Flux and incident wave vector

Let us examine the characteristics of a neutron radiation adapted to the determination of molecular sizes (polymer) of the order of 10 nm. In this case, it is proper to use a neutron beam of wave vector k0 5 nm-1. 

However, the thermalized flux extracted from the source has a very broad distribution of wave vectors. To fulfil the required conditions, one has to select a flux fraction S J with wave vectors centred on k0 in the distribution J0 ((c) 

Let us set the sample under study at the origin of the laboratory system. An incident radiation propagates along OZ and falls on this sample. In certain conditions, which will be examined later, this radiation is scattered in all space directions, with an orientation distribution which depends on the structure of the sample. 

In principle, for the incident radiation, it is sufficient to determine the following parameters 

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