Internal conversion
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Internal conversion or isomeric transition is the act of returning from an excited state by an atom or molecule.
Nuclear Internal Conversion
This is an alternative to gamma-ray emission, which is the usual method for an excited nucleus to return to the ground state. In internal conversion, the excited nucleus transfers its energy directly to one of the most tightly bound electrons causing it to be ejected from the atom. After the ejection of the internal conversion electron the vacancy is filled by another shell electron and the atom ejects one or several x-rays or Auger electrons.
Internal conversion is favored when the energy gap between nuclear levels is small, and is also the only mode of de-excitation for 0+ -> 0+ (ie. E0) transitions. It is the predominant mode of de-excitation whenever the initial and final spin states are the same, but the multipolarity rules for nonzero initial and final spin states do not necessarily forbid the emission of a gamma ray in such a case.
The tendency towards internal conversion can be determined by the internal conversion coefficient, which is empirically determined by the ratio of de-excitations that go by the emission of electrons to those that go by gamma emission.