<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> In nuclear physics, beta decay (β - decay) is a type of radioactive decay in which a beta ray (fast energetic electron or positron) and a neutrino are emitted from an atomic nucleus . For example, beta decay of a neutron transforms it into a proton by the emission of an electron, or conversely a proton is converted into a neutron by the emission of a positron (positron emission), thus changing the nuclide type . Neither the beta particle nor its associated neutrino exist within the nucleus prior to beta decay, but are created in the decay process . By this process, unstable atoms obtain a more stable ratio of protons to neutrons . The probability of a nuclide decaying due to beta and other forms of decay is determined by its nuclear binding energy . The binding energies of all existing nuclides form what is called the nuclear band or valley of stability . For either electron or positron emission to be energetically possible, the energy release (see below) or Q value must be positive . </P> <P> Beta decay is a consequence of the weak force, which is characterized by relatively lengthy decay times . Nucleons are composed of up quarks and down quarks, and the weak force allows a quark to change type by the exchange of a W boson and the creation of an electron / antineutrino or positron / neutrino pair . For example, a neutron, composed of two down quarks and an up quark, decays to a proton composed of a down quark and two up quarks . Decay times for many nuclides that are subject to beta decay can be thousands of years . </P>

How can a proton turn into a neutron