Sources of X rays

FIGURE 7.4. The X radiation emitted when an electron falls from an outer to an inner level, (a) Electronic energy levels involved. Ka = L — K = 2p — s. K0 = M 

There are three accessories used to produce monochromatic radiation metal foil filters, crystal monochromators, and focusing mirrors. An element with atomic number Z can be used as a selective filter for radiation produced by an element of atomic number Z+ 1. For example, a nickel (Z=28) absorption filter, may be used to cut out the Cu KjS (Z=29 for Cu) radiation, leaving only Cu Ka radiation.Not all white radiation, however, is eliminated by this method. Alternatively a single-crystal monochromator may be used. An intense Bragg reflection from the monochromator crystal is used as the incident beam for X-ray diffraction studies. Focusing mirrors, designed to produce a beam that is not only monochromatic but also convergent, may be used. In this case the incident beam is doubly deflected by two perpendicular mirrors. 

Crystals also diffract neutrons, and this fact is useful to the crystal-lographer because it is the nuclei of the atoms that scatter neutrons rather than the electrons (which are the scatterers in X-ray diffraction). Among the difficulties encountered if one wishes to use neutron diffraction, however, are a need for bigger crystals and the uncertainty of availability of time at a nuclear reactor where neutron flux is available. The results of such neutron diffraction studies are particularly valuable for precise location of hydrogen atoms, for the differentiation of atoms of nearly the. same atomic number, and for distinguishing isotopes. 

The area detector - is an electronic device for measuring many diffracted intensities at one time. It is an electronic substitute for film, and is now used, where possible, for crystals of biological macromolecules. It is a position-sensitive detector, and is coupled to an electronic device for recording the data in computer-readable form. The data so recorded include the intensity of a Bragg reflection (diffracted beam) and its precise direction (as a location on the detector). Both types of information are needed for each Bragg reflection so that I(hkl), and sinO/X can be determined. 

Three types of area detectors are presently used in macromolecular crystallographic research (1) multiwire proportional counters, (2) television area detectors, and (3) imaging plates. A multiwire proportional counter consists of an anode between two cathodes these consist of arrays of parallel wires and are arranged perpendicular to each other. The chamber is filled with a gas, consisting partly of xenon which is ionized by the incident X rays, an effect recorded by the detecting device. Television area detectors contain a fluorescent phosphor that produces visible light when hit by an X-ray beam. After intensification, the photons are detected by a television photocathode. 

---

As early as the 1930s X-ray absorption experiments were being carried out using a continuum source of X-rays (the bremsstrahlung mentioned in Section 8.1.1.1), a dispersive... 

Dental x-rays provide valuable information on the health of teeth which cannot be obtained by any other medical imaging modaUty. Dental x-ray procedures use a piece of film placed in the mouth between the tongue and the teeth. A 60 to 70 keV source of x-rays, located outside the mouth, is directed at the film. Metal fillings attenuate x-rays striking the film and therefore appear white in a projection image. Tooth decay appears dark as it attenuates x-rays less than normal tooth enamel. 

Soft x-rays with wavelengths of 1—10 nm ate used for scanning x-ray microscopy. A zone plate is used to focus the x-ray beam to a diameter of a few tens of nanometers. This parameter fixes and limits the resolution. Holographic x-ray microscopy also utilizes soft x-rays with photoresist as detector. With a strong source of x-rays, eg, synchrotron, resolution is in the 5—20-nm range. Shadow projection x-ray microscopy is a commercially estabflshed method. The sample, a thin film or thin section, is placed very close to a point source of x-rays. The "shadow" is projected onto a detector, usually photographic film. The spot size is usually about 1 ]lni in diameter, hence the resolution cannot be better than that. 

The short wavelength of x-rays naturally makes them difficult to focus. Electrons, on the other hand, can rather easily be controlled to give beams a few square microns in cross section, a fact that made possible the x-ray emission electron-microprobe (9.9). Clearly, such a concentrated electron beam striking one side of a suitable thin target can give rise to an x-ray spot on the other, and this spot can be small enough to be regarded as a point source of x-rays. 

