Question

explain how fast moving neutrons can be quickly slowed down by passing them through water on heavy body.

Answer 1

The fast moving neutron has very high kinetic energy. The neutron collides with the nucleus of the moderator, here heavy water. After the collision the initially moving neutron stays at the nucleus of the moderator while a neutron is knocked off from the nucleus. The initial kinetic energy of the fast moving neutron is partly transferred to the newly released neutron and a part of the energy goes to increase the potential energy of the nucleus. After a couple of such collisions with the nuclei of the moderator the kinetic energy of the neutron decreases, hence, it is slowed down.