High-Precision Measurement of the ²³⁸Pu/²³⁹Pu Isotope Ratio by TIMS

With an aim to improve the precision of isotopic standards of special nuclear materials like uranium and plutonium, National Bureau of Standards (NBS) undertook the recertification of isotopic standards prepared between 1960 and 1970. In a recent initiative, Lawrence Livermore National Laboratory (LLNL) in collaboration with New Brunswick Laboratory Program Office (NBL PO) undertaken efforts to recertify SRM 946, SRM 947, and SRM 948. The primary goal of this drive is to lower the uncertainties associated with the isotopic compositions of plutonium standards. The drive is also aims to minimize the uncertainty associated with measurement of the 238Pu content from 2% to ∼0.3%. In this connection, our laboratory at the Fuel Chemistry Division, Bhabha Atomic Research Centre, proposes a novel indicator “minima in the (RMO254/(i + 16))/(RM238/i) curve” (where i = m/z 233 or 235 amu), which is the ratio of the isotope ratio of metal oxides (238M16O/iM16O) to that of metal ions (238M/iM) of uranium in determining the accurate 238Pu content by thermal ionization mass spectrometry. Various enriched uranium spikes have been employed to study the efficacy of the minima in the (RMO(254)/(i + 16))/(RM238/i) curve. With the combination of monitoring the minima in the (RMO(254)/(i + 16))/(RM238/i) curve and the application of the 233U spike (with an isotope abundance of 238U ∼ 0.05%), an external precision of 0.2% has been achieved in the determination of the 238Pu/239Pu isotope ratio for isotopic standard SRM 947