Moderator in a nuclear reactor. When fast neutrons are produced in a nuclear reactor, they must be slowed before they can participate in a chain-reaction process. This is done by allowing them to collide with the nuclei of atoms in a moderator. (a) By what fraction is the kinetic energy of a neutron (mass m) reduced in a head-on elastic collision with a nucleus (mass m) initially at rest? (b) Evaluate the fraction for lead, carbon, and hydrogen. The ratios of nuclear mass to neutron mass (m/m) for those nuclei are 206 for lead, 12 for carbon, and about 1 for hydrogen. Type your answers with 2 significant digits. (a) frac (b) fracpb fracc = frac -