What is 'scattering'?
Scattering is a catch-all term for what happens when a sample 'does something' to a beam of radiation. You probably already use scattering techniques in your everyday research, for example, UV/Vis or IR spectrophotometry, or DLS. These examples use electromagnetic radiation (photons) to probe your sample. In the case of UV/Vis and IR spectrophotometry the sample absorbs photons at particular wavelengths; in the case of DLS the Brownian motion of particles in the sample changes the energy of the photons. These processes are both examples of spectroscopy, the measurement of dynamics, or how atoms and molecules move. The opposite of spectroscopy is diffraction, how a sample is structured. In diffraction the sample simply changes the angle at which the radiation passes through it. This is, of course, the basis of the field of crystallography.
What is SANS?
SANS is essentially low-resolution diffraction using neutron radiation. Here the term low-resolution simply means that the technique does not resolve atomic length scales (for that you can do neutron crystallography!). Instead, it resolves molecular and macromolecular length scales, typically from 1 nm up to 100 nm and beyond (depending on the instrument). These are the same length scales that some refer to as the colloidal size range and others refer to as the 'nanoscale'! Indeed, SANS is not a new technique, it has been around since the 1950's! What is changing is the background of the scientific user community. What was once a preserve of physicists is now attracting increasing numbers of 'soft matter', biomedical, and environmental scientists.
Neutrons vs Light or X-rays
It is possible to do small-angle scattering with photons, and indeed it may be enlightening to do so in certain samples, but there are consequences. If you use light you can only look at structure on length scales between about 0.5 μm and a few mm. This is because light has wavelengths which are several hundred times longer than typical neutron wavelengths. Physics prevents us from measuring anything smaller than the wavelength of the radiation we use: what is called the 'diffraction limit'. Nonetheless some DLS systems, mainly those with moving detectors, can be operated in static light scattering or total intensity mode. In this mode they are performing small-angle light scattering (SALS) measurements. The classic apparatus for SALS was the Sophica 4200™. A few are still in use to this day, albeit with the incandescent light bulb now replaced by a laser.
It is equally possible to perform small-angle X-ray scattering (SAXS). X-rays have comparable wavelengths to neutrons and so measure similar length scales. The difference between SANS and SAXS lies in the way the radiation interacts with the sample; in SANS the neutrons are scattered by the nuclei, but in SAXS (or SALS) the photons are scattered by the electrons. What this provides for is complementary information about the structure of the sample. An analogy might be the difference between viewing the world through a normal camera and viewing it through a thermal imaging camera: the big picture is the same, but different detail stands out. In SAXS, heavier elements always mask the scattering from lighter elements. Not so in SANS.
There are actually many aspects to the complementarity of neutrons and X-rays. Most are beyond the scope of this primer, but are summarised in the accompanying table. However one aspect worth highlighting is that known as 'contrast'. Put simply, contrast is how well some part of the sample shows up when illuminated by the radiation. In SALS the contrast is determined by differences in refractive index. In SAXS it is determined by differences in electron density. And in SANS the contrast is determined by a quantity known as the scattering length density (SLD). The SLD turns out to be very sensitive to variations in the proportion of hydrogen or deuterium in a sample. Thus by simply varying, for example, the proportion of H2O or D2O in an aqueous matrix it is possible to 'contrast match' different components of a sample and essentially make them invisible to neutrons! This can be a huge asset in aiding the understanding of a complex multi-component system.
If you have any experience of SALS or SAXS you will be right at home with SANS because all three techniques use the same equations to analyse their data. The only thing that changes is that contrast term as discussed above.
What can SANS tell me?
In principle, information about:
- size (eg, particle radii, radii-of-gyration, correlation lengths, shell/corona thicknesses, etc)
- size distributions
- shape (eg, are the particles spherical, ellipsoidal, cylindrical, random coil, fractal, etc)
- inter-particle interaction potentials (if present)
- morphological characteristics (eg, are the particles homogeneous, core-shell, multi-shell, porous, etc)
- fractal dimensions
- surface area-to-volume ratios
- orientation (in some instances)
Inter-layer spacings can be readily resolved from layered structures (eg, liquid crystals, vermiculite clays, vesicles, etc).
OK, I'm hooked, how do I get to do some SANS measurements?
Unlike a laser, a practical neutron source is not something that you can have sat in the laboratory. Your samples will have to be measured at a neutron facility. There are several dozen such facilities dotted around the world, with the greatest concentrations in Europe and the USA. Most are co-located at research reactors (because neutrons are produced as a consequence of nuclear fission).
The UK Research & Innovation, through the STFC, contributes to the operation of two neutron facilities, and funds access to them for the researchers they support. These facilities are the reactor at the Institut Laue-Langevin (ILL) in Grenoble, France, and ISIS at the STFC's Rutherford Appleton Laboratory in Oxfordshire, UK. ISIS is a spallation neutron source; a powerful particle accelerator is used to 'chip' neutrons from a metal target! Both of these facilities are free-at-the-point-of-access for anyone engaged in normal academic research. Only if it is intended to keep the results of the research confidential, as might arise in a commercial context, for example, is there a charge for neutron beam time.
ISIS and the ILL both operate 6-monthly calls for experiment proposals which are then subject to competitive peer review, and they also entertain 'rapid access' applications throughout the year for 'hot science'. Rapid access proposals are subject to a modified review process. These experiment proposals are not anything like as onerous as a normal research grant application - the science case is the equivalent of just 2 sides of A4 paper - but the chance of success in the review process is typically only 40-50%, such is the fierce competition for available beam time.
Recognising that these established access mechanisms presented something of a barrier to wider participation by new scientific communities or those simply wanting to make short measurements or feasibility studies, ISIS launched 'Xpress Access'. This is a measure-by-courier service without peer review. The catch, of course, is that the number of samples that can be measured, and the duration of each measurement, is strictly limited. However the intention is that successful express access measurements help encourage more standard proposals.
Dr Stephen King, ISIS
Further reading:
'Neutrons in Soft Matter', (editors) T Imae, T Kanaya, M Furusaka, N Torikai, Wiley (April 2011). ISBN 978-0-470-40252-8
