Scanning Electron Microscopy and X-Ray Microanalysis

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position, which you should test to ensure that the positioning
is reproducible between insertions. Manual mechanisms usu-
ally provide a threaded screw with a manual crank to pull the
detector in and out. The threaded screw will usually have a
pair of interlocking nuts which can be positioned to define a
consistent insertion position. The procedure in the sidebar
below will allow you to set and maintain a constant solid
angle and thus also a consistent detector collection efficiency.

z Sidebar: Setting a Constant Detector-to-Sample
Distance to Maintain Solid Angle

1. Locate the pair of lock nuts on the screw mechanism.
   Move the lock nuts to the inner most position on the
   treaded rod.


2. Insert the detector as close to the sample as possible.
   Ensure that the detector does not touch the interior
   of the microscope. The detector snout must be
   electrically isolated (no conductive path) from the
   interior of the chamber to eliminate noise caused by
   electrical ground loops.


3. Twist the upper lock nut to limit the motion of the
   detector towards the sample. Tighten the lower nut
   to lock the upper nut into position.
   4. Test the reproducibility of the insertion point by
   extracting and inserting the detector and collecting a
   series of spectra. If the characteristic peak intensities
   are reproducible (to much better than a fraction of a
   percent) between insertions, the precision is adequate.

The take-off angle is the angle at which X-rays exit a flat sam-

ple in the direction of the detector. For a flat sample mounted

perpendicular to the electron beam at the optimal working

distance, the take-off angle equals the elevation angle. If the

sample is tilted or the sample surface is at a slightly different

working distance, then the take-off angle can be computed

from the sample tilt, the working distance and the sample-to-

detector distance. This is shown in. Fig. 16.13. Often you

will hear the terms elevation angle and take-off angle used

interchangeably. It is more precise however to think of the

elevation angle as being a fixed property of the instrument/

detector geometry and the take-off angle being dependent

upon instrument-specimen configuration.

z Check 1: Verify the Elevation Angle

It is critical that your quantitative analysis software has the

correct elevation/take-off angle. Matrix correction algorithms

use the take-off angle to calculate the correct absorption

Y

X

Z

Axes

Sample

Elevation angle

Sample tilt

Objective

lens

Optimal work

ing distanc

e

Actual work

ing distanc

e

C

L

ψ:

EDS detec

tor snout

. Fig. 16.13 The take-off angle
is a function of the elevation angle,
the sample tilt and the actual
working distance
16. 3 · Practical Aspects of Ensuring EDS Performance for a Quality Measurement Environment