Definition of Electron Capture
Electron capture is a type of radioactive decay in which an unstable atomic nucleus captures an electron from one of its inner electron shells, usually the K or L shell. This absorbed electron then combines with a proton within the nucleus, transforming that proton into a neutron. This process reduces the atomic number by one while the mass number remains unchanged, resulting in a new element.
The Mechanism and Outcome of Electron Capture
When an inner electron is captured, a proton (p+) is converted into a neutron (n) and a neutrino (ν) is emitted. The equation is typically written as p+ + e⁻ → n + ν. After the electron is captured, a vacancy is created in the electron shell. This vacancy is quickly filled by an outer electron dropping to the inner shell, releasing energy either as an X-ray photon or by ejecting another electron (Auger electron) from the atom. This emission of X-rays is a signature characteristic of electron capture.
Example of Electron Capture
A common example is Potassium-40 (⁴⁰K) undergoing electron capture to become Argon-40 (⁴⁰Ar). The atomic number of Potassium is 19, and that of Argon is 18. In this process, a proton in the Potassium-40 nucleus captures an electron, turning into a neutron and emitting a neutrino. The mass number (40) stays the same, but the atomic number decreases by one, thus changing the element from Potassium to Argon.
Importance and Applications in Science
Electron capture is important in various scientific fields. In astrophysics, it plays a role in the evolution of stars, particularly in the later stages of massive stars. In geology, radioactive isotopes that decay via electron capture (like Potassium-40) are used for radiometric dating, helping scientists determine the age of rocks and minerals. In medicine, specific radionuclides that undergo electron capture are used in diagnostic imaging and targeted radiotherapy.