Internal conversion is a nucleus desexcitation mode which competes with gamma emission. It occurs after a beta or alpha radioactive decay has left the nucleus in an excited state. Internal emission can de viewed as the gamma emission where the gamma vanishes as it interacts with one of the atomic electron of the atom to which it transfers its energy. For that reason, it is also called electronic conversion.
The expelled electron inherits the gamma energy, but it must escape from the forces that bind it to the atom. Once free, its energy is dimininished of its former atomic binding energy. Remembering the shell structure of the atom, the binding energy is that of the layer to which the electron belonged. The energy transferred to the electron is that of the gamma diminished of the characteristic binding energy of the electron on its atomic layer. Both the gamma and the binding energy have well-defined values. Thus the energy of the expelled electron takes as well a series of characteristic values (one for each layer). The internal conversion probability is the largest for electrons belonging to the innermost K layer and decreases rapidly with the outermost layers.
Conversion electrons are characterized by a unique energy, as opposed to the electrons of beta decays whose energy energy vary between 0 and a maximum value, a part of the decay energy being carried by an invisible neutrino.
The electron expulsions are followed by a reorganization of the electron atomic cloud, with the emission of X-rays.
The example displayed of cesium-137 shows the respective weights of gamma emission and internal conversion. 94.7% of beta decays lead to an excited state of the nucleus : 85.1% return to the ground state by emitting an energetic 661.57 keV gamma, while 9.6% returns to stability through internal conversion. The electron conversion energy are a little smaller to that of the gamma.
In the general case, he existence and the energy of conversion electrons remain linked to the gamma rays emitted by the nucleus. Their contribution to the decay energy, which adds up to that of beta electrons, is at most of a few %.
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