X-ray microtomography is a new development of great promise for reconstructing, displaying, and analyzing three-dimensional microstructures. Resolution of around 1 pm has been demonstrated with currently available synchrotron sources of x-rays, x-ray detectors, algorithms, and large-scale computers. The potential for microstructural research in composites, porous materials, and suspensions at this and finer scales appears to be tremendous. 

The silver gray metal can be cut with a knife, although it only melts at 1545 °C (for comparison, iron 1538 °C). It is the rarest of the "rare earths", but is nevertheless more abundant than iodine, mercury, and silver. Thulium has few applications, especially because it is relatively expensive. The element occurs naturally as a single isotope, namely 169Tm (compare bismuth). The artificial, radioactive 170Tm is a transportable source of X-rays for testing materials. Occasionally used in laser optics and microwave technology. 

An alternative and the most generally employed source of X rays for EXAFS experiments is that obtained from synchrotron sources based on electron (or positron) storage rings. 

Figure 14.13. Diagram showing the source of X-ray fluorescence photons.

For many of the analytical techniques discussed below, it is necessary to have a source of X-rays. There are three ways in which X-rays can be produced in an X-ray tube, by using a radioactive source, or by the use of synchrotron radiation (see Section 12.6). Radioactive sources consist of a radioactive element or compound which spontaneously produces X-rays of fixed energy, depending on the decay process characteristic of the radioactive material (see Section 10.3). Nuclear processes such as electron capture can result in X-ray (or y ray) emission. Thus many radioactive isotopes produce electromagnetic radiation in the X-ray region of the spectrum, for example 3He, 241Am, and 57Co. These sources tend to produce pure X-ray spectra (without the continuous radiation), but are of low intensity. They can be used as a source in portable X-ray devices, but can be hazardous to handle because they cannot be switched off. In contrast, synchrotron radiation provides an... 

Name two sources of x-rays, and explain how the x-rays are generated in each case. 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

An EXAFS experimental set-up has three primary components (i) a source of X-rays, (ii) a monochromator (and collimator) and (iii) a detector. Synchrotron radiation is being widely used for EXAFS, but where this facility is not available, a rotating anode source would be suitable. Progress in EXAFS instrumentation has been comprehensively reviewed in the AIP proceedings (1980). 

A drawback of SEXAFS is that synchrotron radiation is needed as a source of X-rays. 

The primary source of x-ray crystal diffract-tion reference data is the above-mentioned ASTM Powder Diffraction File , published by the Joint Committee on Powder Diffraction Standards. This file consists of over 38000 diffraction patterns of crystalline materials including expls and related materials. Scientists in the expls field routinely utilize this source. for the identification of expls and metastable phases by comparing the interplanar d spacings and intensities of exptl phases with those of known phases (Refs 4,10,21 22)... 

The third step in the structure determination is collection of the X-ray diffraction data. This may be done with a diffractometer in which a narrow collimated pencil source of X-rays is aimed at the crystal and the intensities and positions of the diffracted beams are measured automatically. The computer-controlled diffractometer is able to measure the angles to within less than one-hundredth of a degree. If sufficient time is allowed, very weak spots can be counted. Today, diffractometers are more likely to be used for preliminary measurements, while the major data collection is done with an area detector, an... 

It strongly supports that the energy source of X-rays both from an isolated pulsar and its nebula come from the rotating energy loss of the pulsar, and the energy transformation may be simply and directly. Eq.(l) offers a good empirical relation to further investigate their X-ray radiation mechanism. 

A monochromatic (single-wavelength) source of X rays is desirable for crystallography because the diameter of the sphere of reflection is 1/X, and a source producing two distinct wavelengths of radiation gives two spheres of... 

Whatever the source of X rays, the beam is directed through a collimator, a narrow metal tube that selects and reflects the X rays into parallel paths, producing a narrow beam. After collimation, beam diameter can be further... 

Fig. 14.3. Survey X-ray photoelectron spectra for (A) as-deposited diamond and (B) anodically oxidized diamond. The source of X-ray was Mg-Kot.

Figure 4 Regions of the density vs. temperature plane in which the various hydrogen-burning processes are dominant [MAT84c]. The normal CNO cycle occurs in stars slightly larger than the sun. The hot (beta-limited) CNO cycle is particularly important in supermassive stars. The rp-process is important during the thermonuclear runaways on accreting neutron stars which may be the source of X-ray bursts.

---