Neutron scattering lengths and cross sections
TL;DR: The application of thermal neutron scattering to the study of the structure and dynamics of condensed matter requires a knowledge of the scattering lengths and the corresponding scattering and absorption cross sections of the elements as discussed by the authors.
Abstract: The application of thermal neutron scattering to the study of the structure and dynamics of condensed matter requires a knowledge of the scattering lengths and the corresponding scattering and absorption cross sections of the elements. Ln some cases, values for the individual isotopes are needed as well. This information is required to obtain an absolute normalization ofthe scatteredneutron distributions, tocalculate unit-cell structure factors in neutron crystallography, and to correct for effects such as absorption, self-shielding, extinction, multiple scattering, incoherent scattering, and detector efficiency.
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TL;DR: In this article, the authors describe the implementation of real-space refinement in the phenixreal_space-refine program from the PHENIX suite, which makes use of extra information such as secondary-structure and rotamer-specific restraints.
Abstract: This article describes the implementation of real-space refinement in the phenixreal_space_refine program from the PHENIX suite The use of a simplified refinement target function enables very fast calculation, which in turn makes it possible to identify optimal data-restraint weights as part of routine refinements with little runtime cost Refinement of atomic models against low-resolution data benefits from the inclusion of as much additional information as is available In addition to standard restraints on covalent geometry, phenixreal_space_refine makes use of extra information such as secondary-structure and rotamer-specific restraints, as well as restraints or constraints on internal molecular symmetry The re-refinement of 385 cryo-EM-derived models available in the Protein Data Bank at resolutions of 6 A or better shows significant improvement of the models and of the fit of these models to the target maps
1,748 citations
Brookhaven National Laboratory1, Los Alamos National Laboratory2, International Atomic Energy Agency3, Rensselaer Polytechnic Institute4, National Institute of Standards and Technology5, Oak Ridge National Laboratory6, Argonne National Laboratory7, Lawrence Livermore National Laboratory8, Lawrence Berkeley National Laboratory9, North Carolina State University10, University of Michigan11, Institut de radioprotection et de sûreté nucléaire12, TRIUMF13, Rosatom14, Chalk River Laboratories15, Paul Scherrer Institute16, Karlsruhe Institute of Technology17, University of Bucharest18, Joint Institute for Nuclear Research19
TL;DR: The new ENDF/B-VIII.0 evaluated nuclear reaction data library as mentioned in this paper includes improved thermal neutron scattering data and uses new evaluated data from the CIELO project for neutron reactions on 1 H, 16 O, 56 Fe, 235 U, 238 U and 239 Pu described in companion papers.
1,249 citations
TL;DR: The Crystal program as discussed by the authors adopts atom-centered Gaussian-type functions as a basis set, which makes it possible to perform all-electron as well as pseudopotential calculations.
Abstract: The latest release of the Crystal program for solid-state quantum-mechanical ab initio simulations is presented. The program adopts atom-centered Gaussian-type functions as a basis set, which makes it possible to perform all-electron as well as pseudopotential calculations. Systems of any periodicity can be treated at the same level of accuracy (from 0D molecules, clusters and nanocrystals, to 1D polymers, helices, nanorods, and nanotubes, to 2D monolayers and slab models for surfaces, to actual 3D bulk crystals), without any artificial repetition along nonperiodic directions for 0–2D systems. Density functional theory calculations can be performed with a variety of functionals belonging to several classes: local-density (LDA), generalized-gradient (GGA), meta-GGA, global hybrid, range-separated hybrid, and self-consistent system-specific hybrid. In particular, hybrid functionals can be used at a modest computational cost, comparable to that of pure LDA and GGA formulations, because of the efficient implementation of exact nonlocal Fock exchange. Both translational and point-symmetry features are fully exploited at all steps of the calculation, thus drastically reducing the corresponding computational cost. The various properties computed encompass electronic structure (including magnetic spin-polarized open-shell systems, electron density analysis), geometry (including full or constrained optimization, transition-state search), vibrational properties (frequencies, infrared and Raman intensities, phonon density of states), thermal properties (quasi-harmonic approximation), linear and nonlinear optical properties (static and dynamic [hyper]polarizabilities), strain properties (elasticity, piezoelectricity, photoelasticity), electron transport properties (Boltzmann, transport across nanojunctions), as well as X-ray and inelastic neutron spectra. The program is distributed in serial, parallel, and massively parallel versions. In this paper, the original developments that have been devised and implemented in the last 4 years (since the distribution of the previous public version, Crystal14, occurred in December 2013) are described.
1,108 citations
Cites background from "Neutron scattering lengths and cros..."
...Depending on whether the inelastic scattering is coherent or incoherent, different cross-sections have to be considered, which are tabulated and available online (Hudson, 2001; Sears, 1992)....
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TL;DR: Using high-resolution neutron power diffraction technique, the first direct structural evidence is found showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs.
