INTRODUCTION
In
Energy Dispersive X-Ray Fluorescence spectrometry (ED-XRF), the identification
of characteristic lines is performed using detectors that directly measure the
energy of the photons. In energy dispersive X-ray fluorescence
analysis (EDXRF), a solid-state detector is used to count the photons,
simultaneously sorting them according to energy and storing the result in a
multichannel memory. The result is an X-ray energy vs. intensity spectrum. The
range of detectable elements ranges from Be (Z
= 4) for the
light elements and goes up to U (Z = 92) on the high atomic number Z side. In principle, XRF analysis is a multielement analytical
technique and in particular, the simultaneous determination of all the
detectable elements present in the sample is inherently possible with EDXRF. In
WDXRF both the sequential and the simultaneous detection modes are possible. Although energy dispersive detectors generally exhibit poorer
energy resolution than wavelength dispersive analyzers, they are capable of
detecting simultaneously a wide range of energies. The most frequently used
detector in EDXRF is the silicon semiconductor detector, which nowadays can
have excellent energy resolution.
INSTRUMNTATION
An
ED-XRF system consists of several basic functional components, as shown in
Figure
The major components are as follows :
- X- Ray excitation source
- Sample Chamber
- Si (Li) detector
- Preamplifier
- Main Amplifier
- Multichannel Pulse Height Analyzer
The properties and performances of an EDXRF system differ upon
the electronics and the enhancements from the computer software.
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Typical ED-XRF detection arrangement.
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We will
discuss in detail for every component :
1. Excitation Mode
A) Direct Tube Excitation .
Because
of the simplicity of the instrument and the availability of a high photon
output flux by using direct tube excitation, the X-ray fluorescence
spectrometer equipped with an Xray tube as direct excitation source is gaining
more and more attention from manufactures. The spectrometer is more compact and
cheaper compared to secondary target systems. Of course, the drawback is still
the less flexible selection of excitation energy. However, by using an
appropriate filter between tube and sample, one can obtain an optimal
excitation.
The
most popular X-ray tube used in direct excitation ED spectrometer is the side
window tube for reasons of simplicity and safety. With direct tube excitation,
low powered X-ray tubes (< 100 W) can be used. These air cooled tubes are
very compact, less expensive, and only require compact, light, inexpensive, highly
regulated solid state power supplies. In a WD spectrometer, on the other hand,
high-power tubes (3-4 kW) are essential to compensate for the losses in the
crystal and collimator. With the low-power tubes used in ED spectrometer,
better excitation of light elements (i.e. low-Z element), analysis of smaller
samples, small spot analysis, and compact systems can be obtained.
B) Secondary Target Excitation.
The
principle of secondary target excitation was developed to avoid the intense
Bremsstrahlung
continuum from the X-ray tube by using a target between tube and sample.
 |
Schematic illustration of secondary target excitation
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The ratio of the intensity
of the characteristic lines to that the continuum in secondary target
excitation is much higher than that in direct tube excitation because the
continuum part of the excitation spectrum of the secondary target is generated
only by scattering. One can excite various elements efficiently by selecting a secondary target that has
characteristic lines just above the absorption edges of the elements of
interest in the sample. Therefore, secondary target excitation has some obvious
advantages over direct tube excitation: its flexibility for getting an
optimized and near monochromatic excitation providing a better selectivity and
an improved sensitivity. However, to compensate for the intensity losses that
occur at the secondary scatterer, a high-powered tube as used in WD
spectrometers is required; making the whole system more sophisticated and
expensive compared to direct tube excitation setups.
C) Radio Isotopic Excitation.
A
variety of about 30 commercially available radio-isotopic materials can be
chosen for an optimal excitation. The X-rays and/or γ-rays emitted
from these radio-isotopic sources cover a wide range (10 – 60 keV) of
excitation energies. With a high energy source like 241 Am, K lines instead L
lines can be used for quantification in the case of analyzing high-Z rare earth
elements, with considerably less matrix effects and spectrum overlaps.
Sometimes the same idea as in the secondary target excitation is used to avoid
non-photon radiation. A proper design of excitation-detection geometry can
improve greatly the sensitivity and accuracy of the XRF analysis with such
excitation source. The disadvantages of using radioisotopic sources however lie
in their low photon output, intensity decay and storage problems.
2. Detectors
Energy
dispersive X-ray spectrometry is based upon the ability of the detector to
create signals proportional to the X-ray photon energy, therefore, mechanical
devices, such as analyzing crystals, are not required as in wdxrf . Several
types of detectors have been employed, including silicon, germanium and
mercuric iodide .
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Cross section of an Si(Li) detector crystal with p-i-n structure and the
production of electron-hole pair.
