Spectrometer
Please choose a moderate end value of the detector voltage scan to prevent rapid
aging of the detector!
The optimum operating point is about 50 - 100 V beyond the plateau of the curve (fig-
ure 15, page 29). As the multiplier ages, the knee moves to the right and the voltage
must be increased. In general, CEM lifetime (see figure 16, page 30) depends on the
number of counted electrons, thus it will vary as a function of specific application and
environment, but is typically on the order of about one year at 40 hours operation per
week.
For multi channel detection systems each channel should be displayed. The start value
for the detector voltage for counting depends on total number of accumulated counts
(see section 3.5.3.1 “Extended CEM” on page 29) and the threshold level of the
preamplifier. The sensitivity of the preamplifier channels can be varied using a discrim-
inator threshold (the value is factory preset, recommended discriminator voltage see
section 9.1.1 “Discriminator” on page 79). Large differences between the channels can
be equalized by setting the threshold for the channels individually. These differences
may be caused by the varying sensitivities of the CEM’s. Pay attention to the detector
voltage value in the Specification Report sent with the analyzer.
The detector check should be done monthly (choose kinetic energy of 400eV).
28 PHOIBOS
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