Several types of cathode are availa

Several types of cathode are available (Goldstein et al. 2003). The cheapest and most commonly
used electron source is a heated tungsten wire hairpin filament. This thermionic emission source
is heated directly by passing a high current through it and has a typical operating life of up to
about 100 h, but is not as bright (i.e., it has a low electron yield) as other electron sources. A much
brighter electron source is a pointed, single crystal of lanthanum hexaboride (LaB6) that is heated
indirectly. LaB6 emitters have a life expectancy of about 1000 h and they must be operated in a
vacuum that is much higher than that needed for a tungsten wire cathode.