The high-resolution powder diffractometer at the high flux isotope reactor

Neutron powder diffraction is increasingly recognized as one of the most powerful techniques for studying the structural and magnetic properties of advanced materials. Despite the growing demand to study an ever-increasing array of interesting materials, there is only a handful of neutron diffractometers available to serve the US neutron scattering community. This article describes the new high-resolution powder diffractometer that has recently been installed at the High Flux Isotope Reactor in Oak Ridge. The instrument is designed to provide an optimum balance between high neutron flux and high resolution. Due to its versatility the diffractometer can be employed for a large variety of experiments, but it is particularly adapted for refinements of structures with large interplanar spacings as well as of complex magnetic structures. In addition to traditional crystal and magnetic structural refinements, studies of phase transitions, thermal expansion, texture analysis, and ab initio structure solution from powder data can be undertaken.     

Investigations of texture formation in geomaterials by neutron diffraction with high pressure chambers

Abstract

This paper suggests a classification of texture in quartzitic rocks. Possible mechanisms of texture formation and their relation to tectonic processes are discussed.

A scheme for the experimental varification of some models describing the evolution of deformation structures in geological materials in dependence on various erxternal conditions (high pressures, temperatures, various loadings, etc.) acting on a sample is proposed.

To investigate “in situ” the mechanisms of texture formation in rocks, high pressure devices are under construction. They will be built of a special T_i ? Z_r alloy, which has zero coherent scattering lenght and therefore is well suited for neutron diffraction investigations. Two different devices are proposed. The first one allows neutron diffraction measurements of sample volumes up to 4 cm³, a hydrostatic pressure of 1, 5 GPa, a temperature of 300° C, and uniaxial compression up to 50 kN. Pressure temperature and axial load are measured inside the chamber. Besides during the experiment, diagrams of load, elastic wave velocities and acoustic emission will be recorded. The second chamber is designed to investigate the mechanisms of texture formation in polycrystal samples (rocks or their imitations) at temperatures up to 800° C and axial compression with a force of up to 150 kN.     

High-Resolution Neutron Diffractometer Based on the IN-06 Pulsed Neutron Source at the Institute for Nuclear Research,Russian Academy of Sciences

A high-resolution neutron diffractometer has been fabricated on the basis of an IN-06 pulsed neutron source at the Institute for Nuclear Research, Russian Academy of Sciences. The diffractometer incorporates two blocks of detectors with helium neutron counters and time-of-flight (TOF) focusing at scattering angles of 156°–165°. A block of new-type high-efficiency scintillation detectors of thermal neutrons based on a ZnS(Ag)/LiF scintillator and silicon photomultipliers with TOF focusing is developed and tested. Test measurements are performed, and the diffractometer resolution is estimated. The diffraction pattern of nonmagnetic NiCrAl alloy is measured and used to determine the phase composition by means of the Rietveld method. It is demonstrated that the given setup can be applied to phase analysis.     

A simple white beam neutron diffraction technique

White beam neutron diffraction by time-of-flight (TOF) technique has been studied over a number of years as it is believed to be the most convenient method to investigate solids in a fixed geometry. This technique needs a pulsed neutron source and a suitable multichannel, analysing system. The entire system is in general, mechanically quite intricate and expensive. We have investigated an alternative technique to achieve the end result of a constant geometry around the diffracting sample. This involves the use of a single crystal as an analyser to study diffraction pattern from the sample bathed in a white beam and diffracting at any fixed scattering angle. In this paper we report the results of our investigations and have compared this technique with other diffraction techniques. Taking Si, KCl and KNO₃ as typical specimens we have illus. trated the results of our technique and we find that the results are comparable to those obtained by conventional neutron diffraction and TOF diffraction. The technique is simple in mechanical design and data acquisition. It can be easily adapted for high pressure diffraction which is being attempted.     

Double-Q magnetic structures and strong planar anisotropy in tetragonal ErRu2Ge2and ErRu2Si2

Single crystal magnetization measurements and powder neutron diffraction on tetragonal ErRu₂Ge₂ as well as anisotropy of the paramagnetic susceptibility and specific heat measurements on ErRu₂Si₂ are presented. Besides the huge crystal field contribution to the uniaxial anisotropy, which favors the basal plane, a strong in-plane anisotropy is evidenced. From these features and neutron diffraction experiments it is shown that magnetic structures of these materials are double-Q and accordingly non-colinear below their Néel temperature (5.2 and 6.0 K for Ge and Si based compounds, respectively). The magnetic structures induced during the metamagnetic processes are discussed. Received 24 December 1999     

Crystallographic texture analysis of archaeological metals: interpretation of manufacturing techniques

