MJC      Mary Johnson Consulting: How the Electron Microprobe Works

                                                                (in progress)

 

 

What is an electron microprobe?

            This older technique is still best for quantitative analysis of small regions. The surface of a sample is hit by a focused beam of electrons, and gives off X-rays characteristic of the elements present. These X-rays are focused onto X-ray detectors with curved crystals, allowing relative amounts to be counted for specific wavelengths. Additional processing is done to convert X-ray counts to quantitative chemical analyses.

            Electron microprobes are expensive (usually millions of dollars) and somewhat finicky machines, with many moving and electronic parts. Often they required dedicated operators.

 

Parts/how it works:

            The surface of a sample is hit by a focused beam of electrons…

            The electrons are given off from a filament, once typically tungsten metal, and later often replaced by LAB6, lanthanum hexaboride. Both these substances emit electrons when a strong current is run through them. Since the electrons are charged, they can be focused and accelerated through a hole in a charged plate. The assemblage is referred to as an electron “gun.”

            Electrons travel through an evacuated column, shielded from outside magnetic fields, and through various focusing electron optics, so that they will be at a focus at the sample surface. The focusing region at the sample surface is called the spot, and the spot size (diameter) is often measured; however, X-rays are produced from a larger volume than this spot alone, and excitation depth increases with increasing beam voltage.

            The column ends in a central chamber, also in a vacuum, containing the stage on which the sample is held and several detectors

 

The sample gives off X-rays characteristic of the elements present…

To get the best results, the sample being examined must have several characteristics. It must be flat and well-polished. It should be dry and stable to exposure to vacuum. It needs to be conductive, since otherwise it builds up a surface charge that can deflect the incoming electron beam.

Samples may not be stable under exposure to the electron beam; sodium is especially known as a mobile element under electron bombardment. (Three techniques are used to get the best results for sodium: use of a lower-voltage electron beam, use of a larger spot size, and collection of sodium data as early as possible in the analysis.)

How do you know where you are on the sample? Microprobes usually can image in light (either reflected light or cathodoluminescence – luminescence from the electron beam) or in secondary (re-emitted) or backscattered electrons, with appropriate detectors.

To change from one spot to another, either the electron beam can be moved (in two dimensions, usually only over short distances), or the stage can be moved (in three dimensions). Stage positions can be programmed in advance.

 

These X-rays are focused onto X-ray detectors with curved crystals, allowing relative amounts to be counted for specific wavelengths…

X-ray paths cannot be bent like light rays or electrons; X-rays must be diffracted to concentrate them on a detector. Depending on the energy range, specific curved crystals are used to focus these X-rays so that they can be counted. The crystals are tilted to select specific X-ray wavelengths. Traditional curved crystals include KDP (potassium dihydrogen phosphate), PET (pentaerythritol) and TAP (thallium acid phthalate); these are used for X-rays given off by sodium and higher-atomic-number elements.

More recently, layered metal-oxide semiconductors have been grown by ion deposition, and these can be used to focus the lower-energy (longer wavelength) X-rays from the light elements boron, carbon, nitrogen, oxygen, fluorine and argon.

 

Additional processing is done to convert X-ray counts to quantitative chemical analyses…

The number and peak position of X-rays produced by a specific element in a sample depend on several factors. Equipment factors include: the voltage and current of the electrons reaching the sample, the gun-to-sample-to-detector geometry, and the detector efficiency. Sample chemistry factors include the local chemistry of the element in question (Is it a cation? An anion? What ionic charge does it have? Is it in a metal or semi-metal?), the mean atomic number of the sample, and the presence of other elements that can preferentially absorb X-rays from the element in question. Other sample factors include the quality of its polish, thickness of the sample, and the possibility of decomposition under the electron beam.

 

More to come…

Comments? Send them to mlj@cox.net.

 

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