The development of neutron beams for materials science

Physica 137B (1986) 204- 213 North-Holland. Amsterdam

THE DEVELOPMENT OF N E U T R O N BEAMS FOR MATERIALS S C I E N C E C.G. W I N D S O R , M.T. H U T C H I N G S

a n d P. S C H O F I E L D

MatertaL~ Physics and Metallur,w Dwisum A E R E Ilarwell, OXl l OR.4, ~'K

rhe object of experiments undertaken using neutron scattering is traced as progressing from an understanding of the neutron as a probe, to an understanding of solids and liquids of importance to nuclear reactor design, through to the fundamental understanding of condenced matter and eventually to thc problems of industry. The unique information given by neutron beams in the applied field is illustrated by examples from recent research at Harwell on metallurgy, fast ion conductors, nuclear fuels and colloid chemistry. An ideal neutron instrument for materials research in the future is suggested.

1. The changing role of neutron scattering experimeats The diffraction e x p e r i m e n t s we p e r f o r m t o d a y are essentially no different from those Shull perf o r m e d in the 40's. The i n s t r u m e n t s are better now a n d easier to use, but the aim of the e x p e r i m e n t s r e m a i n s the u n d e r s t a n d i n g of the structure of the sample. W h a t has changed, and will c o n t i n u e to change, is our m o t i v a t i o n for p e r f o r m i n g the experiment. The original diffraction e x p e r i m e n t s of Shull and S m a r t in 1949 [1] were directed as much to the basic u n d e r s t a n d i n g of the neutron scattering process as to the p r o p e r t i e s of m a n g a n o u s oxide. F o r neutron scattering this p e r i o d soon passed, helped by the c o m p r e h e n s i v e theoretical f r a m e w o r k d e v e l o p e d by Fermi, Bloch, and H a l p e r n a n d Johnson. A r m e d with this unders t a n d i n g the object of the first e x p e r i m e n t s was the nuclear a n d m a g n e t i c structure of crystals, and Shull's 1951 p a p e r ploughed the first of m a n y furrows through this fertile field [2]. However the n a t u r e of m a n y of the samples studied was to c h a n g e surprisingly soon. The reactors of those early days, as now, belonged to nuclear energy centres. It was natural that the p r o b l e m s central to the nuclear energy i n d u s t r y h a d a growing influence on the choice of experiments. W h e n the first E u r o p e a n neutron diff r a c t o m e t e r was installed by Bacon in 1949 on the B E P O reactor at Harwell, one of his first s a m p l e s

was g r a p h i t e a m o d e r a t o r material for our reactors [3]. His m e a s u r e m e n t s to d e t e r m i n e the electronic structure of g r a p h i t e from differences between X-ray and neutron diffraction could h a r d l y at that time be said to be crucial to Britain's nuclear p o w e r p r o g r a m m e . It was " ' u n d e r l y i n g research" with a high p r o b a b i l i t y of being one day useful to the p r o g r a m m e . The trend to choose " n u c l e a r s a m p l e s " for intensive study was worldwide t h r o u g h o u t the 50's and early 60's and lead to an invaluable fund of k n o w l e d g e on actmides, especially u r a n i u m and its oxides, c l a d d i n g and structural alloys, r a d i a t i o n d a m a g e and defects in m o d e r a t o r materials. The need to u n d e r s t a n d the neutron m o d e r a tion prcxzess, so f u n d a m e n t a l to reactor design, was a cause of the growth of inelastic neutron scattering. The e x p e r i m e n t s were begun on accelera t o r sources d u r i n g the war [4]. However, the possibility of burning, rather than just producing, p l u t o n i u m in the early reactors led to a need to know the effect of the 0.3 eV Pu resonance on the m 6 d e r a t o r t e m p e r a t u r e coefficient, and the design of the first p o w e r reactors with regions of m o d e r a tors at different t e m p e r a t u r e s required k n o w l e d g e of the spatial variation of the thermal neutron spectrum. Detailed calculations in this new field of ' n e u t r o n t h e r m a l i s a t i o n ' [5] required m e a s u r e m e n t of the 'scattering-law" for reactor materials. The need to i n t e r p o l a t e a n d e x t r a p o l a t e these d a t a s t i m u l a t e d the i n t e r p r e t a t i o n of the m e a s u r e m e n t s

