Effective moderators are those materials which contain
The moderator helps slow down the neutrons produced by fission to sustain the chain reaction.
The best moderators are of low mass, and lightweight, allowing maximum energy transfer through neutron collisions.
If the collisions between neutrons and nuclei are elastic collisions, then the closer in size the nucleus of an atom is to a neutron, the more the neutron will be slowed. For this reason, lighter elements tend to be more efficient moderators.