Abstract: UiO-66 is a highly important prototypical zirconium metal–organic framework (MOF) compound because of its excellent stabilities not typically found in common porous MOFs. In its perfect crystal structure, each Zr metal center is fully coordinated by 12 organic linkers to form a highly connected framework. Using high-resolution neutron power diffraction technique, we found the first direct structural evidence showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs. The concentration of the missing-linker defects is surprisingly high, ∼10% in our sample, effectively reducing the framework connection from 12 to ∼11. We show that by varying the concentration of the acetic acid modulator and the synthesis time, the linker vacancies can be tuned systematically, leading to dramatically enhanced porosity. We obtained samples with pore volumes ranging from 0.44 to 1.0 cm3/g and Brunauer–Emmett–Teller surface areas ranging from 1000 to 1600 m2/g, the l...
1,050 citations
TL;DR: Because of the lack of sufficiently bright neutron sources in the foreseeable future, electron microscopy in practice provides the greatest potential for immediate progress.
Abstract: Radiation damage is the main problem which prevents the determination of the structure of a single biological macromolecule at atomic resolution using any kind of microscopy. This is true whether neutrons, electrons or X-rays are used as the illumination. For neutrons, the cross-section for nuclear capture and the associated energy deposition and radiation damage could be reduced by using samples that are fully deuterated and 15N-labelled and by using fast neutrons, but single molecule biological microscopy is still not feasible. For naturally occurring biological material, electrons at present provide the most information for a given amount of radiation damage. Using phase contrast electron microscopy on biological molecules and macromolecular assemblies of approximately 10(5) molecular weight and above, there is in theory enough information present in the image to allow determination of the position and orientation of individual particles: the application of averaging methods can then be used to provide an atomic resolution structure. The images of approximately 10,000 particles are required. Below 10(5) molecular weight, some kind of crystal or other geometrically ordered aggregate is necessary to provide a sufficiently high combined molecular weight to allow for the alignment. In practice, the present quality of the best images still falls short of that attainable in theory and this means that a greater number of particles must be averaged and that the molecular weight limitation is somewhat larger than the predicted limit. For X-rays, the amount of damage per useful elastic scattering event is several hundred times greater than for electrons at all wavelengths and energies and therefore the requirements on specimen size and number of particles are correspondingly larger. Because of the lack of sufficiently bright neutron sources in the foreseeable future, electron microscopy in practice provides the greatest potential for immediate progress.
1,026 citations
References
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01 Jan 1962
TL;DR: In this paper, the mean and dispersion probability of the mean propagation of errors was defined as a function of the normal error function rejection of data goodness of fit further developments standard deviation of the Mean POR method of least squares least squares with several unknowns correlations.
Abstract: Kinds of errors propagation of errors the mean and dispersion probability the meaning of probability permutations and combinations probability distributions the meaning of a probability distribution binomial distribution poisson distribution gauss distribution or normal error function rejection of data goodness of fit further developments standard deviation of the mean propagation of errors method of least squares least squares with several unknowns correlations. Appendices: summary of formulas evaluation of "n" for "o" for binomial distribution derivation of gauss distribution evaluation of normal error integral answers to problems.
461 citations
TL;DR: In this paper, the authors present a summary of the literature up to the end of December 1990 of the available measured neutrons-nuclei scattering lengths with their error bars in a uniform way.
183 citations
01 Jan 1984
94 citations
TL;DR: In this paper, the authors discussed the influence of thermal neutron scattering of short-range order between nuclear spins with no net nuclear polarization on the nuclear spin-spin interactions and their physical applications.
Abstract: Publisher Summary This chapter discusses the nuclear magnetism The chapter discusses the analysis of thermal neutron scattering by solid targets whose nuclear spins are not disordered, and of its physical applications The order of the nuclear spins will refer as a rule to the case when their polarization is nonzero and may in some cases approach unity The origin of the nuclear polarization can be many folds: (i) Thermal equilibrium in an external magnetic field, brought about by the thermal contact between the nuclear Zeeman interaction and the phonons (ii) Forced polarization along an external field by a process known as dynamic nuclear polarization (DNP) (iii) Cooperative ordering of the nuclear spins at sufficiently low temperature under the effect of the nuclear spin-spin interactions (iv) Cooperative ordering of nuclear spins “dressed” with electronic magnetization in Van Vleck ions (v) Thermal equilibrium in the hyperfine field of electronic spins that are in an ordered state The chapter discusses the influence on neutron scattering of short-range order between nuclear spins with no net nuclear polarization Neutron scattering by targets of polarized nuclear spins provides information relevant to different domains of physics First of all, it makes it possible to measure both in magnitude and sign, the spin-dependent part of the neutron scattering amplitude on various nuclear isotopes, which is useful information for practitioners of neutron scattering Second, once this spin-dependent part is known, neutron scattering can be used to measure the nuclear polarization under conditions where this information cannot be obtained by magnetic resonance and to perform studies of solid-state physics
89 citations