Conversion electron
An electron in an atom has a certain probability of being in the nucleus. A metastable nuclear state can then transfer its energy to the electron. If this happens, the electron becomes a free electron with a kinetic energy equal to the energy of the meta stable 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, or alternatively the emission of an Auger electron.
Conversion occurs for the same nuclear decays as gamma decay, and hence competes with that process.