COMPARISON BETWEEN ED-XRF AND WD-XRF

We have discussed about  ED-XRF & WD-XRF in the earlier posts . Now I would like to highlight the major differences between the two X-ray techniques . The most important point of comparison are listed below ::

1. RESOLUTION :

It describes the width of the spectra peaks. The lower the resolution number the more easily an elemental line is distinguished from the nearby X-ray line intensities.

a) The resolution of the WD-XRF system is dependent on the crystal and optics design,particularly collimation, spacing and positional reproducibility. The effective resolution of a WD-XRF system may vary from 20 eV in an inexpensive bench top to 5 eV or less in a laboratory instrument. The resolution is not detector dependant.

Advantage of WD-XRF: High resolution means fewer spectral overlaps and lower

background intensities

b)  The resolution of ED-XRF system is dependent on the resolution of the detector. This can vary from 150 V or less for a liquid nitrogen cooled Si(Li) detector, 150 – 220 eV for various solid state detectors, or 600 eV or more for gas filled proportional counter.

Advantage of ED-WRF: WD-XRF crystal and optics are expensive, and are one more failure mode.

2. SPECTRAL OVERLAPS:

Spectral deconvolutions are necessary for determining net intensities when two spectral lines overlap because the resolution is too high for them to be measured independently.

a) With a WD-XRF instrument with very high resolution (low number of eV) spectral

overlap corrections are not required for a vast majority of elements and applications.

The gross intensities for each element can be determined in a single acquisition.

Advantage WD-XRF: Spectral deconvolutions routines introduce error due to counting statistics for every overlap correction onto every other element being corrected for. This can double or triple the error

b) The ED-XRF analyzer is designed to detect a group of elements all at once. The some type of deconvolutions method must b used to correct for spectral overlaps. Overlaps are less of a problem with 150 eV resolution systems, but are significant when compared to WD-XRF. Spectral overlaps become more problematic at lower resolutions.

3. BACKGROUND: 

The background radiation is one limiting factor for determining detection limits, repeatability, and reproducibility.

a) Since a WD-XRF instrument usually uses direct radiation flux the background in the region of interest is directly related to the amount of continuum radiation within the region of interest the width is determined by the resolution.

b) The ED-XRF instrument uses filters and/or targets to reduce the amount of continuum radiation in the region of interest which is also resolution dependant, while producing a higher intensity X-ray peak to excite the element of interest.

Even, WD-XRF has the advantage due to the resolution. If a peak is one tenth as wide it has one tenth the background. ED-XRF counters with filters and targets that can reduce the background intensities by a factor of ten or more.

4. EXCITATION EFFICIENCY:

Usually expressed in PPM per count-per-second (cps) or similar units, this is the other main factor for determining detection limits, repeatability, and reproducibility. The relative excitation efficiency is improved by having more source x-rays closer to but above the absorption edge energy for the element of interest.

a. WDXRF generally uses direct unaltered x-ray excitation, which contains a continuum of energies with most of them not optimal for exciting the element of interest.

b. EDXRF analyzers may use filter to reduce the continuum energies at the elemental

lines, and effectively increasing the percentage of X-rays above the element absorption edge. Filters may also be used to give a filter fluorescence line immediately above the absorption edge, to further improve excitation efficiency. Secondary targets provide an almost monochromatic line source that can be optimized for the element of interest to achieve optimal excitation efficiency.

WAVELENGTH DISPERSIVE X-RAY FLUORESCENCE (WD-XRF)

INTRODUCTION

Wavelength Dispersive X-Ray Fluorescence Spectrometry (WD-XRF) is the oldest method of measurement of X-rays, introduced commercially in the 1950’s. This name is descriptive in that the radiation emitted from the sample is collimated with a Soller collimator, and then impinges upon an analyzing crystal. The crystal diffracts the radiation to different extents, according to Bragg’s law, depending upon the wavelength or energy of the X radiation. This angular dispersion of the radiation permits the sequential or simultaneous detection of X-rays emitted by elements in the sample. Simultaneous instruments normally contain several sets of analyzing crystals and detectors; one is adjusted for each desired analyte in the sample. These instruments tend to be very expensive, but efficient for the routine determination or preselected elements.

WD-XRF spectrometers are usually larger and more expensive than other spectrometers. Because the analyzing crystal d-spacing determines wavelength sensitivity, they are usually more sensitive than other spectrometers. To overcome losses in X-ray optics of the WD-XRF spectrometers and to maximize primary radiation intensity, X-ray tubes are usually employed. The sample is usually held under vacuum to reduce contamination and avoid absorption of light element characteristic radiation in air.

