Alpha particle penetrating ability. At left is a disc of polonium 210 (red), a radioactive isotope of the element, and at right is a Geiger counter. Between the two is a thin piece of aluminium foil. Polonium 210 emits alpha particles, which are helium nuclei (two protons and two neutrons) emitted by unstable atomic nuclei. They are only able to travel a few centimetres through the air and very few are able to penetrate the aluminium foil. To see this apparatus without the aluminium shield see clip K004/4570.