Isotopic Determination of Uranium in Picomole and Subpicomole Quantities

Abstract

A procedure for the separation of U and measurement of U Isotopes in meteoritic, lunar, and terrestrial samples Is described. Accurate ^(238)U/^(235)U ratios and U concentration measurements can be done on a routine basis at the level of ~ 10^(-13) mol of U by Isotope dilution using a double tracer composed of ^(233)U and ^(238)U. Precise determination of the number of U atoms In a sample can be done at the level of 10^8-10^9 atoms. The technique Involves careful control of the chemical procedures to eliminate laboratory contamination and precise calibration of the mass spectrometer In order to obtain high reproducibility. The ^(238)U/^(235)U ratios were measured on a wide variety of samples which Include bulk meterorltes, lunar samples, coarse-grained Ca-AITI- rlch Inclusions and fine-grained AI-Mg-Na-rich Inclusions from the Allende meteorite, and phosphates from three meteorites. The results show the same Isotopic values within limits of errors as that measured on normal terrestrial U to within 4‰. All of the analyzed meteorite samples are very ancient and several of them show large fractionations of U relative to Th and Nd. These data Imply that ^(247)Cm was not abundant In the early solar system (^(247)Cm/^(235)U « 4 X 10^(-3)) and that the last major "r" process terminated at ~ 10^8 years prior to the formation of the solar system.

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