Investigation of nematic order by coherent neutron scattering

The mathematical relationship between the orientational order parameters and the coherent neutron scattering cross section for a nematic liquid crystal is given. For deuterated para-azoxyanisole the single-molecule part of the cross section is evaluated within the meanfield approximation and combined with experimental results to give information about molecular orientational order in terms of P ₂, P ₄ and P ₆. Both P ₂ and P ₄ are found necessary for describing the molecular order. Discrepancies between experimental and theoretical results are interpreted as possibly reflecting the inadequacy of the meanfield theory of Maier and Saupe.     

Short range order investigation by neutron scattering

Neutron scattering measurements on three Fe-rich samples of the Fe-Al system have been performed and the nuclear diffuse component has been separated.Some results concerning the short range order parameters and the local environment in the iron matrix correlated with the Al concentration are presented.     

Measurement of the coherent neutron scattering length of 3He by neutron interferometry

A precise determination of the coherent scattering length of ³He with a neutron interferometer yields a value a_c = 4.29 ± 0.04 fm. A comparison with varoous theoretical perdictions is made and its relation to the few-body problem is discussed. A combination with other experimental results yield as most probable values for the free singlet- and triplet scattering length a_s = 8.0 ± fm and a_t = 3.05 ± 0.07 fm, respectively.     

Measurement of coherent neutron scattering lengths of gases

With the neutron interferometer, measurements of the coherent neutron scattering lengths by various gases have been performed. We get the following numbers for the bound coherent scattering lengths for atoms (in fm):b _c (H) =?3.64(3),b _c (D)=6.55(8),b _c (He?3)=5.74(7),b _c (He)=3.26(3),b _c (N)=9.30(8),b _c (O)=5.83(5),b _c (Ne) =4.63(4),b _c (Ar)=2.07(2),b _c (Kr)=7.52(6),b _c (Xe)=4.69(4). In some cases greater accuracy is achieved in comparison with recommended values from the literature. The absolute values for hydrogen and deuterium measured with gaseous samples, are about 2% smaller than the values measured with liquids and other reference values.     

Determination of the elastic constants by coherent neutron scattering in polycrystals

It is shown that elastic constants which usually can be determined in single crystals only may be measured in polycrystals by inelastic coherent neutron scattering. Measurements are reported for polycrystalline samples of aluminium, copper and stainless steel. The method is best suited for the determination of the elastic constants connected with transverse phonons.     

Determination of the phonon spectrum from coherent neutron scattering by polycrystals

As proposed by Bredovet al. [2, 3] the phonon spectrum can be obtained approximately from coherent neutron scattering by polycrystals if suitable averages over scattering angles are considered. The accuracy of this method is estimated by comparison with analytical results for simple lattice models (discussed here in particular for Aluminium). The errors are about 5% for low order moments and about 50% near van Hove singularities for “cold” neutrons (wavelength of the order of the nearest-neighbour-distance).     

Strong anisotropic nematic order in liquid crystal polymers: [4] a quasi-elastic neutron scattering study

From quasi-elastic neutron scattering experiments performed in glassy, nematic and isotropic phases, the dynamics of oriented samples of strong anisotropic side-on fixed liquid crystal polymers have been analysed. Using the selective deuteration method, we are able to attribute motions to specific parts of the molecule in the parallel and perpendicular orientations. The motions of the whole macromolecule decrease as soon as the temperature decreases below the isotropic-nematic transition. Nevertheless, the motions of the polymer backbone, compared to the whole polymer dynamics, are systematically reduced, even in the isotropic phase. Moreover, an anisotropy of the motions is revealed, with a reduction in the direction parallel to the orientation. An harmonic character of the vibrational processes is also evidenced. We conclude that the anisotropy of the dynamic corroborates the anisotropy of conformation of the macromolecule (so-called jacketed structure). Received: 29 October 1997 / Revised: 22 January 1998/ Accepted: 11 May 1998     

Self-diffusion in liquid lithium from coherent quasielastic neutron scattering

Using the method of quasielastic coherent neutron scattering by liquid lithium, the temperature dependence of self-diffusion coefficient is investigated and compared with analogous data extracted by the incoherent scattering method.     

