Lab Exercise - Isotopic Ratio of Natural Uranium

Natural uranium consists of three isotopes: ²³⁸U (99,275 %), ²³⁵U (0,720 %) and ²³⁴U (0,0055 %). Isotopes ²³⁸U and ²³⁵U are the starting nuclides of two natural decay series, the uranium and actinium series, respectively. ²³⁴U in turn belongs to the uranium series as ²³⁸U decays through ²³⁴Th and ²³⁴Pa to ²³⁴U. All uranium isotopes are alpha emitters and their activities and isotope ratios can be measured by alpha spectrometry and also by mass spectrometry. The mass fraction of ²³⁸U in uranium is highest (99.3%) and lowest for ²³⁴U (0.006%). The ²³⁵U to ²³⁸U ratio in natural samples is almost always constant while the ²³⁴U to ²³⁸U ratio varies due to fractionation as a consequence of the recoil energy that ²³⁴U receives in radioactive decay. Thus, the ²³⁴U/ ²³⁸U activity ratio in ground waters is often higher than one though the ratio in rock is one due isotopic equilibrium.