Energy-dispersive X-ray Spectroscopy - Emerging Technology

Emerging Technology

There is a trend towards a newer EDS detector, called the silicon drift detector (SDD). The SDD consists of a high-resistivity silicon chip where electrons are driven to a small collecting anode. The advantage lies in the extremely low capacitance of this anode, thereby utilizing shorter processing times and allowing very high throughput. Benefits of the SDD include:

1. High count rates and processing,
2. Better resolution than traditional Si(Li) detectors at high count rates,
3. Lower dead time (time spent on processing X-ray event),
4. Faster analytical capabilities and more precise X-ray maps or particle data collected in seconds,
5. Ability to be stored and operated at relatively high temperatures, eliminating the need for liquid nitrogen cooling.

Because the capacitance of the SDD chip is independent of the active area of the detector, much larger SDD chips can be utilized (40 mm2 or more). This allows for even higher count rate collection. Further benefits of large area chips include:

1. Minimizing SEM beam current allowing for optimization of imaging under analytical conditions,
2. Reduced sample damage and
3. Smaller beam interaction and improved spatial resolution for high speed maps.

In recent years, a different type of EDS detector, based upon a microcalorimeter, has become commercially available. This new model allegedly has the simultaneous detection capabilities of EDS as well as the high spectral resolution of WDS. The EDS microcalorimeter relies highly on two components: an absorber, and a thermistor. The former absorbs X-rays emitted from the sample and converts this energy into heat; the latter measures the subsequent change in temperature due to the influx of heat (in essence, a thermometer). The EDS microcalorimeter has suffered from a number of drawbacks; including low count rates, poor collection efficiencies and small detector areas. The count rate is hampered by its reliance on the time constant of the calorimeter’s electrical circuit. The collection efficiency is a function of the absorber material and remains to be optimized. The detector area must be small in order to keep the heat capacity as small as possible and maximize thermal sensitivity (resolution).