ACCELERATOR MASS-SPECTROMETRY OF PLUTONIUM ISOTOPES

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