Small Angle Neutron Scattering

In small angle neutron scattering we attempt to determine structures in the size range 1-100 nm. These may be surfactant micelles, adsorbed surfactant monolayers, very small crystal nuclei etc. In SANS we can obtain both sizes and compositions in these structures. The size range determines the small scattering angle since available neutrons have wavelengths  of 3-20 Å. By using contrast variation we can produce several scattering patterns from a single chemical composition.   The, generally, differently deuterated components can be assembled to form several samples highlighting different components. For example, in an emulsion we can deuterate such that we highlight only surfactant structures. Small angle x-ray scattering  is often used as an 'extra' contrast, since neutrons and x-rays are scattered differently by a given sample.

We currently use several SANS machines, LOQ at ISIS in the UK, SAND at Argonne in the USA and AUSANS here at home. LOQ is illustrated in the figure above. It operates on the ISIS spallation source which produces pulses of neutrons. Within each pulse there are a range of wavelengths, and as the pulse travels down the flight tube (from the left in the figure) it spreads out — since different wavelengths travel at different speeds, from about as fast as a rifle bullet to a fraction of this. The neutrons pass through the sample, which is 1-2mm in thickness, and some neutrons are scattered, mostly without change in wavelength. Those scattered at small angles are counted by the area counter on the right. The time of arrival shows the speed of the neutron from target source to counter, while the position on the counter gives the angle of scatter. These are combined to give a wavevector, Q,  from which an intensity of scatter versus Q plot can be constructed (I assume here, which is not always so that the sample is isotropic).

We then have an I(Q) for each of several contrasts. As in all scattering experiments these cannot be directly inverted to give structure. We must model the data, proceeding from an assumed structural type, whose scattering can be calculated; fitting that scattering to the data and thus extracting the model's structural parameters. Since neutron scattering produces I(Q) on an absolute scale, not only can we obtain sizes of structures (e.g. radii of micelles, their polydispersity and so on), but we can also obtain compositions. If for example we have the contrast highlighting surfactant only we ca obtain how much surfactant (in g mL^-1) is adsorbed in any monolayer. In other cases we can find how much solvent is absorbed into micelles, the list continues. For more details have a look at our projects on emulsion structure, biological emulsions and oligomer structures in oil.