Neutron probes and high energy X-rays are sources of primary importance for the non-invasive characterization of materials related to cultural heritage. Their employment in the characterization of archaeological metal objects, combined with the recent instrumental and computational developments in the field of crystallographic texture analysis (CTA) from diffraction data proves to be a powerful tool for the interpretation of ancient metal working techniques. Diffraction based CTA, when performed using penetrating probes and adequate detector coverage of reciprocal space, for example using large detector arrays and/or ToF mode, allows simultaneous identification and quantification of crystalline phases, besides the microstructural and textural characterization of the object, and it can be effectively used as a totally non-invasive tool for metallographic analysis. Furthermore, the chemical composition of the object may also be obtained by the simultaneous detection of prompt gamma rays induced by neutron activation, or by the fluorescence signal from high energy X-rays, in order to obtain a large amount of complementary information in a single experiment. The specific application of neutron CTA to the characterization of the manufacturing processes of prehistoric copper axes is discussed in detail. PACS 61.12.-q; 81.05.Bx; 81.40.Ef; 81.70.-q     

Texture development and deformation mechanisms during uniaxial straining of U–Nb shape-memory alloys

The shape-memory effect is well documented in uranium–niobium alloys near the α″–γ^o metastable phase boundary. In situ neutron diffraction measurements during uniaxial loading indicate that U–14?at.%?Nb (in the α″ monoclinic phase field) deforms by stress-induced twin reorientation. Alternatively, U–16?at.%?Nb (initially γ^o tetragonal) undergoes a stress-induced phase transformation to the α″ monoclinic phase. The crystallographic texture of the monoclinic phase of both compositions has been measured and qualitatively interpreted by considering the orientation relationship between the most favoured α′′ variant and the parent phase. In addition, previously published observations of deformation structures within the shape-memory regime of a U–13?at.%?Nb alloy are discussed within the context of the same model.     

Texture development and deformation mechanisms during uniaxial straining of U–Nb shape-memory alloys

The shape–memory effect is well documented in uranium–niobium alloys near the α″–γ^o metastable phase boundary. In situ neutron diffraction measurements during uniaxial loading indicate that U–14?at.%?Nb (in the α″ monoclinic phase field) deforms by stress–induced twin reorientation. Alternatively, U–16?at.%?Nb (initially γ^o tetragonal) undergoes a stress–induced phase transformation to the α″ monoclinic phase. The crystallographic texture of the monoclinic phase of both compositions has been measured and qualitatively interpreted by considering the orientation relationship between the most favoured α′′ variant and the parent phase. In addition, previously published observations of deformation structures within the shape–memory regime of a U–13?at.%?Nb alloy are discussed within the context of the same model.     

High pressure neutron diffraction studies using the Paris-Edinburgh cell

Abstract

Neutron diffraction was until recently confined to pressures below ～ 3 GPa. This restricted range has limited the high-pressure structural information that is available for a wide range of phenomena for which neutron diffraction is the technique of choice. But now the recently-developed Paris-Edinburgh cell can achieve pressures up to ～ 30 GPa with a sample volume large enough to allow accurate structural studies with neutrons. After a period of development of the neutron scattering techniques needed to obtain the best possible results using the cell, a variety of successful structural studies have been performed. These illustrate the value of neutron diffraction in important areas such as locating hydrogen and other low-Z atoms in structures, the measurement of accurate structural pressure dependence and the examination of the changes in atomic thermal motion with pressure.     

Materials Research with X-ray Micro-beams at the Advanced Photon Source

X-ray scattering has had a rich and prominent place in the history of materials research. For samples that are large single crystals, X-ray diffraction provides very detailed, high-quality information about the atomic structure of almost every kind. However, most engineering materials are not a large single crystal, but mixtures of materials or a poly-crystalline material. Still, X-ray diffraction using X-ray beams significantly larger than the structures being investigated has produced much excellent information. Examples of this are powder diffraction, texture analysis, strain analysis, etc. While these types of analyses have been very useful in providing a microscopic understanding, they do not provide the microscopic information that we really want. What we really want to have is this useful X-ray diffraction information for every volume element in our sample; not just the diffraction information about small volume elements, but all of the volume elements, exactly where they are in our sample, and how each one diffracts. This detailed spatial and structural information is what is needed for a full understanding of the source of the properties that we find in engineering materials.     

