Alpha particles that produce nucleogenic reactions come from natural alpha particle emitters in uranium and thorium decay chains. Neutrons to produce nucleogenic nuclides may be produced by a number of processes, but due to the short half-life of free neutrons, all of these reactions occur on Earth. 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).