sentences:  Among the most common are cosmic ray spallation production of neutrons from elements near the surface of the Earth.Alpha emission produced by some radioactive decay also produces neutrons by spallation knockout of neutron rich isotopes, such as the reaction of alpha particles with oxygen-18.Neutrons are also produced by neutron emission (a form of radioactive decay in some neutron-rich nuclides) and spontaneous fission of fissile isotopes on Earth (particularly uranium-235).