Radioactivity is the phenomenon in which certain elements, known as radioactive isotopes, emit energetic particles and electromagnetic radiation as they undergo decay. These emissions include alpha particles (helium nuclei), beta particles (electrons or positrons), and gamma rays (high-energy photons). The rate at which these emissions occur is known as the radioactive decay rate, and it is unique for each isotope. Radioactivity is a natural phenomenon and it also can be induced by human means, such as nuclear reactions, particle accelerators, and nuclear medicine.