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3 Chopper Choppers are usually not standard modules in reactor SANS instruments, even though such devices may allow significantly improved Q-resolution, and in particular may be used for high-resolution time-resolved experiments. The application of choppers, however, usually causes major decrease in neutron flux. , 2008). Spectroscopic time-resolved SANS experiments require in addition a device that modulates the scattering properties of the sample. , 2006). 4 Collimation The neutron beam direction is defined by the collimator, which in principle can be given by just two pinholes, one near the source point and the other near the sample.

The fact that the interaction with neutrons, the scattering length, is sensitive to the nuclear isotopes is the reason for particular interests within soft matter research. Specifically, the scattering cross section of hydrogen is large and comparable to that of other isotopes, in contrast to X-rays, and the scattering power of normal hydrogen, 1 H , and of deuterium, D ¼ 2 H , is significantly different. By specific labeling, it is thereby possible to highlight specific structural properties. The fact that neutrons are magnetic spin-1/2 particles gives the possibility of studying magnetic structures and fluctuations, which is, however, usually not Neutrons in Soft Matter Edited by Toyoko Imae, Toshiji Kanaya, Michihiro Furusaka, and Naoya Torikai Copyright Ó 2011 John Wiley & Sons, Inc.

8). We will then explicitly split the terms in the sum in eq. 27) into the average values and the deviations from the average: X X ds X *   ¼ ðbi bj þ b*i bj À  b*i  ðb*i bj À b*i bj Þei~qðrj Àri Þ : bj Þei~qðrj Àri Þ ¼ b*i  bj ei~q ðrj Àri Þ þ dO i; j i; j i; j ðI:1:32Þ The key of the matter now is to remember that in any sample (except perfect crystals) we have a correlation length x beyond which the atoms are randomly placed and therefore do not contribute to the sums involved here. So in actual fact each sample has to be considered as an ensemble of a large number of independent, distinct “subsamples” of the material studied, and the cross section will be an average over all these subsamples.

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