Geometry

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Manganese resolution
Low energy resolution
Fluorine resolution
Incomplete charge collection
Carbon resolution
Fe55 source
Changes with time
Geometry
Summary

What makes a good detector?

Geometry

To a first approximation, X-rays are emitted equally in all directions, therefore the collection efficiency is governed by the proportion of space intercepted by the detector active area (A). This in turn is proportional to the ‘solid angle’ which would be 2π steradians if all the X-rays in a hemisphere above the specimen were collected. If d is the sample-crystal distance, then solid angle = A/d² is a useful approximation for solid angles less than 0.2 steradians. EDS detectors are available with different sizes of crystals. The crystal size is often measured in area, 5mm², 10mm², 30mm², 50mm² etc. There is a trade off in performance; normally the larger the crystal, the worse will be its resolution, particularly at low energy.

The collection efficiency is more sensitive to the distance between crystal and the sample, and for maximum efficiency the detector must be positioned as close to the sample as possible. A 10mm² crystal at a distance of 5cm will have the same solid angle as a 30mm² crystal at 8.7cm. The distance the crystal will be from the source of X-rays on the sample will depend on the dimensions and design of the detector and also the geometry of the microscope (Fig. 12a). The type of collimator and window assemblies used also affect the solid angle. The requirement for an electron trap places the crystal further from the sample (Fig. 12a), and the need for a support grid for the window reduces the active area A by typically 20% compared to self-supporting thin windows (Fig. 12b). Therefore if count rate is an issue, when considering crystal size it is much more useful to know what the maximum solid angle will be on the microscope to be used, rather than just the crystal area. For most microscopes sufficient solid angle can be achieved with 10mm². In some situations, for example when using a transmission electron microscope, the X-ray signal is very low, and a bigger crystal is used to improve signal collection.

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