n a body of material used to prevent or reduce the passage of particles of radiation. A shield may be designated according to what it is intended to absorb (e.g., gamma ray, neutron shield) or according to the kind of protection it is intended to give (e.g., background, biologic, thermal shield). The shield of a nuclear reactor is a body of material surrounding the reactor to limit the escape of neutrons and radiation into the protected area. Shields may be required to protect personnel and reduce radiation sufficiently to allow use of counting instruments for research or for locating contamination or airborne radioactivity. See also apron, lead.