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X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or
fluorescent) X-rays
from a material that has been excited by bombarding with high-energy X-rays or gamma rays.
The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics
and building materials, and for research in geochemistry,
forensic science and archaeology.
Underlying
physics
Physics of X-ray fluorescence, in a
schematic representation.
When materials are exposed to short-wavelength
X-rays or to gamma rays, ionisation of their component atoms may take place. Ionisation consists
of the ejection of one or more electrons from the atom, and may occur if the
atom is exposed to radiation with an energy greater than its ionisation potential. X-rays and gamma rays can be energetic enough to expel
tightly held electrons from the inner orbitals
of the atom. The removal of an electron in this way renders the electronic
structure of the atom unstable, and electrons in higher orbitals
"fall" into the lower orbital to fill the hole
left behind. In falling, energy is released in the form of a photon, the energy
of which is equal to the energy difference of the two orbitals involved. Thus,
the material emits radiation, which has energy characteristic of the atoms
present. The term fluorescence is applied to phenomena in which the absorption of
radiation of a specific energy results in the re-emission of radiation of a
different energy (generally lower).
Figure 2: Typical energy dispersive
XRF spectrum
Figure 3: Spectrum of a rhodium target
tube operated at 60 kV, showing continuous spectrum and K lines
Characteristic
radiation
Each element has electronic orbitals
of characteristic energy. Following removal of an inner electron by an
energetic photon provided by a primary radiation source, an electron from an
outer shell drops into its place. There are a limited number of ways in which
this can happen, as shown in figure 1. The main transitions are given names: an
L→K transition is traditionally called Kα, an M→K transition is called Kβ, an
M→L transition is called Lα, and so on. Each of these transitions yields a
fluorescent photon with a characteristic energy equal to the difference in
energy of the initial and final orbital. The wavelength of this fluorescent
radiation can be calculated from Planck's Law:
The fluorescent radiation can be
analysed either by sorting the energies of the photons (energy-dispersive
analysis) or by separating the wavelengths of the radiation
(wavelength-dispersive analysis). Once sorted, the intensity of each
characteristic radiation is directly related to the amount of each element in
the material. This is the basis of a powerful technique in analytical chemistry. Figure 2 shows the typical form of the sharp fluorescent
spectral lines obtained in the wavelength-dispersive method (see Moseley's law).
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Primary radiation
In order to excite the atoms, a
source of radiation is required, with sufficient energy to expel tightly held
inner electrons. Conventional X-ray generators are most commonly used, because their output can readily be
"tuned" for the application, and because higher power can be deployed
relative to other techniques. However, gamma ray sources can be used without
the need for an elaborate power supply, allowing an easier use in small
portable instruments. When the energy source is a synchrotron
or the X-rays are focused by an optic like a polycapillary,
the X-ray beam can be very small and very intense. As a result, atomic
information on the sub-micrometre scale can be obtained. X-ray generators in
the range 20–60 kV in order to the K line, which allows excitation of a broad
range of atoms. The continuous spectrum consists of "bremsstrahlung"
radiation: radiation produced when high-energy electrons passing through the
tube are progressively decelerated by the material of the tube anode (the
"target"). A typical tube output spectrum is shown in figure 3.
Dispersion
In energy dispersive analysis, the
fluorescent X-rays emitted by the material sample are directed into a
solid-state detector which produces a "continuous" distribution of
pulses, the voltages of which are proportional to the incoming photon energies.
This signal is processed by a multichannel analyser (MCA) which produces an
accumulating digital spectrum that can be processed to obtain analytical data.
In wavelength dispersive analysis, the fluorescent X-rays emitted by the
material sample are directed into a diffraction grating monochromator. The
diffraction grating used is usually a single crystal. By varying the angle of
incidence and take-off on the crystal, a single X-ray wavelength can be
selected. The wavelength obtained is given by the Bragg Equation:
X-ray
intensity
The fluorescence process is
inefficient, and the secondary radiation is much weaker than the primary beam.
Furthermore, the secondary radiation from lighter elements is of relatively low
energy (long wavelength) and has low penetrating power, and is severely
attenuated if the beam passes through air for any distance. Because of this,
for high-performance analysis, the path from tube to sample to detector is
maintained under high vacuum (around 10 Pa residual pressure). This means in
practice that most of the working parts of the instrument have to be located in
a large vacuum chamber. The problems of maintaining moving parts in vacuum, and
of rapidly introducing and withdrawing the sample without losing vacuum, pose
major challenges for the design of the instrument. For less demanding
applications, or when the sample is damaged by a vacuum (e.g. a volatile
sample), a helium-swept X-ray chamber can be substituted, with some loss of
low-Z (Z = atomic number) intensities.
Chemical
analysis
The use of a primary X-ray beam to
excite fluorescent radiation from the sample was first proposed by Glocker and Schreiber in 1928.[1]
Today, the method is used as a non-destructive analytical technique, and as a
process control tool in many extractive and processing industries. In
principle, the lightest element that can be analysed is beryllium
(Z = 4), but due to instrumental limitations and low X-ray yields for the light
elements, it is often difficult to quantify elements lighter than sodium (Z = 11), unless background
corrections and very comprehensive inter-element corrections are made.
Wafer
detectors
More recently, high-purity silicon
wafers with low conductivity have become routinely available. Cooled by the
Peltier effect, this provides a cheap and convenient detector, although the liquid nitrogen
cooled Si(Li) detector still has the best resolution (i.e. ability to
distinguish different photon energies).
Amplifiers
The pulses generated by the detector
are processed by pulse-shaping amplifiers. It takes time for the amplifier to shape the
pulse for optimum resolution, and there is therefore a trade-off between
resolution and count-rate: long processing time for good resolution results in
"pulse pile-up" in which the pulses from successive photons overlap.
Multi-photon events are, however, typically more drawn out in time (photons did
not arrive exactly at the same time) than single photon events and pulse-length
discrimination can thus be used to filter most of these out. Even so, a small
number of pile-up peaks will remain and pile-up correction should be built into
the software in applications that require trace analysis. To make the most
efficient use of the detector, the tube current should be reduced to keep
multi-photon events (before discrimination) at a reasonable level, e.g. 5–20%.
Processing
Considerable computer power is
dedicated to correcting for pulse-pile up and for extraction of data from
poorly resolved spectra. These elaborate correction processes tend to be based
on empirical relationships that may change with time, so that continuous
vigilance is required in order to obtain chemical data of adequate precision.
Usage
EDX spectrometers are superior to WDX spectrometers in that they are smaller, simpler in
design and have fewer engineered parts. They can also use miniature X-ray tubes
or gamma sources. This makes them cheaper and allows miniaturization and
portability. This type of instrument is commonly used for portable quality
control screening applications, such as testing toys for Lead (Pb) content,
sorting scrap metals, and measuring the lead content of residential paint. On
the other hand, the low resolution and problems with low count rate and long
dead-time makes them inferior for high-precision analysis. They are, however,
very effective for high-speed, multi-elemental analysis. Field Portable XRF
analysers currently on the market weigh less than 2 kg, and have limits of
detection on the order of 2 parts per million of Lead (Pb) in pure sand
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