Isotopic Ratio of Natural Uranium

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Natural uranium consists of three isotopes: ^{238}U (99,275 %), ^{235}U (0,720 %) and ^{234}U (0,0055 %). Isotopes ^{238}U and ^{235}U are the starting nuclides of two natural decay series, the uranium and actinium series, respectively. ^{234}U in turn belongs to the uranium series as ^{238}U decays through ^{234}Th and ^{234}Pa to ^{234}U. 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 ^{238}U in uranium is highest (99.3%) and lowest for ^{234}U (0.006%). The ^{235}U to ^{238}U ratio in natural samples is almost always constant while the^{ 234}U to ^{238}U ratio varies due to fractionation as a consequence of the recoil energy that ^{234}U receives in radioactive decay. Thus, the ^{234}U/ ^{238}U 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 - % |

| 4.47 * 10 | 4.20 | 99.275 |

| 2.47 * 10 | 4.78 | 0.0055 |

| 7.04 * 10 | 4.40 | 0.270 |

| 68.9 | 5.32 |

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