General

Natural uranium is a mixture of isotopes with different nuclear properties. It contains 0.72% atoms of the isotope ²³⁵ U, 99.274 5% atoms of ²³⁸ U and, in radioactive equilibrium with the latter, 0.005 5% of ²³⁴ U. The half-lives of ²³⁵ U and ²³⁸ U are 7.1 × 10 ⁸ and 4.5 × 10 ⁹ years respectively, long enough to explain their presence in the earth's crust.

It is the ²³⁵ U isotope...