The paper copy of this newsletter contained articles from Daresbury which are already on the World Wide Web, so are not reproduced here. They were Protein Crystallography Specialist users Session at the SRS Users Annual meeting and the new CCD detector on SRS station 9.6.
from RAL we have news of neutron and muon User meetings and an overview
of residual stress measurement using neutrons.
The UK Neutron and Muon User meeting was held on 16 and 17 September 1998 with technical sessions at RAL and accommodation at Keble College in Oxford. The Chairman, Philip Withers, Manchester, welcomed users and introduced the first session, 'Scientific Highlights and future opportunities'.
Andrew Taylor, ISIS, described the improving reliability of the accelerator and a collaborative project with Frankfurt, part of the design studies for the 'European Spallation Source', which is hoped will result in much improved reliability by replacing the ISIS aging Cockcroft-Walton set with an RF quadrupole. New instruments delivered are TOSCA and OSIRIS; progressing well are MAPS, GEMS, the PRISMA Guide and the Advanced sample environment. Upgrades are being planned for ENGIN-X, SXD-II and HRPD90. Feedback from users indicates 90% satisfaction with ISIS staff, 80% with the equipment, but much less with the accommodation and food on site and speed of processing user expenses claims. The external review of ISIS was to be presented to the CLRC the next week: Andrew was hoping they would support the 300 mamp upgrade to ISIS and the building of a second target station. The ISIS Annual report for 1998 is now available from the User Liaison Service (email [email protected]) and on the RAL Web site at:
http://www.isis.rl.ac.uk/isis98/
it includes the text and images in the printed version and all 600 experimental reports on a CD-ROM.
Three ISIS developments were described: Chris Frost, (Recent science from HET following the installation of the PSD and plans for MAPS with its 36,000 pixels), Sean Langridge, (Spin polarised reflectivity, multi layer systems, magnetism and polarised neutrons) and Mark Daymond, (Engineering the Future, work on polycrystalline materials, typically metals or ceramics, modelling the action of rivets used in the aerospace industry when one failure amongst the millions used may have fatal consequences, work on standards for measurement and plans for an improved instrument capable of handling large samples, up to one ton).
Alan Leadbetter then described improved facilities at the ILL, Grenoble. IN4 is commissioning with neutrons. Work progresses on the 3He spin filter prototype which is likely to be useful on several beam lines, and also used as a routine tool in hospitals to measure the dynamics of lung function using MRI techniques. Upgrades to instruments include: D17 being converted to be a new reflectometer, D16 a new focussing monochromator, D4 a new large detector, IN8 the primary spectrometer is being rebuilt. The rebuild of the IN5 spectrometer is delayed, they hope to install it in the year 2000. A new 5 year Investment Plan will start next year; users are invited to send their ideas for consideration; proposals received so far include: instruments for polarisation analysis with thermal neutrons, another with an image plate detector optimised for protein crystallography with thermal neutrons and a dedicated strain scanning diffractometer. Phil Withers is leading a bid to the EPSRC for a CRG in this area. Users want more support staff and better documentation preferably on the World Wide Web. Two ILL users described recent research; Ted Forgan spoke on high Tc materials using D22 and a dilution refrigerator. Helmut Schober described the use of high pressure on cold time-of-flight spectrometers with applications in measurement of the order/disorder transition in C60 and the dynamics of the newly discovered phase, ice XII.
Andrew Taylor then gave an update on international activities, XENNI, a 10 member European network on neutron instrumentation, ENSA, the European Neutron scattering Association, support for the IUCr99 in Glasgow, (RAL are arranging one of the satellite meetings and running the electronic abstract submission ) and the sixth Oxford neutron scattering school. There is also the OECD Mega Science Forum, with a neutron source working group about to report. They are likely to suggest a 3 part strategy: maintenance of existing sources, maximise their utilisation and prepare for new facilities of next generation sources.
Jane Nicholson, EPSRC Swindon) spoke on access to facilities at the ILL and ISIS, funding mechanisms and improving the service standards for the ISIS facility.
After lunch the meeting split into parallel subject sessions on the following topics: High Resolution Spectroscopy, Disordered Materials, Excitations, Engineering, Muons, large scale structures and Biology, magnetic structures and non-magnetic structures (this being crystallography of course). There were problems with this large numbers of parallel sessions, some had too few people to have a meaningful discussion, while some users would have liked to attend several sessions at the same time. A Poster session followed in the RAL exhibition area before delegates were taken by bus into Keble College for dinner and overnight accommodation.
Next morning Paolo Radaelli gave the plenary lecture on 'Neutrons and X-rays in CMR research: Stripes and much more'. He studies manganese perovskites because of their rich physics and for the large number of potential applications. Electron microscopy shows stripes with spacing of 8.5 Angstroms on the surface of these materials which are not yet understood.
