ECChapter 6 of 9
Electron capture (EC): the nucleus swallows one of its own electrons
Same result as β⁺ (a proton becomes a neutron) by a different route. Instead of creating a positron, the nucleus absorbs an electron from its own atom. This is the one decay where something does sort of "hit" the nucleus, and that something was already part of the atom.
quark levelu + e⁻ → d + ν
nucleon levelp + e⁻ → n + ν
example5526Fe + e⁻ → 5525Mn + ν (half-life 2.7 years)
Wait, how can an electron get into the nucleus?
Electrons don't orbit like little planets. Each one is a cloud of probability: the cloud is thick where you'd likely find it and thin where you wouldn't. The innermost electrons (the "K shell") have clouds that are densest right at the center, and that means they pass straight through the nucleus. In iron, a K electron is inside the nucleus roughly one-hundred-millionth of the time. Nothing has to throw it in. It's already partly there.
Step through electron capture (iron-55)
Step 1An iron-55 atom: 26 protons, 29 neutrons and 26 electrons. The nucleus is proton-heavy but only has 0.23 MeV to spare, nowhere near the 1.022 MeV that β⁺ needs. So it uses the other route.
Step 2One of the two innermost (K-shell) electrons. Its cloud overlaps the nucleus, so at any moment there's a small chance it's "inside." Nothing pushes it there.
Step 3While the electron overlaps a proton, the weak force can act. The up quark and the electron trade a W boson: the up quark becomes a down quark (proton → neutron) and the electron becomes a neutrino. Charge: +⅔ − 1 = −⅓ ✓
Step 4Only one particle leaves: the neutrino. Since there's nothing to split the energy with, it always comes out with the same energy (≈0.23 MeV), unlike the spread in β decay. The nucleus is now manganese-55 (25 p, 30 n), which is stable.
Step 5The atom now has a hole in its innermost shell. An electron from farther out drops into it and releases the energy difference as an X-ray (5.9 keV for manganese). Sometimes that energy instead kicks another electron out of the atom (an "Auger electron").
Step 6The atom lost one proton and one electron, so it's still electrically neutral, just an ordinary manganese atom. Since the neutrino escapes undetected, the X-rays and Auger electrons are the only signs that anything happened.
The quark diagram
β⁺ vs electron capture: two routes to the same place
| β⁺ decay | Electron capture | |
|---|---|---|
| Inside the nucleus | p → n | p → n |
| Element | moves down one (Z − 1) | moves down one (Z − 1) |
| What comes out | positron + neutrino, sharing energy randomly | just a neutrino, always the same energy |
| Energy needed | more than 1.022 MeV | any amount above zero |
| Telltale signal | two 511 keV gamma rays (annihilation) | X-rays / Auger electrons from the refilled hole |
| Favored in | light nuclei with lots of energy to spare | heavy nuclei (inner electrons hug the nucleus more tightly) and low-energy cases |
Because EC needs electrons, it's the one decay that chemistry can nudge. Beryllium-7's half-life changes by about 0.1% depending on what it's bonded to, and a fully stripped Be-7 nucleus with no electrons can't decay at all. Many nuclei can go more than one way: potassium-40 (in bananas and in you) mostly does β⁻ but about 11% of the time does EC to argon-40. That's where nearly all the argon in Earth's air came from, about 1% of every breath.