Internal Conversion - Mechanism

Mechanism

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 in an s atomic orbital being found in the nucleus) and when this occurs, the electron may couple to the excited state of the nucleus and take the energy of the nuclear transition directly, without an intermediate gamma ray being first produced.

The process of imparting energy from the nucleus to an orbital electron is a quantum process and may be seen as taking place by means of a virtual photon. In that sense the photon involved can be considered as a "virtual gamma ray", which appears as a feature in an equation that describes the process, rather than as a directly measurable emission. 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 (the 1s state, see electron shell), as these two electrons have the highest probability of being found inside the nucleus. However, the s state in the L, M, and N shells (i.e., the 2s, 3s, and 4s states) are also able to couple to nuclear fields and cause IC electrons from these shells (called LMN internal conversion). Ratios of K-shell to other L, M, or N shell internal conversion probabilities for various nuclides have been prepared.

Since the atomic binding energy of the s electron must be supplied in order to eject it from the atom in the internal conversion process, K shell internal conversion cannot happen if the decay energy of the atom is insufficient to do overcome K-shell binding energy. There are a few radionuclides in which the decay energy is not sufficient to convert (eject) a 1s (K) electron, and these nuclides, when they decay by internal conversion, must decay exclusively from the L, M, or N shells (i.e., by ejecting 2s, 3s, or 4s electrons).

After the IC electron has been emitted, the atom is left with a vacancy in one of its electron shells, usually (as noted) an inner one. This hole will be filled with an electron from one of the higher shells and consequently one or more characteristic x-rays or Auger electrons will be emitted, as the remaining electrons in the atom cascade down to fill the vacancy.