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TESTING & MEASUREMENT
Instrument Set Up
Most XRF instruments on the market today belong to two distinct
categories: energy-dispersive (ED) and wavelength-dispersive (WD)
XRF. Within these categories exist numerous configurations of XRF
instrument with differing X-ray sources, detectors, optics and
chamber/system orientations. To give an overview of the topic both
ED-XRF and WD-XRF will be explored though ED-XRF will be the focus
of this article.
WD-XRF
First, WD-XRF systems operate on Bragg’s Law (Equation 1), which
states:
When an X-ray meets the surface of a crystal at an incident angle (θ),
it will also be reflected at the same angle (θ) off the crystal surface.
And, when the path difference (d) is a whole number (n) of the
wavelength, only then would constructive interference occur between
the scattered X-ray beams (Bragg & Bragg, 1914).
The law can be summarised as:
Equation 1: Bragg’s Law equation. Λ is the wavelength of the X-ray, d
is the spacing of the crystal layers (path difference), θ is the incident
angle (angle between the incident ray and the scattered plane and n
is an integer.
The schematic diagram below shows the layout of a typical WD-XRF
instrument (Figure 3). In principle WD-XRF instruments require an
X-ray generator, filter, sample stage, collimator, crystal and detector.
The function of many components is similar in ED-XRF and WD-XRF
instruments, as such in this section only the different components will
be mentioned while the common components will be discussed in the
ED-XRF section. The type of detector used in WD-XRF instruments may
be of the proportional counter variety or silicon-based detectors for
light elements up to iron (Fe), while scintillation type detectors may be
used for heavier elements; the main difference between the ED-XRF
and WD-XRF system detectors is that the WD-XRF detectors only
analyse single wavelengths directed from the crystal, while ED-XRF
detectors analyse the whole spectrum of energies from the sample.
ED-XRF instruments use proportional counter type detectors or
silicon-based detectors and will be discussed further below.
Figure 3:
Schematic
diagram of a
typical WD-XRF
instrument.
X-rays first pass
through a
primary filter
and excite the
electrons in the
sample. The
secondary
X-rays pass
through a
collimator
before reaching the crystal which selectively (according to Bragg’s
Law) sends signals to the detector.
The other major component that is different between the two types of
XRF instrument is the crystal that is used in WD-XRF to selectively
present the detector with wavelengths emerging from the sample. It is
of note that two types of crystal/detector orientations can be used to
detect signals. The first is a fixed crystal/detector geometry, where the
signal is presented to crystals on a rotating turret. Each crystal in the
turret can reflect a number of specific wavelengths to the detector.
Although this set-up is designed for speed it is usually limited in the
number of wavelengths it can transmit. The second set-up involves
mounting the detector on a goniometer which allows it to rotate
around the crystal to interrogate each wavelength sequentially by
modifying the incidence angle between the crystal and the detector.
This however makes instruments with this set-up quite slow in
analysing a sample.
ED-XRF
For ED-XRF instruments the orientation is quite straight forward as the
schematic diagram below shows (Figure 4). The primary radiation
used to elicit a signal from the sample is generated in an XRF
generator.
X-ray Generator
XRF generators are normally designed as a glass chamber housing
the cathode and the target material, from which the X-rays will be
generated, as the anode. Typically, tungsten is used as the target
material as it offers sufficient excitation across a wide range of
wavelengths, but specialist anodes can also be used to elicit excitation
at specific wavelengths. These may include chromium, gold, silver,
aluminium, molybdenum or even rhodium. The glass chamber is
mounted in an oil-filled shield to prevent escape of ionising radiation,
to dissipate the heat generated inside and to maintain stable
temperatures even under heavy use, thus prolonging instrument
longevity. The X-ray required to excite the sample are directed
out through a window usually made from beryllium. ED-XRF
instruments usually require between 10 and 50kV to generate the
primary X-ray beam.
Primary Filters
Figure 4: Schematic
diagram of a typical
ED-XRF instrument
setup. X-rays generated
in the X-ray generator
pass through a primary
filter and a mirror
providing an optical
image, through a
collimator and
onto the sample.
The fluorescence signal
emitted by the sample
is detected by the
detector which is
positioned at a critical
angle to the primary
excitation beam.
The primary X-ray beam first passes through a primary filter, which
modifies the beam to maximise the signal to noise ratio. Filters of
nickel, aluminium and molybdenum are typically used in XRF but
copper or titanium filters at varying thicknesses can also be used for
specialist applications. As an example, aluminium filters are used
in trace analysis of heavy metals, as they subdue background noise
over which the trace amounts of heavy metals may not be detected.
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