Element distribution

To create element distribution images with EDX, so-called ROIs (regions of interest) are selected in the EDX spectrum which correspond to the characteristic x-ray lines in the elements of interest.

All x-ray impulses whose energy falls into such an ROI are allocated to an element and displayed in a gray-scale image or a false color image. The lighter an area appears in the image, the higher the concentration of this element in this area. For the display, it is also possible to summarize several element distribution images in an RGM image.

The element distribution images or mappings can be made in various image resolutions (128x128 pixels to 1024x1024 pixels). Of course, better resolutions also require longer measuring times. To keep the measuring time short and, at the same time, also obtain good statistics, a suitable high counting rate is needed on the EDX detector. This is achieved with high beam currents.

The mappings shown in the pictures were made with a Si(Li) detector with a measuring time (dwell time) of 100ms per measuring point. This means that the total time for such a mapping is around 2 hours.

Higher pulse rates can be processed with a silicon drift detector (SDD). The measuring times required for a comparable element distribution image are reduced to around 10-20 minutes.

The further development of the element distribution images is so-called spectral imaging. Here, an entire EDX spectrum is recorded and saved at each measuring point. This means that, even after the distribution images have been recorded, elements which were not previously considered important can be displayed later or element distributions can also be evaluated semi-quantitatively.

Further references:
Electron probe micro analysis
Scanning electron microscope
Energy dispersive x-ray spectrometry
Wave-length dispersive x-ray spectrometry

  • Figure 1: Element distribution images with EDX using an aluminum casting alloyas an example
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