Neutron Backscattering Technique Research Articles

The NIST high-flux backscattering spectrometer

The neutron-backscattering technique extends the dynamic range of neutron time-of-flight and conventional triple-axis spectrometers down to μeV energies and is ideal for the study of slow motions in complex liquids, jump diffusion, and quantum rotational tunneling. We report on the performance of the new high-flux backscattering spectrometer (HFBS) at the NIST Center for Neutron Research. Compared to other backscattering spectrometers, the HFBS achieves a higher neutron intensity through the use of a device called a phase space transformation chopper and a large analyzer array. A custom-designed Doppler drive for the monochromator provides a triangular velocity profile capable of high-frequency operation, thus extending the dynamic range of the spectrometer by more than a factor of two over similar instruments.

Incoherent quasi-elastic neutron scattering study of highly oriented fibres of copper laurate in the columnar mesophase

Oriented fibres of dicopper tetrakis(dodecanoate) Cu2(COOC11H23)4 were investigated using the neutron backscattering technique both in the columnar liquid crystal phase and in the crystalline phase. Differences in the spectra recorded at various glancing angles in the columnar mesophase, on the time-scale 5 × 10-10 s, were assigned to anisotropic motions of the binuclear units within the columns. In the crystalline phase, on the 10-10 time-scale, the fixed-window technique yielded a determination of the elastic incoherent structure factor.

Critical proton and deuteron spin-lattice relaxation at the phase transition in p-terphenyl.

The spin-lattice relaxation around the order-disorder transition in p-terphenyl has been studied at different frequencies. The data are analyzed in terms of the critical slowing down for the librational fluctuations of the phenyl rings, providing interesting insights on the temperature behavior of the characteristic frequency width ${\ensuremath{\Gamma}}_{c}$ of the central peak in the dynamical structure factor S(q\ensuremath{\rightarrow},\ensuremath{\omega}). This peak has been recently observed in the deuterated crystal through high-resolution neutron backscattering techniques. Values of ${\ensuremath{\Gamma}}_{c}$ are obtained from proton ${T}_{1}$ without adjustable parameters over a wide temperature range. These data are in agreement with the previous estimates from neutron scattering in the deuterated crystal and demonstrate the occurrence of critical slowing down. A critical exponent n=\ensuremath{\gamma}-\ensuremath{\nu}(d-z)\ensuremath{\simeq}1 is derived for T\ensuremath{\rightarrow}${T}_{c}^{+}$, indicating the failure of any mean-field description and suggesting a two-dimensional correlation in the fluctuations. Below ${T}_{c}$ the order parameter, as approximately derived from the ${T}_{1}$ data, exhibits a very rapid rise indicative of a small value of the critical exponent \ensuremath{\beta}, as also observed by superlattice neutron diffraction and as expected for low-dimensional systems. The data for the deuteron spin-lattice relaxation confirm the conclusion from proton ${T}_{1}$ and support the hypothesis of a negligible contribution of quantum tunneling to the critical dynamics driving the phase transition.

Observation of critical slowing down at the structural phase transition in p-terphenyl by high-resolution neutron spectroscopy

The high-energy resolution of the neutron backscattering technique was used to observe the critical slowing down of the orientational fluctuations of phenyl rings in p-terphenyl above the order-disorder phase transition at Tc approximately=180K. The observed relaxation times tau c range from approximately 2*10-11 s at room temperature up to 1.15*10-8 s close to Tc.