205

C.G. Windsor et al. / Neutron beams for materials science

in terms of diffusion, densities of states and excitation spectra, and the field of neutron inelastic scattering to study the dynamics of atomic and molecular motion burgeoned in its own right. Measurements made on the N R X reactor in C a n a d a led to the development of the rotor, rotating crystal, and triple axis spectrometers which have dominated this field since then [6]. Accelerator sources were soon to catch up, with a trend to ever broader ranges of scattering law data which continues today [7]. The underlying importance of moderator materials, especially water, has never ceased. The measurement of the water scattering law with ever greater extent and precision and with ever greater theoretical understanding has been a continual stimulus to inelastic scattering. F r o m the early sixties, the nature of neutron experiments began to change again. In the UK, research workers at universities came to Harwell to use the neutron techniques they were offered there. This led to the establishment of the "joint pro-

g r a m m e " between the Atomic Energy Authority and the then Science Research Council which lasted for 20 years. It enabled some hundreds of researchers to receive training in neutron techniques and to carry on their own programmes. The joint p r o g r a m m e expanded vastly the range of application of neutron beam research, principally into areas of chemical crystallography, molecular spectroscopy, polymer science and biology. In the U S A over approximately the same period, the Brookhaven High Energy Flux Beam Reactor became well established for the study of basic science rather than of nuclear technology. In 1972 came the high flux reactor at the Institute L a u e - L a n gevin in Grenoble which was to open the field to m a n y more users in Europe. It must be said that the very success success of these programmes gave a perception that neutron beams were for basic science and had little to offer of relevance to the materials science concerns of industry. It has been one of the tasks of Harwell's

Table 1 A summary of Harwelrs basic research programme with corresponding applications to the Nuclear Power Programme and to non-nuclear, externally funded, uses of the Harwell facilities. Basic Phase composition Inclusions, voids Residual stress Stress vs strain in fracture Fatigue Creep Texture

Nuclear Pressure vessel steels Cladding Ferritic welds Residual stress in welds Irradiated structural materials

Solid state physics

High temperature disorder Lattice dynamics Non-stoichiometric oxides Active samples

Thermodynamics of UO2 MgO doping of UO2 UO2 ± Irradiated fuel

Solid electrolytes Ceramics

Chemistry

Colloids Sol/gel interactions Porosity Atomic binding to surfaces

Fabrication routes Activity transport Waste encapsulation Corrosion, embrittlement

Detergents Oil additives Catalyst supports Oil shales, coal, clays Cements Catalysts

Glasses

Structure of complex glasses

Waste disposal glasses

Radiography

Real-timeimaging Resonance radiography

Fuel pins Tube reflooding

Metallurgy

Non-nuclear New aerospace materials Single crystal imperfections Autofrettage Residual stress in welds Rolled steel

Turbine blades Engines Catalyst beds Food processing

206

('. G. Bqnd~'or et al. / Neutron heams fi*r matermls" ~'¢temc

neutron beam project over the past few years to demonstrate the falseness of this perception, and to show that industry is willing to sponsor neutron experiments and that neutrons have " c o m e of age" to take their place with other diagnostic and analytical techniques as an essential tool for the metallurgist and materials scientist. How programmes are now changing to embrace these new opportunities are illustrated in table I. The programmes in each column are performed on the same instruments, using identical techniques by the same research workers, but they differ in the aims of the experiment, and in the choice of samples. In basic research the samples are chosen, as in any university materials department, to give the best understanding of the p h e n o m e n a to be investigated. The degree to which these fields also underlie the p r o g r a m m e s on applied work for the nuclear and non-nuclear industry is evident from the table. In many instances the applications have arisen out of existing research, in other cases potential applications have suggested new lines of basic investigation. The samples for the experiments in the nuclear p r o g r a m m e are chosen by the reactor industry to help solve their present and perceived problems. It is the job of the neutron beam scientist to solve these problems with the techniques at his disposal. His input will usually be only one part of a wider effort embracing many techniques. The samples for the experiments in the non-

nuclear area are chosen by industrialists who have to count the commercial benefit. The samples usually come together with a problem they embrittle under heat-treatment, or fracture in service. The scientists must decide which technique, neutron or otherwise, is most likely to lead to the answer. It is a measure of the power of neutron techniques that this sector continues to grow [8].

