high-energy end of the spectrum – the particle range
Here we deal with changes in the atomic nucleus, or with changes in electronic
energy levels relatively ‘deep’ within atoms. The radiation is treated
more as high-energy particles than as waves. In order to use ‘spectro-‘
methods, there must be a way to distinguish the particles by their energies.
While there are instruments which can act like prisms in part of this
range, most measurements use a solid-state detector made of silicon doped
(‘drifted’) with lithium. Germanium is used for particularly high energies
(wavelengths < about 0.3 Å). A voltage is applied to the detector.
An X-ray or gamma ray absorbed in the detector produces electrons (and
‘holes’) which give a pulse of current proportional to the particle energy,
and a pulse height analyzer (PHA) can give a spectrum which is a plot
of the number of pulses versus particle energy.
Elements that are naturally radioactive, or which have been radioactive
by exposure to neutrons (as in neutron activation analysis, NAA) have
well-known half-lives and emit characteristic gamma rays. Gamma ray spectra,
especially if they are recorded at different times, identify and quantitate
many elements. The need for a strong neutron source limits application
X-rays can be used in emission or fluorescence. X-rays are emitted when
atoms are struck by fast-moving electrons, and their wavelengths are characteristic
of the element. This process is most often used in the electron microprobe.
A finely focused beam of electrons is directed onto a point, or scanned
across a sample. Some of the X-rays produced are caught in a solid-state
detector, and produce a spectrum, or generate an image showing the distribution
of a particular element. The detector-analyzer combination does XES (X-ray
Energy Spectrometry) or EDXRA (Energy-dispersive X-ray analysis). These
systems are not sensitive to the very lightest atoms.
In X-ray fluorescence (XRF), a beam of X-rays is directed onto a sample.
They should be ‘monochromatic’, i. e., all should have the same energy.
That can be arranged by selecting the material for the X-ray tube and
the tube voltage, and by removing less energetic X-rays with ‘filters’.
The X-rays absorbed by the sample raise inner electrons of atoms to higher
energy levels, and when they ‘relax’ back to the ‘ground state’ they emit
other X-rays. Those give spectra characteristic of the elements present
and their concentrations.
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