Internal conversion is a radioactive decay process where an excited nucleus interacts with an electron in one of the lower electron shells, causing the electron to be emitted from the atom. Thus, in an internal conversion process, a high-energy electron which appears to be a classical beta particle is emitted from the radioactive atom, but without beta decay taking place. For this reason, the high-speed electrons from internal conversion are by definition not beta particles, since these are defined by their method of production, not their composition. Also, since no beta decay takes place in internal conversion, the element atomic number does not change (i.e., as in gamma radiation, no transmutation of one element to another takes place in this type of radioactive decay).
This internal conversion process is also not to be confused with the more similar photoelectric effect, which also may occur with gamma radiation associated electron emission, in which an incident gamma photon emitted from a nucleus interacts with an electron, expelling the electron from the atom. Gamma photoelectric effect electron emission may also cause high-speed electrons to be emitted from radioactive atoms without beta decay. However, in internal conversion, the nucleus does not first emit an intermediate real gamma ray, and therefore need not change angular momentum or electric moment. Instead, in the internal conversion process, the wavefunction of an inner shell electron penetrates the nucleus (i.e. there is a finite probability of the electron being found in the nucleus) and when this is the case, the electron may couple to the exited state and take the energy of the nuclear transition directly, without an intermediary gamma ray being produced first. Of course, as an electromagnetic process, the process of imparting energy to the electron does take place by means of a virtual photon, but in that sense the photon involved can be considered as a "virtual gamma ray," which never appears except as a feature of an equation, rather than a measurable particle. The kinetic energy of the emitted electron is equal to the transition energy in the nucleus, minus the binding energy of the electron.
Most internal conversion electrons come from the K shell, as this electron has the highest probability of being found inside the nucleus. After the electron has been emitted, the atom is left with a vacancy in one of the inner electron shells. This hole will be filled with an electron from one of the higher shells and subsequently a characteristic x-ray or Auger electron will be emitted.
Internal conversion is favoured when the energy gap between nuclear levels is small, and is also the primary mode of de-excitation for 0+ -> 0+ (i.e. E0) transitions (i.e., where exited nuclei are able to rid themselves of energy without changing electric and magnetic moments in certain ways) with insufficient energy to decay by pair production. It is the predominant mode of de-excitation whenever the initial and final spin states are the same, but the multi-polarity 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.
Krane, Kenneth S. (1988). Introductory Nuclear Physics. J. Wiley & Sons. ISBN 0-471-80553-X.
* Internal conversion coefficient