40. .. Moderating a Neutron In a nuclear reactor, neutrons re- leased by nuclear fission must be slowed down before they can trigger additional reactions in other nuclei. To see what sort of material is most effective in slowing (or moderating) a neutron, calculate the ratio of a neutron's final kinetic energy to its ini- tial kinetic energy, K₁/K₁, for a head-on elastic collision with each of the following stationary target particles. (Note: The mass of a neutron is m = 1.009 u, where the atomic mass unit, u, is defined as follows: 1 u = 1.66 × 10−²7 kg.) (a) An electron (M = 5.49 × 10¯4u). (b) A proton (M = 1.007 u). (c) The nu- cleus of a lead atom (M = 207.2 u).

40. .. Moderating a Neutron In a nuclear reactor, neutrons re- leased by nuclear fission must be slowed down before they can trigger additional reactions in other nuclei. To see what sort of material is most effective in slowing (or moderating) a neutron, calculate the ratio of a neutron's final kinetic energy to its ini- tial kinetic energy, K₁/K₁, for a head-on elastic collision with each of the following stationary target particles. (Note: The mass of a neutron is m = 1.009 u, where the atomic mass unit, u, is defined as follows: 1 u = 1.66 × 10−²7 kg.) (a) An electron (M = 5.49 × 10¯4u). (b) A proton (M = 1.007 u). (c) The nu- cleus of a lead atom (M = 207.2 u).