Conversion electron

A conversion electron is a term used to describe an electron which results from the internal conversion process. Internal conversion is a process in which a nucleus can transfer its energy to an atomic electron. Due to their close proximity to the nucleus, it is typically electrons occupying the K shell which are involved in internal conversion. The previously bound electron subsequently becomes a free electron with a kinetic energy equal to the energy difference of the nuclear states minus the binding energy of the electron. The hole in the electron shell is filled by electrons from other shells thus producing a characteristic X-ray peak. An X-ray of sufficient energy may then cause the emission of an Auger electron.

Competition with gamma decay

The internal conversion process competes with gamma decay. This competition is quantified in the form of the internal conversion coefficient which is defined as$alpha\; =\; e/\{gamma\}$ where $e$ is the rate of conversion electrons and $gamma$ is the rate of gamma-ray emission observed from a decaying nucleus. For example, in the decay of an excited state of the nucleus of ¹²⁵I, 7% of the decays emit energy as a gamma ray, while 93% release energy as conversion electrons. Therefore, this excited state of ¹²⁵I has an internal conversion coefficient of $alpha\; =\; 13.6$. Internal conversion coefficients are observed to increase for increasing atomic number (Z) and decreasing gamma-ray energy.

The energy of the emitted gamma-ray is regarded as a precise measure of the difference in energy between the excited states of the decaying nucleus. However, this is not true in the case of conversion electrons. The energy of a conversion electron is given as $E\; =\; (E\_i\; -\; E\_f)-E\_B$ where $E\_i$ and $E\_f$ are the energies of the nucleus in its initial and final states, respectively, while $E\_B$ is the binding energy of the electron.

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