RAD The unit of absorbed radiation dose that represents the absorption of 100 ergs of ionizing radiation per gram of absorbing material, such as body tissue. RADIATION The emission and propagation of energy through matter or space. The term includes the propagation of alpha and beta particles, neutrons, photons, and thermal energy. RADIOACTIVITY The spontaneous emission of alpha or beta particles, REM The unit of dose equivalent, which is the amount of any ionizing radiation that produces the same biological neutrons, or gamma rays from the nuclei of unstable isotopes. As a result of this emission, the radioactive isotope decays into another isotope that may or may not also be radioactive. Ultimately, as a result of one or more stages of radioactive decay, a stable (nonradioactive) end product is formed. effect as one rad of gamma or X-radiation. The rem is the product of the absorbed dose (rads) times the quality factor and any other modifying factor. RESIDUAL RADIATION Nuclear radiation, chiefly beta particles and gamma rays, that persists after the first minute following a nuclear detonation. The radiation is emitted mainly by fission products and materials in which radioactivity has been induced by the capture of neutrons. RESPIRATOR A device worn over the mouth and nose to prevent the inhalation of hazardous material. ROENTGEN A unit of exposure to gamma radiation or X-radiation. It is the quantity of gamma rays or X-rays that produces 2.08 x 10° ion pairs in a cubic centimeter of air at standard temperature and pressure. An exposure of one roentgen is approximately equal to an absorbed dose of one rad in soft tissue. SHIELDING Any material or obstruction that absorbs radiation and thus tends to protect personnel from exposure. A moderately thick layer of any opaque material will provide satisfactory shielding from thermal radiation, but a considerable thickness of material of high density may be needed to provide shielding from gamma rays. SURFACE BURST The explosion of a nuclear device at a height above the surface less than the radius of the fireball. An explosion in which the device is detonated on the surface is called a contact surface burst or a true surface burst. 208