Improvements in Isotope Ratio Measurements for Nuclear Reference Material Certification using the NBL-modified total evaporation technique

A new technique has been developed to improve precision and accuracy of isotope ratio measurements for nuclear reference material certification. The NBL-modified total evaporation technique combines the advantage of high precision in the major isotope ratio measurement provided by the classical total evaporation technique with the advantage of the multi-dynamic measurement technique that internally corrects for differences in multiple detector bias and efficiency. Using the new technique, isotope ratio data are acquired during the entire sample lifetime similar to the classical total evaporation technique. As a modification, the data acquisition is interrupted for short time intervals on a regular basis to allow for actions such as the peakswitching needed for the multi-dynamic technique, electronic baseline measurements, detection of background due to peak tailing effects and ion beam focussing. Using the new technique, a series of NIST/NBL certified reference materials (CRMs) has been analyzed with improved precision. For the major isotope ratio 235U/238U the relative standard deviation between sample turrets is below 0.01%, based on the analysis of 5-10 samples per turret. If corrected for a small residual mass fractionation using CRM U500, the 235U/238U results of all other analyzed reference materials are well within the 95% confidence interval derived from the certificates. However, if only the precision of the measurement is considered as the uncertainty, significant deviations from the certified 235U/238U ratios appear for some of the NIST/NBL series materials, e.g. a deviation of -0.043% in case of CRM U050. These deviations were confirmed using the classical total evaporation technique. The observed inconsistencies in the 235U/238U ratios of the old NIST/NBL series materials demonstrate the need for a new system of more precisely and more accurately certified nuclear reference materials. The development of the new NBL-modified total evaporation technique has already provided a significant impact on recent certification measurements. For two isotopically identical materials of different chemical form recently undergoing certification, a comparison of results for the major ratio showed excellent agreement on a level of about 0.005% RSD. The 235U/238U ratio was determined with a UF6 gas mass spectrometer and two thermal ionization instruments. This type of cross check using several instruments with different ionization mechanisms underlies the reliability of certification measurements at NBL.