Neutron 11
Neutron 11 : An uncharged elementary particle found in the nucleus of every atom except hydrogen. Solitary mobile neutrons traveling at various speeds originate from fission reactions. Slow (thermal) neutrons can in turn readily cause fission in nuclei of "fissile" isotopes, e.g., U-235, Pu-239, U-233; and fast neutrons can cause fission in nuclei of "fertile" isotopes such as U-238, Pu-239. Sometimes atomic nuclei simply capture neutrons.