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Handbook of X-Ray Spectrometry - CRC Press Book
Allow All Cookies. Applied Spectroscopy Vol. Not Accessible Your account may give you access. Abstract Handheld X-ray fluorescence spectrometry XRF has seen a dramatic increase in use for archaeological projects. PDF Article. References You do not have subscription access to this journal. Cited By You do not have subscription access to this journal. Applied Spectroscopy Sergei G. Kazarian, Editor-in-Chief. Please login to set citation alerts. Equations displayed with MathJax. Right click equation to reveal menu options.
To resolve wavelengths in all regions, different crystals must be used, since crystals with large spacings must be used for long wavelengths but they make the short wavelengths irresolvable at low q Jenkins The system in the diagram utilizes two detectors in series. Most high-energy X-rays pass through it, however, and are counted by the NaI Tl scintillation detector. The gas-flow proportional detector works by placing a high voltage across a volume of gas usually Ar with methane.
An X-ray photon will ionize a number of Ar atoms proportional to its energy. The freed electrons are accelerated in the high voltage, ionizing other Ar atoms and creating an electron cascade which is controlled by the quench gas methane. The freed charges are measured in the circuitry as a voltage pulse whose height is proportional to the energy of the photon that initiated the cascade Jenkins An NaI Tl detector contains a large single crystal of sodium iodide that has been doped with thallium.
This crystal is sealed from light by a Be window. When an X-ray photon enters the crystal, it places primarily the I atoms in an excited state, in numbers again proportional to its energy. These excited states decay exponentially with time, giving off a flash of light or scintillation when they go. The summed intensity of light strikes a photocathode, which releases photoelectrons that are amplified in a discrete dynode detector.
The pulse height measured from this detector is proportional to the energy of the original X-ray photon Jenkins 96, Knoll One may wonder why these detectors need to have any energy resolution at all, since the X-ray energies are supposed to be dispersed by the Bragg crystal. With WDXRF systems, it may be possible to have several detector assemblies placed at fixed angular locations in order to analyze for a few selected elements over and over.
WDXRF spectrometers often offer more flexibility for the researcher as well as very good sensitivities. The detector outputs are also simpler to use directly and do not generally require heavy use of electronics and computer algorithms in order to deconvolute. Disadvantages include the inability to quickly acquire the entire X-ray spectrum for full-element analyses, higher hardware costs, and a larger instrumental footprint when compared to EDXRF systems.
While simpler in terms of the positioning of the detector versus the sample, EDXRF spectrometers require sophisticated electronics and computer software in order to interpret the detector output. Nowadays this is less complicated, though, due to important technological advances in multichannel analyzers and faster computers, and EDXRF is often the technique of choice for fast multielement analyses. Although germanium detectors are utilized, the most common type in service is the Si Li , or lithium-drifted silicon, detector.
A semiconductor detector operates based on the principle that an X-ray photon incident upon the diode material will give up its energy to form electron-hole pairs, the number of which is proportional to the energy of the photon. The high voltage applied across the diode quickly collects the released charge on a feedback capacitor, and the resulting proportional voltage pulse amplified by a charge-sensitive preamplifier.
Overview of X-ray Fluorescence
The output of the preamp is fed to a main amplifier system. The pileup rejector, part of this system, deals with the probable event that two pulses will arrive very close together in time. From this point, the pulse is converted to a digital signal and processed in the multichannel analyzer MCA Jenkins In the MCA, dead time , caused by high counting rates, must be corrected. Peaks in the energy spectrum, once acquired, are subject to a large degree of massaging by the software in the connected computer.
Sophisticated algorithms sense and quantitatively correct for high backgrounds due to Compton scattering from low atomic number matrices Metz Spectrometers that use secondary targets may acquire several energy spectra for each sample, one from each target. Since each target yields better sensitivity in one part of the spectrum, the information from the energy spectra is combined to quantitate each element being analyzed. Accurate quantitative data on the entire mass spectrum may be obtained in a matter of minutes using EDXRF.
For both of the Bruker Tracer instruments we use we have incorporated a secondary target made of thin sheets of copper, aluminum and titanium to optimize the spectra for the analysis of obsidian and any other analyses focusing on elements with with fluorescent energies between about 10 and 20 kV.
