82 APPLIED RADIOACTIVITY 



sulphate. The total thickness of the radium container and the wall of 

 the needle is about 0.5 mm. If platinum is used, this is a sufficient 

 thickness to absorb all beta rays. The internal cylindrical opening 

 has a diameter of 0.6 mm. The diameter of the needle is 1.6 mm. The 

 total overall length of the needle is 36.5 mm. If the radium chamber 

 is to be longer than 25 mm it is good practice to use two or more radium 

 tubes. Usually these contain 1 or 2 mg of radium each. 



In dermatology, fiat " applicators " are often used. They are in the 

 form of very shallow round or square boxes. The radium salt is either 

 placed under silver covers about 0.05 mm thick, to allow for considerable 

 beta-ray transmission, or mixed with a varnish-like base and packed 

 into the shallow opening, to give a uniform thickness of radiating ma- 

 terial. Applicators vary in radium content from 2.5 to 10.0 mg of 

 radium element per square centimeter. 



The intensity of the gamma radiation with increase in distance of 

 either of the above sources does not follow the inverse-square law, since 

 neither of them is a point source of radiation. Available data, however, 

 indicate that the relative intensity varies with size, shape, and changes 

 in distance from the skin. The emission characteristics are reliable, for 

 the quality of the radiation is always the same under identical physical 

 conditions. 



In practice, it has been found that gold radon seeds may be considered 

 point sources. When these seeds are used for interstitial irradiation 

 the amount of radiation passing through a unit area at any tissue depth 

 is modified by absorption and scattering in the tissue. Actually, accord- 

 ing to Quimby [1939], for the distances involved in practice, the absorp- 

 tion and the scattering approximately compensate each other, so that 

 the intensity of radiation within the tissues is about the same for a given 

 source as its intensity at the same distance in air. 



Tissue Reaction to Gamma Radiations 



If a given amount of radiation is administered slowly, either by con- 

 tinuous irradiation of low intensity or in fractions with rest periods of 

 some hours or days between exposures, it is less effective in producing 

 tissue changes than if it is delivered continuously at high intensity. 

 Normal living tissue can tolerate a larger dose if the destructive radiation 

 is administered slowly or administered in fractions with time intervals, 

 and this tolerance is attributable to the recovery or life process of the 

 type of cell radiated. Living tissue possesses dynamic recuperative 

 properties which are not possessed by non-living material. It appears 

 to resist the action of destructive agents. If the destructive radiation 



