Biomedical Engineering Reference
In-Depth Information
3.1.1 Thermionic Sources
These are made from V-shaped metal filaments or tips composed of a single crystal
heated to a high temperature. The electrons extracted from the tip are acceler-
ated toward the sample by the electrical field between the cathode and the anode.
Generally speaking, the thermionic gun does not provide a very coherent beam.
This is due to the fact that the speed, and therefore the kinetic energy, of the emitted
electrons follows a Gaussian distribution, resulting in chromatic aberration.
Tungsten filaments (
T
=
1,346 K) tips require vacuums between 10
-3
and 10
-7
Pa. Lanthanum hexaboride
provides better coherence, resulting from the decreased source size. An LaB
6
filament is commonly used for conventional electron microscopy (Table
3.1)
.
=
3,346 K) and lanthanum hexaboride (LaB
6
) with (
T
3.1.2 Field Emission Guns (FEGs)
FEGs include thermally assisted, cold-cathode field emission guns, Schottky effect
and hot cathode guns.
Table 3.1
Characteristics of different electron sources
Thermal emission
Field emission
Schottky
type
ZrO/W
Thermal
emission
W(100)
Cold
emission
W(310)
W
LaB
6
10
5
(A/cm
2
/sr) at 200 kV
Brightness: 5
×
1
10
1,000
1,000
1,000
Source size
50
µ
m0
µ
m
0.1-1
µ
m
10-100
nm
10-100
nm
Energy window (eV)
2.3
1.5
0.6-0.8
0.6-0.8
0.5-0.7
10
-3
10
-5
10
-7
10
-7
10
-8
Operating
conditions
Vacuum
(Pa)
Temperature
(K)
2,800
1,800
1,800
1,600
300
Emission
Current
(nA)
1-10
50-100
1-10,000
1-10,000
1-10,000
Short-term
stability
(%)
1
1
1
5
7
Long-term
stability
(%/h)
1
3
1
6
5
Current
efficiency
(%)
100
100
10
10
1
Life span (h)
120
1,000
>3,000
>3,000
>3,000
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