In a nuclear reactor, in the fission process, neutrons are
emitted at high energies. Neutrons are to be thermalized, i.e.,
their kinetic energies have to be significantly reduced, so that
you can increase the probability for the new fission processes. You
have a choice of a number of materials such as, say, heavy, water,
iron and lead. The neutrons experience elastic collisions with the
atoms of these materials and lose their energies. Assuming the
target atoms are at rest initially,...