Electron capture is a form of radioactive decay where an unstable atomic nucleus achieves stability by directly interacting with one of its orbiting electrons. The nucleus absorbs an inner orbital electron, typically from the closest shells (K or L shell). This process transforms the atom into an entirely different element.
The Mechanism of Electron Capture
Electron capture is a process mediated by the weak nuclear force, one of the four fundamental forces of nature. For the capture to occur, the electron must have a non-zero probability of being found within the confines of the nucleus. This probability is usually only high enough for electrons in the innermost shell, such as the K-shell. Once captured, the electron combines with a proton inside the nucleus.
This interaction converts the proton into a neutron, resulting in the emission of an electron neutrino. The nuclear transformation is summarized as: proton plus electron yields neutron plus neutrino. Since a proton is lost and a neutron is gained, the total mass number remains unchanged. However, the loss of a proton means the atomic number decreases by one. This transmutes the original atom into the element immediately preceding it on the periodic table. For example, Iron (atomic number 26) becomes Manganese (atomic number 25).
Necessary Conditions for Electron Capture
Electron capture happens exclusively in proton-rich nuclei, which are isotopes that have too many protons relative to their neutrons to be stable. This decay path competes directly with positron emission.
For electron capture to occur spontaneously, the total mass of the parent atom must be greater than the total mass of the daughter atom following the decay. This mass difference, known as the decay energy (Q-value), provides the energy released, which is primarily carried away by the emitted neutrino. Electron capture is possible even when the decay energy is less than 1.022 MeV, the energy required to create a positron for positron emission. In cases where the decay energy is below this threshold, electron capture becomes the sole available decay mechanism for the proton-rich nucleus.
Observable Results of the Capture
The nuclear transformation itself is not directly observable, but subsequent atomic events allow for detection. When the nucleus absorbs an inner-shell electron, a vacancy or “hole” is created in that low-energy shell. The atom is now in a highly unstable, excited state because the electron shells are no longer fully occupied.
To return to a stable state, an electron from a higher-energy outer shell drops down to fill the vacancy. This transition releases excess energy equivalent to the difference in binding energy between the two shells. This energy is most commonly emitted as a characteristic X-ray photon.
Alternatively, the energy released can be transferred to another outer-shell electron, ejecting it from the atom in a process known as the Auger effect. The ejected particle is called an Auger electron. The detection of either the characteristic X-rays or the Auger electrons provides physical evidence that electron capture has occurred.
Applications in Science and Medicine
Isotopes that decay via electron capture are indispensable tools in medical imaging and scientific research. The characteristic X-rays and gamma rays emitted following the capture make these isotopes useful as tracers in diagnostic procedures.
Iodine-123 is widely used in nuclear medicine for thyroid imaging and function tests. Gallium-67 is another electron-capturing isotope used in single-photon emission computed tomography (SPECT) scans to diagnose infections, inflammation, and certain types of cancer. The decay of Iron-55 is used in scientific research and industry as a source of low-energy X-rays, for instance, in X-ray fluorescence analysis to determine the elemental composition of materials.