A conversion electron is an electron which results from interactions with metastable atomic nuclei, which results from radioactive decay processes. A metastable nucleus can transfer its energy to an electron that has a certain probability of being in the nucleus. If this happens, the electron becomes a free electron with a kinetic energy equal to the energy of the metastable state minus the binding energy of the electron. This electron is called a conversion electron. Because of its proximity to the nucleus, the conversion electron usually comes from the K shell. The hole in the electron shell is filled by electrons from other shells thus producing a characteristic X-ray peak. The x-ray may then reproduce the effect and cause the emission of an Auger electron.