Typical uses of WD-XRF include the analysis of oils and fuel, plastics, rubber and textiles, pharmaceutical products, foodstuffs, cosmetics and body care products, fertilizers, minerals, ores, rocks, sands, slags, cements, heat-resistant materials glass, ceramics, semiconductor wafers; the determination of coatings on paper, film, polyester and metals; the sorting or compositional analysis of metal alloys, glass and polymeric materials; and the monitoring of soil contamination, solid waste, effluent, cleaning fluids, sediments and air filters.

PRINCIPLE OF  WD-XRF

WD-XRF spectrometers measure X-ray intensity as a function of wavelength. This is done by passing radiation emanating from the specimen through an analyzing diffraction crystal mounted on a 2θ goniometer. By Bragg’s Law, the angle between the sample and detector yields the wavelength of the radiation:

2d sin θ = nλ ;

where:

d    is the d-spacing of the analyzing crystal,

θ    is half the angle between the detector and the sample,

n    is the order of diffraction.

The analyzing crystal must be oriented so that the crystal diffraction plane is directed in the appropriate direction. Figure shows a simplified schematic of the WD-XRF spectrometer.  A scintillation or flow-proportional detector usually measures the fluoresced radiation. The heights of the resulting pulses are proportional to energy so using a pulse height analyzer (PHA), scattered or undesired diffraction-order X-rays can be ejected. The X-ray beam is usually collimated before and after the analyzing crystal. Each of the components showed in the below (Figure 1)  were be described in the following sections.

1. Schematic description of WD-XRF principle

1. COLLIMATOR MASKS

The collimator masks are situated between the sample and collimator and serve the

purpose of cutting out the radiation coming from the edge of the cup aperture (Figure 2).

The size of the mask is generally adapted to suit of the cup aperture being used.

The masks perform one of the two functions: background reduction and improved

fluorescence (Figure 3).

2.Use of Cu 200 μm filter for cutting off the radiation coming from Rh X-ray tube.

3. Use of Al 100 μm filter for improvement of the ratio peak/background.

2. COLLIMATOR

4. Collimators with different angles of resolution.

Collimators consist of a row of parallel slats (Figure 4) and select a parallel beam of X-rays coming from the sample and striking the crystal. The spaces between the slats determine the degree of parallelism and thus the angle resolution of the collimator.

A 0.077° collimator is adequate for high resolution measurement parameters. Collimators with low resolution (e.g. 1.5 -2.0°) are advantageous for light elements such as Be, B and C (Figure 5). Using a collimator with a low resolution increases then intensity significantly. This enables intensity to be increased without a loss in angle resolution when analyzing light elements.

5. Example of the influence of collimator resolution on the intensity of a light

element.

3. THE ANALYZING CRYSTALS

a. Bragg’s Law

Crystals consist of a periodic arrangement of atoms (molecules) that form the crystal lattice. In such an arrangement of particles you generally find numerous planes running in different directions through the lattice points (= atoms, molecules), and not only horizontally and vertically but also diagonally. These are called lattice planes. All of the planes parallel to a lattice plane are also lattice planes and are at a defined distance from each other. This distance is called the lattice plane distance “d”.

6. Bragg’s Law.

When parallel X-ray light strikes a lattice plane, every particle within it acts as a scattering center and emits a secondary wave. All of the secondary waves combine to form a reflected wave. The same occurs on the parallel lattice planes for only very little of the X-ray wave is absorbed within the lattice plane distance “d”. All these reflected waves interfere with each other. If the amplification condition “phase difference = a whole multiple of the wavelength” (Δλ = nλ) is not precisely met, the reflected wave will interfere such that cancellation occurs. All that remains is the wavelength for which the amplification condition is met precisely. For a defined wavelength and a defined lattice plane distance, this is only given with a specific angle, the Bragg angle (Figure 6).

Under amplification conditions, parallel, coherent X-ray light (1,2) falls on a crystal

with a lattice plane distanced ‘d’ and is scattered below the angle θ (1′,2′). The proportion of the beam that is scattered on the second plane has a difference of ‘ACB’ to the proportion of the beam that was scattered at the first plane. The amplification condition is fulfilled when the phase difference is a whole multiple of the wavelength λ. This results in Bragg’s Law:

2d sin θ = nλ ;

n = 1, 2, 3… Reflection order.

On the basis of Bragg’s Law, by measuring the angle θ, we can determine either the

wavelength λ, and thus chemical elements, if the lattice plane distance ‘d’ is known or, if the wavelength λ is known, the lattice plane –value distance ‘d’ and thus the crystalline structure.

