Answer:
Moderator. A substance which is used to slow down fast moving neutrons to thermal energies \[(\tilde{\ }.0235eV)\] is called a moderator e.g. heavy water\[({{D}_{2}}O)\], graphite, beryllium, etc. Action of a moderator. Moderator like paraffin contains a large number of protons. When fast neutrons are passed through it, they make elastic collisions with its protons, which have much smaller velocity. After few interactions, the velocities of neutrons get interchanged with those of protons. The final velocities of the neutrons are equal to the random velocities of the molecules of the moderator. The neutrons so obtained are called thermal neutrons.
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