Radioactive decay

Radioactive decay, also known as nuclear decay or radioactivity, is the
process by which a nucleus of an unstable atom loses energy by emitting particles
of ionizing radiation. A material that spontaneously emits this kind
of radiation — which includes the emission of energetic alpha particles, beta particles, and
gamma rays — is considered radioactive.

There are many different types of radioactive decay (see table below). A decay, or loss
of energy, results when an atom with one type of nucleus, called the parent
radionuclide, transforms to an atom with a nucleus in a different state, or to a
different nucleus containing different numbers of protons and neutrons. Either of
these products is named the daughter nuclide. In some decays the parent and daughter
are different chemical elements, and thus the decay process results in nuclear
transmutation (creation of an atom of a new element).

The first decay processes to be discovered were alpha decay, beta decay, and
gamma decay. Alpha decay occurs when the nucleus ejects an alpha particle (helium nucleus).
This is the most common process of emitting nucleons, but in rarer types
of decays, nuclei can eject protons, or specific nuclei of other
elements (in the process called cluster decay). Beta decay occurs when the
nucleus emits an electron or positron and a type of neutrino, in a process that
changes a proton to a neutron or the other way around. The nucleus may capture an
orbiting electron, converting a proton into a neutron (electron capture). All of
these processes result in nuclear transmutation.

By contrast, there exist radioactive decay processes that do not result in
transmutation. The energy of an excited nucleus may be emitted as a gamma ray
in gamma decay, or used to eject an orbital electron by interaction with the
excited nucleus in a process called internal conversion. Radioisotopes occasionally
emit neutrons, and this results in a change in an element from one isotope to another.
One type of radioactive decay results in products which are not defined, but appear in
a range of "pieces" of the original nucleus. This decay is called spontaneous fission.
This decay happens when a large unstable nucleus spontaneously splits into
two (and occasionally three) smaller daughter nuclei, and usually emits
gamma rays, neutrons, or other particles as a consequence.

Radioactive decay is a stochastic (i.e., random) process at the level of
single atoms, in that, according to quantum theory, it is impossible to predict
when a particular atom will decay. However, the chance that a given atom will decay
is constant over time. For a large number of atoms, the decay rate for the
collection is computable from the measured decay constants of the
nuclides (or equivalently from the half-lifes).