High velocity electrons are generated under high vacuum
conditions from a filament (usually made of tungsten). These electrons are
focused through a series of electromagnetic lenses into a very narrow beam.
The figure below shows the electron optical
column with the electron gun at the top and the sample on the bottom.
When the electron beam impacts the sample, elements in the
sample emit various types of electrons and X-rays. These are used to image the
sample and obtain chemical compositions. The figure below shows how incident
electrons cause an atom to emit electrons.
When electrons from outer shells of an atom fall in to replace
a discharged electron, they emit characteristic X-rays. These X-rays have
energies specific to the elements from which they were emitted. The amount of
characteristic X-rays emitted by each element in the sample reflects its
concentration.
|
The left figure is an X-ray spectrum of the
mineral albite(NaAlSi3O8).Each peak on
the spectrum represents a
transition with
a characteristic energy.Every element in
albite has its own "fingerprint" of peaks
so we can deduce
which are present. |
Not all X-rays are characteristic of the elements
present in the specimen. Many belong to the background continuum (Bremsstralung)
generated as the beam electrons slow down when they hit the sample. Having
generated a characteristic X-ray from a sample, some sort of detector is needed
to measure it. Two types of X-ray spectrometers are used in electron
microprobes:
EDS measures all X-ray energies simultaneously,
whereas the WDS is used for quantitative compositional analyses. The primary
drawback of EDS is that is has poorer spectral resolution and is of limited
value for quantitative analysis, especially for trace elements.
| The figure on right shows how WDS works. The
source hits a diffraction crystal, which disperses X-rays by Bragg reflection. Most electron microprobes are equipped
with
several crystals of different d-spaces to allow analysis of a wide range
of X-ray wavelengths. |
|
Common crystals are lithium fluoride
(LIF),
pentaerythritol (PET) and thallium acid phthalate (TAP). These crystals can
measure all X-ray wavelengths generated by elements from 11Na to
92U. For lighter elements, wider d-spacing is needed. In this case, soap
films can be used.