After coffee the subject session chairmen presented their reports. Users think that the current mechanism of adding the cost of beam time to their grants discourages applications from new small users and those with ideas for new techniques which are untried, appear expensive, and hence are usually rejected. All groups expressed support for the ISIS intensity upgrade and the second target station, and reservations about the ticket system. There can also be problems in scheduling popular ISIS instruments in relation to the timing of ISIS experimental rounds and there is a need for more flexible 'Direct Access' beam time at ISIS to allow timely experiments on new ideas. Some spoke of the potential advantages to the science <of neutron and photon facilities being on the same site, as they are in Grenoble and in the USA at the Brookhaven National Laboratory. This allows cross-fertilisation between the 2 user groups and means an increasing number of people now use both neutrons and X-rays in their research. The Engineering report included a plea to satisfy the desperate need for user training in this area. The Large Scale Structures report stated that the biological community was poorly represented at this meeting which means that their special needs for sample preparation may go unheeded. They encourage the greater sharing of software amongst users. All groups wanted more and better accommodation on site at ISIS, better transport to the site and improved food especially at weekends.
During the later discussion Alan Leadbetter commented that he believes it wrong that changes in rules for application for beam time at ISIS will mean ILL scientists are no longer eligible to apply. This would impede co-operation between the ILL and ISIS to the detriment of users, and he wanted to know what could be done about this. It is clear that there is no intention for this to happen and the traditional interchanges between scientists at the two facilities should continue.
The meeting ended with short presentations from the poster prize winners, Adrian Hillier, Sue Kilcoyne, Trevor Riley, Miles Hamilton and Abdul Muslim.
Kate Crennell and Chick Wilson
The measurement of stress using neutron diffraction is a rapidly growing field, of great relevance both to the academic materials science world and to the industrial world of structural engineering. Recent developments towards the introduction of an international standard for neutron stress measurement, coupled with European Union funded ventures to bring the technique to a broader range of industry are stimulating an increasing interest in the technique.
History
The techniques required for measuring stress inside a material using neutron diffraction were first developed at the Harwell laboratory in the early 1970's. Since then, both UK and international academic engineering and materials science communities have continued to develop the technique and expand its applicability and usefulness. The UK community is still one of the world leaders in the field, as was evidenced by the construction of a leading rank stress measurement diffractometer at ISIS [1], and by the leading roles played in many related international projects by UK practitioners.
Internal and residual stresses in materials have a considerable effect on material properties, including fatigue resistance fracture toughness and strength. In the early days of neutron stress measurements, experiments were aimed at identifying high stress areas, and likely sources of failure in components or joints (e.g. welds), and thus improving component lifetimes. While this traditional area of interest continues to make up a major fraction of the neutron stress measurements carried out, the technique has diversified, with a strong emphasis on process model validation - leading to improved design to product times and to improvements in both production costs and safety. Academic users of the technique on the other hand, are often interested in investigating fundamentals of materials deformation, that is how the stress state within the material relates to material deformation and ultimately failure.
Introduction to the technique
Relying on the same physics describing the analogous measurement of stress using x-ray diffraction [2], neutrons have two principal benefits compared to x-rays for the engineer or materials scientist.
Firstly, neutrons interact primarily with the nucleus, rather than the electron cloud as x-rays do, so the penetration depth of neutrons is very large compared to x-rays. For example, the penetration depth (1/e) for steel is ~1cm for thermal neutrons, but less than 10 mm for x-rays of comparable wavelength [3]. Hence in neutrons are a probe suitable for bulk average measurements of material properties, compared to x-rays, which useful primarily for measurements of surface layers. For many practical applications, the engineer or material scientist is primarily interested in the bulk properties.
Secondly, since the scattering cross section of neutrons varies considerably across the periodic table, neutrons can be successfully scattered from both very light (e.g. Li, Be) and heavier (e.g. Fe, W) elements, with good penetration depths. In practice, this technique can be used on nearly any crystalline sample.
When a stress measurement is made using neutrons, an incoming neutron beam is diffracted from the sample. From Bragg's law the lattice parameter can be determined, as a function of direction within the sample. The incoming and outgoing beams are collimated [1] so that only a certain region within the sample contributes to the diffraction spectra. By moving the sample around within the beam, the position of the sampling volume within the sample can be scanned, providing a map of atomic lattice plane distances. Alternatively the sample may be held within a loading rig, capable of applying stress or pressure, a furnace or a cryostat. The sample can then be monitored during in situ variation of the thermo-mechanical environment. Accurate measurement of lattice parameter (accuracy of ~3x10-4Å is typical) is required to allow a correct evaluation of strain changes.
