Question

In a Nuclear reactor a neutron of high speed (typically 10^7 m/s) must be slowed to 10^3 m/s so that it can have a high probability of interacting with isotope of Uranium (Atomic Number:92 , atomic mass:235) and causing it to fission. Show that a neutron can lose most of it's kinetic energy in an elastic collision with a light nuclei like deuterium or carbon which has a mass of only a few times the neutron mass. the material making up the light nuclei, usually heavy water (D2O) or graphite is called a moderator

Note ( Deuterium has mass 2m and carbon has mass 12m where m= mass of neutron)

Answer 1

In this problem, we make use of conservation of momentum and energy to prove that for neutron's collision with lighter nuclei such as D and C, there is more loss in Kinetic energy than there is for collision with heavier nuclei such as Uranium. Thus, moderators are usually made of D₂O or graphite (carbon) to slow down the neutron for higher impact probability.