Inelastic Neutron Scattering of H2 in Xerogel

The properties of molecular hydrogen adsorbed in Britesorb were studied through inelastic neutron scattering. We have measured both the rotational energy levels and the momentum distribution at bilayer and nearly full pore fillings. Splitting of the J=1 rotational energy levels is observed for molecular hydrogen adsorbed on the surface, while the rotational properties of the hydrogen adsorbed after monolayer completion is consistent with behavior in the bulk. Additionally, the measurement of the momentum distribution showed that the mean kinetic energy of the molecules in the bilayer is 88 K±7 K. The kinetic energy measured in the nearly full pore was 81 K±6 K, which is consistent with a simple model in which the behavior of the monolayer is dominated by the interaction with the surface of the pore wall but H₂ adsorbed after monolayer completion is bulk like.     

An Inelastic Neutron Scattering Study of H2 in Vycor Glass

The orientational properties of n-H ₂ confined in Vycor have been studied via inelastic neutron spectroscopy. We observe two distinct rotational transitions which we ascribe to H ₂ in the center of the pore and H ₂ strongly bound to the surface. The molecules on the surface can be modeled as rotationally hindered rotors, and a distribution of orientational potentials is extracted.     

Ultra-High Resolution Inelastic Neutron Scattering

Two types of ultra high energy resolution neutron scattering instruments, the backscattering spectrometer and the spin echo spectrometer, are described. Examples of the types of research which can be done with these instruments are given and plans for a cold neutron backscattering spectrometer which will be built in the NIST Cold Neutron Research Facility (CNRF) are discussed. It is hoped that this information will be of use to researchers considering neutron scattering experiments at NIST.     

Inelastic Neutron Scattering Study of Oxygen Intercalated C60

We have studied 0 ₂ intercalated C ₆₀ using both elastic and inelastic neutron scattering. The samples of stoichiometry (0 ₂ ) _x C ₆₀ with x 0.7 show a reduction in the order-disorder transition temperature T ₀ of 20 K with respect to pure C ₆₀ . The neutron diffraction pattern of the low temperature phase can be refined with the structural parameters of pure C ₆₀ whereby an expansion of the host lattice is registered. All 0 ₂ molecules are found on octahedral sites. The phonon spectrum of the host as well as the quasielastic response of the C ₆₀ cages above T ₀ are not visibly influenced by the guest molecules.     

Nuclear Structure Studies With the Inelastic Neutron Scattering Reaction and Gamma-Ray Detection

The (n,n′γ) reaction has been used at the University of Kentucky accelerator facility to examine the detailed structure of a number of nuclei. The advantages of this method are reviewed, and recent developments are described. Examples of unique nuclear structure studies that have been carried out with this method are presented.     

On the Vibrational Spectrum of C60 Measured by Neutron Inelastic Scattering

New experimental data on the vibrational spectrum of C₆₀ have been recorded with neutron inelastic scattering (NIS). These data modify the positions of the lattice modes, which previously were reported at too high frequencies. New information on the frequencies of the intramolecular modes has also been obtained using a better instrumental resolution.     

Inelastic Neutron Scattering Studies of First Order Phase Transitions in bcc Solid 4He

We report inelastic neutron scattering studies of the [110] transverse phonon branches of bcc ⁴He near the bcc-hcp transitions on the solid-liquid coexistence line and near the melting transition. The question behind the experiment was whether these transitions in a quantum solid are in any way different from what one observes in usual materials. In contrast to large softening of the T₁ branch seen near the bcc-hcp transition in group IV metals, we found that the transverse phonons in bcc ⁴He do not soften at all. Altough visual studies of the crystals near the transition are consistent with a martensitic transformation, neutron scattering indicates that the transition in solid ⁴He is different than in metals. Thus, the mechanism of the bcc-hcp transition remains an open question. Similar study done near the melting transition indicates that none of the phonons measured in the present experiment is affected by melting, which rules out a mechanical instability of the bulk as a mechanism of melting. Finally, in addition to the phonons, we observed a new feature at q=0 and at an energy transfer of 1.23meV which we attribute to neutron scattering by point defects. Similarly to the phonons, this feature did not change near any of the phase transitions.     

