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 given atom will decay.[1] However, the chance that a given atom will decay is constant over time. For a large number of identical atoms (of the same nuclide), the decay rate for the collection is predictable to the extent allowed by the law of large numbers, and is easily calculated from the measured decay constant of the nuclide (or equivalently from the half-life).
The 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 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 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, 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 of an excited nucleus. This results in an atom of the same element as before but with a nucleus in a lower energy state. An example is the nuclear isomer technetium-99m decaying, by the emission of a gamma ray, to an atom of technetium-99.
Nuclides produced by radioactive decay are called radiogenic nuclides, whether they themselves are stable or not. There exist stable radiogenic nuclides that were formed from short-lived extinct radionuclides in the early solar system.[2][3] The extra presence of these stable radiogenic nuclides (such as Xe-129 from primordial I-129) against the background of primordial stable nuclides can be inferred by various means. Presently-radioactive nuclides are from three sources: many naturally-occurring radionuclides are short-lived radiogenic nuclides that are the daughters of ongoing radioactive primordial nuclides (types of radioactive atoms that have been present since the beginning of the Earth and solar system). Other naturally-occurring radioactive nuclides are cosmogenic nuclides, formed by cosmic ray bombardment of material in the Earth's atmosphere or crust. Finally, some primordial nuclides are radioactive, but are so long-lived that they remain present from the primordial solar nebula. For a summary table showing the number of stable nuclides and of radioactive nuclides in each category, see radionuclide.
Radioactivity was discovered in 1896 by the French scientist Henri Becquerel, while working on phosphorescent materials. During experiments to see if phosphorescent materials would expose photographic materials through black paper in the manner of the recently-discovered X-rays, which produced fluorescense, Becquerel used a phosphorescent uranium salt and eventually found that it blackened the plate through paper wrapping, in a desk drawer over a weekend, even without application of light, or production of its phosphorescence. These penetrating radiations, accidently discovered emanating from uranium minerals, were first called Becquerel rays.
The SI unit of radioactive activity is the becquerel (Bq), in honor of the scientist. One Bq is defined as one transformation (or decay) per second. Since sensible sizes of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts giving activities on the order of GBq (gigabecquerel, 1 x 109 decays per second) or TBq (terabecquerel, 1 x 1012 decays per second) are commonly used.
No comments:
Post a Comment