Geology Reference
In-Depth Information
Box 6.4 The electron microprobe
the electron microprobe is a scanning electron microscope
adapted to analyse the characteristic X-ray spectra emitted
when the high-energy electron beam strikes a polished
specimen, allowing spatially resolved chemical analyses of
the specimen surface to be obtained. the technique revo-
lutionized petrology when it was introduced in the 1950s.
High voltage supply cable
Electron 'gun'
Gun supply
Electrons from the filament
are accelerated to high
kinetic energy (20 keV).
Aperture at -20 kV
Aperture at 0 V
High-vacuum 'column'
to prevent scattering
of electrons by air
Condenser 'lens':
Electron optics column
Pole piece
Two large coils (acting as
'magnetic lenses') focus the
electron beam on to a fine
spot (< 1 μ m diameter) on
the specimen surface.
Electron beam
(width exaggerated)
trimmed by apertures
Beam scanning coils
Objective 'lens'
To X-ray
To X-ray
High-vacuum in
sample chamber
to prevent absorp-
tion of X-rays by
air molecules.
Final aperture
X-rays from specimen
Specimen stage
( x, y, z drives)
The electron micro-
probe analyzes
crystals in situ in a
petrological section.
The polished surface
is coated with a thin,
earthed film of carbon
to prevent electron
X-ray spectrum
X-rays are generated only at the point of
impact of the finely focussed electron beam
on the specimen surface. Elements in the
sample emit their characteristic wavelength
peaks (cf. Fig. 6.5), whose intensities are
measured by X-ray spectrometers.
Figure 6.4.1 Schematic cross-section showing how an electron microprobe works.
The screening constant , σ, represents the degree to
which inter-electron repulsion counteracts the attrac-
tion of the positive nuclear charge. Z σ can be
regarded as the 'effective nuclear charge' felt by an
individual electron. Moseley, who was the first to
introduce the concept of atomic number, used this
equation to catalogue the chemical elements known
in 1914, and to demonstrate from gaps in the sequence
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