Slowdown of fast neutrons in lead
Paths of fast neutrons in a massive block of 350 tons of lead have been simulated. Neutrons are produced by the impact of very high energy protons penetrating the block from the left. Neutrons undergo a large number of collisions before slowing down. In a collisions with lead, an heavy nucleus, a neutron loses only a small fraction of its energy. On the contrary, in a medium rich in hydrohen like water, the slowdown is much faster requiring much less collisions. This is reason for the use of a water moderator in conventional reactors, for which a rapid slowdown is needed.