454 



JOHN \V. G O W E N 



selected for the intense characteristic radiation that it emits in a 

 limited wavelength region, e.g., a copper anode has characteristic 

 wavelengths near 1.5 A. Gas-filled X-ray tubes can be made self- 

 rectifying, thus requiring only a transformer in their electrical cir- 

 cuit, but the difficulty of maintaining satisfactory air pressure makes 



Fig. 1. Self-rectifying gas type X-ray tube. (Courtesy L. E. Pinney, 47). 



COOLING WATER INLET 



!. -g. \v. ^^^ ■«. vv vt. ^'^ '\ a 



■J. t^t v^ ■■■.■ »L w. V'\ w. ■» ■a.v v-i, w <» ^^^. ^» ^ 



t I r I t J I I -r-r I itriiiiiiiiJii 



rwrritiiiwrriiiiitt r- r-r~^ 



■^ '\'- ^■^. ^\ ■». ^ ^v ^■l. isr 



TUNGSTEN TARGET 

 BERYLLIUM WINDOW 



Fig. 2. Filament type X-ray tube, evacuated and sealed {56). (Courtesy 



Marhlett Laboratories, Inc.) 



gas tubes undesirable for occasional laboratory work except where 

 very large dosage rates are essential to an experiment. Gas tubes 

 have a further limitation in that they will not operate at voltages 

 higher than 50-60 kilovolts. This restricts their use in experimental 

 work since the operating voltage determines the short wavelength 

 limit of the X rays emitted. 



Coolidge type X-ray tubes (see Fig. 2) with sealed vacuum have 

 almost entirely replaced gas-filled tubes for most laboratory work as 



