Some atomic nuclei are unstable. An unstable nucleus gives out radiation as it changes to become more stable. This is called radioactive decay, and it is a random process. The activity of a source is the rate at which its unstable nuclei decay, and it is measured in becquerel (Bq). A detector such as a Geiger-Muller tube records the count-rate, which is the number of decays detected each second.
The nuclear radiation emitted may be an alpha particle, a beta particle, a gamma ray or a neutron. An alpha particle is two protons and two neutrons, the same as a helium nucleus. A beta particle is a high speed electron ejected from the nucleus when a neutron turns into a proton. A gamma ray is electromagnetic radiation from the nucleus.
The three main types differ in how far they travel and how strongly they ionise. Alpha has the shortest range in air, only a few centimetres, and is stopped by a sheet of paper, but it has the greatest ionising power. Beta travels about a metre in air and is stopped by a few millimetres of aluminium. Gamma has the longest range, is only reduced by thick lead or concrete, and has the weakest ionising power. This is why the best source for a job depends on how far the radiation must pass through a material.
