RAY TECHNOLOGY

Analytical x-ray instruments ate used to characterize materials in several different ways. As with medical x-ray instmments there are analytical instmments that can produce images of internal stmctures of objects that are opaque to visible light. There are instmments that can determine the chemical elemental composition of an object, that can identify the crystalline phases of a mixture of soHds, and others that determine the complete atomic and molecular stmcture of a single crystal. These ate the most common appHcations for x-ray iastmments. 

The determination of particle size and stmctural iaformation for fibers and polymers, and the study of stress, texture, and thin films are appHcations that are growing ia importance and can be examined with x-ray iastmments. 

Because of recent advances ia hardware, particularly detector hardware, and computer software, x-ray instmments have become very powerful. Problems that could not be solved several years ago can be solved with the newer iastmmentation. Also the instmments have become much more automatic so that the mote routine problems can be solved much faster than a few years ago. 

There are several ways to produce x-rays that have the range of energies appropriate for the various appHcations discussed here. The most commonly used methods ate discussed below. 

Unlike for synchrotron radiation, the maximum iatensity of x-rays from an x-ray tube is limited by how fast heat can be removed from the target to prevent its melting. In a conventional sealed tube, the target is stationary, relatively small, and must be continually cooled with water. In a rotating anode tube, the target is larger and is continually rotated so that the heat can be distributed over a larger surface. With such a tube the amount of heat, and hence. 

Fluoroscopic screens allow direct examination of X-ray images by the eye. (Zn, Cd)S Ag is used almost exclusively (molar ratio of ZnS CdS, 60 40 particle size 20-40 pm). The emission spectrum corresponds well to the spectral sensitivity of the eye. 

The fluoroscopic screen consists of a substrate (cardboard, plastic sheet), which is first covered with a reflective layer of MgO or TiOz. The phosphor layer is applied by pouring, after dispersion in a binding agent based on acetylcellulose. 

Intensifying screens consist of cardboard or plastic sheets (e.g., polyester) as substrates. A reflective layer (e.g., TiOz) is first applied to this and then the phosphor-binder layer. 

Storage phosphors include Y202S Eu3+, Bi3 + GdOBr Sm3 + LaOBr Tb3 +, Bi3 + and the barium fluorohalides. 

In the optoelectronic X-ray image intensifier (Fig. 86), [5.427], the X-ray phosphor screen (input screen) is in direct optical contact with a photocathode that converts the luminance distribution of the X-ray screen into an electron-density distribution. The liberated electrons are accelerated in an electric field between the photocathode and an anode (20-30 kV) and are focused by electron lenses onto a second phosphor screen (output screen), where conversion of the electron image to a visible image takes place. 

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Analyses of alloys or ores for hafnium by plasma emission atomic absorption spectroscopy, optical emission spectroscopy (qv), mass spectrometry (qv), x-ray spectroscopy (see X-ray technology), and neutron activation are possible without prior separation of hafnium (19). Alternatively, the combined hafnium and zirconium content can be separated from the sample by fusing the sample with sodium hydroxide, separating silica if present, and precipitating with mandelic acid from a dilute hydrochloric acid solution (20). The precipitate is ignited to oxide which is analy2ed by x-ray or emission spectroscopy to determine the relative proportion of each oxide. 

The clad plate is x-rayed perpendicular from the steel side and the film contacts the aluminum. Radiography reveals the wavy interface of explosion-welded, aluminum-clad steel as uniformly spaced, light and dark lines with a frequency of one to three lines per centimeter. The waves characterize a strong and ductile transition joint and represent the acceptable condition. The clad is interpreted to be nonbonded when the x-ray shows complete loss of the wavy interface (see X-ray technology). 

Materials characterization techniques, ie, atomic and molecular identification and analysis, ate discussed ia articles the tides of which, for the most part, are descriptive of the analytical method. For example, both iaftared (it) and near iaftared analysis (nira) are described ia Infrared and raman SPECTROSCOPY. Nucleai magaetic resoaance (nmr) and electron spia resonance (esr) are discussed ia Magnetic spin resonance. Ultraviolet (uv) and visible (vis), absorption and emission, as well as Raman spectroscopy, circular dichroism (cd), etc are discussed ia Spectroscopy (see also Chemiluminescence Electho-analytical techniques It unoassay Mass specthot thy Microscopy Microwave technology Plasma technology and X-ray technology). 

A detailed account is given in Reference 20. The techniques giving the most detailed 3-D stmctural information are x-ray and neutron diffraction, electron diffraction and microscopy (qv), and nuclear magnetic resonance spectroscopy (nmr) (see Analytical methods Magnetic spin resonance X-ray technology). 

Sihcon has strong optical emission lines at 251.6113 and 288.1579 nm that can easily be detected by emission spectrography and that give sensitivities in the 1—100-ppm range. For nondestmctive analysis, either x-ray diffraction or x-ray fluorescence may be used (see Spectroscopy X-ray technology). 

Because of its extreme insolubiUty, barium sulfate is not toxic the usual antidote for poisonous barium compounds is to convert them to barium sulfate by administering sodium or magnesium sulfate. In medicine, barium sulfate is widely used as an x-ray contrast medium (see Imaging TECHNOLOGY X-RAY technology). It is also used in photographic papers, filler for plastics, and in concrete as a radiation shield. Commercially, barium sulfate is sold both as natural barite ore and as a precipitated product. Blanc fixe is also used in making white sidewall mbber tires or in other mbber appHcations. 

Analysis. Many analytical procedures calling for determination of molecular stmcture are aided by crystallization or requite that the unknown compound be crystalline. Methodologies coupling crystalliza tion and analytical procedures will not be covered here (see X-RAY TECHNOLOGY)... 

Computed tomography (CT) scan A series of x-ray scans taken from different angles and then compiled by computer to show a cross-section of a body part of interest a method of body imaging that uses x-ray technology to create cross-sectional images of a person s body. 

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