SIMULTANEOUS ISOTOPIC ANALYSIS OF URANIUM AND PLUTONIUM BY THERMAL IONIZATION MASS-SPECTROMETRY COUPLED TO A VARIABLE MULTICOLLECTION DETECTION SYSTEM

Simultaneous isotopic analysis of uranium and plutonium employing a th ermal ionisation mass spectrometer coupled to a variable multicollecti on Faraday cup detector system is reported. Factors such as the U/Pu r atio in the sample, filament currents during sample de-gassing and dat a acquisition have been investigated to arrive at optimum conditions f or analysis. A simple correction to the observed 235/238 peak ratio is necessary to account for the Pu-238 isotope S contribution to U-238. The precision and accuracy achievable in the measurement of isotopic r atios is better than 0.2%. An attempt was also made to show that urani um and plutonium vaporise predominantly through their oxides and not t hrough direct metal evaporation when the samples are loaded from dilut e nitric acid medium. This might explain the earlier appearance of Uions in preference to Pu+ ions in the mass spectra of all the U/Pu mix tures investigated.