The radioactive decay of uranium is one of the most significant processes in nuclear physics and geochronology. Naturally occurring uranium consists primarily of two long-lived isotopes, and , each of which initiates a distinct decay chain (or "series") that terminates at a stable isotope of lead.
The Uranium Series, also known as the Radium Series, begins with Uranium-238. This isotope has a half-life of approximately 4.47 billion years. Through a sequence of alpha radiation and beta radiation decays, it transmutates into several significant intermediate radionuclides, including:
The chain finally terminates at stable Lead-206. Because is the most abundant isotope of uranium (99.27%), this series is the primary source of terrestrial radioactivity.
The Actinium Series begins with Uranium-235, which has a half-life of 704 million years. Although is much less abundant than , it is of critical importance due to its fissile nature. Key intermediates in this chain include:
This series terminates at stable Lead-207.
In a closed geological system, the intermediate decay products eventually reach a state called Secular Equilibrium. In this state, the activity of each "daughter" product is equal to the activity of the "parent" Uranium-238. This principle is vital for uranium - undeground mining, as it allows specialists to estimate the total radiological hazard.