292 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 9 



focus the beam to the smallest possible angular spread. Numerous com- 

 binations of electrodes have been used to achieve these effects, and two 

 examples are shown in Figs. 69 and 70. 



Permanent gases and vapors of volatile liquids are introduced directly 

 into the ionization chamber through capillary tubes in which the flow is 



Fig. 70. Ion source developed by Nier. Gas introduced at G is ionized in the chamber / 

 by stream of electrons emitted from filament F. The electron stream is defined by slot C 

 and by the magnetic field produced by permanent magnets. Ions are accelerated and 

 focused into a narrow beam by slit system S. Electrical leads are brought into the vacuum 

 tube through Kovar seal K. [A. O. Nier, Rev. Sci. Instruments, 18, 398 (1947).] 



regulated by a valve or throttling device [9-11]. Solid substances can be 

 analyzed usually by one of several methods listed below. 



Sublimation or Distillation: Some metals, e.g., lithium, calcium, aluminum, 

 and magnesium, and certain compounds of less volatile elements, usually in 

 the form of solids, can be distilled from a hot filament or furnace in or near 

 the ionization chamber. 



Sputtering: Substances that sputter readily will frequently give a measur- 

 able spectrum when a small quantity is inserted in the ionization chamber 



