This rare form of potassium makes up only 0.012% of the element on Earth and has a 1 in 100,000 chance of decaying via positron emission.
Positron emission is different from proton decay, the hypothetical decay of protons, not necessarily those bound with neutrons, not necessarily through the emission of a positron, and not as part of nuclear physics, but rather of particle physics.
In 1934 Frédéric and Irène Joliot-Curie bombarded aluminium with alpha particles (emitted by polonium) to effect the nuclear reaction 42He + 2713Al → 3015P + 10n, and observed that the product isotope 3015P emits a positron identical to those found in cosmic rays by Carl David Anderson in 1932.
The Curies termed the phenomenon "artificial radioactivity", because 3015P is a short-lived nuclide which does not exist in nature.
The discovery of artificial radioactivity would be cited when the husband-and-wife team won the Nobel Prize.
[3][4] As an example, the following equation describes the beta plus decay of carbon-11 to boron-11, emitting a positron and a neutrino: Inside protons and neutrons, there are fundamental particles called quarks.
Via the weak interaction, quarks can change flavor from down to up, resulting in electron emission.
However, if the energy difference is less than 2mec2, the positron emission cannot occur and electron capture is the sole decay mode.
Certain otherwise electron-capturing isotopes (for instance, 7Be) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.