RADIUM SALTS AS A SOURCE OF GAMMA RADIATION 81 



Quimby's [1928] data shows how the intensity decreased with distance 

 from such a seed buried in tissue. 



Some of Quimby's illustrations serve to show how the empirical data 

 represented by this curve can be used. 



Suppose that a preliminary experiment shows that a seed of 4 mc 

 will bleach butter to a radius of 3.5 mm. How many millicuries will 

 bleach butter to a radius of 5 mm? A distance from the center of the 

 seed marked la on the curve is the radius of a full bleaching dose, here 

 called 100 per cent intensity. Then la = 3.5 mm, or 5 mm = 1.43a. 

 The curve shows that, at the distance 1.43a, bleaching of 46 per cent is 

 produced. Thus x millicuries are necessary to produce 100 per cent 

 bleaching when 4 mc produces 46 per cent bleaching. Hence z/4 

 = 100/46, or Z =8.7 mc. 



It has been found that 110 mc-hr produces an erythema at 10 mm. 

 What is the erythema dose at a distance of 6 mm? Here la = 10 mm; 

 6 mm = 0.6a. The curve shows that 300 per cent intensity is equal 

 to 0.6a. Hence z/110 = 100/300, or x = 36.6 mc-hr. 



It has been found that a seed of 5 mc produced necrosis in a rabbit 

 muscle at a distance of 3. 1 mm. How far will a tube of 8 mc be effective? 

 Ans. 3.85 mm. 



Radium Salts as a Source of Gamma Radiation 



Radium sulphate may also be used as a source of biologically effective 

 gamma radiation. It was commercially available before 1911 at £20 

 to £25 per milligram. In 1911 it was quoted at £18. In America it 

 could be obtained for $120 per milligram in 1913 and for about $100 in 

 1914. With the discovery of the Canadian deposits, the price sank to 

 $70 in 1933. Recently it could be obtained at $20 per milligram. 



Fig. 11-10. Diagram of a typical radium needle. L, overall length of needle 

 36.5 mm; R, length of radium chamber 25 mm; E, length of eye 5 mm; P, length of 

 point 6.5 mm. 



For interstitial radiation the salt is placed in a capillary needle-shaped 

 tube containing from 1 to 25 mg of radium element. The radium tube 

 is then placed as a core in the hollow cylindrical needle, 1 to 6 cm long, 

 made of platinum or Monel metal. 



The construction of a typical needle is shown in Fig. 11-10. The 

 hollow metal cylindrical tube contains the cell filled with the radium 



