Activity and half-life calculations

The amount (or activity) of a radioactive isotope repeatedly falls by half in equal amounts of time. A radioactive half-life graph shows the amount (or activity) of a radioactive isotope plotted against time. In 2 half-lives the amount (or activity) of a radioactive isotope falls to ½ × ½ = ¼ as much. In 3 half-lives the amount (or activity) of a radioactive isotope falls to ½ × ½ × ½ = 1/8 as much, and so on. Radioactive isotopes with short half-lives decay quickly, emitting most radiation over a short period of time.