Radioactive decay and nuclear radiation

Some nuclei are unstable. They decay at random by giving out radiation, becoming more stable. Activity is the rate of decay, measured in becquerels (Bq): 1 Bq is one decay per second. A Geiger–Muller tube detects radiation; the count rate is the number of decays recorded per second.

Alpha particles are two protons and two neutrons (a helium nucleus). They are strongly ionising but stopped by paper or a few centimetres of air. Beta particles are fast electrons made when a neutron turns into a proton; they are stopped by a few millimetres of aluminium. Gamma rays are electromagnetic waves from the nucleus; they are weakly ionising and need thick lead or concrete to absorb them. In alpha decay the mass number falls by 4 and the atomic number by 2. In beta decay the mass number is unchanged and the atomic number rises by 1.