Masses of the descendants of uranium 238 at equilibrium
The law of radioactive equilibrium states that the activities of the descendants of uranium 238 must be equal to the activity of the ancestor nucleus: 12.4 million becquerels per tonne. One of the consequences of this equilibrium law is that the ratio between the masses in which the descendants are present is proportional to the ratio that exists between their half-lives. These periods are very diverse and all much smaller than the period of uranium 238 (4.47 billion years), and the activities are also far smaller: uranium 234 has an activity of 54 grams per tonne, thorium 230 has an activity of 16g and radium’s activity is only of 0.34g.