WHAT IS RADIO ACTIVITY

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.

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.^[1] 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).

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 (or parent radioisotope^{[note 1]}), 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 daughte rnuclide. 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 different 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. Highly excited neutron-rich radioisotopes (formed as the product of other types of decay) occasionally lose energy by emitting neutrons, and this results in a change in an element from one isotope to another. Another 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 generally immediately emits gamma rays, neutrons, or other particles as a consequence.

For a summary table showing the number of stable nuclides and of radioactive nuclides in each category, seeradionu clide. There exist 34 mildly radioactive elements on Earth that are primordial nuclides, still decaying from the formation of the solar system (well known examples are uranium and thorium). Another 50 or so radionuclides can be detected in decay chains resulting from the primordial nuclides (such as radium and radon), and also new cosmogenic processes (for example carbon-14). Radion uclides can also be produced artificially e.g. using particle accelerators or nuclear reactors, with about 650 of these characterized with half-lives over an hour, and several thousand more characterized with even shorter half lives. See list of nuclides for a list by half life.