2. Why use neutrons?

The greatest overall advantage of neutrons over other probes, electromagnetic or charged particles, for the study of materials is their penetration, which means that true bulk measurements can be made with little or no sample preparation. A further consequence is that the neutron beam is little attenuated by sample containment. This means that the samples can be contained in furnaces, cryostats, pressure cells or in special chemical environments. It is therefore possible to study the effects of varying these conditions, often being able to observe changes with time. Examples of such in situ studies are given below. In other respects the information obtained with neutron beams is similar but complementary to other diffraction and spectroscopic techniques. As exanapies we shall now consider several topics from the main areas of the neutron beana programme at Harwell. tlowever it should be explained that Harwell, although perhaps the first laboratory to

Fig. 1. Dynamic neutron radiographs of tube re-flooding The sequence sho'as the advance of the liquid front of water pumped into an iron tube electrically heated to 6000(`. and condensed droplets ahead of the front.

C.G. Windsor et aL / Neutron beams for materials science

seek applied contract work, is now by no means unique in this respect. Many other laboratories are now establishing programmes of industrial sponsored research.

207 i

--

l

I

I

101

g o

I I

3. N e u t r o n r a d i o g r a p h y o

Neutron radiography uses the penetrating power of the neutron to 'see inside' metal components. At Harwell, imaging techniques were developed under the basic research programme to enable real-time viewing of the flow of hydrogeneous material, which gives high attenuation, within metal casing. For sufficiently low energy neutrons, there is no Bragg scattering from the metal and the penetration of neutrons is greatly enhanced. The facility on the DIDO reactor at Harwell, probably the world's best, now provides a service for a wide range of nuclear and non-nuclear applications [9]. A recent study has been of the behaviour of cold water pumped into an iron tube at 600°C. This is of importance to the problem of reflooding a reactor in a loss of-coolant accident. Fig. 1 shows a sequence of stills from this video which shows how the water boils and eventually recondenses.

|

I

200

2C

a

E

10

100

o

j,

g

a l

l

6O0 Ageing temperature (°C)

400

I

I

J

4. Metallurgy The use of neutron beams in metallurgy covers both microstructural and mechanical properties. In the former, neutrons are providing useful information to complement and supplement that obtainable using the electron microscope. Whereas the electron microscope gives a direct visual display of the microstructure, with neutrons a quantitative analysis of the defect size distribution and volume fraction can be made over the sampling volume. The four main areas of research in metallurgy are

[10]: (1) The study of phase composition of alloys. (2) The size and distribution of voids and precipitates, using small angle scattering. (3) The measurement of internal strain. (4) The measurement of texture. A particular example of the potential of neu-

It

20

10

t • 400

500 Ageing teml~roture (%1

z~ 600

Fig. 2. (a) Mean precipitate diameter and volume fraction for maraging steels aged for 10 h at various temperatures (solid circles). (b) Volume fraction of transformed austenite in maraging steels aged for 10 h at the temperatures shown. Closed circles - neutron diffraction; crosses - X-ray data.

