Many-Particle Cooperative Scattering of X-Rays in the Two-Crystal Spectrometer

It has been emphasized by Guinier and Fournet [1] that low-angle x-ray scattering patterns should theoretically exhibit a relatively high, extremely narrow central maximum because the entire irradiated sample will scatter coherently in the same phase at scattering angles very close to zero. This effect is very small outside of a central region where h=(4π/λ)sin(Φ/2) is less than 2π/d, where d is the average dimension of the sample perpendicular to the incoming beam of x-rays [2]. The magnitude of d is almost always so large that the above effec t occurs at angles too small to be observed. Parratt et al. [3] have called this and similar effects "many-atom cooperative scattering," or "ultra-small-angle" scattering. In this letter, I shall adopt the term "many-particle cooperative scattering" and abbreviate it as "m.p.c. scattering."