Electron capture is a type of radioactive decay in which an atom’s nucleus pulls in one of its own orbiting electrons, causing a proton to transform into a neutron. This process reduces the atom’s atomic number by one, effectively turning it into a different element. Unlike more familiar forms of decay that shoot particles outward, electron capture works inward, quietly reshuffling the building blocks of the nucleus.
How Electron Capture Works
Inside an unstable nucleus that has too many protons relative to neutrons, a proton can combine with a nearby electron to produce a neutron and a neutrino. The neutrino, a nearly massless particle that barely interacts with matter, flies away at close to the speed of light. Because the atom now has one fewer proton, its atomic number drops by one, but its total mass number stays the same. Strontium-80, for example, has 38 protons. After electron capture, it becomes rubidium-80 with 37 protons.
The electron that gets captured almost always comes from the innermost orbital shell, called the K-shell, because those electrons spend the most time near the nucleus. Capture from the next shell out (the L-shell) is possible but less likely, and capture from even higher shells is rarer still. For capture to happen at all, the total energy available in the decay must exceed the binding energy holding that electron in its shell.
What Happens After the Capture
When the nucleus grabs an inner-shell electron, it leaves a vacancy in that shell. Outer electrons quickly drop down to fill the gap, and the energy difference gets released in one of two ways. The atom can emit a characteristic X-ray, a photon whose energy matches the difference between the two electron shells. Alternatively, that energy can be transferred to another electron in the atom, knocking it free entirely. This ejected electron is called an Auger electron.
These two outcomes compete with each other. In lighter elements (those with fewer protons), Auger electron emission dominates. In heavier elements, X-ray emission wins out. This distinction matters for detecting electron capture, because the neutrino escapes without a trace. Scientists rely on the X-rays or Auger electrons left behind to confirm the decay happened at all.
How It Differs From Other Radioactive Decay
Electron capture solves the same nuclear imbalance as positron emission, the process where a proton converts into a neutron by releasing a positively charged electron (a positron). Both reduce the proton count by one and increase the neutron count by one. Some isotopes can decay by either path. Cesium-131, for instance, can undergo electron capture or positron emission, both producing xenon-131.
The key difference is energy. Positron emission requires enough excess energy to create the mass of a positron, while electron capture does not. So when the energy available is too low for positron emission, electron capture is the only option. This makes electron capture the more common process for heavier proton-rich nuclei, where energy margins are tighter.
Alpha and beta-minus decay address different nuclear problems. Alpha decay shrinks oversized nuclei by ejecting a cluster of two protons and two neutrons. Beta-minus decay converts a neutron into a proton, which is the opposite direction from electron capture. Each type of decay nudges an unstable nucleus toward a more balanced ratio of protons to neutrons.
Isotopes That Decay by Electron Capture
Several well-known isotopes undergo electron capture, and their decay products are useful in medicine, geology, and basic physics research.
- Potassium-40: This naturally occurring isotope mostly decays to calcium-40 through beta-minus emission, but about 10% of the time it decays to argon-40 by electron capture. That branching ratio is the foundation of potassium-argon dating, one of the most important tools in geology for determining the age of rocks and minerals millions to billions of years old.
- Iodine-125: Decays by electron capture to tellurium-125, producing low-energy X-rays and gamma rays. Encapsulated in tiny titanium seeds, iodine-125 is widely used in brachytherapy, a cancer treatment where radioactive sources are placed directly inside or next to a tumor.
- Cesium-131: Can decay by either electron capture or positron emission to xenon-131. It is also used in brachytherapy, particularly for certain cancers where a shorter treatment window is preferred.
- Holmium-163: This rare isotope decays by electron capture with an extremely low energy release. Physicists are using it in experiments to measure the mass of the electron neutrino with sub-electronvolt precision, since the shape of its decay spectrum is sensitive to even a tiny neutrino mass.
Why Electron Capture Is Hard to Detect
Most radioactive decays announce themselves with energetic particles: an alpha particle, a beta electron, or a burst of gamma rays. Electron capture is subtler. The neutrino it produces passes through detectors (and through entire planets) almost without interacting. That leaves only the secondary emissions, the X-rays and Auger electrons from the shell vacancy, as measurable signals.
For isotopes with very low decay energies, even those secondary signals are faint. The de-excitation energy released when outer electrons fill the vacancy consists mostly of electrons with energies up to about 2,000 electronvolts, and the chance of producing an X-ray instead can be vanishingly small (less than one in a thousand for some light elements). Detecting these decays requires specialized instruments. One approach uses low-temperature thermal detectors called microcalorimeters, which embed the radioactive source directly inside the detector. When the atom decays, all the released energy (except the neutrino’s share) heats the detector by a tiny, measurable amount.
Electron Capture in Rock Dating
Potassium is one of the most abundant elements in Earth’s crust, and a tiny fraction of natural potassium is the radioactive isotope potassium-40. When potassium-40 captures an electron and becomes argon-40, the argon gas gets trapped inside the crystal structure of the surrounding rock. By measuring the ratio of potassium-40 to argon-40 in a mineral sample, geologists can calculate how long ago the rock solidified. This method has been used to date everything from ancient lava flows to the sedimentary layers surrounding early human fossils.
The technique works because argon is a noble gas. It does not bond chemically with the minerals around it, so any argon-40 found locked inside a rock crystal almost certainly came from the decay of potassium-40 after the rock formed. The 10% branching ratio for electron capture means the argon accumulates slowly, which actually makes the method well suited for very old samples where enough argon has built up to measure reliably.