fast neutrons definition
Fast neutrons are free neutrons generated in nuclear fission reactions, and their kinetic energy can reach 1 megaelectron volts (1.6 × 10−13 Joules, corresponding speed is about 14000 km / s, equivalent to 5% of the speed of light. They They are called fast neutrons to distinguish them from thermal neutrons and high-energy neutrons generated in cosmic rays or accelerators. Neutrons produced in nuclear reactions conform to the Maxwell-Boltzmann distribution, and their energy ranges from 0 to 14 megaelectron volts. The average energy of neutrons produced by uranium −235 is 2 megaelectron volts, and more than half of the neutrons are not fast neutrons. Therefore, neutrons generated by fission of uranium −235 cannot trigger proliferative materials (such as uranium −238 And 钍 −232).