium isotope ratios in nitric acid was started in December 2005. Four samples of 2.5mg uranium, ranging from depleted up to slightly enriched uranium, are sent to more than 85 participating laboratories, originating mainly from the nuclear safeguards and the isotope geochemistry area, and using a variety of techniques such as alpha-spectrometry, TIMS, ICP-MS, AMS, RIMS, etc. As observed during several REIMEP campaigns organized during the past 10 years, REIMEP 18 is designed to show the present state of uranium isotope measurements and gives the opportunity for participating laboratories to evaluate their own performance, to identify possible problems and to improve their own measurement procedures. Due to instrumental improvements in measurement techniques and instrumentation continuously going on over the years, measurement campaigns such as REIMEP are an important and very much appreciated way to achieve an ongoing careful quality control on an international level. Certification measurements at IRMM were performed using recently upgraded techniques for high precision and high accuracy uranium isotope ratio measurements. 235U/238U measurements were performed using a UF6 gas source mass spectrometer, calibrated using synthetic isotope mixtures. 234U/238U measurements, even down to values of 5.5x10-5 (natural equilibrium value) were performed on a TRITON TIMS, using 1012 O Faraday cup amplifiers for the detection of 234U in order to improve the signal-to-noise-ratio, so without the need to use ion counting. 236U/238U measurements were performed on a TRITON TIMS using a procedure in which 236U was detected using an ion counter and which has been validated using IRMM's special synthetic mixtures with 236U/238U ratios of 10-6, 10-7 and 10-8. Details of the sample preparation and certification will be given as well as comparative results of the measurements made by all ca. 70 participating laboratories worldwide. The results of the REIMEP 18 campaign confirm in general the excellent capability of laboratories in measuring isotopic abundances of uranium, but also identify some problems regarding the measurements of the minor isotope ratios 234U/238U and 236U/238U.JRC.D.4-Isotope measurement