Radioisotopes are radioactive isotopes of an element. They can also be defined as atoms that contain an unstable combination of neutrons and protons, or excess energy in their nucleus. The unstable nucleus of a radioisotope can occur naturally, or as a result of artificially altering the atom. In some cases, a nuclear reactor is used to produce radioisotopes, in others, a cyclotron. Nuclear reactors are best-suited to producing neutron-rich radioisotopes, such as molybdenum-99, while cyclotrons are best-suited to producing proton-rich radioisotopes, such as fluorine-18. Atoms with an unstable nucleus regain stability by shedding excess particles and energy in the form of radiation. The process of shedding the radiation is called radioactive decay. The radioactive decay process for each radioisotope is unique and is measured with a time period called a half-life.