# Electron capture

**Electron capture** (EC, also called K-capture or inverse beta decay) is a radioactive decay mode in which an atomic nucleus absorbs one of its own inner-shell electrons, converting a proton into a neutron and emitting an electron neutrino: \( p + e^{-} \rightarrow n + \nu_{e} \). The daughter nucleus has an atomic number one lower than the parent and an unchanged mass number.<sup>[1](https://openstax.org/books/chemistry-atoms-first-2e/pages/20-3-radioactive-decay)</sup> The process competes with positron emission, which produces the same nuclear change, and it is the only available mode for proton-rich nuclides whose decay energy is too small to create a positron.<sup>[2](https://giophysics.com/university-physics/topics/beta-decay-fermi-theory/)</sup>

| Key fact | Detail |
|---|---|
| Nuclear change | A proton combines with a captured inner-shell electron to form a neutron; atomic number decreases by one, mass number is unchanged.<sup>[1](https://openstax.org/books/chemistry-atoms-first-2e/pages/20-3-radioactive-decay)</sup> |
| Energy threshold | Positron emission requires an atomic mass difference above 1.022 MeV; below that, electron capture proceeds and β⁺ is forbidden.<sup>[2](https://giophysics.com/university-physics/topics/beta-decay-fermi-theory/)</sup> |
| Which electron | Capture occurs from all shells when energetically allowed but primarily from the K shell; s\(_{1/2}\) electrons have appreciable wavefunction overlap with the nucleus.<sup>[3](https://www.researchgate.net/publication/321939375_Radioisotopes_the_Kirk-Othmer_Encyclopedia)</sup><sup> • </sup><sup>[4](https://english.ihep.cas.cn/nw/ue/201811/t20181121_290326.html)</sup> |
| Atomic signature | The vacancy left in the shell is filled by an outer electron, emitting a characteristic X-ray or an Auger electron.<sup>[1](https://openstax.org/books/chemistry-atoms-first-2e/pages/20-3-radioactive-decay)</sup> |
| Benchmark isotope | ⁷Be decays 100% by electron capture with a half-life of about 53 days; its measured EC Q value is 861.963(23) keV.<sup>[5](https://koethgroup.umd.edu/curie/)</sup><sup> • </sup><sup>[6](https://doi.org/10.1103/PhysRevC.109.L022501)</sup> |
| Neutrino spectrum | Unlike β or β⁺ decay, the neutrino energy and nuclear recoil energy in a capture branch are sharply defined, not continuous.<sup>[7](https://www.cambridge.org/core/books/an-introduction-to-nuclear-physics/beta-decay-and-gamma-decay/CB9FC0F19AC186932931A0BE90CF3714)</sup> |
| Environmental sensitivity | The ⁷Be capture rate varies by up to 0.38% between host materials; the ⁴⁰K rate shows no measurable environmental dependence within ±1%.<sup>[8](https://www.sciencedirect.com/science/article/pii/S0370269301010978)</sup> |

## Mechanism

Capture is possible only if the electron wave function overlaps the nucleus. This is the case for electrons in \( ns_{1/2} \) orbitals and, through the relativistic lower component, in \( np_{1/2} \) orbitals; electrons with nonzero orbital angular momentum have vanishing wavefunction at the origin, so their contribution is small.<sup>[4](https://english.ihep.cas.cn/nw/ue/201811/t20181121_290326.html)</sup> Because inner electrons spend more time near the nucleus, capture occurs from all atomic shells when the decay energy is well above the K-shell binding energy, but primarily from the K shell; if the energy is below the K binding energy, only higher shells can contribute.<sup>[3](https://www.researchgate.net/publication/321939375_Radioisotopes_the_Kirk-Othmer_Encyclopedia)</sup>

