Diffraction of electrons, neutrons and X-rays

As a simple example, a diatomic molecule such as N2 with a certain N-N distance could act as a diffraction object with two scattering centers. The situation is very much analogous to a double-slit experiment with visible light. Those parts of the two spherical waves that emerge from the two sUts (or the two atoms) in phase lead to constructive interference. The diffraction angle G under which the conditions 

Interference patterns (thick Unes) formed by the superposition of waves of the same wavelength hut different phases (thin and dashed lines) (a) same phase, (b) slightly out of phase, (c) almost opposing phases, and (d) exactly opposing phases. The double arrows represent the phase differences. 

Diffraction geometry for constructively interfering waves emerging from a double slit. 

The principles of diffraction are more or less the same, no matter what is being diffracted, so in the following sections we will concentrate on the most pertinent diffraction technique used to study each particular phase. However, it is important to be aware that neutrons and electrons are predominantly diffracted at the atomic nuclei, whereas X-rays interact mainly with the electrons. Consequently, electron and neutron diffraction enable nuclear positions to be determined, whereas X-ray diffraction gives the positions of centers of electron density - which is not quite the same thing, in particular when the electron distribution about the nuclei is aspherical. The consequences of this difference will be discussed in the following sections. 

Variation of scattering power (the scattering amplitude f) with atomic number for (a) X-rays and (b) neutrons, (c) Variation with angle (expressed as sin 9/X) for X-ray and neutron scattering by carbon. 

---