Conversion, Internal

of gamma radiation, a phenomenon observed in the transition of an excited atomic nucleus to a lower energy state when the energy liberated is not radiated as a γ-quantum but is transferred directly to one of the electrons of the same atom, so that a conversion electron is emitted instead.

Conversion electrons may be emitted from the various shells of an atom, and they are correspondingly designated as K, L, M, and so forth. The energy of the electron is equal to the difference between the energy of the converted nuclear transition and the binding energy of the shell from which it is emitted. Measurement of the energy spectra of the conversion electrons makes it possible to determine the energy of the transitions and their multipolarity.

The probability of internal conversion with respect to the probability of a transition with the emission of a γ-quantum is characterized by the internal conversion ratio, which is defined as the ratio of the flux intensity of the conversion electrons (total or for a particular electron shell) to the intensity of the γ-radiation for the given nuclear transition. The internal conversion ratio is calculated according to quantum field theory, allowing for the shielding of the nuclear charge by electrons in the other atomic shells and taking into account the finite dimensions of the nucleus.

The internal conversion ratio varies over a broad range (10³ to 10^˗4), depending on the energy and multipolarity of the nuclear transition, nuclear charge, and the shell in which the internal conversion takes place. The lower the energy, the higher the multipolarity; and the greater the nuclear charge, the greater will be the ratio.

For transitions between nuclear states with zero spins, the emission of γ-quanta is absolutely forbidden and nuclear transition can only occur by means of internal conversion. Comparison of experimental internal conversion ratios with those computed theoretically is one of the fundamental methods of determining the multipolarities of transitions and the quantum characteristics (spins and parities) of nuclear states.

At nuclear transition energies higher than twice the energy of an electron at rest (Ɛ₀ > 2mc² = 1.022 MeV), internal conversion can occur with the formation of an electron-positron pair (pair conversion), the probability of which increases with the transition energy. The spectra of the electrons and positrons of a conversion pair are continuous, and the total kinetic energy of the electron and positron is equal to Ɛ₀ — 2mc². A special case of pair conversion is internal conversion with the emission of monoenergetic positrons. This occurs when an electron of a pair is captured by any shell, of the same atom, that has been cleared as a result of a previous nuclear conversion.