This provides the basis for two measuring techniques for the quantitative and qualitative determination of chemical elements (XRF) and crystalline structures (molecules, XRD), depending on whether the wavelength λ or the 2d-value is identified by measuring the angle θ.

• In X-ray diffraction (XRD) the sample is excited with monochromatic radiation of a known wavelength (λ) in order to evaluate the lattice plane distance as per Bragg’s equation.
• In XRF, the ‘d’-value of the analyzer crystal is known and we can solve Bragg’s equation for the element characteristic wavelength (λ).

b.  Reflections of Higher Orders

Figures 7a and 7b illustrate the reflections of the first and second order of one wavelength below the different angles θ1 and θ2. Here, the total reflection is made up of the various reflection orders (1, 2 …, n). The higher the reflection order, the lower the intensity of the reflected proportion of radiation generally is. How great the maximum detectable order is depends on the wavelength, the type of crystal used and the angular range of the spectrometer.

7 a.  First order reflection: λ = 2 d sin θ1.

7 b.  Second order reflection: 2λ = 2 d sin θ2.

It can be seen from Bragg’s equation that the product of reflection orders ‘n = 1; 2; ..’

and wavelength ‘λ’ for greater orders, and shorter wavelengths ‘λ* < λ’ that satisfy the

condition ‘λ* = λ/n’, give the same result.

Accordingly, radiation with one half, one third, one quarter etc. of the appropriate

wavelength (using the same type of the crystal) is reflected below the identical angle θ:

1λ = 2(λ / 2) = 3(λ / 3) = 4(λ / 4) = ……......

As the radiation with one half of the wavelength has twice the energy, the radiation with one third of the wavelength three times the energy etc., peaks of twice, three times the energy etc. can occur in the pulse height spectrum (= energy spectrum) as long as appropriate radiation sources.

c.  Crystal Types

The wavelength dispersive X-ray fluorescence technique can detect every element

above the atomic number 4 (Be). The wavelengths cover the range of values of four

magnitudes: 0.01 – 11.3 nm. As the angle θ can theoretically only be between 0° and 90° (in practice 2° to 75°), sinθ an only accept values between 0 and +1. When Bragg’s equation is applied:

  0 < nλ/2d = Sinθ < 1

 This means that the detectable element range is limited for a crystal with a lattice plane difference ‘d’. Therefore a variety of crystal type with different ‘2d’ values is necessary to detect the whole element range (from atomic number 4).

Besides the ‘2d’ values, the following selection criteria must be considered when a

particular type of crystal is to be used for a specific application:

• Resolution
• Reflectivity (→ intensity)

Further criteria can be:

• Temperature stability
• Suppression of higher orders
•       Crystal fluorescence.

d.  Dispersion, Line Separation

The extent of the change in angle Δθ upon changing the wavelength by the amount Δλ (thus: Δθ/Δλ) is called “dispersion”. The greater the dispersion, the better is the separation of two adjacent or overlapping peaks. Resolution is determined by the dispersion as well as by surface quality and the purity of the crystal.

Mathematically, the dispersion can be obtained from the differentiation of the Bragg

equation. 

Δθ /Δλ =  n/2dsinθ

It can be seen from this equation that the dispersion (or peak separation) increases as the lattice plane distance ‘d’ declines.

e. Synthetic Multilayers

8.  Diffraction in the layers (here: Si/W) of a multilayer.

Multilayers are not natural crystals but artificially produced ‘layer analyzers’. The lattice plane distances ‘d’ are produced by applying thin layers of two materials in alternation on to a substrate (Figure 8). Multilayers are characterized by high reflectivity and a somewhat reduced resolution. For the analysis of light elements the multilayer technique presents an almost revolutionary improvement for numerous applications in comparison to natural crystals with large lattice plane distances.

4.    DETECTORS

When measuring X-ray, use is made of their ability to ionize atoms and molecules, i.e. to displace electrons from their bonds by energy transference. In suitable detector materials, pulses whose strengths are proportional to the energy of the respective X-ray quants are produced by the effect of X-ray. The information about the X-ray quarts energy is contained in the registration of the pulse height. The number of X-ray quants per unit of time, e.g. pulses per second (cps = counts per second, KCps = kilocounts per second), is called their intensity and contains in a first approximation the information about the concentration of the emitting in the sample. Two main types of detectors are used in wavelength dispersive X-ray fluorescence spectrometers: the gas proportional counter and the scintillation counter.

a. Gas Proportional Counter

9: A gas proportional counter.

The gas proportional counter comprises a cylindrical metallic tube in the middle of which a thin wire (counting wire) is mounted. This tube is filled with a suitable gas (e.g. Ar+ 10% CH4). A positive high voltage (+U) is applied the wire. The tube has a lateral aperture or window that is sealed with a material permeable to X-ray quants (Figure 9).

