Neutron methods
H/D Conrast Variation
The particular strength of neutron scattering in the study of soft-matter systems is based on the possibility of generating specific scattering contrast by using different isotopes of the same element. Soft-matter systems use almost exclusively the replacement of light hydrogen (H) by the heavy hydrogen isotope, deuterium (D). The scattering properties of these two isotopes are drastically different; a deuterated molecule in a protonated environment (or vice versa) is clearly visible.
In combination with appropriate chemistry, one can investigate otherwise unattainable structural and dynamic properties of individual components of complex systems.
An elementary classical example is the elucidation of the shape (conformation) of a long, flexible polymer molecule in the melt. Without variation in the contrast of some of the polymer molecules, the shape of the individual molecule remains virtually invisible, as it is indistinguishable whether one molecule segment belongs to the same or another molecule (all look the same). Now, when a molecule is marked by H / D replacement, one can track the shape lengthwise through the other "colour" of the associated segments. The scattering intensity corresponds to the conformation of the molecule under consideration. In this way, in the pioneering period of neutron scattering and soft-matter systems, this question was clearly clarified by neutron small-angle scattering (KWS or SANS) on a mixture of isotope-labelled and "normal" polymers: the shape of the polymer chain corresponds to that of a so-called Gaussian tangle (quasi like a 3D random walk).
In a further step, the analysis of the change in speed of the neutrons during the scattering (inelastic or quasi-elastic scattering) can be used to infer the mobility of this Gaussian coil or other structures which each contribute to the scattering intensity. For this one uses then preferably the neutron spin-echo spectroscopy (NSE).
Isotope-labelled structures occurring side by side in a sample (e.g., an aqueous solution) can be viewed separately if the so-called scattered-length density of the solution is adjusted by isotope mixing (here, H2O: D2O).
Topics
- Neutron small angle scattering
- Neutron spin echo spectroscopy
- Neutron back scattering spectroscopy
- Neutron reflectometry and SANS, NSE and grazing incidence
- Other neutron techniques