Energy-dispersive X-ray spectroscopy

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Energy dispersive X-ray spectroscopy (EDX or EDS) is a method used to determine the energy spectrum of X-ray radiation. It is mainly used in chemical analysis, in an X-ray fluorescence spectrometer (especially portable devices), or in an electron microprobe (e.g. inside an scanning electron microscope).

The detector is a semiconductor detector, usually a Silicon Drift Detector or a silicon crystal doped with lithium (Si(Li) detector). The semiconductor is polarised with a high voltage; when an X-ray photon hits the detector, it creates electron-hole pairs that drift due to the high voltage. The electric charge is collected, it is like charging a capacitor; the increment of voltage of the condensator is proportional to the energy of the photon, it is thus possible to determine the energy spectrum. The condensator voltage is reset regularly to avoid saturation.

To reduce the electronic noise, the detector is cooled by Peltier effect or best by liquid nitrogen.

In recent years a new type of EDS detector has become commercially available based on a superconducting microcalorimeter. This microcalorimeter spectrometer has the simultaneous detection capabilities of the EDS combined with the high spectral resolution of the WDS. Unlike the semiconductor EDS the microcalorimeter measures the temperature change caused by the absorption of the x-ray photon in the detector, as such the detector must be maintained at ultra low temperatures (~100mK) by the use of liquid helium and/or an adiabatic demagnetisation refrigerator (ADR). In essence the microcalorimeter is an efficient absorber well coupled to a very sensitive thermometer. The microcalorimeter EDS has suffered from a number of drawbacks compared with conventional detectors which scientists are now addressing, these include; low count rates, poor collection efficiencies and small detector areas. The count rate is connected to the thermal time constant of the calorimeter, i.e. it is equivalent to the "RC time" constant of an electrical circuit. The collection efficiency has to do with the absorber materials and the small area follows from the need to keep the heat capacity as small as possible in order to increase thermal sensitivity. These drawbacks have been overcome somewhat by the use of arrays of detectors and x-ray focusing optics.

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