What is electron capture?
Electron capture is a type of radioactive decay in which a proton inside an unstable nucleus captures an inner-shell electron (usually from the K shell) and is converted into a neutron. As a result, the atomic number decreases by one, while the mass number remains unchanged.
Electron capture occurs when a proton captures an inner-shell electron and converts into a neutron, decreasing atomic number without emitting a positron.
Electron capture occurs in proton-rich nuclei, similar to beta plus decay. However, unlike β⁺ decay, no positron is emitted. Instead, the captured electron combines with a proton to form a neutron and a neutrino.
This process often leaves the daughter nucleus in an excited state, which then releases excess energy through gamma emission or internal conversion.
Understanding the physics
In proton-rich nuclei, the imbalance between protons and neutrons makes the nucleus energetically unstable. To restore a more favourable neutron-to-proton ratio, one proton can convert into a neutron. In electron capture, this occurs when a proton interacts with an orbital electron via the weak nuclear force.
The reaction can be written conceptually as:
p + e− → n + ν
The captured electron typically comes from the innermost atomic shell (the K shell). Because an inner-shell electron is removed, the atom is left with an electron vacancy. This vacancy is filled by electrons from higher energy levels, leading to the emission of characteristic X-rays or Auger electrons.
Electron capture competes with beta plus decay. However, β⁺ decay requires a minimum energy of 1.022 MeV to create the positron–electron pair. If the energy difference between parent and daughter nuclei is insufficient for positron emission, electron capture becomes the only available decay pathway.
Unlike β⁺ decay, electron capture does not produce annihilation photons, and therefore it is not used for PET imaging. However, the subsequent gamma emissions may still be suitable for detection in gamma camera imaging.
Where this matters clinically
Some radionuclides used in diagnostic imaging decay via electron capture rather than positron emission. Understanding this distinction helps explain why certain isotopes are suitable for SPECT imaging but not PET, and why characteristic X-rays may contribute to radiation dose.