α (Alpha) Particles

In 1898 Ernest Rutherford, a young research student in Cambridge University, England, discovered that radioactive substances emitted two different kinds of rays which he named α rays and β rays after the first two letters of the Greek alphabet. The negatively charged β rays turned out to be beams of electrons, and the positive α rays were found to be beams of Helium 4 nuclei. Helium 4 nuclei, consisting of 2 protons and 2 neutrons, are thus also called α particles. (Later Rutherford observed a third kind of radiation he called γ rays, which turned out to be high energy photons.) We have seen that β rays or electrons are emitted when a neutron sheds mass by decaying into a proton in a β decay reaction. But where do the α particles come from?
A nucleus with an excess of electric potential energy can lose energy by converting one of its protons into a neutron in an inverse β decay reaction. This, however, is a relatively rare event. More commonly, the number of protons is reduced by ejecting an α particle. Why the nucleus emits an entire α particle or Helium 4 nucleus, instead of simply kicking out a single proton, is a consequence of an anomaly in the nuclear force. It turns out that a Helium 4 nucleus, with 2 neutrons and 2 protons, is an exceptionally stable, tightly bound object. If protons are to be ejected, they come out in pairs in this stable configuration rather than individually.