C. Andreani

Neutron diffraction methods for the study of residual stress fields

Abstract A technique is described for the measurement of the residual stress tensor averaged over a specified volume within a component. The method involves measurement of small changes in lattice spacing using high resolution neutron diffraction. The stress is inferred from these measurements of the strain, and the theory of the relationship between the two quantities is described, including the effects of crystalline anisotropy. The various types of high resolution neutron diffractometer suitable for the work are described. Experimental results validating the method are given for a simple bent bar of mild steel of known strain, a plastically strained mild steel bar, and a mild steel tube of known torsional strain. Examples of the method in practical use are given by a cracked fatigue test specimen, a double-V test weld and a weld joining a tube to a plate. A more detailed example is the anisotropic response of a polycrystalline sample under elastic and plastic strain; this is illustrated by measurements...

Measurement of momentum distribution of light atoms and molecules in condensed matter systems using inelastic neutron scattering

Studies of single-particle momentum distributions in light atoms and molecules are reviewed with specific emphasis on experimental measurements using the deep inelastic neutron scattering technique at eV energies. The technique has undergone a remarkable development since the mid-1980s, when intense fluxes of epithermal neutrons were made available from pulsed neutron sources. These types of measurements provide a probe of the short-time dynamics of the recoiling atoms or molecules as well as information on the local structure of the materials. The paper introduces both the theoretical framework for the interpretation of deep inelastic neutron scattering experiments and thoroughly illustrates the physical principles underlying the impulse approximation from light atoms and molecules. The most relevant experimental studies performed on a variety of condensed matter systems in the last 20 years are reviewed. The experimental technique is critically presented in the context of a full list of published work. It is shown how, in some cases, these measurements can be used to extract directly the effective Born–Oppenheimer potential. A summary of the progress made to date in instrument development is also provided. Current data analysis and the interpretation of the results for a variety of physical systems is chosen to illustrate the scope and power of the method. The review ends with a brief consideration of likely developments in the foreseeable future. Particular discussion is given to the use of the VESUVIO spectrometer at ISIS. Contents PAGE 1. Introduction 378 2. Theoretical basis of measurements 381   2.1. The impulse approximation and the neutron Compton profile 381   2.2. Validity of the impulse approximation and corrections at finite q 384   2.3. Properties of the dynamic structure factor SIA (q ω) in the impulse approximation 389   2.4. Extracting the atomic momentum distribution from the neutron Compton profile 390   2.5. Determination of effective Born–Oppenheimer potentials 394 3. Theoretical momentum distributions of atoms 395   3.1. Maxwellian regime and atoms in harmonic solids 395   3.2. Quantum systems and weakly quantum systems 397   3.3. Fermi and Bose systems 398   3.4. Molecular systems 399   3.5. Polyatomic molecules 401 4. An exact calculation: liquid H2 and D2 403 5. Experimental technique 408   5.1. Direct and inverse geometry spectrometers for DINS measurements 408   5.2. The VESUVIO spectrometer 409   5.3. The resonance filter configuration 411   5.4. The resonance detector configuration 415   5.5. Extracting the neutron Compton profile from observations 417 6. Review of existing measurements 420   6.1. Liquid and solid 4He 420   6.2. Liquid and solid 3He 428   6.3. Liquid 4He–3He mixtures 431   6.4. Liquid para-H2, ortho-D2 and N2 436   6.5. Hydrogen sulphide 444   6.6. Water and ice 447   6.7. Single crystal measurements: the example of KDP (KH2PO4) 453 7. Conclusions and perspectives 457   7.1. Applications in physics 459   7.2. Applications in chemistry 460   7.3. Applications in biology 460   7.4. Technological applications 461 Acknowledgments 461 Appendix A: The intensity deficit problem 462 References 463

Measurement of internal stress within bulk materials using neutron diffraction

Abstract Neutron diffraction measures stress through the small changes in atomic lattice parameters caused by strain. The method is similar to X-ray diffraction with the vital difference that a thermal neutron beam penetrates several centimetres in most materials allowing measurements within bulk samples. Ways of achieving the high resolution necessary for quantitative measurements are described. Results are presented for the strains in a mild steel bar subjected to known elastic stresses, and on the internal stresses in a deformed bar.

VESUVIO: a novel instrument for performing spectroscopic studies in condensed matter with eV neutrons at the ISIS facility

The VESUVIO project aims to provide unique prototype instrumentation at the ISIS-pulsed neutron source and to establish a routine experimental and theoretical program in neutron scattering spectroscopy at eV energies. This instrumentation will be specifically designed for high momentum, , and energy transfer inelastic neutron scattering studies of microscopic dynamical processes in materials and will represent a unique facility for EU researchers. It will allow to derive single-particle kinetic energies and single-particle momentum distributions, n(p), providing additional and/or complementary information to other neutron inelastic spectroscopic techniques.