A Polarized Neutron Scattering Study of the Normal Phase in YBa2Cu3O6.95

Polarized neutron scattering with polarization analysis has revealed negligible magnetic scattering up to 30 meV in the normal phase of YBa₂Cu₃O_6.95 indicating the absence of local moments.     

Inelastic Scattering in Normal and Superconducting Raman Response

We use the nearly antiferromagnetic Fermi liquid model to illustrate the effect of strongly anisotropic inelastic scattering on the electron Raman response. Both normal and superconducting states are considered, accounting fully for the momentum and frequency dependence of the resulting self-energy. Special attention is paid to the low frequency of the response and an to ³ crossover in B _1g . By analogy to the infrared conductivity case, effective Raman scattering times are extracted, and their temperature and frequency dependences are studied and are compared with equivalent optical quantities. Impurities, within a unitary scattering approximation, are introduced and found to also have a strong characteristic signature on the Raman response.     

Study of Undersaturated C60 Solutions in Cs2 by Small Angle Neutron Scattering

We measured precisely, using the Small Angle Neutron Scattering (SANS) technique, the radius of the carbon nuclei shell of the C₆₀ molecule, in spite of its small value. For this purpose, controlled undersaturated solutions of C₆₀ in CS₂, where theC₆₀ molecules are individually distributed, were studied in the room temperature range. Absolute measurements also allowed us to extract from the contrast in scattering length the partial molecular volume of the molecule C₆₀ in solution in CS2.     

Study of Undersaturated C60 Solutions in Cs2 by Small Angle Neutron Scattering

Abstract

We measured precisely, using the Small Angle Neutron Scattering (SANS) technique, the radius of the carbon nuclei shell of the C₆₀ molecule, in spite of its small value. For this purpose, controlled undersaturated solutions of C₆₀ in CS₂, where theC₆₀ molecules are individually distributed, were studied in the room temperature range. Absolute measurements also allowed us to extract from the contrast in scattering length the partial molecular volume of the molecule C₆₀ in solution in CS2.     

Momentum Distribution of Liquid 3He: Simulation of Deep Inelastic Neutron Scattering Experiments with the VESUVIO Spectrometer

The recent experimental determination of the ³ He single-particle mean kinetic energies in solid hcp, bcc and dense liquid phases has allowed to perform an accurate test of the single-particle dynamical properties derived from the existing theoretical models. A Monte Carlo simulation of a Filter-Difference neutron spectrometer for Deep Inelastic Neutron Scattering with enhanced resolution performance is presented. It aims to study the feasibility of an accurate experimental determination of the ³ He momentum distributions in pure liquid and in ³ He- ⁴ He liquid mixtures.     

Outline of Neutron Scattering Formalism

Neutron scattering formalism is briefly surveyed. Topics touched upon include coherent and incoherent scattering, bound and free cross-sections, the Van Hove formalism, magnetic scattering, elastic scattering, the static approximation, sum rules, small angle scattering, inelastic scattering, thermal diffuse scattering, quasielastic scattering, and neutron optics.     

Neutron Scattering and Ordering of Nuclear Spins

The ordering of nuclear spins takes place at extremely low temperatures because of the weakness of the interactions between their magnetic moments. Neutron scattering is the traditional and unique method to study the collective ordering of magnetic moments. The combination of the problem (nuclear spin ordering) and the method (neutron scattering) leads to new insight into the many body behaviour of the nuclear spin system and into some of the long standing general questions of the cooperative behaviour of magnetic systems. We describe neutron scattering studies on the nuclear spin systems in Cu and Ag at nano- and subnanokelvin temperatures as an example of a very successful international collaboration based on the work performed at the Low Temperature Laboratory, in the Helsinki University of Technology in Otaniemi. The collaboration was initiated by Olli V. Lounasmaa.     

