Radioactive decay (also known as nuclear decay or radioactivity ) is the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame ) by emitting radiation , such as an alpha particle , beta particle with neutrino or only a neutrino in the case of electron capture , gamma ray , or electron in the case of internal conversion. A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission , or more rarely, proton emission.
The decaying nucleus is called the parent radionuclide (or parent radioisotope [note 2] ), and the process produces at least one daughter nuclide. Except for gamma decay or internal conversion from a nuclear excited state , the decay is a nuclear transmutation resulting in a daughter containing a different number of protons or neutrons (or both). When the number of protons changes, an atom of a different chemical element is created.
By contrast, there are radioactive decay processes that do not result in a nuclear transmutation. The energy of an excited nucleus may be emitted as a gamma ray in a process called gamma decay , or that energy may be lost when the nucleus interacts with an orbital electron causing its ejection from the atom, in a process called internal conversion.