X-ray microanalysis in the electron microscope Tutor: Peter Harris The aims of this course are: ● to explain how X-rays are generated in the electron microscope ● to describe how X-rays are analysed using energy dispersive and wavelength dispersive spectrometers and to highlight possible errors and limitations. ● to introduce the technique of X-ray mapping Click on the buttons to move between slides. (Make sure you are in “Slide Show” mode) Electron Microscopy Lab

Generation of X-rays: Background Interaction of electrons with the strong electric field of the nucleus, as shown on the right, produces background X-rays, or Bremsstrahlung. These X-rays have a continuous range of energies, and are of no value in analysing the elemental composition of the specimen. The appearance of the background in a typical X-ray spectrum is shown below.

Generation of X-rays: Characteristic X-rays The X-rays which are used in microanalysis are the characteristic X-rays. The way these are produced is illustrated on the right. Firstly, an incident electron ejects an electron from an inner shell of an atom in the specimen. An electron from a higher energy shell then “fills the hole” left by the ejected electron, and this results in the emission of an X-ray photon.

Generation of X-rays: Characteristic X-rays Most elements give a series of peaks in the spectrum, not just a single peak. This is because there are a number of possible electronic transitions which produce X-rays. This is illustrated on the right for the case of copper. It should be noted that X-ray spectrometry does not give information about the chemical bonding (i.e. oxidation states) of elements – it just indicates which elements are present.

Detection of X-rays: Energy dispersive spectrometer The energy dispersive X-ray (EDX) spectrometer is by far the most common way of detecting and analysing X-rays in electron microscopy. The most important part of the spectrometer is the detector (labelled “Crystal” in the diagram on the right). Most energy dispersive X-ray spectrometers employ an Si(Li) detector. This is described in the next slide.

Detection of X-rays: Si(Li) detector This consists of a 3–5 mm thick silicon junction diode with a bias of -1000 V across it. The central part is a lithium-drifted silicon crystal. When an X-ray photon passes through this crystal, electron-hole pairs are produced, and this causes a voltage pulse. To obtain sufficiently low conductivity, the detector must be maintained at low temperature using liquid-nitrogen.

Pulse processing As noted on the previous slide, X-rays entering the Si(Li) detector produce a voltage pulse which is then passed into a pulse processor. This assigns the pulse, according to its energy, to a channel in a multichannel analyser to create the spectrum. voltage pulse pulse processor X-ray

Pulse processing Process time 6 Process time 1 It is important to understand that the processor can only process one pulse at a time. The amount of time the processor takes to process the pulse is called the process time (or sometimes the dead time). The longer the process time, the better will be the resolution of the spectrum. This is illustrated on the right where it can be seen that a longer process time (6) enables two adjacent peaks to be resolved.

Detection of X-rays: Wavelength dispersive spectrometer The wavelength dispersive X-ray (WDX) spectrometer operates in a different way to the EDX spectrometer. In this case, a crystal is used to diffract the X-rays into the detector.

Detection of X-rays: Wavelength dispersive spectrometer The crystal only diffracts those X-rays that satisfy Bragg's Law, so that a single wavelength is passed on to the detector. The crystal is rotated, enabling X-rays with a range of wavelengths to be detected. WDX analysis is much slower than EDX, so it is usual to select a few elements to be analysed rather than sweeping the whole energy range. Bragg diffraction

Comparison of EDX and WDX spectrometry The higher resolution of WDX spectrometry compared with EDX is illustrated on the right. The grey peaks were obtained using WDX, the yellow peak using WDX. As well as having greater resolution, WDX also has lower detection limits than EDX. The precise detection limits depend strongly on the element to be analysed, and the nature of the matrix, but useful “ball-park” figures are as follows: EDX – approximate detection limit 0.1% by weight WDX – approximate detection limit 0.01% by weight Note that neither EDX or WDX are ultra-sensitive techniques.

Comparison of EDX and WDX spectrometry Although WDX has higher resolution and greater sensitivity than EDX, there are some significant disadvantages. As already mentioned, WDX is much slower than EDX because the spectrum is acquired sequentially. Also, because of the geometry of the detector, WDX cannot be used at low magnifications. It should also be noted that for WDX, standard samples need to be run for all elements to be analysed. This is not the case for EDX, where the intensities can be simulated: “standardless analysis”.

Further information The recommended book for this course is "Electron microscopy and analysis", by Goodhew, Humphreys and Beanland. Chapter 6 gives a good introduction to microanalysis. Peter Harris and other members of EMLab staff will be happy to answer your questions.