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The Microanalysis System

What makes a Good Detector?

The Pulse Processor

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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?

Detector specifications based on tests using an Fe⁵⁵ radioactive source

Testing the performance of an EDS detector using a radioactive source is convenient for a detector manufacturer because it can be done without having to mount a detector on an electron microscope column. However, Fe⁵⁵ source specifications don’t necessarily guarantee the performance when mounted on a column and collecting X-rays emitted during electron bombardment. They are also limited in scope and do not reveal some important aspects of detector performance, in the real situation of electron microscope based X-ray microanalysis.

The reason that the resolution of a detector is traditionally specified for Manganese Kα X-rays at 5.895 keV, is because this is the energy of the most intense X-ray line emitted by the Fe⁵⁵ source (Fig. 10).

One useful feature of the Fe⁵⁵ source is the absence of the continuum or bremsstrahlung X-rays that would be generated in an electron microscope. Therefore, in an Fe⁵⁵ spectrum the very low intensity background at lower energies than the characteristic Mn X-ray peaks is due to incomplete charge collection. The most common measurement of ICC is the peak: background ratio comparing the height of the MnKα peak to the average background between 0.9-1.1keV (Fig. 10). Events appearing at 1keV correspond to MnKα photons where 83% of the charge has not been collected, whereas events appearing at say 4keV have lost 32% of the charge. Although the original IEEE standard suggested measurement at several energies, manufacturers have most commonly used 1keV. As shown in Fig. 10, 1keV is typically the lowest part of the ICC background and a low value at this energy (high peak:background ratio) does not guarantee that charge collection will be good for the smaller charge losses that contribute to the tail or plateau on the low energy side of a peak. Fe⁵⁵ peak:background only gives a general indication of charge collection efficiency, but does not ensure good peak shapes or give any indication of ICC for low energy X-rays. Incomplete charge collection is most important where X-rays are strongly absorbed and only penetrate a short distance into the crystal, for example just above the absorption edge of the elements making up the crystal (1.84keV for Si(Li) detectors), and for very low energy X-rays (less than 0.5keV). The CKα resolution measurement on the microscope represents a more useful and sensitive measurement of incomplete charge collection than peak:background ratios measured with an Fe55 source. Low values for CKα FWHM guarantee excellent charge collection, and good peak shapes at all energies.

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