Characteristics of Electron Source
- Practical Electron Microscopy and Database -
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Apart from a long lifetime, the main requirements of the electron source are high brightness (electron current density per solid angle) and small spread of the kinetic electron energy. This requires materials with a small work function.

Table 4963. Operating parameters and characteristics of electron sources

Type of source
Tungsten
thermionic
LaB6
thermionic
Schottky
emission
Cold field
emission
Material
W
LaB6
ZrO/W
W
ds (µm)
≈40
≈10
≈0.02
≈0.01
ΔE (eV)
1.5
1.0
0.5
0.3
φ (eV)
4.5
2.7
2.8
4.5
T (K)
1700-2700
1800
1800
300
E (V/m)
Low
Low
≈108
>109
Je (A/m2)
≈104
≈106
≈107
≈109
β (Am-2/sr-1)
≈109
≈1010
≈1011
≈1012
Vacuum (Pa)
<10-2
<10-4
<10-7
≈10-8
Lifetime (hours)
100
1000
104
104
Effective area Larger Smaller from a crystal vertex    
Coherence Lower Higher    

** ds -- Effective (or virtual) source diameter; ΔE -- Energy spread of the electron beam; φ -- Work function; T -- Operating temperature; E -- Electric field; Je -- current density of electron beam; β -- Electron-optical brightness at the cathode.

 

 

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