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SPI Microanalysis Standards

Further information about Faraday Cups

A Faraday Cup allows a beam of charged particles (electrons, ions) to be measured accurately. It collects all the particles which enter it, and leads them to an ammeter, or more correctly a micro-ammeter. Most electron beam instruments have a Faraday Cup by which one can measure the electron beam current in the column, but this is not the same as the current which strikes the sample. When electrons hit the sample, some are back-scattered, and some are absorbed. The relative proportion of back- scattered and absorbed electrons depends on the nature of the sample material. Light materials have low back-scatter coefficients; heavy materials, like gold, back-scatter a large proportion of the beam.

The true reading of the current reaching the sample can only be determined if the back-scattered proportion is reduced to near zero, thus forcing all the electrons to be absorbed by the sample. A Faraday Cup accomplishes this by trapping the electrons inside a cavity from which they have little chance of escaping. In the SPI Faraday Cup, which is included with every standard mount we produce, the electron beam is directed through a 200 micrometer aperture in a piece of platinum. Below this aperture is a relatively large stainless steel cavity in which electrons may bounce around, ultimately coming to rest in the walls, whence they are conducted away through the sample to the sample current meter. The cup has a screw thread which allows it to be inserted and removed easily using a hex Allen key.

Why should one want to know the current incident on the sample? Really this is a quantity which should be reported when analyses are made, not only for comparative purposes from day to day, but also to ensure that the instrument is functioning correctly. If you know the true current, measured by Faraday Cup, and you also know the absorbed current in a particular material, let us say X, then the difference is the back-scattered current, which is a function of the atomic number, Z, of X. Hence Faraday Cup measurements allow you to determine the mean Z of a sample independently of knowing its chemical composition.

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Friday October 03, 2008
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