Particle radiation can be emitted from an unstable atomic nucleus (radioactive decay) in the form of a positively charged Alpha particle (α), a positively or negatively charged (the latter being more common) Beta particle(β),a neutron, or Gamma rays (γ). Gamma rays are a form of electromagnetic radiation, but exhibit particle-like properties (see photon) due to their high energy. Other forms of particle radiation include neutrons, positrons and neutrinos.
Radiation is often separated into two categories, ionizing and non-ionizing, to denote the energy and danger of the radiation. Ionization is the process of removing electrons from atoms, leaving two electrically charged particles (ions) behind. Some forms of radiation like visible light, microwaves, or radio waves do not have sufficient energy to remove electrons from atoms and hence, are called non-ionizing radiation. The negatively charged electrons and positively charged nuclei created by ionizing radiation may cause damage in living tissue. The term radioactivity generally refers to the release of ionizing radiation.