208

('. G. Windsor et al. / Neutron beam.~ for materials scten( e

at a lower temperature the same amount of hardening phase was precipitated, but without the simultaneous reversion to austenite which occurs at the higher temperatures and which has an adverse effect on the strength of the alloy [11]. Another example of the usc of small angle scattering is an on-going study for CEGB, of copper clustering in irradiated pressure vessel steels [12]. The technique is used also for basic fracture

tron diffraction in studying heat treatment of alloys is given by a study of maraging steels carried out in conjunction with electron microscopy studies at Risley Nuclear Laboratories (fig. 2). Maraging steels are hard steels which are conventionally aged at about 480°C to produce a hardening phase. At Harwell, we were able to show, by studying the growth of these precipitates in a furnace in situ on the spectrometer, that bv ageing for a longer time

~

T

u

b

e

,,u0e H

0,ock

~Tail

off region

(a) as welded Tension 1500 1000

{ ~.~

/,,,-Weld

D

C~

.E_

0 J

-~ -1000 t-

-2000 Compression

J TaiFoff ) ) )) )~~-)~~- ~ - ~

--4,-- Weld --~-- Haz (plate)

Tube plate Haz Weld

C D E F H I K L Positions around circumference

TubeHaz

Fig. 3. Residual strain measurements on an Evaporator Tube Weldment. Note the rapid oscillation in the strain observed near the ""tail-off" region of this single pass weld [13].

C.G. Windsor et al. / Neutron beartt~for materials science

research on, for example, the development of micro-voids around crack tips and the microstructure of weld metal. The second major area in the metallurgy programme is the use of neutron diffraction to measure internal strain in components, from which it is possible to deduce the stress state of the material [13]. The technique is finding an important application in the study of residual stress, a topic of major concern in the design of structures, and in particular in welding. This is an area where neutron diffraction is particularly important as a non-destructive probe since conventional destructive techniques, such as hole-drilling, may have an unknown and unpredictable effect on the strain to be measured. While X-ray diffraction can measure surface strains only to some tens of microns in depth, neutrons can penetrate a few centimetres into steel and enable strain variation to be measured to a resolution of a few millimetres. The basic research programme has established the viability of the technique, and measurements on test-pieces, such as plastically bent bars, welds (see fig. 3) and fatigue cracks, are contributing greatly to our understanding of the distribution of stress and strain in steels and other alloys. Establishment of this technique is resulting in an increasing amount of funded research work both for nuclear and non-nuclear industry. An additional dimension to this aspect of the programme is the use of the neutron measurements to validate ultrasonic measurements of residual stress, which have the potential of providing a portable technique. Such measurements, as well as requiring an understanding of the stress-strain relations, also have to allow for 'texture' or preferred orientation of grains in the material. Theory has shown that texture effects may be eliminated by combining different ultrasonic velocities [14]. These methods can be validated by measurement on samples where the texture is known, and this can be measured directly in bulk material by neutron diffraction. 5. Defect studies

Defects in an otherwise perfect crystal lattice may, in addition to possible small angle scattering,

209

give rise to coherent diffuse scattering which yeilds information on the relative position of defective atoms within a disordered volume of the lattice. Such information may be combined with the mean time-averaged occupancy of sites in the unit cell obtained from Bragg diffraction to build up a model of the defect state. The first studies were made on irradiated graphite using cold neutron transmission spectrometry as long ago as 1955 [15]. Later, cold neutron small angle scattering was used to define the nature of the defects in graphite [16]. Clustering of impurities in metals, or atoms in alloys, have been the traditional object of research using diffuse scattering. However, during the past decade the technique has been used to study the ionic disorder in solid electrolytes, or fast ion conductors, in which thermally induced defects are created which diffuse through the lattice. This dynamic disorder gives rise to coherent diffuse scattering which is quasi-elastic, and is best studied by a triple-axis spectrometer, or a time-of-flight spectrometer, in order to separate it from scattering from phonons and from purely elastic scattering. If the diffusing ions have an incoherent crosssection an additional separation of coherent and incoherent scattering must be made. This can be difficult in practice. The incoherent quasi-elastic diffuse scatttering yields direct information on the nature of the diffusion of individual ions, and so its experimental isolation is important. At Harwell work in this area has concentrated on the class of solid electrolytes which have the fluorite crystal structure. The advantage of studying these materials is that their structure is simple and can be modelled theoretically with relative ease. A large range of either vacancy or interstitial doping is possible. The materials are used in a range of devices, and their properties are related to those of uranium dioxide and other nuclear fuels. An extensive study, carried out in collaboration with Oxford University and Riso National Laboratory, has used diffraction, coherent and incoherent quasi-elastic diffuse scattering, and inelastic scattering, to investigate the nature of the dynamic anion lattice disorder [17]. This occurs at a temperature of about four-fifths of the melting temperature of the pure materials and is accompanied by a peak in the heat capacity and an increase in ionic

