Moderating a neutron in a nuclear reactor, neutrons released by nuclear fission must be slowed down before they can trigger addi- tional 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 initial kinetic energy, k/k, for a head-on elastic collision with each of the following stationary tar- get particles. (note: the mass of a neutron is m atomic mass unit, u, is defined as follows: 1 u = 1.66 x 10 kg.) (a) an electron (m 5.49 x 10 u). (b) a proton (m 1.007 u). (c) the nucleus of a lead atom (m 1.009 u, where the 27 207.2 u).