The invention refers to a device for measurement of the thickness of a first layer, comprising one or more sublayers, on a second layer of a metal sheet by X-ray fluorescence analysis comprising means for generating and directing a beam of polychromatic primary X-rays, said beam being able to penetrate into the first and second layers for converting primary X-rays into chemical element specific fluorescent X-rays by means of absorption of primary X-rays and re-emission of fluorescent X-rays by the chemical element, and further comprising means for detecting element specific fluorescent X-rays and determining an intensity thereof. The means for detection have been placed at an angle with respect to the primary beam of X-rays in dependence of the chemical element from which the fluorescent X-rays are to be detected. Herewith an improvement of the efficiency of detection is achieved, and the measurement time is reduced accordingly. Hence, a device is provided with which alloys with a low concentration of fluorescent elements can now be analysed, for determining the thickness of a cladding.

 

CHAPTER ONE
1.0                                                        INTRODUCTION
Quantitative X-ray fluorescence analysis of most common materials (major ores and alloys such as aluminium and zinc) is affected by several adverse factors that influence accuracy of element determinations. Most attention over the past two decades or so was focussed on inter-element interactions and their mathematical corrections using various fundamental-parameters methods. However, spectral interference plays a role, which is very often not properly recognized and has even been neglected. Spectral interference considered important in the authors’ practice was identified and characterized for 38 elements in the range B(5) – Bi(83). Selected examples demonstrate the magnitude of the analytical error occurring if spectral interference has not been corrected. An example of a complex application involving analysis of 26 elements in aluminum alloys is shown. The standard error obtained in the calibration process is compared for three cases: inter-element correction alone, inter element and spectral correction, and no correction at all. The effect of spectral correction on standard error obtained in the calibration process is assessed.
In modern quantitative XRF analysis of materials it is necessary to apply matrix corrections that compensate for the interactions among the elements and their radiation. Most attention over the past two decades or so was focused on inter-element interactions and their mathematical corrections using various fundamental-parameters methods. However, wavelength spectral interference plays a role, which is very often not properly recognized and has even been neglected. Spectral interference is defined as a condition when radiation corresponding to a specific analyst line and radiation of another line enter the detector at the same time. Another type of spectral interference originating during the pulse-height distribution analysis is not dealt with in this work.
Unlike atomic spectra that are characterized by a large number of spectral lines the X-ray characteristic radiation of elements is relatively simple. Low atomic number elements are characterized by just a few major spectral lines. Middle atomic number and heavy elements have an increased potential for spectral interference. Nevertheless, the number and magnitude of possible spectral interference is limited and can be handled mathematically. This work deals with spectral interference that we managed to come across in our practice. The paper does not pretend to be a complete document on spectral interference in XRF analysis. Although the work does not cover all spectral interference, it would apply to most common materials encountered in daily practice. Another objective was to demonstrate on selected examples the magnitude of analytical error occurring if spectral interference was not corrected.

1.1                                               OBJECTIVE OF THE STUDY
X-ray fluorescence is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays. The aim of this study was to evaluate the effect of the modulation of the radiation spectrum with the use of X-ray on aluminium and zinc.

1.2                                           APPLICATIONS OF THE STUDY

X-Ray fluorescence and is absorption rate is used in a wide range of applications, including

• research in igneous, sedimentary, and metamorphic petrology
• soil surveys
• mining (e.g., measuring the grade of ore)
• cement production
• ceramic and glass manufacturing
• metallurgy (e.g., quality control)
• environmental studies (e.g., analyses of particulate matter on air filters)
• petroleum industry (e.g., sulfur content of crude oils and petroleum products)
• field analysis in geological and environmental studies (using portable, hand-held XRF spectrometers)

X-Ray fluorescence is particularly well-suited for investigations that involve

• bulk chemical analyses of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) in rock and sediment
• bulk chemical analyses of trace elements (in abundances >1 ppm; Ba, Ce, Co, Cr, Cu, Ga, La, Nb, Ni, Rb, Sc, Sr, Rh, U, V, Y, Zr, Zn) in rock and sediment – detection limits for trace elements are typically on the order of a few parts per million

1.3                                           SIGNIFICANCE OF THE STUDY

X-Ray fluorescence is particularly well-suited for investigations that involve:

• bulk chemical analyses of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) in rock and sediment
• bulk chemical analyses of trace elements (>1 ppm; Ba, Ce, Co, Cr, Cu, Ga, La, Nb, Ni, Rb, Sc, Sr, Rh, U, V, Y, Zr, Zn) in rock and sediment.

1.5                                            LIMITATIONS OF THE STUDY

In theory the XRF has the ability to detect X-ray emission from virtually all elements, depending on the wavelength and intensity of incident x-rays. However, X-ray fluorescence is limited to analysis of fluorescent effect and absorption of x-ray radiation aluminium and zinc.

• In practice, most commercially available instruments are very limited in their ability to precisely and accurately measure the abundances of elements with Z<11 in most natural earth materials.
• XRF analyses cannot distinguish variations among isotopes of an element, so these analyses are routinely done with other instruments
• XRF analyses cannot distinguish ions of the same element in different valence states, so these analyses of rocks and minerals are done with techniques such as wet chemical analysis or Mossbauer spectroscopy.
• relatively large samples, typically > 1 gram
• materials that can be prepared in powder form and effectively homogenized
• materials for which compositionally similar, well-characterized standards are available
• materials containing high abundances of elements for which absorption and fluorescence effects are reasonably well understood

In most cases for rocks, ores, sediments and minerals, the sample is ground to a fine powder. At this point it may be analyzed directly, especially in the case of trace element analyses. However, the very wide range in abundances of different elements, especially iron, and the wide range of sizes of grains in a powdered sample, makes the proportionality comparison to the standards particularly troublesome. For this reason, it is common practice to mix the powdered sample with a chemical flux and use a furnace or gas burner to melt the powdered sample. Melting creates a homogenous glass that can be analyzed and the abundances of the (now somewhat diluted) elements calculated.

 

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