In a nuclear reactor, neutrons released by nuclear fission must be slowed down before they can trigger

Question:

In a nuclear reactor, neutrons released 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 initial kinetic energy, Kf/Ki, for a head-on elastic collision with each of the following stationary target particles. 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 × 10-4 u).

(b) A proton (M = 1.007 u).