Accelerator mass spectrometry of plutonium isotopes

The feasibility of measuring plutonium isotope ratios by accelerator mass spectrometry has been demonstrated. Measurements on a test sample of known composition and on a blank showed that isotope ratios could be determined quantitatively, and that the present limit of detection by AMS is similar to 10(6) atoms of plutonium. For Pu-239, this limit is at least two orders of magnitude lower than that practicable by alpha-spectrometry. In addition, Pu-240/Pu-239 ratios were measured for four samples for which the combined activity of the two isotopes had been determined previously by alpha-counting. All measurements of plutonium isotope ratios entailed injection of PuO- into the 14UD accelerator operating at 3.5 MV, gas stripping, and analysis of the 7(+) charge state after acceleration, Plutonium ions at 28 MeV were detected in a longitudinal-field ionisation chamber with an energy resolution of 3%. Using uranium oxide as a surrogate for plutonium oxide, it was shown that UO- was the predominant negative ion and that the probability for its formation and extraction was 0.3%.