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How does the electron microprobe work?

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Schematic drawing of electron microprobe instrument.

Electrons are produced by heating a filament similar to the filament in a light bulb (A). These electrons are then formed into a beam by accelerating them down a column at very high voltages, typically 15 to 20 thousand volts. The electrons pass through lenses that condense the beam (B), remove aberrations (C) and focus the beam (D). When the electrons arrive at the sample (E) the beam is focused into a spot much smaller than 0.001 millimeter in diameter. Upon entering the sample, the electrons interact with the atoms in the sample in what is called the interaction volume, causing X-rays to be produced. Each element produces X-rays with characteristic energies. These X-rays can then be counted by reflecting them through a crystal (F) and sending them on to a detector (G). By counting the X-rays generated by each element in the sample and comparing that number to the number of X-ray generated by a standard of known composition, it is possible to determine the chemical composition of a spot one one-thousandth of a millimeter in diameter with great accuracy.

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