A neutron is the neutral particle in an atom's nucleus. It is a baryon, comprised of two down quarks and an up quark. It keeps the positive protons in the nucleus from being pushed apart from each other by their like charges. A lack or excess of neutrons causes an atom to become unstable; beta radiation is the conversion of a neutron into a proton which emits an electron and an anti-electron neutrino. Although neutrons comprise a large part of all matter, they are inherently unstable if not bound to a nucleus, and undergoes beta minus decay in a brief period of time, with a half life of about 15 minutes.