Electron Capture

Electron capture is a mode of radioactive decay in which a proton-rich atomic nucleus absorbs one of its own inner-shell electrons, typically from the K or L shells. This interaction converts a proton into a neutron within the nucleus through a weak charged-current process, accompanied by the emission of an electron neutrino. As a result, the atomic number decreases by one, the neutron number increases by one and the mass number remains unchanged.
Electron capture serves as an important decay mechanism for nuclides in which proton excess cannot be resolved by positron emission, especially where the available energy is insufficient to permit the creation of a positron. It is also an alternative decay mode for nuclides that possess enough decay energy for positron emission, as both processes stem from the same underlying weak interaction.

Mechanism of the Process

In electron capture, an inner atomic electron is drawn into the nucleus, where it participates directly in a weak interaction. An up quark within a proton transforms into a down quark, producing a neutron and emitting a neutrino. Because the neutrino carries essentially all of the decay energy, it is emitted with a single characteristic energy, and the recoil momentum of the daughter nucleus is similarly characteristic.
If the daughter nucleus is created in an excited state, it subsequently relaxes to its ground state. This relaxation often involves emission of a gamma-ray photon, though internal conversion may occur instead, transferring energy to one of the atom’s remaining electrons.
As the captured electron leaves a vacancy in an inner shell, electrons from higher energy levels drop down to fill this vacancy. This process gives rise to characteristic X-ray photons. In some cases, excess energy is transferred to another atomic electron, which is then ejected from the atom in a process known as the Auger effect. Consequently, the atom may become a positively charged ion following the decay.

Relation to Beta Decay

Electron capture is frequently discussed alongside beta decay because both are governed by the weak nuclear force. In positron emission, a proton transforms into a neutron with emission of a positron and a neutrino. Electron capture represents a complementary process, replacing the emitted positron with an absorbed orbital electron. For isotopes whose decay energy is less than approximately 1.022 MeV—the rest-mass energy of an electron–positron pair—positron emission is energetically forbidden, leaving electron capture as the sole means of converting a proton to a neutron.
Examples of pure electron capture include isotopes such as rubidium-83, which decays to krypton-83 exclusively by this mechanism, as the available energy is insufficient to support positron emission.

Historical Background

The theoretical basis for electron capture was first outlined by Gian-Carlo Wick in 1934 and developed further by Hideki Yukawa and contemporaries. Experimental confirmation occurred in 1937 when Luis Walter Alvarez observed K-shell electron capture in vanadium. Alvarez later extended his studies to gallium and other nuclides, establishing the phenomenon as a distinct form of nuclear decay. The identification of the process helped clarify the symmetry between weak interactions in nuclear transformations and contributed to the broader theoretical framework of beta decay.

Reaction Characteristics

Electron capture involves interactions between an inner-shell electron and a proton within the nucleus mediated by W and Z bosons. The electron originates from the atom itself rather than from an external source. Equivalent reaction diagrams typically illustrate the transformation of an up quark into a down quark through the exchange of an intermediate boson.
Radioisotopes that decay solely by electron capture can exhibit inhibited decay when fully ionised, as there are no bound electrons available for capture. This condition is believed to arise in certain astrophysical environments, such as supernova ejecta, where newly synthesised nuclei may remain fully stripped of electrons. In some cases, full ionisation can enable alternative decay pathways, such as bound-state beta decay, where decay proceeds into an unoccupied electron orbital of the daughter nucleus.
Chemical bonding can influence the rate of electron capture in small degrees when the probability of finding electrons near the nucleus changes. Notably, beryllium-7 shows measurable variation in half-life between metallic and insulating environments, attributed to changes in electron density at the nucleus. Electrons in s-orbitals, which possess a non-zero probability density at the nucleus, contribute more significantly to electron capture processes than electrons in p or d orbitals, which have nodal structures that reduce their likelihood of nuclear proximity.

Trends and Occurrence Across the Periodic Table

Isotopes lighter than the stable nuclides of a given element often decay by electron capture due to their relative abundance of protons. Conversely, isotopes heavier than the stable forms typically undergo beta minus decay, emitting electrons. Electron capture is most common among heavier, neutron-deficient nuclides where mass differences between parent and daughter atoms are small and where positron emission may be energetically restricted.
When the energy release in a nuclear transition is positive yet below the threshold for positron formation, electron capture becomes the only pathway for the decay. This situation arises frequently among medium- to high-atomic-number elements with proton-rich isotopic forms.

Common Electron-Capture Radionuclides

Several radionuclides decay exclusively through electron capture and do not emit positrons. These isotopes commonly serve as calibration standards and experimental sources in nuclear and atomic physics. Their decay produces characteristic X-ray and Auger electron emissions associated with atomic shell restructuring following electron capture.