X-ray mapping using a multiple-EDS (DUAL) detectors

X-ray mapping (XRM) is an extremely useful problem solving tool. However, the two major problems for energy dispersive spectroscopy are interpretation of results under non ideal conditions (strong overlap and small peak size relative to background), and the time required to obtain a good quality 256x256 pixel map (1 to 3 hours). XRM has been considered a slow technique, claiming many hours of SEM time and often relegated to out-of-hours (overnight) mapping. With the development of high count rate silicon high resolution drift detectors (SDD) [1, 2] and multi-detector systems [3], the time required to acquire XRM decreases. With a single EDS detector at 20kcps output, a good 512x512 quantitative map can be obtained in around 4 to 8 hours for major elements (>10wt% evenly distributed) and minor elements (>1wt% localised). With the development of the SDDs, X-ray mapping at count rates of 100kcps to 1000kcps output are possible, thus allowing 256x256 X-ray maps in around 3 to 15 minutes, depending on the count rate. The accuracy of the EDS results and the quality of the X-ray maps depends on the elements present in the sample. If there are no overlaps between elements in your sample, then these higher count rate detectors can be used to map qualitatively. For quantitative work, it is still not desirable to use these detectors at the higher count rate, as the spectra are of poor resolution (190 to 250eV) with other artefacts. It is now possible to quantitatively map using multi-detectors and combined detectors such as WDSEDS, hence not only improving count rate capability but also improving the ability to map trace elements very accurately [3]. Mapping with multiple detectors enables better quality maps to be obtained through use of lower beam currents, thus improving the map quality as well as minimizing other problems such as beam drift. The combination of new detectors and multi-detector systems now opens up new possibilities in high speed X-ray mapping. Using a single detector at higher count rates can lead to large beam currents, higher pulse pile up, loss of energy resolution, gain and zero drift, higher dead times, specimen damage, specimen charging and poorer image resolution.