In this study, published in Physical Review Research, the combination of polarization analysis with the unique capabilities of the LET spectrometer was used to distinguish the incoherent and coherent dynamics of deuterated water (D2O) for the first time. Incoherent scattering is due to the diffusion of single D2O molecules, whereas coherent scattering, according to the so-called viscoelastic model, is due to the interaction of the D2O molecules with their neighbouring molecules, as well as diffusion. By using polarized neutrons, the researchers were able to distinguish the diffusive motions of the D2O molecules from the other dynamical contributions.
Being able to separately analyse the incoherent and coherent contributions is important because the two often coincide in neutron scattering data, making it challenging to tell them apart. It is often assumed during studies of deuterated materials that the incoherent scattering is insignificant, but this study has shown that this is not necessarily the case.
By being able to measure each contribution unambiguously with polarized neutrons, the researchers were able to verify molecular dynamics simulations of D2O at intermediate length scales where the incoherent and coherent scattering intensities are roughly equal; something that was not previously possible. This offers new insights into the origins of density fluctuations occurring at these mesoscopic length scales in water, and the validity of using the viscoelastic model for glass-forming liquids.
Neutron polarization analysis uses the spin of the neutron to separate different contributions to the overall scattering signal. From a technical point of view, performing high resolution neutron spectroscopy with polarization analysis on spectroscopic measurements has been a long-standing goal in the neutron scattering community. While several previous attempts had been made, either the performance or usability of the equipment have prevented its widespread adoption.
The three-year project to build the LET polarized option (PLET) involved a large number of ISIS staff across several divisions, including scientists, engineers, software engineers, sandwich students and technicians. The combination of a supermirror neutron polarizer in the incident beam, and a 3He spin-filter analyser in the scattered beam, enabled the experiments to have high counting rates and uncompromised resolution over the whole (and huge!) LET detector array.
Professor Arantxa Arbe of the Centro de Fisica de Materiales in San Sebastian, Spain, who led this research said, “Resolving coherent and incoherent contributions at intermediate length scales in a system like D2O was probably the most demanding experiment one could design to demonstrate the maturity of the technique. The performance of PLET is beyond any expectation."
This study highlights the importance of measuring the incoherent and coherent contributions separately in accurately analysing and interpreting data. Because of the significant remaining nuclear spin incoherent scattering, it may not be sufficient to deuterate a sample to measure its collective dynamics and, conversely, it may not be sufficient to deuterate the hidden component in a mixture in order to highlight the signal from the incoherent component.
This experiment leads the way for future studies, well beyond the study of water and other liquids, extending to soft materials, biological systems, and energy materials. For example, polarization analysis could be used in battery materials to separate the scattering due to diffusion of lithium or sodium ions, from the scattering due to other atoms in the sample, or to separate the scattering of proteins and lipidic components from the cellular media.
Further information
The full paper can be found online, open access, at: https://journals.aps.org/prresearch/pdf/10.1103/PhysRevResearch.2.022015