Microscopic structure of low temperature liquid ammonia: A neutron diffraction experiment

Neutron diffraction measurements on liquid ammonia at two temperature states on the liquid‐vapor coexistence curve are presented. Three isotropic mixtures of hydrogenated and deuterated ammonia have been studied and the three radial atom–atom distribution functions extracted at 213 K and 273 K. The comparison of the experimental results with the partial radial distribution functions for orientationally uncorrelated molecules, shows some evidence for hydrogen bonding in liquid ammonia but this is much less pronounced than that found in water under ambient conditions.

Facility for fast neutron irradiation tests of electronics at the ISIS spallation neutron source

The VESUVIO beam line at the ISIS spallation neutron source was set up for neutron irradiation tests in the neutron energy range above 10MeV. The neutron flux and energy spectrum were shown, in benchmark activation measurements, to provide a neutron spectrum similar to the ambient one at sea level, but with an enhancement in intensity of a factor of 107. Such conditions are suitable for accelerated testing of electronic components, as was demonstrated here by measurements of soft error rates in recent technology field programable gate arrays.

YAP scintillators for resonant detection of epithermal neutrons at pulsed neutron sources

Recent studies indicate the resonance detector (RD) technique as an interesting approach for neutron spectroscopy in the electron volt energy region. This work summarizes the results of a series of experiments where RD consisting of YAlO3 (YAP) scintillators were used to detect scattered neutrons with energy in the range 1–200 eV. The response of YAP scintillators to radiative capture γ emission from a 238U analyzer foil was characterized in a series of experiments performed on the VESUVIO spectrometer at the ISIS pulsed neutron source. In these experiments a biparametric data acquisition allowed the simultaneous measurements of both neutron time-of-flight and γ pulse height (energy) spectra. The analysis of the γ pulse height and neutron time of flight spectra permitted to identify and distinguish the signal and background components. These measurements showed that a significant improvement in the signal-to-background ratio can be achieved by setting a lower level discrimination on the pulse height at ab...

Direct Measurement of Competing Quantum Effects on the Kinetic Energy of Heavy Water upon Melting

Even at room temperature, quantum mechanics plays a major role in determining the quantitative behavior of light nuclei, changing significantly the values of physical properties such as the heat capacity. However, other observables appear to be only weakly affected by nuclear quantum effects (NQEs); for instance, the melting temperatures of light and heavy water differ by less than 4 K. Recent theoretical work has attributed this to a competition between intra- and intermolecular NQEs, which can be separated by computing the anisotropy of the quantum kinetic energy tensor. The principal values of this tensor change in opposite directions when ice melts, leading to a very small net quantum mechanical effect on the melting point. This Letter presents thefirst direct experimental observation of this phenomenon, achieved by measuring the deuterium momentum distributionsn(p) in heavy water and ice using deep inelastic neutron scattering (DINS) and resolving their anisotropy. Results from the experiments, supplemented by a theoretical analysis, show that the anisotropy of the quantum kinetic energy tensor can also be captured for heavier atoms such as oxygen. (The iceberg image in the Table of Contents and Abstract graphics was used with permission of the NOAAs National Ocean Service, 2012 (http://commons.wikimedia.org/wiki/ File:Iceberg_-_NOAA.jpg).)

Single-crystal diamond detector for time-resolved measurements of a pulsed fast-neutron beam

A fast-neutron detector for time-resolved beam measurements at spallation neutron sources is presented. The device features a p-type/intrinsic/metal Schottky barrier structure where the active (intrinsic) detection layer is a 150 μm thick single-crystal diamond obtained by chemical-vapour deposition. Coupling to fast front-end electronics preserves the excellent timing properties of the device as demonstrated in tests performed at the ISIS spallation neutron source in UK. The device represents a novel approach in the field of pulsed fast-neutrons spectroscopic techniques. It will find immediate application in localized (mm resolution) fast-neutron fluence measurements required by neutron irradiation experiments at ISIS also envisaging its use for spectrum measurements.

A resonant detector for high-energy inelastic neutron scattering experiments

Results on the application of the resonant detector (RD) for epithermal neutron scattering in an unexplored kinematical region are presented. The RD is based on resonance radiative neutron capture for energy analysis of the scattered neutrons in an inverse geometry time of flight spectrometer. Application of the RD to detection of epithermal neutrons at very low scattering angles allows access to an unexplored scattering kinematical region, the High-energy Inelastic Neutron Scattering (HINS) region, of low wave vector (3A−1<q<10A−1) and high energy transfers (0.1eV<ω<10eV). Results of HINS measurements from polycrystalline ice are presented.