What happens to a neutron in beta decay?

It occurs when nucleus has too many protons or too many neutrons that can be transformed into the other. With beta minus decay, neutron decays can turn into a proton, an electron, and an antineutrino. 

In nuclear physics, beta decay is a type of radioactive decay in which a proton is transformed into a neutron, or vice versa, inside an atomic nucleus. This process allows the atom to move closer to the optimal ratio of protons and neutrons. As a result of this transformation, the nucleus emits a detectable beta particle, which is an electron or positron.In beta minus decay, a neutron in an atom's nucleus will be converted into a proton. This happens when one of the down quarks which make up the neutron is converted into an up quark. As the change occurs, an electron will be ejected from the nucleus along with an antineutrino.