Non-destructive characterization of compositional and textural properties of Etruscan bronzes: a multi-method approach

A combination of conventional analytical techniques, such as X-ray fluorescence (XRF) and scanning electron microscopy-electron probe microanalysis (SEM-EPMA), with novel applications of neutron scattering were employed for a non-destructive study of 6th century BC Etruscan bronze plates discovered almost two centuries ago in a princely chamber tomb in Umbria, Italy. The pieces were used to richly decorate a ceremonial carriage, two war chariots and some furniture. Analytical investigations have been carried out to provide the essential information to correctly assign several fragments in order to recompose the original plates. Analytical responses from XRF and SEM-EPMA, although indicative, were strongly affected by surface alteration and contamination. Rietveld analysis of neutron diffraction profiles emerged for its powerful capability to provide extensive non-destructive, high sensitivity information on bulk alloy composition and phase quantification allowing meaningful comparison among the pieces for the reconstruction of the original plates. In addition, strain and texture analyses demonstrated the capability of the technique to achieve a non-invasive characterization of manufacturing procedures. PACS 61.12.Ld; 81.40.Ef; 81.05.Bx     

利用252 Cf快裂变室测量BC501A液闪探测器的相对探测效率和响应函数

本文利用²⁵²Cf快裂变室和多参数数据采集系统,逐事例的同时记录了自发裂变中子和瞬发伽马的飞行时间(TOF),脉冲形状甄别(PSD)和反冲能量(RE,裂变中子是通过测量反冲质子;瞬发伽马是通过测量康普顿反冲电子)三维信息.详细介绍了通过离线数据分析完全扣除三维信息中的伽马事例贡献,以获得Φ50.8 mm×50.8 mm的BC501A液闪探测器的相对探测效率和响应函数的方法.在不通过探测器响应函数进行数据转换的条件下,利用中子的能量直接确定了中子的有效测量阈值.得到的中子 关键词： 252Cf快裂变室')" href="#">²⁵²Cf快裂变室 BC501A液闪探测器 相对探测效率 响应函数     

Pulsed neutron powder diffraction at high pressure by a capacity-increased sapphire anvil cell

A new design of opposed anvil cell for time-of-flight neutron powder diffraction was prepared for use at advanced pulsed sources. A couple of single-crystal sapphire sphere anvils and a gasket of fully hardened Ti–Zr null alloy were combined to compress 35 mm³ of sample volume to 1 GPa and 11 mm³ to 2 GPa of pressures, respectively. A very high-quality powder diffraction pattern was obtained at Japan Proton Accelerator Research Complex for a controversial high pressure phase of methane hydrate. The counting statistics, resolution, absolute accuracy and d-value range of the pattern were all improved to be best suitable for precise structure refinement. The sample is optically accessible to be measured by Raman and fluorescence spectroscopy during and after compression. The current cell will be an alternative choice to study hydrogenous materials of complex structures that are stable at the described pressure regime.     

Textures and physical properties of marbles deformed at 20 – 250°c

Abstract

The experimental equipment at beam line 7A of the IBR-2 pulsed reactor at JINR, which includes a texture diffractometer and a high-pressure device, is used to characterize textural and structural changes of marbles in dependence upon directed stress, temperature and time. It was shown, that texture changes as a consequence of heating are not valid, in contrast, long-acting load even at moderate stresses led to significant texture modification. The neutron diffraction data are also used to demonstrate its usefulness for the calculation of the thermal expansion coefficient a of calcite, and to calculate the lattice stresses induced by temperature and mechanical stresses.     

Field induced magnetic structures in hexagonal HoAlGa

A neutron diffraction study on a single crystal of the HoAlGa compound under magnetic field applied along the c easy magnetization direction has been performed. The different field induced structures of the multistep metamagnetic process, as well as their thermal evolution, have been determined. Whereas the component of the propagation vector in the hexagonal plane is always (1/3, 1/3), along c it is 1/2 or 1/6 at low temperature and 1/2- τ with τ = 0.02, i.e. incommensurate, at high temperature. The stability of different structures of the H - T phase diagram can be discussed in terms of exchange interactions in a compound with a huge uniaxial anisotropy.     

Modified Bridgman anvils for high pressure synthesis and neutron scattering

ABSTRACT

A simple modified Bridgman design for large volume pressure anvils usable in the Paris-Edinburgh (PE) press has been demonstrated at Oak Ridge National Laboratory Spallation Neutron Source. The design shows advantages over the toroidal anvils typically used in the PE press, mainly rapid compression/decompression rates, complete absence of blow-outs upon drastic phase transitions, simplified cooling, high reliability, and relative low loads (～40 tons) corresponding to relatively high pressures (～20?GPa). It also shows advantages over existing large-volume diamond cells as sample volumes of ～2–3?mm³ can be easily and rapidly synthesized. The anvils thus allow sample sizes sufficient for in situ neutron diffraction as well as rapid synthesis of adequate amounts of new materials for ex situ analysis via total neutron scattering and neutron spectroscopy.     

