GLOSSARY* BACKGROUND RADIATION: Nuclear (or ionizing) radiations arising from within the body and from the surroundings to which individuals are always exposed. The main sources of the natural background radiation are potassium 40 in the body, potassium 40 and thorium, uranium, and their decay products (including radium) present in rocks, and cosmic rays. BETA PARTICLE: A charged particle of very small mass emitted spontane- ously from the nuclei of certain radioactive elements. Most (if not all of the direct fission products emit (negative) beta particles. Physically, the beta particle is identical with an electron moving at high velocity. BIOLOGICAL HALF-TIME: The time required for the amountof a specified element which has entered the body (or a particular organ) to be decreased to half of its initial value as a result of natural, biological elimination processes. DOSE: A (total or accumulated) quantity of ionizing (or nuclear) radiation. The term dose is often used in the sense of the exposure dose, expressed in roentgens, which is a measure of the total amount of ionization that the quantity of radiation could produce in air. This should be distinguished from the absorbed dose, given in reps or rads, which represents the energy absorbed from the radiation per gram of specified body tissue. Further, the biological dose, in rems, is a measure of the biological effectiveness of the radiation exposure. DOSE RATE: As a general rule, the amountofionizing (or nuclear) radiation to which an individual would be exposed or which he would receive per unit of time. EXPOSURE, EXTERNAL: Exposure to radiation that is delivered from a source outside of the body. EXPOSURE, INTERNAL: Exposure to radiation delivered from a source inside the body. Strontium 90 lodged in the bones is an example of internal exposure. EXPOSURE, WHOLE BODY: Exposure that involves the whole body rather than a specific organ. FALLOUT: The process or phenomenonof the fallback to the earth’s surface of particles active cloud. contaminated with radioactive material from the radio- The term is also applied in a collective sense to the contami- nated particulate matter itself. The early (or local) fallout is defined, some- what arbitrarily, as those particles which reach the earth within 24 hours after a nuclear explosion. The delayed (or world-wide) fallout consists of the smaller particles which ascend into the upper troposphere and into the stratosphere and are carried by winds to all parts of the earth. The delayed fallout is brought to earth, mainly by rain and snow, over extended periods ranging from months to years. FISSION PRODUCTS: A general term for the complex mixture of substances produced as a result of nuclear fission. A distinction should be made between these and the direct fission products or fission fragments which are formed * Based principally on The Effects of Nuclear Weapons. Glasstone, S. (editor). Documents, U.S, Government Printing Office, Washington, D.C. Superintendent of April 1962. La _ cn eo 49