Sunday, 1 December 2013

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.


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