The energy available to the decay, the Q value, is the atomic mass difference between parent and daughter less only the keV-scale binding energy of the vacated shell, because the parent's own electron is consumed and no particle must be created.<sup>[2](https://giophysics.com/university-physics/topics/beta-decay-fermi-theory/)</sup> That energy is shared between the neutrino and the recoil of the daughter atom, with the neutrino carrying almost all of it.<sup>[9](https://www.tpscollegepatna.org/book/Nuclear%20Physics%20%28%20PDFDrive%20%29.pdf)</sup>

## Relation to positron emission

Electron capture and positron emission both convert a proton to a neutron and both occur in proton-rich nuclides below the band of stability, but their energy requirements differ. [Positron emission](https://www.edgepedia.org/positron-emission) is charged 2\( m_{e}c^{2} \) = 1.022 MeV, the energy to create the positron and to shed the daughter atom's excess electron; capture pays neither cost. In the window \( 0 < \Delta Mc^{2} < 1.022 \) MeV, capture proceeds and β⁺ is flatly forbidden.<sup>[2](https://giophysics.com/university-physics/topics/beta-decay-fermi-theory/)</sup> ⁷Be sits in that window at 0.862 MeV and decays by capture alone.<sup>[2](https://giophysics.com/university-physics/topics/beta-decay-fermi-theory/)</sup> When both modes are energetically allowed, which one dominates is difficult to predict and depends on kinetic factors, with the branch requiring the smaller activation energy more likely.<sup>[1](https://openstax.org/books/chemistry-atoms-first-2e/pages/20-3-radioactive-decay)</sup> In general, positron emission predominates at high transition energies and low atomic number, while capture predominates at low transition energies and higher atomic number.<sup>[10](https://www.sciencedirect.com/topics/physics-and-astronomy/electron-transition)</sup>

A second distinction is spectral. In ordinary beta decay the emitted electron or positron takes any energy up to the maximum, giving a continuous spectrum; in electron capture only the neutrino and the recoiling nucleus leave, so the neutrino energy and recoil energy are sharply defined for each branch.<sup>[7](https://www.cambridge.org/core/books/an-introduction-to-nuclear-physics/beta-decay-and-gamma-decay/CB9FC0F19AC186932931A0BE90CF3714)</sup>

## Atomic aftermath

The captured electron leaves a vacancy in an inner shell. An outer electron drops into the vacancy and the released energy emerges either as a characteristic X-ray or as an Auger electron, in which the transition energy ejects another atomic electron instead of producing a photon.<sup>[1](https://openstax.org/books/chemistry-atoms-first-2e/pages/20-3-radioactive-decay)</sup><sup> • </sup><sup>[11](https://www.hyperphysics.phy-astr.gsu.edu/hbase/Atomic/auger.html)</sup> These X-rays and Auger electrons carry the atomic excitation energy of the daughter and are the experimentally observed signature of the decay; their energies and intensities can be calculated for specific nuclides such as ⁵⁵Fe.<sup>[12](https://www.nst.sinap.ac.cn/article/id/1770)</sup>

## Environmental dependence

Because the decay rate depends on the electron wavefunction density at the nucleus, it can in principle be perturbed by the atom's surroundings. Measurements of ⁷Be implanted in graphite, boron nitride, tantalum, and gold found decay-rate differences of up to 0.38% between hosts, attributed partly to differing electron affinities of the hosts; other experiments have shown 1 to 2% variations in different chemical compounds.<sup>[8](https://www.sciencedirect.com/science/article/pii/S0370269301010978)</sup><sup> • </sup><sup>[5](https://koethgroup.umd.edu/curie/)</sup> For ⁴⁰K measured in four chemical compounds and in solution, no influence of the environment was found within a ±1% precision, a result relevant to potassium-argon dating.<sup>[8](https://www.sciencedirect.com/science/article/pii/S0370269301010978)</sup> A precision study of the electron-capture decay of ⁹⁷Ru at 19 K and at room temperature found no temperature dependence, showing that claims of large environmental shifts cannot be generalized across electron-capture decays.<sup>[13](https://arxiv.org/abs/0910.4338)</sup>

