Neutrons and Materials Science
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ution function directly, in terms of a parameter q, which is related to the change of momentum of the neutron when it is scattered through a given angle. (See Figure 3.) By comparison with électrons, which are charged, both x-rays and neutrons are only weakly scattered by most materials. Since weak scattering means weak signais, this would at first appear to be a disadvantage; in ail but the most perfect crystals, however, x-rays and neutrons are singly scattered and are consequently more readily susceptible to quantitative interprétation than électron scattering. A principal advantage of neutrons over x-rays is that the weak neutron-nucléon interaction means that neutrons can travel large distances through most materials without being absorbed, and this high penetrability is a great asset. Real materials are often of interest because they contain mixed phases, for example, and samples must be large enough to ensure obtaining a représentative measurement. In addition, information is often needed about material properties under conditions of processing or end use. This is usually straightforward, since neutron beams easily penetrate furnaces, cryostats, and shear or pressure cells. More than 95% of a neutron beam is transmitted through 5 mm of aluminum, for example, compared with less than 0.1% of an x-ray beam. A second advantage is that the neutron scattering power of a given élément is not simply related to atomic number but has about the same magnitude throughout the periodic table. This makes neutron scattering an extremely powerful technique for studying light atom structures, such as aerospace materials, or compounds containing both light and heavy atoms such as métal hydrides. Radiation of any type is scattered by changes in refractive index, with the magnitude of the change being
known as the contrast. For light or x-rays, the refractive index varies as the square root of the électron density. For neutrons, the quantity analogous to électron density is the scattering amplitude density, where the nuclear scattering amplitude is a measured quantity with the dimensions of length.1 Some examples of neutron scattering amplitudes are given in Table I, where another feature will also be noted: scattering power may vary dramatically between différent isotopes of the same élément. The différence between hydrogen and deuterium is especially important in studies of organic materials, where one type of molécule (or part of a molécule) may be labeled by deuteration. The importance of this technique of contrast variation will be described in the article by Wignall and Bâtes, elsewhere in this issue. This same technique is also very important in the study of biological materials. An interesting variation on the thème of contrast variation is neutron résonance radiography. Neutron radiography is a well-known technique that is used in much the same way as x-radiography. Objects as diverse as ancient Egyptian urns, internai combustion and jet engines, and fighter aircraft wings are routinely examined by this method. The techn
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