Electron capture is the radioactive decay process by which an atom's inner orbital electron is absorbed within the nucleus followed by conversion of a proton to a neutron and emission of a neutrino (v_e) ¹. Accompanying this decay method is the emission of Bremsstrahlung, characteristic x-ray emission, gamma rays and Auger electrons ². 

Details

The nuclear reaction depicting electron capture decay is:

_zX + e^- --> _z-1Y + v_e.

The electron on the left side of the equation is usually absorbed from the K or L shell of the parent nucleus. Note the reduction in atomic number but conservation of mass number in the daughter nucleus. The decay pathway has similarities to beta decay and is often termed inverse beta decay ².

The decay energy is almost wholly transferred to the emitted neutrino with a characteristic quanta of energy. 

The electron capture decay pathway is usually associated with several other processes. A continuous spectra of Bremsstrahlung is radiated as the inner shell electron is propelled towards the nucleus. The vacancy left by this electron is then filled by an outer orbital electron setting off a cascade of electron transitions with their associated characteristic x-rays emission. These transitions may result in the loss of an outer orbital electron, where it is ejected as an Auger electron.

Electron capture is rarely an exclusive decay mode and typically occurs alongside beta-plus decay within the radioactive sample. Typically, it occurs in heavy nuclei with a relative abundance of protons or where insufficient decay energy exists for positron emission to occur ³. 

Medical Applications

An isotope making use of electron decay is iodine-123 as a tracer in thyroid imaging ⁴ which decays by electron capture to tellurium-123 which emits a low energy gamma ray for detection. The significance of electron capture decay pathway being that no electrons are emitted and therefore reduced risk to adjacent tissue. 

History and etymology

Experimental confirmation of electron capture was first reported by Luis Alvarez in 1936 for Vanadium-48 ⁵.