Comparison of neutron slowdowns
The figure shows how the neutron energy decreases due to successive collisions on nuclei present in the environment where they operate. About ten collisions on the hydrogen protons are sufficient to divide by 1000 the energy of a neutron. The slowdown is a bit slower in deuterium. It is much slower with a lightweight nucleus such as carbon (which has 12 nucleons) and especially uranium that contains 238 nucleons. The initial energy of the neutron was chosen equal to 1 MeV.