The interpretation of polycrystalline coherent inelastic neutron scattering from aluminium

A new approach to the interpretation and analysis of coherent inelastic neutron scattering from polycrystals (poly-CINS) is presented. Here we describe a simulation of the one-phonon coherent inelastic scattering from a lattice model of an arbitrary crystal system. The one-phonon component is characterized by sharp features e.g. determined by boundaries of the (Q, omega) regions where one-phonon scattering is allowed. These features may be identified with the same features apparent in the measured total coherent inelastic cross-section, the other components of which(multiphonon or multiple scattering) show no sharp features. The parameters of the model can then be relaxed to improve the fit between model and experiment. This method is of particular interest where no single crystals are available. To test the approach, we have measured the poly-CINS for polycrystalline aluminium using the MARI spectrometer (ISIS) because both lattice dynamical models and measured dispersion curves are available for this material. The models used include a simple Lennard-Jones model fitted to the elastic constants of this material plus a number of Embedded Atom Method (EAM) force fields. The agreement obtained suggests that the method demonstrated should be effective in developing models for other materials where single crystal dispersion curves are not available

MCViNE -- An object oriented Monte Carlo neutron ray tracing simulation package

MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo (MC) neutron ray-tracing program that provides researchers with tools for performing computer modeling and simulations that mirror real neutron scattering experiments. By adopting modern software engineering practices such as using composite and visitor design patterns for representing and accessing neutron scatterers, and using recursive algorithms for multiple scattering, MCViNE is flexible enough to handle sophisticated neutron scattering problems including, for example, neutron detection by complex detector systems, and single and multiple scattering events in a variety of samples and sample environments. In addition, MCViNE can take advantage of simulation components in linear-chain-based MC ray tracing packages widely used in instrument design and optimization, as well as NumPy-based components that make prototypes useful and easy to develop. These developments have enabled us to carry out detailed simulations of neutron scattering experiments with non-trivial samples in time-of-flight inelastic instruments at the Spallation Neutron Source. Examples of such simulations for powder and single-crystal samples with various scattering kernels, including kernels for phonon and magnon scattering, are presented. With simulations that closely reproduce experimental results, scattering mechanisms can be turned on and off to determine how they contribute to the measured scattering intensities, improving our understanding of the underlying physics.Comment: 34 pages, 14 figure

Adaptive heterogeneous parallelism for semi-empirical lattice dynamics in computational materials science.

With the variability in performance of the multitude of parallel environments available today, the conceptual overhead created by the need to anticipate runtime information to make design-time decisions has become overwhelming. Performance-critical applications and libraries carry implicit assumptions based on incidental metrics that are not portable to emerging computational platforms or even alternative contemporary architectures. Furthermore, the significance of runtime concerns such as makespan, energy efficiency and fault tolerance depends on the situational context. This thesis presents a case study in the application of both Mattsons prescriptive pattern-oriented approach and the more principled structured parallelism formalism to the computational simulation of inelastic neutron scattering spectra on hybrid CPU/GPU platforms. The original ad hoc implementation as well as new patternbased and structured implementations are evaluated for relative performance and scalability. Two new structural abstractions are introduced to facilitate adaptation by lazy optimisation and runtime feedback. A deferred-choice abstraction represents a unified space of alternative structural program variants, allowing static adaptation through model-specific exhaustive calibration with regards to the extrafunctional concerns of runtime, average instantaneous power and total energy usage. Instrumented queues serve as mechanism for structural composition and provide a representation of extrafunctional state that allows realisation of a market-based decentralised coordination heuristic for competitive resource allocation and the Lyapunov drift algorithm for cooperative scheduling

Parallel Computational Modelling of Inelastic Neutron Scattering in Multi-node and Multi-core Architectures

This paper examines the initial parallel implementation of SCATTER, a computationally intensive inelastic neutron scattering routine with polycrystalline averaging capability, for the General Utility Lattice Program (GULP). Of particular importance to structural investigation on the atomic scale, this work identifies the computational features of SCATTER relevant to a parallel implementation and presents initial results from performance tests on multi-core and multi-node environments. Our initial approach exhibits near-linear scalability up to 256 MPI processes for a significant model

Monte Carlo simulation of neutron scattering by a textured polycrystal

A method of simulating the neutron scattering by a textured polycrystal is presented. It is based on an expansion of the scattering cross sections in terms of the spherical harmonics of the incident and scattering directions, which is derived from the generalized Fourier expansion of the polycrystal orientation distribution function. The method has been implemented in a Monte Carlo code as a component of the McStas software package, and it has been validated by computing some pole figures of a Zircaloy-4 plate and a Zr-2.5Nb pressure tube, and by simulating an ideal transmission experiment. The code can be used to estimate the background generated by components of neutron instruments such as pressure cells, whose walls are made of alloys with significant crystallographic texture. As a first application, the effect of texture on the signal-to-noise ratio was studied in a simple model of a diffraction experiment, in which a sample is placed inside a pressure cell made of a zirconium alloy. With this setting, the results of two simulations were compared: one in which the pressure-cell wall has a uniform distribution of grain orientations, and another in which the pressure cell has the texture of a Zr-2.5Nb pressure tube. The results showed that the effect of the texture of the pressure cell on the noise of a diffractogram is very important. Thus, the signal-to-noise ratio can be controlled by appropriate choice of the texture of the pressure-cell walls

Single Crystal to Polycrystal Neutron Simulation

The holy grail for inspection of manufactured parts is being able to place an arbitrary part in a measurement system that generates a 3-D map of grain size, orientation, and strain within the part at 10μm resolution. Current measurement capabilities are far from this ideal and development of models, instruments and algorithms is needed to reach this ideal. Over the past two decades the technique of Bragg-edge neutron transmission along with computed tomography algorithms has materialized as a potential technique to obtain three-dimensional maps within the bulk of materials. To date, these techniques have been applied only to simplistic three-dimensional strains without consideration of texture. In this work, a new approach to modeling Neutron Bragg-edge transmission is investigated. The basic principle of the Bragg-edge transmission technique is the measurement of transmission of cold and thermal neutrons through polycrystalline materials. The spectral signatures of the transmission are based on the sample’s-crystal symmetry, and atomic parameters. The shape, position, and relative magnitude of these Bragg-edge spectral signatures contain information about grain size, grain orientation, and the average strain within the sample that is collinear with the incident beam. The focal point of this thesis is the development of a new neutron Bragg-edge transmission simulation code in which the user can define distributions for grain size, mosaic distribution per grain, grain orientations (texture), and general three-dimensional strain on the grain systems of the sample. A theoretical neutron cross-section calculation for single crystals dependent on crystallographic description of the sample, granular topology, and the strain state of the grain is applied to each crystal in the defined distribution to model the Bragg-edge effect in polycrystalline materials. The cross-section calculation is implemented using the python scripting language and the simulation tool is used to investigate the transmission spectrum of single crystals and polycrystalline materials. In order to verify the transmission spectrum,simulations spectra are compared to neutron transmission taken at the VULCAN instrument at the Spallation Neutron Source. Comparison of the simulation spectra to those found in literature are also presented