78 APPLIED RADIOACTIVITY 



through the top of a fireproof lead-lined safe to the purification appara- 

 tus. The safe may be 32 by 18 by 16 in., with 5-in. walls. 



The whole purification plant is first well exhausted by a quick-action 

 vacuum pump. Then the radon gas accumulates over the solutions and 

 spreads into the lead-covered glass chamber S. The mercury trap T\ 

 must be lowered so as to allow the gas to fill the chambers A, B, C, D. 

 When T\ is raised the gas in A is pushed into the purification chambers 

 B, C, D. Here the accumulated and compressed gases are passed over 

 caustic potash to remove carbon dioxide. The chamber C contains a 

 heated tungsten filament surrounded by a well-oxidized copper gauze. 

 When the gases pass through this chamber the oxygen and hydrogen 

 combine to form water vapor, which is removed by the phosphorus 

 pentoxide tube D. This purification process takes about one-half hour. 



The purified gas is then allowed to expand into trap T 2 , from where 

 it is compressed into the capillary tubes. 



One gram of radium in solution produces about 15 cc of mixed gas 

 per day. It is well known that the alpha rays rapidly decompose water. 

 As a result the gas contains about 1 part radon to 2500 parts of hydrogen 

 and oxygen. The hydrogen and oxygen must be removed in order to 

 provide a sufficiently active radon supply to be compressed into the 

 capillary tubes. 



Formerly, when the capillary tubes were made of glass, the wall thick- 

 ness was 0.5 mm, and the tubes were placed in hollow silver needles for 

 radiation therapy. The present practice is to compress the radon into 

 gold capillary tubing without glass lining. The gold tubes, cut into 

 4- or 5-mm sections, are the " seeds " used in radium therapy. Their 

 external diameter is about 0.75 mm with wall thickness 0.3 mm. Plati- 

 num seeds from 5 to 6 mm long are also employed; their radioactive 

 strength varies from 1 to 5 mc. 



These seeds are standardized three and a half hours after they have 

 been sealed off at the radon plant. In this time the decomposition 

 products RaA, RaB, and RaC have reached their " transient " equilib- 

 rium (Table 11-13) with the radon, after which the activity decays 

 with the decay period characteristic of radon. The activity of the 

 seeds follows the usual decay law (see Table 11-14), decreasing in activ- 

 ity 22.08 per cent by the end of the first day and dropping to half strength 

 in 3.825 days. A common hospital practice is to have 20 to 50 seeds 

 available and to calibrate them daily when not in use. 



Radon Seed Implants 



If the seed is introduced into an incision in tissue, when its calibrated 

 strength is 1 mc, then Table 11-14 can be consulted to determine its 



