442 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



thimble ionization chamber: A small cylindrical or spherical ionization chamber 

 usually with walls of organic material. ' 



thin-window counter tube, thin-wall counter tube : Counter tubes in which a portion 

 ol the enclosure is of low absorption to permit the entry of short-range radiation 



threshold dose: The minimum dose that will produce a detectable degree of any 

 given effect. -^ 



tissue dose : Dose received by a tissue in the region of interest. In the case of 

 X rays and gamma rays, tissue doses are expressed in roentgens. At the present time 

 there IS no generally accepted unit of tissue dose for other ionizing radiations In 

 radiobiological studies it is customary to think of the tissue dose in terms of the energy 

 absorbed per gram of tissue. Several units related to the roentgen have been sug- 

 gested, such as the rep. ' 



tissue-equivalent ionization chamber: Ionization chamber in which the material of 

 the walls, electrode, and gas are so selected as to produce ionization essentially equiv- 

 alent to that characteristic of the tissue under consideration. In some cases it is suffi- 

 cient to have only tissue-equivalent walls, and the gas may be air, provided the air 

 volume IS negligibly small. The essential requisite in this case is that the contribution 

 to the ionization in the air made by ionizing particles originating in the air is negligible 

 compared with that produced by ionizing particles characteristic of the wall material 



tissue-equivalent material : Material made up of the same elements in the same pro- 

 portions as they occur in some particular biological tissue. Such material is important 

 in the construction of ionization chambers for neutron measurement In some cases 

 the equivalence may be brought about sufficiently closely without exact duplication of 

 the elemental composition of the tissue. 



tolerance dose : A term based on the assumption that an individual can receive such 

 aldose of radiation without any harmful effects. It is now superseded by permissible 



trace: (a) A minute quantity. (6) A small quantity detectable only by special 

 techniques, (c) An adjective pertaining to such minute quantities 



trace chemistry: The chemical behavior of a substance present in a system in 

 imponderable amount or in very minute concentration. Trace chemical behavior is 

 observable with radioactive nuclides free or nearly free of nonradioactive isotope and 

 may be markedly different from the behavior of the same material in ordinary amount, 

 and concentrations. Knowledge of trace chemical behavior may therefore be of 

 great importance in the handling of radioactive materials. Characteristic features of 

 trace chemical behavior include the prominence of surface effects (see adsorption) 

 mcorporation of the trace material in bulk precipitates (see carrier), and dissociation 

 (e.g., the stability of bromine atoms in aqueous solutions of trace bromine) 



tracer, isotopic : (a) The isotope or nonnatural mixture of isotopes of an element 

 which may be incorporated into a sample to make possible the observation of the 

 course of that element, alone or in combination, through a chemical, biological, or 

 physical process. The observations may be made by measurement of radioactivity 

 (in the case of radioactive tracer) or of isotopic abundance, e.g., by mass-spectro- 

 graphic or density measurement, (b) Material in which isotopic tracer has been 

 incorporated. 



track : Visual manifestation of the path of an ionizing particle in a cloud chamber or 

 nuclear emulsion. 



transmutation : (a) Any process in which a nuclide is transformed into a different 

 nuclide. (6) More specifically, the transformation of a nuchde into a nuclide of a 

 different element by a nuclear reaction. 



trauma : Injury. 



