While non-ionizing radiation and x-rays are a result of electron transitions in atoms or molecules, there are three forms of ionizing radiation that are a result of activity within the nucleus of an atom. These forms of nuclear radiation are alpha particles (α-particles), beta particles (β-particles) and gamma rays (γ-rays).
Alpha particles are heavy positively charged particles made up of two protons and two neutrons. They are essentially a helium nucleus and are thus represented in a nuclear equation by either α or . See the Alpha Decay page for more information on alpha particles.
Beta particles come in two forms: and . particles are just electrons that have been ejected from the nucleus. This is a result of sub-nuclear reactions that result in a neutron decaying to a proton. The electron is needed to conserve charge and comes from the nucleus. It is not an orbital electron. particles are positrons ejected from the nucleus when a proton decays to a neutron. A positron is an anti-particle that is similar in nearly all respects to an electron, but has a positive charge. See the Beta Decay page for more information on beta particles.
Gamma rays are photons of high energy electromagnetic radiation (light). Gamma rays generally have the highest frequency and shortest wavelengths in the electromagnetic spectrum. There is some overlap in the frequencies of gamma rays and x-rays; however, x-rays are formed from electron transitions while gamma rays are formed from nuclear transitions. See the Gamma Rays page for more information on gamma rays.