Precision measurement of the coherent neutron scattering length of 3He through neutron interferometry

Improved knowledge of the real part of the neutron scattering length of ³He is important for further development of nuclear few-body theory, as well as for a thorough understanding of neutron scattering off quantum liquids. The real part of the bound incoherent neutron scattering length b_i' has recently been measured directly with an experimental uncertainty of better than 1% by means of spin echo spectrometry. The uncertainty of the more fundamental bound multiplet scattering lengths b_±' is thus limited by today's 1.2% uncertainty of the spin-independent coherent part b_c'. Employing the skew-symmetric perfect crystal Si-interferometer at the S18 experimental site at ILL, Grenoble, we have re-measured the real part of the bound coherent neutron scattering length b_c' of ³He. Our result b_c' = 6.010(21)fm exhibits a significant deviation compared to the latest accepted value b_c' = 5.74(7)fm (H. Kaiser, H. Rauch, G. Badurek, W. Bauspiess, U. Bonse, Z. Phys. A 291, 231 (1979)). Including the known value of the incoherent neutron scattering length, we obtain new values for the real parts of the free singlet and triplet scattering lengths, a_-' = 7.573(30)fm and a₊' = 3.480(18)fm. Our result contravenes by more than 7 standard deviations the measurement of the same physical quantity that has recently been performed by a group at NIST in a very similar experiment (P.R. Huffman, D.L. Jacobson, K. Schoen, M. Arif, T.C. Black, W.M. Snow, S.A. Werner, Phys. Rev. C 70, 014004 (2004)) which yielded b_c' = 5.853(7)fm.     

Study of the glass transition order parameter in amorphous polybutadiene by incoherent neutron scattering

The temperature dependent elastic incoherent scattering from a glass forming polybutadiene was studied using high resolution neutron spectroscopy. This elastic scattering measures directly the non-ergodicity order parameter of the glass transition. We observed an anomalous decrease of this scattering setting in around 30 K below the thermodynamic glass transition,T _g , the temperature dependence of which is in agreement with the square root ofT prediction of the mode coupling approach. The critical temperature of 220 K lies about 30 K aboveT _g . The missing elastic intensity reappears as inelastic scattering in the 1 meV range. Within the eV resolution of the backscattering spectrometer no quasielastic scattering can be detected up to 20 K aboveT _g . The observed inelastic scattering may be interpreted as resulting from a continous shift of the density of states towards low frequencies as a consequence of a general softening of the structure.     

Self-diffusion in liquid lithium and lead from coherent quasi-elastic neutron scattering data

The temperature dependence of the self-diffusion coefficient of liquid lithium in the temperature range of 500–830 K has been investigated using coherent quasi-elastic neutron scattering. Good agreement with the results obtained from incoherent quasi-elastic neutron scattering experiments traditionally used for this purpose has been achieved, as well as with the corresponding literature data. In addition, one temperature point (623 K) has been obtained for the self-diffusion coefficient of liquid lead, which is a pure coherent scatterer.     

Temperature measurements by coherent Rayleigh scattering

We demonstrate, for the first time to our knowledge, the utility of coherent Rayleigh scattering (CRS) for temperature measurements in low-density gases and weakly ionized plasmas by measuring the translational temperature of neutral argon in a glow discharge. By analysis of the near-Gaussian spectral profile of the CRS signal, we determine temperatures with an uncertainty of 相似文献   

Precision neutron interferometric measurement of the nd coherent neutron scattering length and consequences for models of three-nucleon forces

We have performed the first high precision measurement of the coherent neutron scattering length of deuterium in a pure sample using neutron interferometry. We find b(nd)=(6.665+/-0.004) fm in agreement with the world average of previous measurements using different techniques, b(nd)=(6.6730+/-0.0045) fm. We compare the new world average for the nd coherent scattering length b(nd)=(6.669+/-0.003) fm to calculations of the doublet and quartet scattering lengths from several modern nucleon-nucleon potential models with three-nucleon force (3NF) additions and show that almost all theories are in serious disagreement with experiment. This comparison is a more stringent test of the models than past comparisons with the less precisely determined doublet scattering length of (2)a(nd)=(0.65+/-0.04) fm.     

Oscillatory diffuse scattering study by time-of-flight neutron scattering

The oscillatory profile of diffuse scattering of ordered crystal PbF₂ is observed by time-of-flight (TOF) neutron scattering measurement. The theoretical expression of diffuse scattering intensity including the correlation effects among the thermal displacements of atoms is used to analyze the diffuse scattering profile of PbF₂. The theoretical expression is effective for the analysis of the diffuse scattering data by the TOF measurement.     

Phonon density of states of tetracyanoethylene from coherent inelastic neutron scattering at Dhruva reactor

Inelastic neutron scattering experiments to determine phonon density of states of coherent scattering samples of polycrystalline complex solids are generally intensity-limited and therefore are feasible only at high flux facilities. Phonon density of states of the monoclinic phase of tetracyanoethylene at 300 K, obtained using the medium resolution triple axis spectrometer at the new Indian medium flux reactor Dhruva are reported here. The raw data is converted to the “neutron weighted” phonon density of states by applying suitable corrections. Comparison made with results from a theoretical calculation based on a semirigid molecule model of lattice dynamics is fair. Results from Dhruva are also consistent with that obtained (to be published) at the high flux pulsed neutron source (ISIS) of the Rutherford Appleton Laboratory in United Kingdom.