Energy-dispersive X-ray spectroscopy

Energy dispersive X-ray spectroscopy (English energy dispersive X - ray spectroscopy, EDX, EDRS or EDS, energy dispersive X and analysis, EDA called ) is an entity belonging to X-ray spectroscopy measurement method of material analysis. It excites the atoms in the sample by an electron beam of uniform energy, they then emit X-ray radiation of a specific to that element energy of the characteristic X-rays. This radiation provides information on the elemental composition of the sample.

Origin of the X-ray emission

For the emission of characteristic X-rays in the sample initially the atom must be excited. This can be done through the bombardment with electrons (e.g., by scanning electron microscopy ) or by X-ray irradiation (X-ray fluorescence). Here, an electron from an inner shell is ejected. Such a state is unstable, and the resulting "hole" is filled immediately by a higher energy electron from a higher atomic orbital. The difference in energy is released in the form of an X-ray quantum. Characterized the resulting X-radiation characteristic of the transition, and the atom, that is the element.

For an element different transitions are allowed, depending on which shell comes from the higher-energy electron and in which energy state ( cup) which is to fill the "gap ". This results in X-ray quanta that are marked with Ka ..., Kβ, L?, . The energy of an X-ray line ( position of the line in the spectrum ) is an indicator of which element it is. The "strength " of the line depends on the concentration of the element within the sample.

Further formed by deceleration of electrons in Coulomb the nuclei bremsstrahlung, which constitutes the continuous background of the EDX spectrum.

Operation of the detector

The detector measures the energy of each incoming X-ray photon. Is an X-ray photon is absorbed in the sensitive area of the detector, so there arise the electron -hole pairs whose number is proportional to the energy of the photon. By statistical effects in the detector and electronic noise, there is a broadening of the natural line width, hence some types of detectors need to be cooled. The typical energy resolution of EDX detector is located at 120-140 eV.

As a detector, different types of a semiconductor detector for use. Are typical of the Si ( Li) detector, the silicon drift detector and detectors of high-purity germanium.

EDX spectra and their evaluation

EDX spectrum in which signal intensity is plotted as a function of the energy of the X-ray quanta. EDX spectrum of the element consists of specific peaks and the broad non-specific background, which is generated by bremsstrahlung.

Peak identification, peak superposition and peak deconvolution

For most items, there are several lines in the spectrum. When assigning lines must be checked whether all lines of an element are present, and whether their intensities in the correct ratio to each other. This possible peak overlaps are to be considered with other elements. This is especially important in the development of peak, if there is an overlay of signals from different elements. Alternatively, an additional measurement with the higher -resolution wavelength dispersive X-ray spectroscopy ( WDX ) could be performed.

Quantitative Analysis

The quantitative analysis of EDS spectra will depend on many factors, such as absorbance, fluorescence, Probenkippung, excitation energy. The detection limit for most elements with atomic number greater than ten ( ie from sodium) with roughly 0.1-0.2 wt - % can be estimated. For lower atomic number elements, the detection limit is dramatically worse. Laboratory studies show theoretically all elements with atomic number greater than four ( ie from boron).

Lateral resolution of the analysis

The local accuracy of a measurement in the scanning electron microscope is limited by the penetration depth of the electron beam in the material. The incidence of the beam on the material it is scattered in the sample so that the emitted X- rays are produced in a pear-shaped volume with a diameter of 0.1-2 microns. The size of the excitation bulb is smaller in materials with higher density and at lower acceleration voltage. If the accelerating voltage is too small, however, peaks of higher power can no longer excited and the corresponding elements are no longer detected.

A higher spatial resolution can be achieved when the EDX detector is combined with a grid, but with a transmission electron microscope (TEM ): As for the TEM analysis, the sample is prepared as a very thin strip ( <100 nm), can the incident electron beam is not as widely spread in volume. In addition, the electrons are due to the much higher acceleration voltage much less scattered. The resolution is then determined only by the diameter of the electron beam and is less than 1 nm can still artifacts due to secondary excitations by the scattered electrons or X-ray quanta (X-ray fluorescence ) on the rest of the sample on the holder, on microscope parts or the detector occur.

Due to the relatively large range of x-ray radiation in matter of the analyzed range when excited by X-radiation (X-ray fluorescence) in the millimeter to centimeter range.

Application

EDX detectors are used, for example, in the following analysis methods:

• SEM - EDX: combination with a scanning electron microscope for elemental analysis on a microscopic scale. The excitation takes place by electrons. Due to the widespread use of this method EDX is often used as shorthand for SEM -EDX. The combination of imaging scanning procedure in the SEM with elemental analysis (EDX ), it is also possible to take element distribution images.

• TEM -EDX: combination with a transmission electron microscope ( TEM ) and elemental analysis on a microscopic scale. The excitation takes place by electrons. EDX is an essential addition to EELS method of analytical transmission electron microscopy, operating at inherently often also in raster mode (see scanning transmission electron microscope).

• X-ray fluorescence analysis: In an energy-dispersive X-ray fluorescence spectrometer (EDXRF, Eng energy dispersive X -ray fluorescence spectrometer, EDXRF. ), The excitation by X-rays and it comes in the sample in the emission of X-rays on the principle of fluorescence. This method allows large-scale analysis of compact specimens.

Comparison with wavelength dispersive X-ray spectroscopy

A wavelength-dispersive X-ray spectroscopy is an alternative (WDS or WDX ). EDX allows the simultaneous measurement of the entire X-ray spectrum of the analyzed sample location and thus the simultaneous analysis of all detectable elements, which results in a significant time - or speed advantage. In contrast, an order of magnitude, the detection sensitivity is better with WDX and at the same time a significantly higher spectral resolution of the X-ray spectrum is achieved.