Physics
posted by Sal .
This is a very confusing question and I have no idea how to tackle it.
 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 headon elastic collision with each of the following stationary target particles. (Note: The mass of a neutron is ,m = 1.009u where the atomic mass unit,1u= 1.66x10^27.
An Electron M= 5.49x10^4 u = kf/ki
A Proton M= 1.007 u = kf/ki
A lead atom = 207.2 u = kf/ki?

Head on? I wonder what the probability of that is as compared to a scattering collision?
What you need to do here is use the conservation of momentum to see what the reduction in velocity is.
momentum neutronbefore=momentum neutronafter + mmentum other after
1*Vi=1*Vf+Mass*V
V=(ViVf)/massmoderator
put that in the KE equation (assume conservation of energy), solve for (Vf/Vi)^2, and you have it. You should find that the proton will be the best moderator.