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radioactive decay

n. (context physics English) Any of several processes by which unstable nuclei emit subatomic particles and/or ionizing radiation and disintegrate into one or more smaller nuclei.

radioactive decay

n. the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation [syn: decay, disintegration]

Radioactive decay

Radioactive decay, also known as nuclear decay or radioactivity, is the process by which the nucleus of an unstable atom loses energy by emitting radiation, including alpha particles, beta particles, gamma rays and conversion electrons. A material that spontaneously emits such radiation 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. The chance that a given atom will decay never changes, that is, it does not matter how long the atom has existed. The decay rate for a large collection of atoms, however, can be calculated from their measured decay constants or half-lives. This is the basis of radiometric dating. The half-lives of radioactive atoms have no known lower or upper limit, spanning a time range of over 55 orders of magnitude, from nearly instantaneous to far longer than the age of the universe. A radioactive source emits its decay products isotropically.

There are many different types of radioactive decay (see table below). A decay, or loss of energy from the nucleus, results when an atom with one type of nucleus, called the parent radionuclide (or parent radioisotope), transforms into an atom with a nucleus in a different state, or with a nucleus containing a different number of protons and neutrons. The product is called the daughter nuclide. In some decays, the parent and the daughter nuclides are different chemical elements, and thus the decay process results in the creation of an atom of a different element. This is known as a nuclear transmutation.

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 in the case of cluster decay specific nuclei of other elements. Beta decay occurs when the nucleus emits an electron or positron and a neutrino, in a process that changes a proton to a neutron or the other way about. The nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. All of these processes result in a well-defined nuclear transmutation.

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 be used to eject an orbital electron by its interaction with the excited nucleus, in a process called internal conversion. Highly excited neutron-rich nuclei, formed as the product of other types of decay, occasionally lose energy by way of neutron emission, resulting in a change of an element from one isotope to another.

Another type of radioactive decay results in products that are not defined, but appear in a range of "pieces" of the original nucleus. This decay, called spontaneous fission, happens when a large unstable nucleus spontaneously splits into two (and occasionally three) smaller daughter nuclei, and generally leads to the emission of gamma rays, neutrons, or other particles from those products.

For a summary table showing the number of stable and radioactive nuclides in each category, see radionuclide. There exist twenty-nine chemical elements on Earth that are radioactive. They are those that contain thirty-four radionuclides that date before the time of formation of the solar system, and are known as primordial nuclides. Well-known examples are uranium and thorium, but also included are naturally occurring long-lived radioisotopes such as potassium-40. Another fifty or so shorter-lived radionuclides, such as radium and radon, found on Earth, are the products of decay chains that began with the primordial nuclides, and ongoing cosmogenic processes, such as the production of carbon-14 from nitrogen-14 by cosmic rays. Radionuclides may also be produced artificially in particle accelerators or nuclear reactors, resulting in 650 of these with half-lives of over an hour, and several thousand more with even shorter half-lives. See this list of nuclides for a list of these, sorted by half life.

Usage examples of "radioactive decay".

Operate the detector for a period just long enough that there is a fifty-fifty chance that one radioactive decay will be recorded.

The weak force engages in more miscellaneous tasks, mostly to do with controlling the rates of certain sorts of radioactive decay.

Instead, Voyager relies on a small nuclear power plant, drawing hundreds of watts from the radioactive decay of a pellet of plutonium.

There is, of course, a chance that some vital subsystem will fail tomorrow, but as far as the radioactive decay of the plutonium power source is concerned, the two Voyager spacecraft should be able to return data to Earth roughly through the year 2015.

The ancient Terrans held a simplistic view of planetary genesis still accepted today in most respects Thev believed that planets were formed by accretion of smaller fragments dust, gas and ices remaining about a star after or during stellar formation As each planet grew, its interior warmed through gravitational contraction with heat released by radioactive decay The surfaces and mten ors of Neta-class (modern terminology) plan ets soon became molten After about one eon sufficient cooling occurred to form a solid crust Convection in the planet's mol ten mtenor caused continued breakup of the crust and formation of new crust i^mubt like scum floating on the surface of a bubbling pot of molten fanmetal) As cooling contin ued the crust thickened to the pomt where it became stable and a basically permanent feature This typically occurred .

However, the mechanism will need retuning at regular intervals, as natural radioactive decay will alter compositions unpredictably.

The actual amount is gradually decreasing due to radioactive decay, but is being replenished through food intake, or through photosynthesis in the case of plants.

Different kinds of radioactive decay-based geological stopwatches run at different rates.

The age of the remains could be accurately calculated from the radioactive decay present and compared--”.

His thought-experiment made the cat's state of health dependent on radioactive decay -- a quantum phenomenon which can only be expressed as a probability.

In 1913, the British chemist Frederick Soddy (1877-1956), advanced the isotope concept based on his studies of the elements produced in the course of radioactive decay.