GLOSSARY OF SELECTED TEKM8 IN NUCLEAR SCIENCE 433 



exposure of personnel; usually dental-size X-ray film, worn on the person and fre- 

 quently combined with an identification badge. 



fission products: The nuclides produced by the fission of a heavy-element nuclide 

 such as U-^^ or Pu'-'^ Thirty-five fission-product elements from zinc through gad- 

 olinium have been identified from slow-neutron fission. 



fluorescence : The emission of radiation of particular wavelengths by a substance 

 as a result of absorption of radiation of shorter wavelength. This emission occurs 

 essentially only during the irradiation. 



flux: (a) For electromagnetic radiation, the power, or energy per unit time, passing 

 through a surface. (6) For particles or photons, the number passing through a surface 

 per unit time, (c) As commonly used in nuclear physics, the product nv, where n is 

 the number of particles per unit volume and v is their mean velocity. 



free-air ionization chamber: An ionization chamber in which a delimited beam of 

 radiation passes between the electrodes without striking them or other internal parts 

 of the equipment. The electric field is maintained perpendicular to the electrodes m 

 the collecting region; as a result, the ionized volume can be accurately determined 

 from the dimensions of the collecting electrode and the limiting diaphragm. This is 

 the basic standard instrument for X-ray dosimetry, at least within the range from 



5 to 400 kv. 



free water : The amount of water removed in drying a solid to its equilibrium water 



content. 



gamma ray (7 ray) : A quantum of electromagnetic radiation emitted by a nucleus, 

 each such photon being emitted as the result of a quantum transition between two 

 energy levels of the nucleus. Gamma rays have energies usually between 10 kev and 

 10 Mev, with correspondingly short wavelengths and high frequencies. They are 

 often associated with alpha and beta radioactivity, following transitions that leave the 

 product nuclei in excited states; in general, they are more penetrating than alpha or 

 beta particles. They also occur in isomeric transitions and in many induced nuclear 



gas counter (radiation) : A counter in which the sample is prepared in the form of a 

 gas and introduced into the counter tube itself. 



gas-flow counter (radiation) : A counter in which an appropriate atmosphere is 

 maintained in the counter tube by allowing a suitable gas to flow slowly through the 



volume. . 



Geiger counter: Historically, a "point counter." By popular usage, a Geiger- 

 Mliller counter tube, or such a tube together with its associated electronic equipment. 



Geiger-Miiller counter tube, Geiger-Miiller tube. A gas-filled chamber usually con- 

 sisting of a hollow cylindrical cathode and a fine wire anode along its axis. It is oper- 

 ated with a voltage high enough so that a discharge triggered by a primary ionizing 

 event will spread over the entire anode until stopped by the reduction of the field by 



space charge. j u +1 



genetic effect of radiation : Inheritable changes, chiefly mutations, produced by the 

 absorption of ionizing radiations. On the basis of present knowledge these effects arc 

 purely additive, and there is no recovery. , , - ., 



geometry factor (radiation) : The average solid angle at the source subtended by the 

 aperture or sensitive volume of the detector, divided by the complete solid angle (47r). 

 Frequently used loosely to denote counting yield or counting efficiency. 



half -life (radioactive) : See radioactive half-life. 



half-thickness : The thickness of any given absorber which will reduce the intensity 

 of a beam of radiation to one-half its initial value. It may be expressed in units ol 

 length or of mass per unit area. 