Neutron and Raman Scattering Studies of Hgba2CuO4+δ

The HgBa₂CuO_4+δ sample was characterized by Neutron diffraction and magnetic measurements. Both of the measurements indicate a high purity of the sample. Raman measurement was performed on a HgBa₂CuO_4+δ compound of T_c = 96 K. The apical oxygen vibration at 592 cm^?1 was found to show (a) an above T_c anomaly, and (b) frequency hardening and linewidth broadening below the superconducting phase transition. The latter is attributed to the coupling of the phonon to the electronic excitation and related to the opening of the superconducting gap below the phonon frequency.     

Small-Angle Neutron Scattering Studies of Vesicle Stability

Small-angle neutron scattering (SANS) was used to investigate the structure of mixed colloids of egg yolk phosphatidylcholine (EYPC) with the bile salt, cholylglycine (CG), in D₂O as a function of pressure (P) and temperature (T). At atmospheric pressure, the system forms an isotropic phase of mixed, single-bilayer vesicles (SLVs). Increasing the external hydrostatic pressure brought about significant changes in particle morphology. At T=25°C, application of a pressure of 3.5 MPa resulted in collapse of the SLVs. A further increase in P, up to 51.8 MPa, resulted in a transition from a phase of ordered (stacked), collapsed vesicles to one of stacked, ribbon-like particles. A similar collapse of the vesicles was observed at a higher temperature (T=37°C) with increasing P, but at this temperature, no ribbon phase was found at the highest pressure explored.     

中国先进研究堆中子散射谱仪建设现状和展望

中子具备不带电、穿透力强、可鉴别同位素、较之X射线对轻元素灵敏、具有磁矩等优点,因此中子散射技术作为一种独特的、从原子和分子尺度上研究物质结构和动态特性的表征手段,在多学科交叉领域发挥着不可替代的作用.中国先进研究堆(CARR)上的中子散射工程将充分利用该研究堆为中子散射工程提供的高通量、具有冷中子源、切向水平孔道等有利条件,预计一期将分别建造高分辨中子粉末衍射仪、中子反射谱仪、应力测量衍射谱仪、中子三轴谱仪、中子小角散射谱仪、中子四圆衍射仪、中子高强度粉末谱仪、中子织构测量仪、冷中子照相谱仪和热中子照相谱仪等10台谱仪及装置.瞄准世界先进水平,建立起研究手段配套、覆盖中子散射研究的主要领域、强调封应用又具备必要的基础研究能力的中子散射设备,将为生命科学、材料科学、物理、化学、地矿、环境等各种学科及工程技术方面提供先进的中子散射技术.其最终目标足建屯起符合我国国情、其综合研究能力与世界先进水平接轨的中子散射国家重点实验室,使之成为我国21世纪中子散射研究和应用的中心及人才培养基地,以期为我国国防工业和国民经济建设服务.     

High-Precision Determination of the Neutron Coherent Scattering Length

The neutron coherent scattering length b_c has been determined interferometrically to an uncertainty of about 5 × 10⁻⁵ by measuring the nondispersive phase. We propose improving the uncertainty to about 10⁻⁶ by optimizing various parameters of the interferometric experiment. Any uncertainty in the b_c determination arising from possible variations in the constitution of the ambient air can be eliminated by performing the experiment in vacuum. When such uncertainty is attained, it becomes necessary to account for the neutron beam refraction at the sample-ambient interfaces, to infer the correct b_c from the observed phase. The formula for the phase used hitherto is approximate and would significantly overestimate b_c. The refractive index for neutrons can thus be determined to a phenomenal uncertainty of about 10⁻¹².     

Investigation of Metallic and Ceramic Materials by Small-Angle Neutron Scattering

Small-angle neutron scattering measurements on a double-crystal spectrometer with perfect monochromator and analyzer crystals were used to follow microstructural changes in the aluminum alloy VD-17, refractory alloy ZhS-6, and dispersion-hardened zirconia-based ceramics with yttria additions. The measurements were carried out by the Warren method, involving determination of the small-angle neutron attenuation coefficient. The experimentally determined total macroscopic small-angle scattering cross section is correlated with different parameters characterizing external influences on the material: the number of loading cycles, heat-treatment temperature, and fracture stress. The potential of the proposed approach for optimizing processing technologies is discussed.