Radioactivity and half-life

In short: Some atomic nuclei are unstable and decay of their own accord. As they do so they emit radiation; this is called radioactivity.

In more detail: When a single nucleus decays is pure chance. With very many nuclei, however, it is possible to say exactly how long it takes for half of them to decay: the half-life. It is fixed for each kind of atom (each isotope) and cannot be changed by heat, pressure or chemical reactions.

In detail

• Discovery: Henri Becquerel discovered the radiation from uranium in 1896. Shortly afterwards Marie and Pierre Curie found the elements polonium and radium.
• Alpha radiation: It consists of helium nuclei and is stopped by a sheet of paper or the outer layer of skin. It is dangerous above all when radioactive substances are inhaled or swallowed.
• Beta radiation: It consists of fast electrons or positrons and penetrates further, but is blocked by a few millimetres of aluminium.
• Gamma radiation: It is high-energy electromagnetic radiation. Shielding requires thick layers of lead or concrete.
• Half-lives: They range from fractions of a second to billions of years. Iodine-131 has about eight days, carbon-14 about 5,730 years and uranium-238 around 4.5 billion years.
• Natural radiation: We are constantly exposed to low levels of radiation, for example from the noble gas radon in the ground, cosmic rays and radioactive potassium in our own bodies.
• Dating: Because the half-life is fixed, the proportion of carbon-14 can be used to date wood, bone or fabric remains, going back up to about 50,000 years.
• Medicine and technology: Radioactive substances are used in cancer therapy, in imaging examinations and in materials testing. Nuclear power stations use nuclear fission to generate electricity.
• Units and basics: The becquerel counts decays per second; the sievert describes the effect on the body (atom-und-molekuel, periodensystem-grundlagen, stromerzeugung-und-kraftwerke).