Electron microprobes can be used as cameras to photograph
specimens beyond the capabilities of ordinary optical microscopes.
The pictures appear very real, as if a regular camera photographed them.
The resolution of these images are a function of particle emission instead
of the light radiation we're familiar with. Particles detected by the electron
microprobe to form the secondary electron image are called secondary electrons,
and are ejected from the sample as a result of inelastic collisions with beam electrons.
They are detected using a secondary electron detector and are displayed on a scanning TV display.
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Secondary electron images show surface
topography. The left picture is a secondary electron image of the
inside of an eggshell.
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Backscattered electrons (BSE) are primary electrons emitted as a
result of elastic collisions with specimen electrons. BSE emission
intensity is a function of the specimen's atomic number. The higher
the atomic number, the brighter the signal. For example, minerals with 26Fe will
appear brighter than those containing 12Mg. Backscattered electron images are
obtained exactly the same way as secondary electron images.
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The right image is a BSE image of a Himalayan
garnet with quartz and zircon inclusions. The garnet is the large cracked grain making
up most of the picture, the quartz inclusion is the darker oval and the zircon grains are the
bright white splotches scattered throughout the garnet.
Scale bar= 200 mm= 0.2 mm.
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The electron microprobe can also be used to
create X-ray element maps showing the relative concentrations of elements
within a mineral. The electron beam is scanned across the sample, stopping at
regular intervals to count the number of X-rays within a predefined energy
window arriving at the detector. For example X-rays that fall in the range of
1.64-1.84ev may be used to map silicon, which has a Ka peak at 1.74ev. The
number of counts at each stop (or pixel) in the image can be displayed as a map.
By maximizing the number of points at which X-rays are counted, and by scanning
the beam many times over the sample, high resolution can be produced. These maps
shows the concentration differences not detectable using backscattered electron
images.
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This X-ray element map is half of a Himalayan garnet
showing the
distribution of manganese.
This garnet
would look entirely gray in
backscattered electron imaging.
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