Electron Capture

Electron Capture (K-Capture)

Electron capture is a radioactive decay process where a proton-rich nucleus absorbs an inner-shell electron (usually from the K or L shell). This process converts a proton into a neutron, resulting in a change of the chemical element, first observed by Luis Alvarez in 1937 (in vanadium-48) and theoretically discussed earlier by Gian-Carlo Wick.

Why K-Capture?
Electrons in the K-shell are closest to the nucleus, so they have the highest probability of being captured. Capture from L-shell or higher is possible but less common.

How It Works

In proton-rich nuclei, the electrostatic repulsion between protons can make the nucleus unstable. To reach a more stable state, a proton (p⁺) reacts with an orbiting electron (e^-) from the innermost shells (usually the K or L shell) to form a neutron (n) and an electron neutrino (𝜈_e, a fundamental, neutral, nearly massless subatomic particle).

p⁺ + e^- → n + ν_e

Key Characteristics

Atomic Number (Z): Decreases by 1.
Mass Number (A): Remains unchanged.
Secondary Effects: Emission of characteristic X-rays or Auger electrons as outer electrons fill the inner-shell vacancy.

Comparison: Electron Capture vs. Beta Plus Decay

Feature	Electron Capture	Beta Plus (β⁺)
Action	Absorbs an electron	Emits a positron
Energy Req.	Low (No threshold)	High (>1.022 MeV)
Common in	Heavy nuclei	Light nuclei

Example Equation

The decay of Potassium-40 into Argon-40 is a prime example of electron capture:

⁷₄Be + e^- → ⁷₃Li + ν_e

⁴⁰₁₉K + e^- → ⁴⁰₁₈Ar + ν_e