Within a chemically closed system, the process of radioactive decay of U and Th results in a state of equilibrium being reached, whereby each intermediate daughter product in the chain is being formed at the same rate as it decays. Daughter products with long half-lives are more abundance than those with short half—lives. Approximately 10 6 years are required for the 238 U series to reach a state of equilibrium, whereas 40 years are sufficient for the 232 Th series.

There are two defined types of equilibrium in a radioactive decay series. Stable radioactive equilibrium. Temporary”unstable” radioactive equilibrium. In the stable radioactive equilibrium, the condition is: λ1N 1 =λ 2 N 2 = λ 3 N 3 = =λ i N i Where λN is the number of decays/unit time, and the decay constant λ 1 <<λ I, where λ I represents any decay constant in the series, and λ 1 of the mother element in the disintegration series very small (approximately zero), i e. not rabidly decaying, i.e. the ratio of the elements is stable with time in the series. For example A → B → C → O → atoms/sec atoms/sec atom/sec atoms/sec In the temporary (unstable) radioactive equilibrium, the ratio of the elements is not a constant of time, i.e. N 1 /N 2 + constant (t)

Disequilibrium in radioactive decay series: Under surface and near-surface geological conditions, a closed chemical system may not exist. The process of a weathering provides for both the introductions and removal of material, i.e. of parent and daughter products of the decay series of U and Th especially the former. This causes disequilibrium in the disintegration series, and the intermediate and end products will not be present at any one point in the proportions predicted by the law of radioactive decay. Under these conditions, a measurement of the abundance of a decay product will not necessarily provide a reliable measurement of the abundance of the parent element.

The degree of disequilibrium is strongly dependent upon climate, topography and surface hydrology. Recognition of disequilibrium is also dependent on the sample volume studied. A small hand specimen is much more likely to show extreme disequilibrium than will on a large bulk sample, or an in situ measurement on a large volume of material.