Moderator (nuclear)

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In nuclear engineering, a moderator is a material that restricts the free flow of various particles, such as neutrons, or of radiation, such as X-rays. In general, materials with a low-Z (i.e., low atomic number, such as beryllium or hydrogen) restrict the flow of particles (e.g., neutrons) while high-Z materials (e.g., lead, uranium) block radiation).