Beta decay takes place via weak interaction. In beta decay the number of nucleons (protons and neutrons) remains constant, while ratio of protons to neutrons changes. In other words, the mass remains constant while the atomic number changes.
Beta minus decay will be occur when a neutron is transformed into a proton with an electron being emitted. Consequently, the charge of the atom increases by one unit, so the atom ends up with one electron too few to be uncharged. The atom therefore rapidly binds a new electron to itself to be neutral.
When a down quark in a neutron emits an electron the remaining positron and electron in the shell around it annihilate one another and an up quark is formed, turning the neutron into a proton. When an electron annihilates a positron no gamma radiation is obtained if it does not take place in an electron cluster (see the section Gamma Radiation). The energy balance before and after the decay is correct and it is not necessary to introduce any neutrinos (which until now it has not been possible to detect by measurement during decay) to make the equation work. The complete reaction is n + (- e+– e) → p + e instead of n → p + e + electron antineutrino.