The first SANS instruments utilizing long flight paths, long wavelength neutrons from a reactor cold source and position sensitive detectors were developed in Europe (Julich, Grenoble, Pisa). Small angle neutron scattering instruments should really be called low-Q instruments (Q being the momentum transfer which for low scattering angles θ is given in terms of the neutron wavelength λ as Q=2πθ/λ). Low Q can be realized either through the use of small angles or high wavelengths. In order to obtain small angles, good collimation and good area detector resolution are needed. Good collimation is achieved through the use of long neutron flight paths before and after the sample. SANS instruments on continuous neutron sources use velocity selectors to select a slice
17
of the (often cold) neutron spectrum while time of flight SANS instruments use the whole spectral distribution with careful timing between the source chopper and the detector to separate out the various wavelength frames. In this last case (TOF instruments) the maximum length of an instrument is determined by the pulse frequency so as to avoid frame overlap problems (the slowest neutrons of a pulse should not interfere with the fastest neutrons of the next pulse).