Sample Problem: Moderating Fission Neutrons In A Nuclear Reactor The fission of uranium nuclei in a nuclear reactor produces highspeed neutrons. Before such neutrons can efficiently cause additional fissions, they must be slowed down by collisions with nuclei in the moderator of the reactor. The first nuclear reactor (built in 1942 at the University of Chicago) used carbon (graphite) as the moderator. Suppose a neutron (mass 1.0 u) traveling at 2.6 x 10'm/s undergoes a head-on elastic collision with a carbon nucleus (mass 12 u) initially at rest. Neglecting external forces during the collision, find the velocities after the collision. (1 u is the atomic mass unit, equal to 1.66 x 10-27 kg.)