CHAPTER 5 



Radioactivity; Biological Tracers 



Our sensory data, even with complex equipment, consists of flashes of 

 light, of the rates of discharge of an electroscope, of audible clicks or totals 

 from an automatic counter, of tracks of liquid particles in a small chamber, 

 of the deposit of silver grains on a photographic film, of heat evolved, of 

 certain color changes. From these simple observations scientists have al- 

 ready created a complex and exciting description of particles far too small 

 to be seen directly (Miner, Shackelton, and Watson. 3 ) 



INTRODUCTION 



Properties of the Emanations 



In 1897, we entered the golden age of nuclear physics. It was then that 

 Becquerel, experimenting with pitchblende, which is fluorescent, acciden- 

 tally discovered a new and exciting emanation from the material. The 

 emanation was rather penetrating (through his desk-top), and darkened 

 some photographic plates kept in a drawer below. The Curies extracted the 

 element which gave rise to the activity — radium — and called the emanation 

 "radium-activity/' from whdch we derive the modern name, radioactivity. 

 Chapter 4 has already described how three components were isolated from 

 one another by Rutherford, and named alpha, beta, and gamma rays. The 

 relevant properties of each as determined from scattering experiments, etc., 

 are gathered in Table 5- 1 . 



It is the penetrating properties of these radiations with which we are now 

 primarily concerned. However, to understand penetrating properties of 

 radiations from any radioactive source, we must first understand their origin 

 (i.e., in the atomic nucleus) and their absorption, as well as the methods 

 used to detect them, to identify them, and to measure their energy. 



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