3.4.2 Detecting negative ions
HV-
HV+
CP 400
+3.1kV
HV 702
-
+
0...-3.5kV
Fig. 6:
Detecting negative ions
Procedure
► With HV 702, apply +3.1 kV to the 1st dynode (HV-) for detection of negative ions (-3.1 kV for pos-
itive ions).
– The positive end (HV+) is thus connected to up to 6.6 kV.
► Instead of the EP 422 electrometer pre-amplifier, use the CP 400 ion counter.
– With it, you can detect from 1 impulse per 10 seconds (this equals an ion current of 10
-20
A) to
107 impulses per second. In this case, the post-acceleration energy is independent of the op-
erating voltage and thus of the set amplification.
3.4.3 SEV 218
The SEV 218 corresponds to the SEV 217, additionally it has a conversion dynode that is separate from
the dynode chain. This conversion dynode is connected to a fixed high-voltage source that is separate
from the operating voltage of the SEM (-6.3 kV from plug CD of the HV 702). The post-acceleration is
therefore independent of the set amplification. The high post-acceleration significantly reduces the mass
discrimination during the conversion. Other advantages also occur during ratio measurements (e.g. iso-
topes) with high dynamics.
HV-
HV+
EP
CD
...-6.3kV
HV 702
-
+
0...-3.5kV
CD
EP 422
Fig. 7:
SEV 218
3.5 Overview of the variants
3.5.1 Cathode materials
Tungsten (W)
is the standard cathode material. Thanks to the very low vapor pressure, it is particularly
suitable for UHV applications. However, at high carbon concentrations, tungsten forms carbides in the
gas mixture to be measured, which leads to unstable emission conditions (CO
2
cycle).
Yttriated iridium (YOx-Ir)
is relatively insensitive to air ingress, as iridium does not form oxides. The
emission temperature of oxide cathodes is lower than that of tungsten. Reactions with residual gas are
weaker because the ion source temperature remains low. The susceptibility to contamination can be
stronger when substances with a low vapor pressure are admitted.
Product description
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