Radioactive decay

Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting ionizing particles (ionizing radiation). The emission is spontaneous, in that the nucleus decays without collision with another particle or atom. The decay is a stochastic (i.e. random) process on the level of single atoms, in that according to quantum theory it is impossible to predict when a given atom will decay.[1] However, given a large number of identical atoms (nuclides) the decay rate for the collection is predictable.

The decay, or loss of energy, results from an atom with one type of nucleus, called the parent radionuclide, transforming to an atom with a nucleus in a different state, or a different nucleus, either of which is named the daughter nuclide. Often the parent and daughter are different chemical elements, and in such cases the decay process automatically results in nuclear transmutation. In an example of this, a carbon-14 atom (the "parent") emits radiation (a beta particle, antineutrino, and a gamma ray) and transforms to a nitrogen-14 atom (the "daughter"). By contrast, in nature there exist two types of radioactive decay processes (gamma decay and internal conversion decay) that do not result in transmutation, but only decrease the energy in an excited nucleus, resulting in an atom of the same element as before with a nucleus in a lower energy state. An example is the nuclear isomer technetium-99m decaying, by emission of a gamma ray, to an atom of technetium-99.

Nuclides produced as daughters are also called radiogenic nuclides, whether they themselves are stable or not. A number of naturally occurring radionuclides are short-lived radiogenic nuclides that are daughters of radioactive primordial nuclides. Other naturally-occurring radioactive nuclides are cosmogenic nuclides, formed by cosmic ray bombardment of material in the Earth's atmosphere or crust. For a summary table showing the number of stable nuclides and of radioactive nuclides in each category, see Radionuclide.

The SI unit of activity is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of GBq (gigabecquerel, 1 x 109 decays per second) or TBq (terabecquerel, 1 x 1012 decays per second) are commonly used. Another unit of radioactivity is the curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with of one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 1010 Bq. The use of Ci is presently discouraged by the SI.

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