We have developed a world-renowned set of obsidian calibration standards that we have used to calibrate our own instruments and Bruker now runs this calibration on all portable XRF instruments heading out to museums and archaeologists. With this calibrations it is possible to acquire quantitative concentrations for many elements that are comparable to data acquired by mosre costly and destructive neutron activation analysis NAA. The tube voltage can be varied up to 45 kV, although we generally analyze the obsidian with a setting of 40kV.
The secondary target, or filter, primarily used includes a 6 mil thick sheet of copper used to block X-rays below about 20kV a 2 mil sheet of titanium added to remove the secondary copper X-rays and a 12 mil sheet of aluminum to absorb the titanium X-rays. Sample preparation is highly variable depending on the matrix and goals of the analysis.
Most of the materials we analyze obsidian, metals, and ceramic paints do not require any sample preparation. The choice of sample preparation depends on the nature of the X-ray beam relative to the sample. For example, a piece of obsidian that is 1 cm thick and has a clean, flat surface will provide ideal results. As sample sget smaller, thinner, or less homogenous it is necessary to understand the nature of the X-ray beam and how it interacts with the sample.
This small beam is fine for homogenous materials, but heterogenous material such as crystalline rocks and tempered pottery may need to be analyzed multiple times in numerous areas to generate a representative average composition.
The small beam size is ideal for isolating specific painted elements on the surface of ceramics and also aids in the analysis of very small obsidian artifacts. Perhaps even more important than the area of the beam is the depth of analysis. As a general rule, the higher up the energy spectrum, the greater the depth of X-ray penetration in the sample.
For example, the analysis of iron 6. In thick homogenous samples this depth of analysis makes little difference, but if samples are thinner, it effects to resulting spectrum in different ways depending on the specific sample thickness and particular element of interest.
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Ferguson in press addresses a number of approaches to quantitative analysis of thin samples. The ability to analyze samples without destructive sample preparation procedures has been a great advancement for archaeologists. We can now analyze large and valuable artifact assemblages that would have been off-limits to destructive proceedures. However, for non-archaeological applications of XRFthe most common method of sample prep is pelletizing, which can be made to work for most matrices that can be ground into an homogeneous powder, including soil, minerals, and dried organic matrices such as tissues or leaves.
Difficult grinding is accomplished with a hard agate mortar and pestle but many samples can be adequately homogenized by placing into a hard plastic vial, adding a plastic mixing ball, and violently shaking in a mixer mill. A powdery binder containing cellulose, starch, polyvinyl alcohol or other organics is usually weighed in and blended thoroughly with the sample, and the resulting mixture added to a deformable aluminum cup.
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Buhrke p. Here particle size and homogeneity play a big factor. This is due to the variance in X-ray penetration depths with energy Jenkins Particles may be inhomogeneous also, having a different surface composition than their bulk.
For example, copper sulfides may become partially oxidized at the surface, causing the relative absorption for Cu K lines to differ from that of the L lines. The L line photons will not penetrate as deeply and will tend to be emitted more from the oxide layer. One way to get around sample grinding is to fuse the sample at high temperatures with sodium or lithium tetraborate and then to pour this glass-like mixture into a mold Buhrke: Chemical reactions occur within the melt which dissolve particles and create a homogeneous liquid that hardens upon cooling.
The disadvantages to this technique include the additional time to prepare the melt and the possibility of the sample reacting with even inert crucible materials such as platinum. Homogeneous solid samples such as metals may be machined and smoothed to form disks. Whatever type of preparation is done, the surface roughness of the sample should be taken into account.
A rough surface causes the penetration layer to look heterogeneous to the spectrometer. Currently XRF spectrometry is very widely applied in many industries and scientific fields. The steel and cement industries routinely utilize XRF devices for material development tasks and quality control. Anzelmo Part 1 NIST utilizes XRF as one technique to quantitatively analyze and acceptance-test many of its standard reference materials SRMs , from spectrometric solutions to diesel fuel to coal to metal alloys Sieber The plastics industry is looking at a modified XRF spectrometer as an on-line wear monitor, taking advantage of its ability to detect particles of worn-off metal in extruded plastic pieces Metz Polish scientists are accomplishing XRF analyses on very thin films by placing the source and detector at very low angles with respect to the sample.