ROENTGEN ABSORPTION HISTOSPECTROSCOPY 



135 



Roentgen Tube for Secondary Excitation. When the element 

 whose line radiation is desired cannot be soldered to the anode, the 

 oxide or the powdered metal must be used. However, the difficulties 

 of suitably incorporating these powders in the surface of the anode 

 led Engstrom to the use of secondary excitation outside of the high 

 vacuum. The resulting reduction in intensity was compensated in 

 some measure by the use of greater wattage and by obviating the 

 passage of the radiation through a window or membrane. 



The roentgen tube is illustrated diagrammatically in Figure 23. 

 The brass body (A) is 90 X 90 X 100 mm. The cathode filament 

 consists of a flat platinum spiral with an oxide coating. An iron 

 cylinder surrounds the filament to direct the electron stream. The 

 tube is evacuated through H, which is 42 mm. in diameter. The 

 surface of the anode {B) forms a 12° angle, and the porcelain tube 



Fig. 24. Spectrograph for microanalysis. 

 From Engstrom (1946) 



C insulates the anode. Adjustment of the anode position under 

 vacuum is obtained by a bellows arrangement (D). Water-cooling 

 is employed for the body of the tube, the cathode, anode, and slit G. 

 An aluminum foil window ( 9 fi thick.) is placed over the slit, and a 

 plug (P) is used to hold the element whose line radiation is desired. 

 This plug and all of the vacuum connections are sealed with rubber. 

 The composition of the anode is chosen to give the greatest yield of 

 secondary radiation from P. Hevesy has shown that the greatest 

 yield of characteristic radiation results when the incident linp radi- 



