Half-life, contamination and irradiation

Radioactive decay is random: we cannot predict which nucleus will decay next, but we can predict the behaviour of a large number. The half-life is the time it takes for the number of unstable nuclei (or the activity) to fall to half. After one half-life, half remains; after two, a quarter; after three, an eighth. Half-life can be read from a graph of activity against time.

Irradiation is exposure to radiation from an outside source; the object does not become radioactive. Contamination is unwanted radioactive atoms on or inside something, which keep irradiating it until they decay or are removed. Inside the body, alpha sources are most dangerous because they are strongly ionising; outside the body, beta and gamma are more dangerous because they can penetrate the skin. Precautions include keeping sources in lead boxes, using tongs, limiting time and keeping a distance.