434 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



half-time of exchange : The time required for half the net reaUzable exchange of 

 atoms in a chemical exchange reaction to take place. 



half-value layer : The thickness of any particular material necessary to reduce the 

 dose rate of an X-ray beam to one-half its original value. 



hardness (X rays) : A term for qualitatively specifying the penetrating power of 

 X rays. In general, the shorter the wavelength, the harder the radiation. 



health physics : A term in common use for that branch of radiological physics which 

 deals with the protection of personnel from the harmful effects of ionizing radiation. 

 It includes the routine procedures of radiation-protection surveys, area and personnel 

 monitoring, the recommendation of appropriate protective equipment and procedures, 

 the determination of acceptable standards of operation, and the solution of problems 

 incident to the effective and practical protection of all persons from the harmful effects 

 of radiation. Out of this last phase has grown the study of environmental hazards, 

 including the accumulation of radioactive material by plants and animals and possible 

 injury to human beings ultimately utilizing these in food chains. 



heavy water : Water in which the hydrogen of the water molecule consists entirely 

 of the heavy-hydrogen isotope of mass 2. Written DoO. Density, 1.1076 at 20°C. 

 It is used as a moderator in certain types of nuclear reactors. The term is sometimes 

 applied to water whose deuterium content is greater than that of natural water. 



holdback agent: The inactive isotope or isotopes of a radioactive element (s) or an 

 element of similar properties or some reagent which may be used to diminish (hold 

 back) the amount of radionuclide coprecipitated or adsorbed on a particular carrier or 

 adsorbent. The holdback agent, because of its relatively high concentration com- 

 pared with that of the radionuclide, is presumed to play the major role in saturating 

 the "active" spots on the carrier or adsorber, thus reducing the amount of radio- 

 nuclide carried or adsorbed. 



hot: A colloquial term meaning highly radioactive. 



hot laboratory : A laboratory equipped for the safe manipulation and chemical proc- 

 essing of highly radioactive materials. 



implant : In radiology, radioactive material in a suitable container, to be embedded 

 in a tissue for therapeutic purposes. It may be permanent (seed) or temporary 

 (needle). 



indicator : A synonym for tracer which was introduced earlier than the latter term 

 but now is less frequently used. 



induced radioactivity: Radioactivity that is produced by nuclear reactions. See 

 reaction, nuclear. 



insensitive time (counters) : See dead time. 



integral dose (volume dose) : A measure of the total energy absorbed by a patient 

 or any object during exposure to radiation. According to British usage, the integral 

 dose for X or gamma rays is expressed in gram-roentgens. 



integrating dose meter: Ionization chamber and measuring system designed for 

 determining the total radiation administered during an exposure. In medical radiol- 

 ogy the chamber is usually designed to be placed on the patient's skin. A device may 

 be included to terminate the exposure when it has reached a desired value. 



intensitometer : A device for determining relative X-ray intensities during radiog- 

 raphy in order to control exposure time. 



intensity (of radiation) : The amount of energy per unit time passing through a unit 

 area perpendicular to the line of propagation at the point in question. Often this term 

 is used incorrectly in the sense of dose rate. 



interchange : The mixing of tracer and added isotopic carrier such that the two par- 

 ticipate to the same degree in any chemical reaction, showing that mixing has occurred 

 in whatever chemical forms the tracer may have been originally distributed. 



