When an electron beam is transmitted through a very thin film of matter an electron diffraction pattern is formed by electrons which are elastically scattered in the crystalline regions. Electrons which are inelastically scattered, together with electrons which are elastically scattered in noncrystalline regions, form a diffuse background, and this tends to mask the diffraction pattern. When thicker specimens are used the number of scattered electrons increases but the contrast of the diffraction pattern decreases because the pattern is formed only by singly scattered electrons whilst the background includes all plurally scattered electrons.
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