THE MEASUREMENT OF RADIATION 



G. J. NEARY, M.A., Ph.D. 



Physics Department, Mount Vernon Hospital and 



The Radium Institute, ISorthwood, Middlesex 



Introduction 



A COMPREHENSIVE discussion of the whole of the vast 

 field which might be implied in the above title is clearly 

 out of the question here, so the present remarks will be 

 arbitrarily confined to the subject of ionizing radiation, around 

 , which most interest is centered in the present context, leaving 

 aside entirely the question of ultraviolet, infrared, and "short 

 wave" radiations, which are of no less importance in biology 

 and therapy. 



By "ionizing radiations." we mean those types of radiation 

 which in their interaction with matter are able, by virtue of 

 their high intrinsic energy, actually to disrupt the individual 

 atoms or molecules by the splitting-off of an electron. The 

 electron thus set free quickly attaches itself to some other mole- 

 cule, and so, dispersed among the normal electrically neutral 

 molecules, there appear positively and negatively charged mole- 

 cules or clusters known as ions, which may exist independently 

 in the medium for considerable lengths of time, and endow it 

 with the property of electrical conductivity. 



If left to themselves, the ions will gradually neutralize each 

 other, but the exact status quo may not be restored, for obviously 

 the chance that various types of atomic and molecular rearrange- 

 ment, i.e., chemical change, will occur is considerable. It is 

 believed that such changes caused by ionization are the more 

 immediate causes of the biological efifects produced. On the 

 other hand, by the application of sufficiently large electric field, it 



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