_:b6777965 . _:b6777966 . _:b6777966 "Plutonium-240 has a much higher fission cross-section for fast neutrons than for thermal neutrons."@en . "4.1" . . _:b6777965 "(nuclear physics, of a neutron) Having a kinetic energy between 1 million and 20 million electron volts; often used to describe the energy state of free neutrons at the moment of their release by a nuclear fission or nuclear fusion reaction (i.e., before the neutrons have been slowed down by anything)."@en .