GLOSSARY OF SELECTED TERMS IN NUCLEAR SCIENCE 435 



ion: Any electrically char}2;cd particle of molecular, atomic, or nuclear size, 



ion density: The luimher of ion pairs per unit volume. 



ion exchange: A chemical process iiivolviuj; tlie reversible interchange of ions 

 between a solution and a particular solid material such as an ion-exchaiige resin, con- 

 sisting of a nuitrix of insoluble material interspersed witii (ixeti ions of opposite charge. 



ionization : The process whereby a neutral atom or molecule is split into positive and 

 negative ions. 



ionization chamber: An instrument designed to measure the quantity of ionizing 

 radiation in terms of the charge of electricity associatetl with the ions produced within 

 a defined volume. This is usually done by measuring the quantity of electricity in the 

 form of ions carried to the chamber electrodes when a suitable potential difference is 

 estal)lished In^tween them. 



ionization density: Xumber of ion pairs per unit volume. 



ionization path (or track) : The trail of ion pairs produced by an ionizing particle in 

 its passage through matter. 



ionizing radiation: Any electromagnetic or particulate radiation capable of produc- 

 ing ions, directly or indirectly, in its passage through matter. 



irradiation : Exposure to radiation. One speaks of radiation therapy, but of irradia- 

 tion of the patient. 



isomer, nuclear : One of two or more nuclides having the same mass muiiber A and 

 atomic number Z, but existing for measurable times in different states. The state 

 of lowest energy is the ground state. Those of higher energies are metastable states. 

 To indicate the metastable isomer, the letter m is added to the mass number in the 

 symbol for the nuclide; thus, Br'*"'". 



isomeric transition : The process by which a nuclide decays to an isomeric nuclide 

 (i.e., one of the same mass number and atomic number) of lower quantum energy. 

 Isomeric transitions, often abbreviated IT, proceed by gamma-ray and/or internal- 

 conversion electron emission. 



isotope : (a) One of several nuclides having the same number of protons in their 

 nuclei and hence belonging to the same element, but differing in the number of neu- 

 trons and therefore in mass number A. (6) A synonym for isotopic tracer, (c) A 

 radionuclide or a preparation of an element with special isotopic composition as an 

 article of commerce, so called because of the principal use of such materials as isotopic 

 tracers, {d) In common usage, a synonym for nuclide. 



isotope-dilution analysis : A method of chemical analysis for a component of a mix- 

 ture based on the addition to the mixture of a known amoimt of labeled component of 

 known specific activity, followed by isolation of a quantity of the pure component and 

 measurement of the specific activity of that sample. 



isotope effect : The effect of the difference in mass between isotopes on the rate 

 and /or equilibria of chemical transformations. 



isotopic exchange : (a) A process whereby atoms of the same element in two different 

 molecules or in different sites in the same molecule exchange places, {h) The transfer 

 of isotopically tagged species, without net chemical reaction, from one chemical form 

 or valence state of an element to another. This may come about by exchange of 

 tagged atoms, by exchange of other atoms in the chemical complex, or by transfer of 

 electrons. 



K-electron capture: Electron capture from the K shell l)y the micleus of the atom. 



kilocurie : 1000 c; symbol kc. 



kilo-electron-volt: 1000 ev; symbol kev. 



labeled compound: A compound consisting, in part, of labeled molecules. By 

 observations of radioactivity or isotopic composition, this comjjound or its fragments 

 may be followed through physical, chemical, or liiological processes. 



