M.W. Johnson

Strain imaging by Bragg edge neutron transmission

The Bragg edges appearing in the transmitted time-of-flight spectra of polycrystalline materials have been recorded using a two-dimensional array of detectors. Subsequent analysis has enabled maps of the elastic strain to be produced.

An optimum design for a time-of-flight neutron diffractometer for measuring engineering stresses

A method is described for optimizing the design of a time-of-flight neutron diffractometer designed to measure lattice parameters. Such diffractometers are now used extensively by engineers and materials scientists for measuring strain within metallic and ceramic components. The method presented relies on the identification of a figure of merit (FOM) that accurately describes the performance of such an instrument. For the first time, an FOM for an instrument exhibiting non-Gaussian peak shapes is described, and the methods by which this FOM may be maximized are described. Although the instrument described is based on the time-of-flight technique, the FOM derived may equally well be used to optimize a reactor-based instrument. While measuring peak position is a straightforward example, it is shown that similar figures of merit may be found for other peak shape parameters, and thus other types of instrumentation.

The precision of diffraction peak location

Much of crystallography is concerned with the accurate location of the centre of diffraction peak profiles. Simple analytical expressions are derived for estimating the precision of diffraction peak location that can be achieved for Gaussian diffraction peaks with a flat background, in terms of the standard deviation, integrated intensity and peak height (H) to background (B) ratio. Two formulations are derived using standard methods: one for the case of very low background, the other for significant backgrounds. It is found that in cases of significant background, peak position is less well determined by a factor of [1+2(21/2)B/H]1/2 compared with the case of no background. The applicability of the expression has been demonstrated by Monte Carlo simulation of Gaussian profiles and by the analysis of real data collected at a large number of neutron and synchrotron sources, largely as part of the VAMAS TWA20 project. While the solution is presented for Gaussian peak shapes, it is believed to be approximately correct for a wide range of other common diffraction peak shapes (Lorentzian, Voigtian etc.). The method is applied to the assessment of the variation in optimal measuring time as a function of the depth of the gauge volume for residual strain scanning measurements.

Neutron strain scanning using a radially collimated diffracted beam

Abstract Although neutron diffraction provides a unique tool for the sub-surface measurement of elastic strain within engineering components, the technique is characterised by slow rates of data acquisition. The use of a radially collimated diffracted beam for defining the gauge volume enables large, position-sensitive detectors (PSD) to be used, with the acceleration of the data collection process. As a result, focusing collimators are being used or are being considered for use on many new and existing neutron strain scanning instruments. The gauge volume is of key importance when measuring strain or stress by any technique. In this paper we introduce a formalism for quantifying the gauge volume and a phase space representation for its visualisation. These are used to describe and analyse the performance of radial (sometimes loosely called ‘focusing’) collimation systems in general, and to compare their performance with traditional and cheaper aperture-based volume definition methods. It is shown that radial collimation of the diffracted beam is an essential companion to a PSD at pulsed sources if a high level of spatial discrimination is to be achieved and can be of value at constant flux sources. Geometrical aberrations, which have previously been well documented for slit gauge definition systems, are a necessary consequence of the gauge definition process and give rise to apparent strains when scanning through a surface. Knowing the radial collimator geometry, the geometrical shifts in the peak positions can be predicted, and the shifts corrected for to provide accurate residual strain measurements, even near surface.

Bragg Edge Determination for Accurate Lattice Parameter and Elastic Strain Measurement

The transmission spectrum of thermal neutrons through a polycrystalline sample displays sudden, well-defined increases in intensity as a function of neutron wavelength. These steps, known as Bragg edges, occur at the point at which the neutron wavelength exceeds the Bragg condition for coherent scattering from the respective lattice planes, and can be easily observed using the time-of-flight method. Accurate location of these edges and determination of their magnitude and shape can be used to extract information about the stress state, texture and phases present in the material. This paper describes a method for analysing these edges singly and collectively, using a Pawley-type refinement. Furthermore, experimental trials are presented which demonstrate the utility of the method for the accurate measurement of lattice spacings, and thus strain. These trials include measuring the lattice parameter in Cu/Zn alloys as a function of Zn content, and the determination of elastic strain of an iron rod under tensile/compressive straining. In the former case the results are compared with Bragg diffraction peak measurements made on HRPD and in the latter case with conventional strain gauge measurements.

Pixelated neutron scintillation detectors using fibre optic coded arrays

Abstract Over a number of years a variety of neutron detectors have been developed at ISIS incorporating fibre optic coded arrays viewing ZnS 6 Li scintillator. This technique has proved to be extremely versatile in matching detector characteristics to neutron instrumentation requirements. These detectors have generally consisted of linear detector arrays using low order fibre optic codes. Recently a number of detectors have been developed using quad coincident coded arrays and offering true two-dimensional position resolution. Detector characteristics and the advantages of such detector arrangements for neutron scattering instrumentation are discussed.

An experimental test of a neutron silicon lens

Abstract We describe the first results obtained from tests of a new design of neutron lens. The Neutron Silicon Lens consists of multiple layers of different thickness, each layer a single crystal silicon wafer coated in neutron supermirror. An optimal stacking arrangement allows, in principle, the reflection of all neutrons incident on the lens within the appropriate angular divergence range. The construction utilised for a practical lens is described. We show that a focussing effect is achieved and the results are analysed with a view to future improvements to the lens design.

The determination of a stress-free lattice parameter within a stressed material using elastic anisotropy

A method for the calculation of a stress-free lattice parameter from the analysis of diffraction data from stressed material is discussed, utilizing the elastic anisotropy of the material. The technique is demonstrated using data obtained during a uniaxial tension test on untextured austenitic (face-centred cubic) steel. The uncertainty in the calculated lattice parameter for various choices of number of diffraction peaks and different number of stress levels available for the calculation is considered. It is shown that when all the data are within the elastic regime, an accurate evaluation of the reference lattice parameter can be made. When some data are in the plastic regime, a more limited evaluation is possible. The use of plots of lattice parameter against Γhkl [= (h2k2 + h2l2 +k2l2)/(h2 + k2 + l2)2] as a method for monitoring plasticity as well as freedom from deviatoric stress is demonstrated.

Silicon APS detectors for neutron scattering

In this paper we review neutron detector types available for neutron scattering and demonstrate the need for highly pixellated ‘space-time’ neutron detectors. We then indicate how they might be produced using silicon active pixel sensors.

The neutron silicon lens: a new lens design for thermal neutrons

Abstract The development of thermal neutron lenses would enable more efficient and effective neutron scattering instruments to be built. Such lenses would have many applications in both the primary and scattered beams on neutron instruments, and would lead to immediate improvements where the sample to be illuminated is small, as in high pressure or engineering strain scanning instruments. In this paper we describe a proposal for new design of a cylindrical neutron lens based on the use of multiple neutron mirrors supported and separated by silicon wafers. Details of the new design may also be found in patent application no. 9909052.4 at the UK patent office.