21 ()

( ".G. lVind~or et al. / N e u t r o n hewm" /}~r matertal~" ~'ciencc

conductivity to near that of the ionic melt. The alkali earth halides have such transitions at about 1000 K, and the experimental results suggested a model of the lattice disorder in terms of fluctuating clusters of defective anions which can account quantitatively for many of the observed properties. In the case of strontium chloride, quasi-elastic incoherent scattering shows that the chlorine ions hop directly between regular anion sites [181. Molecular dynamics calculations are giving new insights into the interpretation of these data [19]. A recent development at Harwell of a furnace capable of operating in a ncutron beam at temperature up to 3000 K (fig. 4) has enabled measuremetats on uranium dioxide itself to be made. These have shown that oxygen ion disorder occurs at temperatures above 2000 K which is very similar to that in the halides [20], and these data may be used to estimate the effect of the disorder on thermodynamic and mechanical properties of the

oxide fuel under extreme conditions postulated in reactor accident assessment. Inelastic scattering measurement of lattice vibrational excitations have provided data on the elastic constants of uranium dioxide, which have been used to correct previously accepted values based on extrapolation from lower temperatures.

6. Colloid chemistD ('olloid systems include those in which matter is finely dispersed in a host material with particlc sizes from 1 nm to 1 /~m. Examples include emulsions, foams, aerosols, smokes, ultrafine powders and particles dispersed in liquids known as sols. In this state of subdivision, properties become dominated by the high interfacial surface area of the system. An understanding of colloid and surface chemistry is thus important in numerous

Fig. 4. Furnace used to study defects in uranium and other oxides at temperatures up to 3000 K. The furnace is shown mounted on the sample table of the triple-axis spectrometer on the right of the picture. The PLUTO reactor is in the background.

C.G. Windsor et al. / Neutron beams for materials science

technological processes and environmental phenomena. Neutron scattering techniques have recently proved particularly valuable in studies of oxide colloids and oxide-water interfaces, which are relevant to sol-gel processes, aqueous corrosion, surface contamination and radio-activity transport mechanisms, for example. Because neutron beams have considerably more penetrating power than X-rays and light, studies of colloids in concentrated systems and in special containment, giv-

211

ing high temperature or pressure, are possible. This unique capability has been exploited in our programmes on the mechanisms of sol-gel processes and in studies of oxide colloids in water at high temperature [21]. Sol-gel processes were originally developed for the production of reactor fuels, but have many other applications where metal oxide ceramics of controlled composition and microstructure are required, for example as adsorbents, catalysts and coatings. Small angle neutron scattering can pro-

100

dZ

50

o

28 Doys

x

17 Hours

,(o

+

6 Hours

: ~+

0

l, Hours

++

A

53 Minufes

o

~2.s - _

A

c~ L.

+ i

E

? Minufes o

10

Powder

Surfoce Scattering

'E

S'O

-

-

dZ aO-~

dC~ P~

dE ot 0.-3.2 dQ

1"0-

0"5 0"01

I

1

I

0"1

1"0

10 a ( nm -1)

OPC

CEMENT 0 ~ d Z l d ~

vs

Fig. 5. The evolution of the small-angle scattering pattern of CSH cement gel in 'Ordinary Portland Cement'. The scattering vector Q is proportional to the scattering angle, and d o / d R is the intensity. The increasing scattering at the right-hand of the figure shows the growth of the gel structure, while the left-hand side arises from the hydration and break-up of the surface of the original dry powder. This figure is a compilation of data collected at Harwell and at the lnstitut Laue-Langevin [22].

