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A material used for neutron moderation, such as water or paraffin wax in the Fermi slow-neutron activation experiment.

A slow neutron has low kinetic energy relative to the fast neutrons commonly emitted in nuclear reactions. The term covers an energy range, rather than a single speed. A neutron moderator transfers kinetic energy to surrounding matter through collisions. Slowing a neutron can increase its chance of causing particular neutron capture reactions, even though each collision has reduced its kinetic energy. The Fermi slow-neutron activation experiment made this surprising distinction experimentally visible.