## Applications

**Dating and medicine.** [Potassium-40](https://www.edgepedia.org/potassium-40) decays by positron emission and electron capture to argon-40 with a half-life of 1.25 billion years, and the Ar-40:K-40 ratio in crushed rock yields its age.<sup>[14](https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_-_Atoms_First_1e_(OpenSTAX)/20:_Nuclear_Chemistry/20.3:_Radioactive_Decay)</sup> [Thallium-201](https://www.edgepedia.org/thallium-201), which decays by electron capture with a 73-hour half-life, is used for heart and artery scans and cardiac stress tests.<sup>[14](https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_-_Atoms_First_1e_(OpenSTAX)/20:_Nuclear_Chemistry/20.3:_Radioactive_Decay)</sup>

**Solar neutrinos.** The gallium radiochemical detectors rely on the inverse process, neutrino capture on ⁷¹Ga producing ⁷¹Ge, whose cross section is anchored by the known electron-capture lifetime of ⁷¹Ge. The BEST experiment at the [Baksan Neutrino Observatory](https://www.edgepedia.org/baksan-neutrino-observatory) irradiated about 47 metric tons of liquid gallium with a chromium-51 source and found the ⁷¹Ge yield 20% to 24% lower than expected, consistent with the earlier gallium anomaly seen by SAGE and GALLEX; no signs of neutrino oscillation between the target's inner and outer zones were observed, and the origin of the anomaly remains unexplained.<sup>[15](https://www.energy.gov/science/np/articles/testing-gallium-anomaly)</sup>

**Neutrino mass.** The sharply defined endpoint makes low-Q electron capture a route to the electron-neutrino mass. In the capture of ¹⁶³Ho to ¹⁶³Dy the Q value is only 2.8 keV, so energy conservation restricts capture to the 3s\(_{1/2}\) and 3p\(_{1/2}\) orbits and higher; the daughter's deexcitation energy, measured with a bolometer, has an endpoint at Q minus the neutrino rest mass. The determination is considered possible but difficult, with about two orders of magnitude of background reduction still needed, and three experiments are pursuing it: ECHo ([Heidelberg](https://www.edgepedia.org/heidelberg), Mainz, Tübingen), HoLMES (Milan), and NuMEX (Los Alamos).<sup>[4](https://english.ihep.cas.cn/nw/ue/201811/t20181121_290326.html)</sup> Atomic many-body calculations have also identified ¹⁵⁹Dy electron capture as a candidate for sub-eV neutrino mass determination.<sup>[16](https://arxiv.org/abs/2106.06626)</sup>

**Precision Q values and sterile neutrino searches.** Penning-trap mass spectrometry at the LEBIT facility measured the ⁷Be → ⁷Li electron-capture Q value as 861.963(23) keV, three times more precise than any previous determination; the improved value supports experiments that measure the recoiling ⁷Li nucleus as a signature for beyond-Standard-Model neutrino physics using ⁷Be-doped superconducting sensors.<sup>[6](https://doi.org/10.1103/PhysRevC.109.L022501)</sup> Theoretical work shows that a sterile neutrino emitted in capture would reduce the daughter's recoil energy, so the difference in recoil energies between the ground-state and excited-state decay channels of ⁷Be could indicate sterile-neutrino radiation.<sup>[17](https://www.mdpi.com/2076-3417/15/17/9502)</sup>

**Evaluated data.** Calculated capture probabilities, subshell capture-to-positron ratios, and spectra for electron-capture decays are produced by the BetaShape program, part of the IAEA's ENSDF Analysis and Utility Programs and of the Decay Data Evaluation Project.<sup>[18](https://github.com/IAEA-NSDDNetwork/BetaShape)</sup> The standard reference treatment of the theory, capture probabilities, capture ratios, EC/β⁺ ratios, and radiative electron capture remains the 1977 review by W. Bambynek, a nuclear metrologist at the Bureau Central de Mesures Nucléaires (EURATOM, Geel), and colleagues in *Reviews of Modern Physics*.<sup>[19](https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.49.77)</sup>

