Radiation, as photons or charged particles, that is released as an unstable (radioactive) atomic nucleus decays. There are three types of radiation. Alpha particles are helium nuclei, comprising two protons and two neutrons; they carry a positive charge and a sheet of paper can prevent them penetrating tissue. Beta particles are emitted when a neutron decays to a proton and an electron, or a proton decays to a neutron and a positron (anti-electron); an aluminium sheet will prevent them penetrating tissue. Gamma radiation is a high-energy form of electromagnetic radiation (photons) and is very penetrating.