The Measurement of Radiation 177 



remains a very useful and often simpler alternative to the ioniza- 

 tion method when high accuracy is unnecessary — for example, 

 in the recording of stray radiation in questions of staff protec- 

 tion. Other methods, such as chemical methods, change of color 

 or fluorescence of salts, selenium cells, etc., proved unsatisfactory 

 and are of historical interest only. 



Again, comparative studies have been made by using some 

 standard biological test material, for example, Drosophila eggs, 

 but it is clear that far greater importance attaches to the more 

 fundamental problem of relating biological effects to the radia- 

 tion producing them, evaluated in precise physical terms. The 

 radiations hitherto most commonly met with are X-rays and 

 the gamma rays of radium, and they will, of necessity, occupy 

 most of our attention. 



X- and Gamma Radiation 



Quantum Character and Interaction with Matter 



These radiations are different examples of essentially the 

 same type of radiation, and it may not be out of place to 

 state briefly some of the most important facts relating to their 

 interaction with matter. The radiation is electromagnetic in 

 character, propagated with the speed of light. For our purpose, 

 it is best to concentrate on the quantum character of the radia- 

 tion, i.e., the energy of the beam of radiation is concentrated 

 in discrete units rather like a hail of bullets, the amount per 

 unit being given by Einstein's equation 



E=hv 



c 



where h is Planck's universal constant, and v = — , where v, X 



and c are the frequency, wave length and velocity of the radia- 

 tion, the latter also being a universal constant. These quanta, 

 or photons, interact with matter in several different ways : 



1. ''Unmodified/' or Thomson scattering. A quantum is 

 merely deflected from its course without loss of energy by an 



