Vacuum Technology Know How - Triumf

Detectors
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Table 4.2: Detectors and their attributes
PrismaPlus
Faraday / C-SEM
Maximum possible
pressure, Faraday
10
Maximum possible
pressure, SEM / C-SEM
Maximum measuring
speed / amu
Bake-out temperature (max)
Counting operation
Detection of positive ions
Detection of negative ions
Counter option
-3 mbar
10 -5 mbar
2 ms
300
No
Yes
No
No
HiQuad with SEM 217
Faraday / SEM
10 -4 mbar
10 -5 mbar
125 μs
400
Yes
Yes
Yes
Yes
HiQuad with SEM 218
Faraday / SEM with
conversion dynode
10 -4 mbar
10 -5 mbar
125 μs
400
Yes
Yes
No
Yes
Extremely fast measurements are possible with the aid of secondary electron multipliers.
As can be seen from Table 4.2, the measuring speeds are significantly higher than with a
Faraday cup.
In addition to operation as current amplifiers, discretely designed SEMs are also suitable
as ion counters. Extremely low count rates of 1 ion per 10 s can be attained with this
configuration. High count rates are also possible, producing an extremely broad dynamic
range by comparison with operation as a current amplifier.
In the counting mode, the speed of the SEM serves as the upper limit of the dynamic range.
With a pulse width of 20 ns, non-linearity begins at a count rate of 106 events per second.
Given its pulse width, the SEM must be suitable as a counter.
What all secondary electron multipliers have in common is that they are restricted to operating
at pressures of less than 10 - 5 mbar. At pressures of more than 10 - 5 mbar, the layer of
water on the dynodes can lead to pyrolysis in operation, and thus to premature aging.
Due to the high voltages involved, gas discharges that could destroy the SEM can occur
at high pressures.
A C-SEM (Figure 4.17) is a continuous secondary electron multiplier, in which ions trigger an
electron avalanche through secondary electron emissions. It consists of a glass tube whose
interior is coated with a conductive layer that has high resistance and a low work function.
High voltage is applied to the layer in order to obtain a uniform voltage gradient throughout
the length of the tube. Ions from the quadrupole system are routed to the conversion dynode
and generate secondary electrons that trigger an electron avalanche in the tube. Current
amplification factors of 106 are attained at an amplification current of 2.5 kV.
Here, too, amplification and dark current govern the signal-to-noise ratio, and the maximum
current / dark current ratio of 106 the current amplification factor. Thanks to a C-SEM
arrangement that is slightly offset relative to the axis of the quadrupole, both a Faraday cup
as well as a C-SEM can be used next to one another in the analyzer, with changeover from
one detector to the other even being possible when necessary.
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