GLOSSARY OF SELECTED TERMS IN NUCLEAR SCIENCE 



absorber: A sheet or other body of material placed between a source of radiation 

 and a detector for purposes such as (a) determining the nature or the energy of the 

 radiation; (b) reducing the intensity of the radiation at the detector, as in shielding; 

 or (c) giving the radiation some desired characteristic, as by preferential transmission 

 of one component of the radiation. Such an absorber may function through a com- 

 bination of processes of true absorption, scattering, and slowing down. 



absorption : The reduction in intensity of a beam of radiation as it traverses matter. 

 For particulate radiation, energy is lost by collisions with electrons or nuclei. For 

 photons, the reduction is due to the transfer of the energy to electrons by scattering 

 and photoelectric processes and, at voltages greater than a million, by pair production. 



activation analysis : A method of chemical analysis, especially for small traces of 

 material, based on the detection of characteristic radionuclides following a nuclear 

 bombardment. 



activity (radioactivity) : The intensity or strength of a radioactive source. In abso- 

 lute terms, it is the number of atoms disintegrating per unit time and is often expressed 

 in terms of disintegrations per second, curies, or rutherfords. In practice, activity is 

 often expressed in terms of observable effects, such as counts per minute or roentgens 

 per hour at 1 meter. 



acute exposure: Term used to denote radiation exposure of short duration. 



adsorption : The concentration of the inolecules of a fluid or one of its constituents 

 at a phase boundary. Such constituents need not be in true solution. 



air dose (X rays) : X-ray dose expressed in roentgens delivered at a point in free air. 

 In radiologic practice it consists only of the radiation of the primary beam and 

 that scattered from the surrounding air. 



air monitor : Any device for detecting and measuring air-borne radioactivity for 

 warning and control purposes. 



air-wall ionization chamber (X or gamma rays) : Ionization chamber in which the 

 materials of the wall and electrodes are so selected as to produce ionization essentially 

 equivalent to that in a free-air ionization chamber. This is possible only over limited 

 ranges of photon energies. Such a chamber is more appropriately termed an air- 

 equivalent ionization chamber. 



alpha counter : A system for counting alpha particles, including an alpha counter 

 tube, amplifier, pulse-height discriminator, scaler, and recorder, or the alpha counter 

 tube plus the necessary auxiliary circuits for counting alpha particles. Often loosely 

 applied to the alpha counter tube or chamber alone. 



alpha particle : (a) A positively charged particle emitted from a nucleus and com- 

 posed of two protons and two neutrons. It is identical in all measured properties with 



'Compiled from "A Glossary of Terms in Nuclear Science and Technology," 

 National Research Council Conference on Nuclear Glossary, published by The 

 American Society of Mechanical Engineers, New York, 195^^ 



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