An element-specific imaging technique utilizes the element-specific fluorescence X-rays that are induced by primary ionizing radiation. The fluorescence X-rays from an element of interest are then preferentially imaged onto a detector using an optical train. The preferential imaging of the optical train is achieved using a chromatic lens in a suitably configured imaging system. A zone plate is an example of such a chromatic lens; its focal length is inversely proportional to the X-ray wavelength. Enhancement of preferential imaging of a given element in the test sample can be obtained if the zone plate lens itself is made of a compound containing substantially the same element. For example, when imaging copper using the Cu La spectral line, a copper zone plate lens is used. This enhances the preferential imaging of the zone plate lens because its diffraction efficiency (percent of incident energy diffracted into the focus) changes rapidly near an absorption line and can be made to peak at the X-ray fluorescence line of the element from which it is fabricated. In another embodiment, a spectral filter, such as a multilayer optic or crystal, is used in the optical train to achieve preferential imaging in a fluorescence microscope employing either a chromatic or an achromatic lens.
CHAPTER ONE
1.0 INTRODUCTION
X-ray fluorescence (XRF) spectrometry is an elemental analysis technique with broad application in science and industry. XRF is based on the principle that individual atoms, when excited by an external energy source, emit X-ray photons of a characteristic energy or wavelength. By counting the number of photons of each energy emitted from a sample, the elements present may be identified and quantitated.
Henry Moseley was perhaps the father of this technique, since he, building on W.C. Röentgen’s discovery of the high-energy radiation dubbed X-rays, built an X-ray tube which he used to bombard samples with high-energy electrons. Moseley in 1912 discovered a mathematical relationship between the element’s emitted X-ray frequency and its atomic number. In 1925 Coster and Nishina were the first to use primary X-rays instead of electrons to excite a sample. After Glocker and Schreiber were the first to perform quantitative analysis of materials using XRF in 1928, detector technology had to catch up in order to make the technique practical, which didn’t begin to happen until the 1940’s. The 1950’s saw the first commercially produced X-ray spectrometers. In 1970, the lithium drifted silicon detector was developed, and this technology is still in use today (Jenkins 1988: 51-53).
Modern XRF instruments are capable of analyzing solid, liquid, and thin-film samples for both major and trace (ppm-level) components. The analysis is rapid and usually sample preparation is minimal or not required at all.
The identification of elements by X-ray methods is possible due to the characteristic radiation emitted from the inner electronic shells of the atoms under certain conditions. The emitted quanta of radiation are X-ray photons whose specific energies permit the identification of their source atoms. To understand this phenomenon, we must first look at how X-rays are generated.
When an electron beam of high energy strikes a material, one of the results of the interaction is the emission of photons which have a broad continuum of energies. This radiation, called bremsstrahlung, or “braking radiation”, is the result of the deceleration of the electrons inside the material.
Another result of the interaction between the electron beam and the material is the ejection of photoelectrons from the inner shells of the atoms making up the material. These photoelectrons leave with a kinetic energy (E-φ) which is the difference in energy between that of the incident particle (E) and the binding energy (φ) of the atomic electron. This ejected electron leaves a “hole” in the electronic structure of the atom, and after a brief period, the atomic electrons rearrange, with an electron from a higher energy shell filling the vacancy. By way of this relaxation the atom undergoes fluorescence, or the emission of an X-ray photon whose energy is equal to the difference in energies of the initial and final states. Detecting this photon and measuring its energy allows us to determine the element and specific electronic transition from which it originated (Jenkins 1988: 4-6, Anzelmo 1987 Part 1). Herein lies the basis for XRF spectrometry, where elements may be quantitated based on the rate of emission of their characteristic X-rays from a sample that is being excited.
Any of the electrons in the inner shells of an atom can be ejected, and there are various electrons in the outer shells that can “drop” to fill the void. Thus there are multiple types of allowed transitions that occur which are governed by the laws of quantum mechanics, each transition having its own specific energy or line (Jenkins 1988: 6). The three main types of transitions or spectral series are labeled K, L, or M, corresponding to the shell from which the electron was initially removed. K series lines are of the highest energy, followed by L and then M. Within the series, the specific transitions are denoted by the subscripts α, β, γ, etc. to denote which upper energy shell was involved in the relaxation and finally a numerical subscript to indicate the quantum state within that upper energy shell. For example, the Mo Kα1 transition yields a photon of wavelength 0.071 nm. (Jenkins 1988 p. 4) It is important to note that only the very highest resolution spectrometers could resolve Kα1 and Kα2 lines, so for practical purposes in X-ray spectrometry only the Kα line would be mentioned (Skoog 1998: 275).
Fluorescence, however, is not the only process by which an excited atom may relax. It competes with the Auger effect, which results in emission of a second photoelectron to regain stability. The relative numbers of excited atoms that fluoresce are described by the fluorescence yield, which increases with increasing atomic number for all three series (Jenkins 1988: 6).
High energy electrons are not the only particles which can cause ejection of photoelectrons and subsequent fluorescent emission of characteristic radiation. High-energy X-ray photons can create the same effect, allowing us to excite a sample with the output of an X-ray tube or any source of photons of the proper energy. In fact, in some applications of XRF spectrometry, X-rays from a tube are used to excite a secondary fluorescer, which emits photons that in turn are used to excite the sample.
When X-rays impinge upon a material, besides being absorbed, causing electron ejection and subsequent characteristic photon emission, they may also be transmitted or scattered. When an X-ray is scattered with no change in energy this is called Rayleigh scattering, and when a random amount of energy is lost the phenomenon is Compton scattering. Scattered X-rays are usually problematic in XRF, creating high levels of background radiation (Anzelmo 1987 Part 1).
Since only the inner electron shells are involved in the emission of X-rays, the wavelengths are independent (within our ability to measure) of the state of chemical bonding, which involves the outer-most electron shells only. One exception to this rule involves low-Z elements with fewer electrons. The overall lack of chemical shifts allows the analyst to determine the elemental composition of the sample, whether the elements are present in their pure forms or as compounds (Skoog 1998: 275).
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 copper.
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
1.4 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
- 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.
1.5 SCOPE OF THE STUDY
When materials are exposed to short-wavelength X-rays or to gamma rays, ionization of their component atoms may take place. Ionization consists of the ejection of one or more electrons from the atom, and may occur if the atom is exposed to radiation with an energy greater than its ionization potential. X-rays and gamma rays can be energetic enough to expel tightly held electrons from the inner orbitals of the atom. The removal of an electron in this way makes the electronic structure of the atom unstable, and electrons in higher orbitals "fall" into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two orbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy.
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