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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
|
|