Uses of radiation, fission and fusion

Radiation has many uses. Gamma-emitting tracers with short half-lives are swallowed or injected to show how organs work. Gamma rays from outside the body (radiotherapy) kill cancer cells, and they sterilise medical equipment and food. Beta sources monitor the thickness of paper and aluminium foil. Smoke alarms use alpha sources, which are stopped by smoke particles. Choosing a source means matching the type of radiation and half-life to the job.

Nuclear fission is the splitting of a large unstable nucleus, such as uranium-235, after it absorbs a neutron. It forms two smaller nuclei, two or three neutrons and gamma rays, and releases a lot of energy. The neutrons can trigger a chain reaction, which is controlled in a reactor by control rods. Nuclear fusion joins two light nuclei, such as hydrogen, to form a heavier nucleus; it powers the stars. Background radiation comes from rocks, cosmic rays and medical X-rays.