The Measurement of Radiation 179 



imparted to fast electrons, the so-called "corpuscular emission." 

 It is these swift secondary electrons which actually ionize and 

 excite the atoms and molecules of the medium. 



The Concept of Quality 



The "quality" of a beam of radiation refers to its intrinsic 

 characteristics such as wave length, or quantum energy. It may 

 be investigated exactly by spectrographic methods (crystal 

 diffraction ) or by measurements of the energy of the secondary 

 electrons produced in matter. A quick practical method, par- 

 ticularly useful for approximate results with heterogeneous 

 beams, is to measure the absorption or attenuation of the radia- 

 tion in some suitable standard substance, from which an average 

 or effective wave length of the radiation may be estimated. 

 Thus, it is usual to quote the half-value layer (HVL) of a 

 given beam of X-rays in aluminum, or copper, i.e., the thickness 

 of material required to reduce the "intensity" (dose rate, see 

 below) to one half. 



By suitable developments of these principles, it is possible in 

 some cases to form an estimate of the effective wave length 

 of the diffuse radiation produced during the passage of a beam 

 through matter. Thus, the measurement of "quality" is achieved 

 by the application of familiar physical ideas and need not be 

 dealt with here in detail. It may be mentioned in passing, that 

 the particular aspect of quality of greatest biological significance 

 is the spacing of the ions along the tracks of the ionizing par- 

 ticles, the "ion density." As the energy of the ionizing particle 

 becomes less, the shorter the interval between successive ions. 



The Concept of Quantity or Dose 



When we come to the question of "quantity," it is necessary 

 to break new ground. Normally, "amount" of radiation is ex- 

 pressed in terms of intensity, defined as quantity of energy 

 flowing through unit area of the beam per unit time, but any 

 arbitrary measure of "amount" related to this, however in- 