212

('. G. 14'ind.~or el a t / N e u t r o n heam~" fi~r material~" ~ w m c

vide information on the size and shape of the colloidal particles. More detailed analysis of the scattering from concentrated sols provides information on the ordering of particles and the nature of the inter-particle forces. This mformation is important for an understanding of the stability of concentrated colloids, and the mechanism of the conversion of sol to gel. Another aspect of the programme at Harwell on colloids is the study of cement--gel, the calcium-silicate hydrate (CSH) gel which binds the cement and gives it strength. ' I n situ' studies have been carried out on a variety of cement matcrials, in which the time evolution of the small angle scattering has been followed after mixing of the dry powder with water. This is illustrated in fig. 5. From the scattering distribution information about the evolution of the structure of the CSH gel and the resulting fine-porosity of the cement may be obtained [22].

7. Glasses The study of the atomic structure of glassy materials is, like the nuclear fuels area, another example of the benefit of longer-term research. Starting from a study of the arrangements of atoms

t

in 'simple" borate and silicate glasses, it is now possible to build up a picture of the complex boro-silicate glasses required as matrix material for active-waste disposal and to examine modifications by the inclusion of waste materials. In particular, neutron diffraction may be used to detect crystallization of simulated waste materials under extreme treatments of the glass. In another investigation, the structure of naturally occurring glasses was studied and compared to man-made materials. These results have enabled deductions to be made about the thermal history of the natural glasses [23].

8. Looking to the future As physical measurement techniques, such as electron microscopy, X-ray diffraction or infra-red spectrometry, have been taken up by industry', the instruments have become commercially available, more efficient, and easier to use. Neutron instruments differ from these in not being portable, but as we have seen, this does not stop them being available commercially'. The efficiency increase from Shull's first diffractometer to the lastest USA Japanese powder diffractometer recently installed on the Oak Ridge reactor will not see the

To Stress bank

Sm

!er mg 8m

order

bank

Texture bank

lm

Fig. 6. The neutron beam materials muhi-facility proposed for installation on the Spallation Neumm Source at the Rutherford Appleton Laboralorv.

C G. Windwr et al. / Neutron beams for materials science

end of the story [24]. Its m o n o c h r o m a t o r still only selects a p e r c e n t or so of the incident neutrons from the reactor, its large d e t e c t o r counts only a p e r c e n t or so of the scattered neutrons a n d it only p e r f o r m s diffraction. W h a t electron m i c r o s c o p e t o d a y p e r f o r m s only transmission microscopy? Electron diffraction, scanning microscopy, energy dispersive X-ray analysis, A u g e r analysis, a n d energy loss s p e c t r o s c o p y can all be p e r f o r m e d on the s a m e instrument, even if not simultaneously. C a n a n e u t r o n i n s t r u m e n t have the same versatility? Fig. 6 shows an outline of a materials multifacility p r o p o s e d by Harwell for installation on the U K S p a l l a t i o n N e u t r o n Source. It uses a g o o d fraction of the incident wavelength b a n d from 0 to 8 A wavelength to record s i m u l t a n e o u s l y the mic r o s t r u c t u r e from small angle scattering, the defect structure from diffuse scattering, the phases present from diffraction, the strain from 90 ° high resolution diffraction, the texture from azimuthal diffraction, and the s a m p l e diffusional and vibrational m o t i o n s from quasi-elastic a n d beryllium filter analysers. It brings the full p o w e r of neutrons b e a m s to b e a r on the s a m p l e to measure all the quantities discussed in this p a p e r simultaneously. It w o u l d be especially useful for e x a m i n ing phase t r a n s f o r m a t i o n p h e n o m e n a and time d e p e n d e n t effects. So far, i n d u s t r y has not yet found the need to use the elegant techniques involved in neutron i n t e r f e r o m e t r y - a subject which owes a great deal to S h u l r s ingenuity. However, silicon crystals, the m a t e r i a l used for most n e u t r o n interferometers, are currently the principal starting material for m a n y devices in the s e m i c o n d u c t i n g industry. The layered structures possible with gallium and a l u m i n i u m a r s e n i d e layers, with their tailored b a n d gaps, are likely to be the devices of the future. These too are a l r e a d y being investigated by neutron reflection techniques. It m a y well be that S h u l r s very f u n d a m e n t a l area of neutron scattering is the next to ' m a k e the grade' a n d to be used for materials research. I n d e e d it is the c o m p l e t e u n p r e d i c t a b i l i t y of the u l t i m a t e o u t c o m e of fundamental research which provides one of its m a j o r fascinations.

