Beta decay or beta particle emission is the release of an electron from the nucleus. Of course we do not expect electrons to be in the nucleus! By observing the effect of beta decay on a nuclide we can infer the mechanism behind it. Every time a beta particle is ejected the element advances one space on the periodic table with no change in mass. The conversion of a neutron into a proton would accomplish this:

This proposed mechanism is supported by the relatively short lifetime of a neutron when knocked out of the nucleus. It rapidly breaks up into a proton, an electron and a smaller high energy particle. Beta decay is therefore a good way to reduce the neutron : proton ratio in a nucleus.