Ultra Small Angle Neutron Scattering

In small angle neutron scattering we attempt to determine structures in the size range 1-100 nm. In Ultra Small Angle Scattering the range is 50-20000 nm. This region spans the gap between SANS and optical methods. As in SANS we can obtain both sizes and compositions in structures at this scale. Such structures can be voids, cracks, very small crystals, or as in our case aggregates of surfactant lamellar phases.  The size range determines the small scattering angle since the neutrons used have wavelengths about 4 Å. With such wavelengths, the neutron scattering angles need to extend to VERY small angles — about 1/1000 of a degree. 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. In optical microscopy the contrast between these and water structures may be so small as to render the surfactant structures invisible. This is one advantage of USANS.

We have been using the USANS machine at NIST in the USA. This machine, named BT-5, operates in an angle dispersive mode. The incoming and exit beams must be very highly collimated to measure scattering angles down to 1/1000°. This is accomplished by use of 2 "triple bounce" monochromators. In these a slot is cut in a large single crystal of silicon. The neutron beam is Bragg diffracted 3 times while  bouncing down this slot. In between the monochromators the neutrons pass through the sample, which is often 1-2mm in thickness, and some neutrons are scattered, mostly without change in wavelength. Those scattered at ultra small angles are counted by a counter after passing through the second monochromator. The major limitation is at high Q where neutron flux is a problem. So a slit geometry beam is used, which requires the large cells illustrated above. Great care is also taken to reduce background neutrons to about 1 counted every 100 seconds. The I(Q) obtained appertains to a slit geometry. This can be deconvoluted to give an I(Q) from a hypothetical pinhole geometry before modelling. 

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. crack dimensions), but we can also obtain compositions. In our case we have been examining droplet size distributions and surfactant aggregates of micron scale in emulsions.