Isotopic Ratio of Natural Uranium

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.



Isotope

Half-life [y]

Alpha Energy [MeV]

Mass - %

238U

4.47 * 109

4.20

99.275

234U

2.47 * 105

4.78

0.0055

235U

7.04 * 108

4.40

0.270

232U

68.9

5.32