Pulsed neutron sources, such as ISIS, provide data which is analogous to energy dispersive x-rays. If we consider Bragg's equation, at a nuclear reactor or monochromatic x-ray source, we use a single wavelength. Changes in d-spacing are monitored as changes in the diffraction angle. Alternatively, we can fix the scattering angle q, and instead monitor changes in d-spacing as changes in wavelength. This is possible since de Broglie's relation links the neutron's wavelength to its velocity. The velocity can be determined simply by measuring the time taken for a neutron to travel a known distance - from creation in the spallation event, to detection after scattering. The major advantage of the pulsed neutron technique over reactor sources, is that an entire diffraction spectra is produced for a fixed detector angle. This means multiple phases can be easily monitored, and experimental apparatus to make measurements at a fixed sample orientation is much simpler (e.g. windows within high pressure cells or furnaces).
Increasing demand and international standards
Industrial
demand has led to two European funded networks, containing members from
industry, academia and neutron facilities. RESTAND, a
network run by the JRC in the Netherlands aims to establish best practice
for neutron measurements. TRAINSS, a network run by ISIS
in the UK, aims to introduce industries to the technique and train them in
measurement practice and interpretation. Companies in these networks
include British Aerospace, Rolls Royce, Nuclear Electric, Mercedes,
Volkswagen, Fiat, Peugeout/Citroen, Pieraleisi, Sintec, SNCF, AEA Technology
and Schunk. Both networks have already successfully carried out
measurements.
The VAMAS TWA20 initiative, headed by Imperial College, London, is made up mostly of academics and representatives from facilities around the world; it is working to define an international standard for stress measurement using neutrons. As well as detailing best practice for measurements and data interpretation, the project will involve four different round robin samples which will be measured at each neutron facility. Each is typical of a problem found in real measurements: high strain gradients, multiple phases, through surface strain measurements, and large compositional variations. The first two round robin samples were successfully completed by October 1998 by all facilities with good agreement. The third is being measured now; the fourth will be distributed next year.
Responding to the demand
The present facilities available to the UK community are already heavily oversubscribed and in great demand. These include the ENGIN instrument at ISIS; it is located on the PEARL beamline which is shared between the engineering programme ( on ENGIN ) and the high pressure programme (HiPr). At the ILL neutron reactor at Grenoble, the UK community has access to time through the peer review system, to the D1A beam-line, which is also available for engineering studies for 50% of the time. The UK community has responded in two ways to the great demand for experimental time. In the short term, a proposal has been submitted to the EPSRC for a Collaborative Research Group project to run D1A at the ILL 100% of the time for an engineering program. This will quickly provide more access for the UK academic community for relatively small capital cost and the funding of two instrument scientists However, UK users want a facility closer to home. Unlike many other industrial countries (e.g. Germany, France, Sweden, Denmark, Netherlands, Czech Republic and Hungary) there is no longer a neutron scattering reactor facility in the UK. The UK community therefore relies primarily on ISIS where demand for time at ISIS is very high, including heavy demand from outside the European Community. While the ENGIN instrument at ISIS is still world leading, it was constructed in the early 1990's on an existing beamline as a compromise and development instrument. An order of magnitude improvement over the existing instrument can be achieved by building an optimised instrument on a dedicated beamline. A university consortium has therefore submitted a proposal to the EPSRC for ENGIN-X, a dedicated stress measurement diffractometer. ENGIN-X will be able to measure stresses in real engineering components of up to 1 ton in weight, scan tiny sampling volumes (sub-millimetre) within materials, and apply realistic loads and temperatures to test pieces. As well as providing sufficient capacity to meet the growing demands of industry, ENGIN-X will open up new avenues of academic research, allowing faster time resolution of time-dependent processes (e.g. thermo-mechanical fatigue), greater spatial resolution of high stress gradients (e.g. barrier coatings), study of stresses deeper within materials (larger components), and more complete studies, whether this be 2D mapping rather than 1D scans of spatial variation, or more complete parametric studies.
A training network has been proposed to help cope with the increasing demand from academic users. This will allow novice users to accompany more experienced ones on experiments at neutron facilities where they will learn by example, helping with the experiments and discussing the results and their interpretation. Help with proposal writing will also be given.
Anyone interested in using neutrons for stress measurement, or any other applied applications, is encouraged to contact Mark Daymond, email: [email protected]. Tel: +44 (0)1235 445414.
Mark R. Daymond, ISIS facility
References
1. ENGIN - A new instrument for engineers. Johnson, M.W., Edwards, L., and Withers, P.J.,Physica B: Condensed Matter, 1997. 234-236: p. 1141-1143.
2. Residual Stress - Measurement by Diffraction and Interpretation. Noyan, I.C. and Cohen, J.B., Materials Research and Engineering, ed. B. Ilschner and N.J. Grant. 1987, New York: Springer-Verlag. 272.
3. Industrial Applications of Neutron Scattering Hutchings, M.T. and Windsor, C.G., ,
Ch 25 in Neutron Scattering, K. Skold and D.L. Price, Editors. 1986, Academic Press: Orlando, Fla. p. 405-482.
Editor's Note: a diagram of ENGIN was printed in 'Crystallography News' June 1996 issue 57 p48,49
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