Some comments on “relation entre structure et conductivite ionique basse temperature Ag3PO4”

The structural and conductivity data on silver orthophospate described recently by Deschizeaux-chèuy et al. are considered in the context of earlier work on the same subject. The results of powder neutron (PND) and single crystal X-ray diffraction (SXD) analyses of Ag₃P0₄ provide a consistent picuture of how the I-Ag₃P0₄ structure evolves with temperature. A comparison of independent analyses of II-Na₃P0₄ by PND and SXD demonstrates that solution and refinement of the structures of even highly disordered superionic materials can, in certain cases, be accomplished using powder diffraction data.     

LS301快中子探测器的效率刻度

主要介绍了利用252Cf标准裂变中子能谱刻度快脉冲LS301型液闪中子探测器的探测效率的实验方法和结果, 简要介绍了实验数据的处理过程, 得到了阈值分别为0.5, 0.7, 1.0和1.6 MeV, 以及中子能量在10 MeV以下的探测器效率, 并对测量结果进行了误差分析。同时为了检验实验结果的准确性, 将实验结果与理论计算结果进行比较, 两者在不确定度范围内符合得很好。Neutron detection efficiency of LS301 fast neutron detector was calibrated by measuring the neutron energy spectrum of 252Cf source, which has a standard fission neutron spectrum. A low mass, fast ionization chamber is used as the fission fragments detector in the time of flight(TOF) spectrometer and afforded the start signal of neutron flight. The stop signal was offered by the anode of LS301. A measured TOF spectrum was turned to the neutron energy spectrum which will be compared with the standard one. Consequently, the fast neutron detection efficiency of LS301 was obtained. Calibration procedures of experimental and data processing was given. Relative detection efficiencies were obtained separately for threshold settings of 0.5, 0.7, 1.0 and 1.6 MeV for neutron energies under 10 MeV. Experimental results accorded with theoretical efficiency curves which were calculated with the Monte Carlo code NEFF50.     

High-pressure neutron diffraction studies at LANSCE

The development of neutron diffraction under extreme pressure (P) and temperature (T) conditions is highly valuable to condensed matter physics, crystal chemistry, materials science, and earth and planetary sciences. We have incorporated a 500-ton press TAP-98 into the HiPPO diffractometer at the Los Alamos Neutron Science Center (LANSCE) to conduct in situ high-P–T neutron diffraction experiments. We have developed a large gem-crystal anvil cell, ZAP, to conduct neutron diffraction experiments at high P. The ZAP cell can be used to integrate multiple experimental techniques such as neutron diffraction, laser spectroscopy, and ultrasonic interferometery. More recently, we have developed high-P low-T gas/liquid cells in conjunction with neutron diffraction. These techniques enable in situ and real-time examination of gas uptake/release processes and allow accurate, time-dependent determination of changes in crystal structure and related reaction kinetics. We have successfully used these techniques to study the equations of state, structural phase transitions, and thermo-mechanical properties of metals, ceramics, and minerals. We have conducted researches on the formation/decomposition kinetics of methane, CO₂ and hydrogen hydrate clathrates, and hydrogen/CO₂ adsorption of inclusion compounds such as metal–organic frameworks (MOFs). The aim of our research is to accurately map out phase relations and determine structural parameters (lattice constants, atomic positions, atomic thermal parameters, bond lengths, bond angles, etc.) in the P–T–X space. We are developing further high-P–T technology with a new 2000-ton press, TAPLUS-2000, and a ZIA (Deformation-DIA type) cubic anvil package to routinely achieve pressures up to 20 GPa and temperatures up to 2000 K. The design of a dedicated high-P neutron beamline, LAPTRON, is also underway for simultaneous high-P–T neutron diffraction, ultrasonic, calorimetry, radiography, and tomography studies. Studies based on high-pressure neutron diffraction are important for multidisciplinary sciences, particularly for theoretical/computational modeling/simulations.     

Neutron Diffraction Study of the p - T Phase Diagram for Erbium

Electrical resistivity measurements performed on a single crystal of erbium as a function of temperature and hydrostatic pressure have provided a preliminary p - T phase diagram. The results have been interpreted in terms of a model for the magnetic structures of Er deduced from neutron diffraction studies at ambient pressure. This model predicted the existence of a magnetic structure with a wave vector of Q =2/7 c * at 4.2 K, when the applied pressure is larger than 3 kbar. This paper reports a neutron diffraction study of erbium made in the temperature range of 4 to 100 K, at pressures between 0.5 and 6 kbar. We have observed the predicted suppression of the low temperature conical ferromagnetic phase and the emergence of a new phase with Q =8/33 c *. The neutron diffraction measurements has enabled us to identify the various phases that develop from the cycloidal phases previously observed at atmospheric pressure.