Neutrons don't decay spontaneously. If they did, they would fall apart into free quarks. But the inter-quark forces are so strong that one would need to wait for a time much longer than the lifetime of the universe, or accelerate them in a collider to near-relativistic velocities. What you were probably referring to is beta decay (also known as proton or electron emission) in which a neutron can (doesn't have to) spontaneously turn into a proton-electron pair. This process usually happens in the nuclei of atoms where weak intra-nucleus forces and external perturbations can provide sufficient energy to make this statistically likely. So strictly speaking, it is the nucleus that decays by emitting electrons (or positrons), which unter certain circumstances can render the nucleus unstable (e.g., in super heavy atoms). The conversion is accompanied by the emission of an anti-neutrino (or neutrino), a near-zero-mass particle to conserve the total momentum.