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The
solid state, lithium-drifted silicon detector, Si(Li), was developed and
applied to Xray detection in the 1960’s. Early 1970’s, this detector was firmly
established in the field of X-ray spectrometry, and was applied as an X-ray
detection system for scanning Electron Microscopy (SEM) as well as X-ray
spectrometry. The principal advantage of the Si(Li) detector is its excellent
resolution.
Si(Li)
detector can be considered as a layered structure in which a lithium-drifted
active region separates a p-type entry side from an
n-type side. Under reversed
bias of approximately 600 V, the active region acts as an insulator with an
electric field gradient throughout its volume. When an X-ray photon enters the
active region of the detector, photoionization occurs with an electron-hole
pair created for each 3.8 eV of photon energy. Ideally, the detector should
completely collect the charge created by each photon entry, and result in a
response for only that energy. In reality, some background counts appear
because of the energy loss in the detector. Although these are kept to a
minimum by engineering, incomplete charge collection in the detector is a
contributor to background counts. In the X-ray spectrometric, important region
of 1 – 20 keV, silicon detectors have excellent efficiency for conversion of
X-ray photon energy into charge. Some of the photon energy may be lost by
photoelectric absorption of the incident X-ray, creating an excited Si atom
which relaxes to yield an Si Kα
X-ray. This X-ray may escape from the detector, resulting in an
energy loss equivalent to the photon energy; in the case of Si Kα, this is
1.74 keV. Therefore, an escape peak 1.74 keV lower than the true photon energy
of the detected X-ray may be observed for intense peaks. For Si(Li) detectors,
these are usually a few tenths of one percent, and never more than 2%, of the
intensity of the main peak.
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| The Si(Li) detector schematic |
Resolution of an energy dispersive X-ray spectrometer is normally
expressed as the Full Width at Half Maximum amplitude (FWHM) of the Mn X-ray at
5.9 keV. The resolution will be somewhat count rate dependent. Commercial
spectrometers are supplied routinely with detectors which display approximately
145 eV (FWHM @ 5.9 keV). The resolution of the system is a result of both
electronic noise and statistical variations in conversion of the photon energy.
Electronic noise is minimized by cooling the detector, and the associated
preamplifier with liquid nitrogen (Figure). In many cases, half of the
peak width is a result of electronic noise.
3. Pulse Height Analysis
The
X-ray spectrum of the sample is obtained by processing the energy distribution
of X-ray photons which enter the detector. A single event of one X-ray photon
entering the detector causes photoionization and produces a charge proportional
to the photon energy. Numerous electrical sequences must take place before this
charge can be converted to a data point in the spectrum.
When
an X-ray photons enters the Si(Li) detector, it is converted into an electrical
charge which is coupled to a Field Effect Transistor (FET). The FET, and the
rest of the associated electronics which make up the preamplifier, produce an
output proportional to the energy of the X-ray photon. Using a pulsed optical
preamplifier, this output is in the form of a step signal. Because photons vary
in both energy and number per unit time, the output signal, due to successive
photons being emitted by a multielement sample, resembles a staircase with
various step heights and time spacing. When the output reaches a predetermined
level, the detector and the FET circuitry is reset to its starting level, and
the process repeated.
The preamplifier stage integrates each detector charge signal to
generate a voltage step proportional to the charge. This is then amplified and
shaped in a series of integrating and differentiating stages. Owing to the
finite pulse-shaping time, in the range of microseconds, the system will not
accept any other incoming signals in the meanwhile (dead time), but extend its
measuring time instead. In a further step the height of these signals is
digitized as a channel number (analog-to-digital converter, ADC), stored to a memory
(multichannel analyzed, MCA) and finally displayed as a spectrum, where the
number of counts reflects the respective intensity. In a more modern approach,
the output signals of the preamplifier are digitized directly, which can
increase the throughput of the system significantly.
4. Energy Resolution
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| Mn-Kα spectrum and calibrated pulser |
The energy resolution of the EDXRF spectrometer determines the
ability of a given system to resolve characteristic X-rays from
multiple-element samples and is normally defined as the full width at half maximum (FWHM) of the pulse-height distribution measured for
a monoenergetic X-ray. A conventional choise of X-ray energy is 5.9 keV,
corresponding to the Kα energy of Mn. Figure II.6 shows a typical
pulse-height spectrum of Mn-Kα X-rays simultaneously with a
calibrated pulser. The purpose of the pulser measurement is to monitor the
resolution of the electronic system independent of any peak broadening due to
the detector itself. Typical state-of the art detectors Si(Li) and Ge(HP)
achieve 130 to 170 eV, but depends strongly on the size of the crystal. The
smaller the crystal, the better is the resolution.
