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Radioactive decay is the process by which radionuclides decay, emitting ionizing radiation. Such nuclear reactions involve a change in the composition of the nucleus, in contrast to chemical reactions which involve only an exchange or sharing of electrons.

There are forces in the nucleus that oppose each other, notably the strong force holding protons and neutrons to each other and the electrostatic force of protons repelling other protons. In many arrangements of protons and neutrons the electrostatic force causes instability in the nucleus causing it to decay. It will continue to decay until it reaches a stable combination. Nearly all decay products are themselves radioactive, giving rise to decay chains which eventually end in a stable nuclide.

The observed forms of decay are alpha decay, beta decay, electron capture, neutron emissionNeutron emission is a type of radioactive decay in which a neutron is simply ejected from the nucleus. An example isotope which emits neutrons is Beryllium-13. See also neutron radiation Radioactivity., positron emissionPositron emission is a type of beta decay, referred to as "beta plus" (β+). In beta plus decay, a proton is converted to a neutron via the weak nuclear force and a beta plus particle (a positron) and a neutrino are emitted. Isotopes which emit positr, proton emissionProton emission (also known as proton radioactivity) is a type of radioactive decay in which a proton is ejected from the nucleus. Proton emission can occur from high-lying excited states in a nucleus following a beta decay, in which case the process is k, and spontaneous fissionSpontaneous fission is a form of radioactive decay characteristic of very heavy isotopes, and is theoretically possible for uranium and thorium, or indeed for any atomic nucleus whose mass is greater than or equal to 100 amu ( ruthenium). In practice, how. The latter five forms of decay tend to be rare for longer-lived radioisotopes, so only alpha and beta decay are seen in the decay chains of naturally occurring radioactive materials.

Neutron emission and spontaneous fission are the most important reasons for the difficulty of manufacturing a nuclear bomb from lower grades of plutoniumPlutonium is a radioactive, metallic, chemical element. It has the symbol Pu and the atomic number 94. Its atomic weight is 244. 06, its density 19,800 kg/m3. It is the element used in most modern nuclear weapons. The most important isotope of plutonium i.

Radioactive decay is observed astronomically in supernovaeRemnant of Kepler's Supernova, SN 1604. A supernova is a type of stellar explosion which appears to result in the creation of a new star upon the celestial sphere. Nova" is Latin for "new"). The "super" prefix distinguishes this from a nova, which also in. The light curve of a supernova is generated via the decay of radioactive nickel into iron.

Many radionuclides have several different observed modes of decay, each with its own probability. Bismuth-212, for example, has three.

All radioactive decay is also associated with emission of gamma radiation in varying degrees.

This decay has been deemed by some utterly random, and has been used by some in random number generation.

In terms of chemical kinetics, most instances of radioactive decay can be said to be first order -- that is, the half life of a decaying isotope will stay constant regardless of how much time has passed.

The decay of an atom is said to be spontaneous as one can only determine the probability of decay and not predict when an individual atom will decay. All the atoms of a particular isotope have the same probability of disintegrating in a given time. Therefore, a sample of radioactive material containing many millions of atoms will, on average, always disintegrate at the same rate. This rate at which the material changes is expressed in terms of the half-life, the time required for one half the atoms initially present to disintegrate, which is constant for any particular isotope.

Half-lives of radioactive materials range from fractions of a second for the most unstable to billions of years for those which are only slightly unstable. Decay is said to occur in the parent nucleus and produce a daughter nucleus. Decay from a parent to a daughter nucleus may produce alpha, beta particles, and neutrinos. Gamma radiation may be produced as the nucleus is de-excited but this is only after the alpha or beta decay has taken place. Radioactive decay results in a mass loss, which is converted to energy (the disintegration energy) according to the formula E = mc². Often, the daughter nucleus is also radioactive, and so on down the line for several successive generations of nuclei until a stable one is finally reached. The three such naturally occurring series are shown in the following table: