Sec. 9.8] 



MASS SPECTROGRAPHS 



291 



current or direct current. Collimation of the electron stream is obtained 

 with the aid of small electro- or permanent magnets which provide a field 

 of at least several hundred gauss in the region of the ionization chamber. 

 Electrons emitted from the filament move freely along the lines of force but 

 have low mobility in a transverse direction. On the other hand, ions, because 

 of their greater mass, can cross the field with relative ease when leaving the 

 chamber. Electrons, therefore, leave the filament only in the direction of 



^ 





0, 



-f IT 



Fig. 69. Simple ion source. Gas introduced through the tube G is ionized in the chamber / 

 by the stream of electrons (vertical broken line) emitted from the filament F maintained at 

 75 to 150 volts negative with respect to /. The electron stream is defined by the magnetic 

 field // and collimating slot C. Ions that diffuse to the exit slit are accelerated and focused 

 by slits S. [W. Siri, Rev. Sci. Instruments, 18, 540 (1947).] 



the anode or electron catcher to which they are accelerated by a potential 

 of about 100 volts. In nearly all cases, the total electron emission is less 

 than 1 ma. Of the fraction of electrons that enter the chamber through the 

 collimating slot, a large proportion either is lost to the walls by repeated 

 collisions with gas molecules and ions or is captured by ions. The remaining 

 electrons pass through the chamber to be collected at the anode. Filaments 

 are operated emission limited and require good thermal and emission regula- 

 tion to ensure stable and consistent operation since the ion-beam current is a 

 rather sensitive function of the emission current. 



The slit system through which the ion beam passes serves both to accelerate 

 the ions to their final velocity before entering the analyzing fields and to 



