When did natural radioactivity appear ?

Radioactivity is a natural phenomenon that has existed every since atoms were formed several billion years ago, at the beginning of the universe. In the context of the Big Bang theory, we can say that radioactivity appeared a fraction of a second after the universe was formed.

The cosmic explosion created a massive amount of matter, in accordance with Einstein's famous E=mc² equation. Atomic nuclei, made up of neutrons and protons, began forming a few tens of seconds after the explosion. Given the countless number of possible combinations of neutrons and protons, a great many different atoms were formed.

However, only a minority of them were stable. The others (~radionuclides) were radioactive and destined to decay at some time to form other nuclei, many of which were also radioactive and would decay to form still others. This process continues until a stable element is reached. The decay of these radionuclides is always accompanied by radiation, the external manifestation of radioactivity.