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 thermal ionisation mass spectrometer coupled to a variable multicollection Faraday cup detector system is reported. Factors such as the U/Pu ratio in the sample, filament currents during sample de-gassing and data acquisition have been investigated to arrive at optimum conditions for 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 ratios is better than 0.2%. An attempt was also made to show that uranium and plutonium vaporise predominantly through their oxides and not through direct metal evaporation when the samples are loaded from dilute nitric acid medium. This might explain the earlier appearance of U+ ions in preference to Pu+ ions in the mass spectra of all the U/Pu mixtures investigated.