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The size of the nucleus
A fast electron sees the nucleus only through its charge. Its wavelength 2πħc/E, , is of the size of the nucleus, so the scattered wave carries a diffraction pattern, like light behind a disc. In the plane-wave Born approximation (PWBA) the minima sit where the form factor F(q), the Fourier transform of the charge density, passes through zero. A bigger nucleus or a faster electron moves them to smaller angles.