Oxford Instruments INCA Tip 1: November 2005

All modern Si(Li) detectors suffer from a little known problem of ice build up on the detector crystal. Ice forms on the cold crystal due to the migration and condensation of any water molecules in the detector vacuum. With today's low vacuum SEM's working with increasingly "dirtier" vacuums, the potential for this type of contamination is greatly increased.
	These contaminants will cause preferential absorption and a drop off in the sensitivity for low energy X-rays. There will also be degradation of the overall resolution of the detector.

One way to check for ice on your detector is to collect a spectrum from a piece of Cr metal at 20kV, and look at the L lines around 0.5keV. The L line spectrum consists of the Ll line at 0.5keV, and the La line at 0.571keV. The La 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 CrLa 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 La line should be higher than the Ll line (Fig.1). On a detector which has ice built up on the crystal the Ll line will be higher (Fig. 2).

This crystal contamination is an unavoidable consequence of the polymer window used in today's detectors, with which all detector manufacturers have to deal with. Significantly, Oxford Instruments is the only EDS manufacturer that has a patented conditioner that allows this crystal contamination to be removed without having to remove the detector from the microscope (as you would with other systems). This saves you valuable time and effort, keeps your detector in optimum condition, and more importantly, ensures that your results are consistent and accurate.

We recommend you check for ice build up using the test described above. If this test reveals the presence of ice then you should condition your detector.

Next month's Tip will explain the simple steps you need to follow to condition your detector.

If you would like to find out how to do this now, details are available on our web site at: http://x-raymicroanalysis.com/INCAtips

Fig. 1. Detector with no ice on the crystal
Fig. 1. Detector with no ice on the crystal

Fig. 2. Detector with ice on the crystal
Fig. 2. Detector with ice on the crystal