70 APPLIED RADIOACTIVITY 



When a flat radium salt " applicator " possessing a cover impervious 

 to alpha rays is applied to the skin under clinical conditions, it is of 

 importance to know the filter thickness that must be used to remove all 

 the low-velocity beta particles in order to control the therapeutic effects. 



It has been found that 1 mm of aluminum reduces a 100 per cent 

 electron intensity to 1.3 per cent, indicating that probably only the 

 high-velocity electrons from RaB and RaC are coming through. The 

 data of Table II-5 obtained by E. H. Quimby [1939] show that for all 

 practical purposes a 2-mm brass, 1.0-mm silver, 1.0-mm lead, 0.5-mm 

 gold, or 0.5-mm platinum filter will remove all the beta rays emitted 

 by a radium source. 



Energy of Gamma Radiations 



Radium will attain its radioactive equilibrium with its products in 

 about one month. Under these conditions it will emit about 88.8 per 

 cent of the radioactive energy as alpha rays, 4.5 per cent as beta rays, 

 and 6.7 per cent as gamma radiation. The short-wavelength gamma 

 radiation is primarily due to the disintegration of RaB and RaC. Of 

 these two, the gamma radiations from RaB are much more easily ab- 

 sorbed than those from RaC. The gamma radiations from RaD are 

 also comparatively soft, and moderate filtering removes them completely. 



TABLE II-6 



Gamma Rays from RaB — > RaC. Most Intense Radiations 



Composite results from Ellis and Aston, Proc. Roy. Soc, A 129, 180, 1930. 



• Gamma radiations are comparable to x-rays but of shorter wavelength. 

 Table II-6 shows some of their wavelengths and their equivalent ener- 

 gies in electron volts. They range from 50 to 1400 kv. An x-ray tube 