An X-ray quant penetrates the window into the counter’s gas chamber where it is absorbed by ionizing the gas atoms and molecules. The resultant positive ions move to the cathode (tube), the free electrons to the anode, the wire. The number of electron-ion pairs created is proportional to the energy of the X-ray quant. To produce an electron-ion pair,approx. 0.03 keV are necessary, i.e. the radiation of the element boron (0.185 keV) produces approx. 6 pairs and the K-alpha radiation of molybdenum (17.5 keV) produces approx. 583 pairs. Due to the cylinder geometric arrangement, the primary electrons created in this way see an increasing electrical field on route to the wire.

The high voltage in the counting tube is now set so high that the electrons can obtain enough energy from the electrical field in the vicinity of the wire to ionize additional gas particles. An individual electron can thus create up to 10.000 secondary electron-ion pairs. The secondary ions moving towards the cathode produce measurable signal. Without this process of gas amplification, signals from boron, for example, with 6 or molybdenum with 583 pairs of charges would not be able to be measured as they would not be sufficiently discernible from the electronic noise. As amplification is adjustable via high voltage in the counting tube and is set higher for measuring boron than for measuring molybdenum. The subsequent pulse electronics supply pulses of voltage whose height depends, amongst other factors, on the energy of the X-ray quants.

b.  Scintillation Counters

10.   Scintillation counter including photomultiplier.

The scintillation counter, “SC”, used in XRF comprises a sodium iodide crystal in which thallium atoms are homogeneously distributed ‘NaI(Tl)’. The density of the crystal is sufficiently high to absorb all the XRF high energy quants. The energy of the pervading X-ray quants is transferred step by step to the crystal atoms that then radiate light and cumulatively produce a flash. The amount of light in this cintillation flash is proportional to the energy that the X-ray quant has passed to the crystal. The resulting light strikes a photocathode from which electrons can be detached very easily. These electrons are accelerated in a photomultiplier and, within an arrangement of dynodes, produce so-called secondary electrons giving a measurable signal once they have become a veritable avalanche (Figure 10). The height of the pulse of voltage produced is, as in the case of the gas proportional counter, proportional to the energy of the detected X-ray quant.

c.  Pulse Height Analysis (PHA), Pulse Height Distribution

If the number of the measured pulses (intensity) dependent on the pulse height is displayed in a graph, we have the ‘pulse height spectrum’. Synonymous terms are: ‘pulse height analysis’ or ‘pulse height distribution’. As the height of the pulses of voltage is proportional to the X-ray quants energy, it is also referred to as the energy spectrum of the counter (Figure 11a and 11b). The pulse height is given in volts, scale divisions or in ‘%’ (and could be started in keV after appropriate calibration). The “%”-scale is defined in such a way that the peak to be to be analyzed appears at 100 %.

11.a.    Pulse height distribution (S) Gas proportional counter

11.b .   Pulse height distribution (Fe) Scintillation counter.

12.    Pulse height distribution (Fe) with escape peak.

If argon is used as the counting gas component in gas proportional counters, an

additional peak, the escape peak (Figure 12), appears when X-ray energies are irradiated that are higher than the absorption edge of argon.

The escape peak arises as follows:

The incident X-ray quant passes its energy to the counting gas thereby displaying a K electron from an argon atom. The Ar atom can now emit an Ar Kα1,2 X-ray quant with an energy of 3 keV. If this Ar-fluorescence escapes from the counter then only the incident energy minus 3 keV remains for the measured signal. A second peak, the escape peak that is always 3 keV below the incident energy, appears in the pulse height distribution.

When using other counting gases (Ne, Kr, Xe) instead of argon, the escape peaks appear with an energy difference below the incident energy that is equivalent to the appropriate emitted fluorescence radiation (Kr, Xe). Using neon as the counting gas component produces no recognizable escape peak as the Ne K-radiation, with energy of 0.85 keV, is almost completely absorbed in the counter. Also, the energy difference to the incident of 0.85 keV and the fluorescence yield are very small.

ENERGY DISPERSIVE X-RAY FLUORESCENCE (ED-XRF)

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 :

1. X- Ray excitation source
2. Sample Chamber
3. Si (Li) detector
4. Preamplifier
5. Main Amplifier
6. Multichannel Pulse Height Analyzer

The properties and performances of an EDXRF system differ upon the electronics and the enhancements from the computer software.   

Typical ED-XRF detection arrangement.

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

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 .

Cross section of an Si(Li) detector crystal with p-i-n structure and the

production of electron-hole pair.

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.

 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

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.