213

References [ll C.G. Shull and J.S. Smart, Phys. Rev. 76 (1949) 1256. [2] C.G. Shull, W.A. Strauser and E.O. Wollan, Phys. Rev. 83 (1951) 313. [3] G.E. Bacon, Acta Cryst. 5 (1952) 492. [4] C.P. Baker and R.F. Bacher, Phys. Rev. 59 (1941) 332. [5] P.A. Egelstaff and M.J. Poole. Neutron Thermalisation (Pergamon, New York, 1969). [6] B.N. Brookhouse, Inelastic Scattering of Neutrons (IAEA, Vienna, 1961) p. 113. [7] C.J. Kirouac, W.E. Moore, L.J. Esch, K.W. Seeman and M.L. Yeater, Neutron Thermalisation and Reactor Spectra, vol. 1 (IAEA, Vienna 1968) p. 389. [8] C.G. Windsor, A.J. Allen, M.T. Hutchings, C.M. Sayers, R.N. Sinclair, P. Schofield and C.J. Wright, Neutron Scattering in the "90s (1AEA, Vienna, 1985) p. 575. [9] J. Barton, First World Conference on Neutron Radiography, San Diego (1984). [10] G.F. Slattery and C.G. Windsor, The Metallurgist Feb. (1983) 6. [11] C.G. Windsor, R.N. Sinclair, S. Faulkner, V. Rainey and G.F. Slattery, Microstructural Characterisation of Materials by Non-Microscopical Techniques, Proc. 5th Int. Symp. Riso (1984) p. 583. [12] R.B. Jones and J.T. Buswell, CEGB rep. TPRD/B/0262/ N83 (1983). [13] A.J. Allen, M.T. Hutchings, C.G. Windsor and C. Andreani, Adv. in Physics 34 (1985) 445. [14] C.M. Sayers, J. Phys. DI7 (1984) L179. [15] J.J. Antal, R.J. Weiss and G.J. Dienes, Phys. Rev. 99 (1955) 1081. [16] D.G. Martin and R.W. Henson, Phil. Mag. 9 (1964) 659. [17] M.T. Hutchings, K. Clausen, M.H. Dickens, W. Hayes, J.K. Kjems, P.G. Schnabel and C. Smith, J. Phys. C: Solid State Phys. 17 (1984) 3903. [18] M.H. Dickens, W. Hayes, P. Schnabel, M.T. Hutchings, R.E. Lechner and B. Renker, J. Phys. C: Solid State Phys. 16 (1983) LI. [19] M.J. Gillan, Proc. NATO ARW, Norwich, July 1984, Physica 131B (1985) 157. [20] K. Clausen, W. Hayes, J.E. Macdonald, R. Osborn and M.T. Hutchings, Phys. Rev. Lett. 52 (1984) 1238. [21] J.D.F. Ramsay, R.G. Avery and L. Benest, Farday Discuss. Chem Soc. 76 (1983) 53. [22] A.J. Allen and D. Pearson, Microstructural Characterisation of Materials by Non-Microscopical Techniques, Proc. 5th Int. Symp. Riso (1984). [23] A.C. Wright, J.A.E. Desa, R.A. Weeks, R.N. Sinclair and D.K. Bailey, J. Non-Cryst Solids 67 (1984) 35. [24] Solid State Division Progress report, ORNL Oak Ridge Tenn. (1984).