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What
makes a good detector?
Detector
performance changes with time
A major cause of detector
degradation over time is the build up of contaminants that absorb X-rays
before they can be detected by the crystal. Common examples include the
condensation of oil on the collimator or detector window and ice forming on
the face of the crystal. These contaminants will cause preferential
absorption and a drop-off in sensitivity for low energy X-rays.
Ice forms on
the cold crystal due to the migration and condensation of any water
molecules in the detector vacuum. Two sources of water vapor exist:
impurities present during manufacture, and molecules that migrate through
the window when the detector is exposed to high vapor pressures. Modern
manufacturing techniques mean that when installed, a detector vacuum should
be free of water molecules. In SEMs where variable vacuum or environmental
modes are used, the detector spends time in conditions where water is
present in the microscope chamber. Some types of polymer thin window, which
are predominant in modern EDS detectors, have been shown to degrade and
become porous under conditions where water molecules are present.
A gradual
decrease in low energy sensitivity over time will result in a decrease in
the height of peaks at low energy. This can be checked by monitoring the
relative height of K and L lines from a transition metal element. The ratio
of L to K line heights from pure nickel measured at 20kV is a common test
used. A more sensitive test for the presence of ice on a crystal is to look
at the L spectrum from pure Cr. The L line spectrum consists of the Ll line
at 0.5keV, and the Lα line at 0.571keV. The Lα line is on the high energy
side of the oxygen absorption edge (energy 0.531keV) whilst the Ll line is
on the low energy side. Therefore CrLα X-rays are much more efficiently
absorbed by ice than CrLl X-rays. On a detector with little or no ice on the
crystal face the Lα line should be higher than the Ll line (Fig. 11b). On a
detector which has ice built up on the crystal the Ll line will be higher
(Fig. 11a).
The ice must be removed to regain the light element
sensitivity of a detector. This can be done in two ways. If the detector is
thermally cyclable it can be allowed to warm up. When the crystal starts to
warm the ice will sublime and the vapor will disperse into the vacuum. When
the detector is cooled down again the water vapor will normally condense
within the dewar because this area cools down first.
This technique is
time-consuming, requiring the liquid nitrogen reservoir in the dewar to be
exhausted which can take a number of days, or to speed up the process, the
detector can be removed from the column and the nitrogen poured out. Some
detectors have a built-in heating circuit called a conditioner. This circuit
warms up the crystal enough to sublime off any ice. Conditioning can be done
whilst the detector is cooled down on the microscope column, and can remove
any ice in as little as 2 hours.
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