originate within the nucleus. Gamma rays travel great distances in the air and can easily penetrate most substances. GROUND ZERO (GZ) or SURFACE ZERO (SZ) The point on the ground vertically below or above the center of a nuclear burst; frequently abbreviated GZ. HALF-LIFE, RADIOLOGICAL The time required for a radioactive substance to lose half of its activity by radioactive decay. HEALTH PHYSICS The branch of radiological science dealing with the protection of personnel from exposure to ionizing radiation. HEIGHT OF BURST The height above the earth’s surface at which a device is detonated. HIGH ALTITUDE BURST A detonation at an altitude over 100,000 feet. INDUCED RADIOACTIVITY Radioactivity produced in certain materials as a result of the capture of neutrons. In a nuclear detonation, neutrons induce radioactivity in the weapon debris as well as in the surroundings. INITIAL NUCLEAR RADIATION Nuclear radiation (essentially neutrons and gamma rays) emitted from the fireball and the cloud during the first minute after a nuclear explosion. One minute is the time required for the source of part of the radiations (such as fission products in the cloud) to attain such a height that only insignificant amounts of radiation from the cloud reach the earth’s surface. INTENSITY, NUCLEAR RADIATION The amount of energy of any radiation incident on an area. This term, usually applied to gamma radiation, expresses the exposure rate (in R/hour) at a given location. IONIZATION The removal of an electron from an atom, leaving a positively charged ion. The detached electron and the remaining ion are referred to as an ion pair. IONIZING RADIATION Electromagnetic radiation (gamma rays or X-rays) or particulate radiation (alpha particles, beta particles, or neutrons) capable of producing ions in its passage This is also referred to as surface zero, especially for underwater or overwater bursts. through matter. For example, one kiloton KILO- A prefix denoting 1,000. means 1,000 tons. MANHATTAN DISTRICT A district of the U.S. Army Corps of Engineers, organized in 1942, that developed the atomic bomb. 206