P- 40 CURTIS: From 1939 to 1945 (Planes and Pilots, 3) by Anis ElBied, Daniel Laurelut

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Also, in contrast to X-ray scattering, some neutron scattering factors are negative (which implies a 180 phase shift after scattering). This is the case for the isotopes 2H and 7Li. Since most (isotopes of) elements have positive scattering factors, a negative scattering can be advantageous when attempting to locate these atoms in difference Fourier maps. A handful of isotopes even possess an “imaginary” component (which implies a phase shift between 0 and 180 after scattering). Neutron scattering factors vary over a smaller range of relative values than X-ray scattering factors do.

For a Bragg-Brentano diffractometer in the y/2y setup (Fig. 15b), the X-ray tube remains fixed while the specimen plate varies as y and the detector varies as 2y. In the y/y setup (Fig. 15a), the X-ray tube varies with y, the specimen remains fixed (although it can rotate about an axis perpendicular to the plate or about the goniometer axis), and the detector varies as y. W. 14 Preferred orientation effects in powder photographs. ) A. Shape orientation. Needle-like crystals of PbO. B. Shape orientation.

16) where m/r is referred to as the “mass absorption coefficient”, r is the sample density, and x is the thickness of the sample through which the beam passes. The mass absorption coefficients are a function of the element and the wavelength. For a given material, the mass absorption coefficient is a weight average of the individual elements that comprise the sample. 29 shows how m/r varies with wavelength for cobalt. The wavelength of CuKa radiation is only slightly shorter than the wavelength at the absorption edge of cobalt.