## References

1. [Radioactive Decay, Chemistry: Atoms First 2e, OpenStax](https://openstax.org/books/chemistry-atoms-first-2e/pages/20-3-radioactive-decay)
2. [Beta Decay, Neutrinos & Fermi's Theory, University Physics IV](https://giophysics.com/university-physics/topics/beta-decay-fermi-theory/)
3. [Radioisotopes, Kirk-Othmer Encyclopedia (ResearchGate copy)](https://www.researchgate.net/publication/321939375_Radioisotopes_the_Kirk-Othmer_Encyclopedia)
4. [A. Faessler, Can the electron-neutrino mass be determined by electron capture? IHEP lecture abstract, 2018](https://english.ihep.cas.cn/nw/ue/201811/t20181121_290326.html)
5. [CURIE: Cryogenic Ultrahigh vacuum Radioactive Ion Experiment, Koeth Research Group, University of Maryland](https://koethgroup.umd.edu/curie/)
6. [R. Bhandari et al., First direct ⁷Be electron-capture Q-value measurement, Phys. Rev. C 109, L022501 (2024)](https://doi.org/10.1103/PhysRevC.109.L022501)
7. [W. N. Cottingham and D. A. Greenwood, Beta decay and gamma decay, An Introduction to Nuclear Physics, Cambridge University Press](https://www.cambridge.org/core/books/an-introduction-to-nuclear-physics/beta-decay-and-gamma-decay/CB9FC0F19AC186932931A0BE90CF3714)
8. [E. B. Norman et al., Influence of physical and chemical environments on the decay rates of ⁷Be and ⁴⁰K, Physics Letters B 519, 15–22 (2001)](https://www.sciencedirect.com/science/article/pii/S0370269301010978)
9. [Nuclear Physics, Graduate Texts in Physics (PDF copy)](https://www.tpscollegepatna.org/book/Nuclear%20Physics%20%28%20PDFDrive%20%29.pdf)
10. [Electron Transition, ScienceDirect Topics](https://www.sciencedirect.com/topics/physics-and-astronomy/electron-transition)
11. [Auger Effect, HyperPhysics, Georgia State University](https://www.hyperphysics.phy-astr.gsu.edu/hbase/Atomic/auger.html)
12. [Zhou, Wu, Huang, Calculations of energies and absolute intensities of Auger electrons and X-rays arising from electron capture decay, Nuclear Science Techniques 16, 90–94 (2005)](https://www.nst.sinap.ac.cn/article/id/1770)
13. [Half-life of the electron-capture decay of ⁹⁷Ru: precision measurement shows no temperature dependence, arXiv:0910.4338](https://arxiv.org/abs/0910.4338)
14. [Radioactive Decay, Chemistry LibreTexts (OpenStax, Atoms First)](https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_-_Atoms_First_1e_(OpenSTAX)/20:_Nuclear_Chemistry/20.3:_Radioactive_Decay)
15. [Testing the Gallium Anomaly, U.S. Department of Energy Office of Science](https://www.energy.gov/science/np/articles/testing-gallium-anomaly)
16. [¹⁵⁹Dy electron capture: a strong new candidate for neutrino mass determination, arXiv:2106.06626](https://arxiv.org/abs/2106.06626)
17. [Recoil Energy in Electron Capture Beta Decay and the Search for Sterile Neutrinos, Applied Sciences 15, 9502 (2025)](https://www.mdpi.com/2076-3417/15/17/9502)
18. [BetaShape: Calculation of beta and electron capture decays, IAEA NSDD Network, GitHub](https://github.com/IAEA-NSDDNetwork/BetaShape)
19. [W. Bambynek et al., Orbital electron capture by the nucleus, Rev. Mod. Phys. 49, 77 (1977)](https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.49.77)

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*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Nuclear physics › Radioactivity and nuclear decay › Decay modes › Electron capture*

*Initially written Sep 17, 2026 · Reviewed: Sep 17, 2026; Oct 15, 2026 · Edited: Sep 17, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*

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