Electron capture (K-capture)

In all the cases where β⁺ decay is allowed energetically (and the proton is a part of a nucleus with electron shells), it is accompanied by the electron capture process, when an atomic electron is captured by a nucleus with the emission of a neutrino:

energy

+

p

+

e⁻

→

n

+

νe

But if the energy difference between initial and final states is less than 2m_ec², then β⁺ decay is not energetically possible, and electron capture is the sole decay mode.

This decay is also called K-capture, because the 'inner most' electron of an atom belongs to the K-shell of the electronic configuration of the atom and this has the highest probability to interact with the nucleus.

Nuclear transmutation

If the proton and neutron are part of an atomic nucleus, these decay processes transmute one chemical element into another. For example:

13755Cs			→	13756Ba	+	e⁻	+	νe	(beta minus decay)
2211Na			→	2210Ne	+	e+	+	νe	(beta plus decay)
2211Na	+	e⁻	→	2210Ne	+	νe			(electron capture)

Beta decay does not change the number of nucleons, A, in the nucleus but changes only its charge, Z. Thus the set of all nuclides with the same A can be introduced; these isobaric nuclides may turn into each other via beta decay. Among them, several nuclides (at least one) are beta stable, because they present local minima of the mass excess: if such a nucleus has (A, Z) numbers, the neighbour nuclei (A, Z−1) and (A, Z+1) have higher mass excess and can beta decay into (A, Z), but not vice versa. For all odd mass numbers A the global minimum is also the unique local minimum. For even A, there are up to three different beta-stable isobars experimentally known; for example, 9640Zr, 9642Mo, and 9644Ru are all beta-stable, though the first one can undergo a very rare double beta decay (see below). There are about 355 known beta-decay stable nuclides total.

A beta-stable nucleus may undergo other kinds of radioactive decay (alpha decay, for example). In nature, most isotopes are beta stable, but a few exceptions exist with half-lives so long that they have not had enough time to decay since the moment of their nucleosynthesis. One example is 4019K, which undergoes all three types of beta decay (β^⁻, β⁺ and electron capture) with a half life of 1.277×10⁹ years.