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RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



the nucleus of a helium atom. (6) By extension, the nucleus of a helium atom {Z = 2, 

 A = 4), especially when it is in rapid motion, as when artificially accelerated. 



alpha ray : A synonym for alpha particle. 



anemia: A condition in which the blood is deficient either in ciuantity or in quality. 

 The deficiency in quality may consist in diminution of the amount of hemoglobin or in 

 diminution of the number of red blood corpuscles. 



anticoincidence circuit : A circuit with two input terminals which delivers an output 

 pulse if one input terminal receives a pulse, but delivers no output pulse if pulses are 

 received by both input terminals simultaneously or within an assignable time interval. 



artificial radioactivity : (a) A term used to denote the phenomenon of radioactivity 

 produced by particle bombardment or electromagnetic irradiation, {h) The radio- 

 activity of synthetic nuclides. 



atomic energy: In popular usage, nuclear energy released in sufficient quantity to 

 be of engineering interest. 



atomic mass : The mass of a neutral atom of a nuclide. It is usually expressed in 

 terms of the physical scale of atomic masses, that is, in atomic mass units. The 

 atomic mass unit, amu, is exactly one-sixteenth of the mass of a neutral atom of 

 the most abundant isotope of oxygen, 0'«; 1 amu = 1.657 X IQ-^^ g = 931 Mev = 

 0.999728 awu. 



atomic number: An integer Z that expresses the positive charge of the nucleus in 

 multiples of the electronic charge e. In present theory, it is the numl)er of protons in 

 the nucleus and is also equal to the number of electrons outside the nucleus of the 

 neutral atom. 



atomic weight : The weighted mean of the masses of the neutral atoms of an element 

 expressed in atomic weight units. Unless otherwise specified, it refers to a naturally 

 occurring form of the element. The atomic weight unit, awu, is exactly one-sixteenth 

 of the weighted mean of the masses of the neutral atoms of oxygen of isotopic composi- 

 tion found in fresh lalce or rain water; 1 awu = 1.660 X lO^^^ g = 1.000272 amu. 



autoradiogram (radioautogram) : Record of radiation from radioactive material in 

 an object, made by phicing its surface in close proximity to a photographic emulsion. 

 average life (mean life) : The average of the individual lives of all the atoms of a 

 particular radioactive substance. It is 1.443 times the radioactive half-life. 



background : Ever-present effects in physical apparatus above which a phenomenon 

 must manifest itself in order to be measured. Background can take various forms, 

 depending on the nature of the measurement. In electrical measurements of radio- 

 activity and nuclear phenomena, the term usually refers to those undesired counts or 

 currents which arise from cosmic rays, local contaminating radioactivity, insulator 

 leakage, amplifier noise, power-line fluctuations, and so on. In nuclear work and 

 photographic emulsions, the term refers to developable grains unrelated to the tracks 

 under investigation. 



background counts: (a) Counts caused by any agency other than the one that it is 

 desired to detect. (6) Counts caused by radiation coming from sources external to 

 the counter tube other than the source being measured, or by radioactive contamina- 

 tion of the counter tube itself. 



backscatter : Scattering of radiation in a generally backward direction. In radiation 

 therapy, it is applied particularly to radiation scattered back to the skin from under- 

 lymg tissues. In the assay of radioactivity, it applies to the scattering of particles 

 into the measuring device by the material on which the sample is mounted. 

 barn: A unit of area for luiclear cross section; 1 barn = 10"^^ cm^. 

 beta decay, beta disintegration: Radioactive transformation of a nuclide in 

 which the atomic number changes by ±1 and the mass number remains unchanged. 



