Neutron

Neutron scattering techniques as a tool for non- destructive testing G. Apostolopoulos, K. Mergia and A.G. Youtsos Institute of Nuclear Technology and Radiation Protection, National Center for Scientific Research “Demokritos”, Athens, Greece [email protected] Abstract – Neutron scattering techniques, such as neutron diffraction, small angle neutron scattering and neutron reflectometry consist very powerful tools for the non-destructive study of materials. At the Greek Research Reactor, GRR-1, a number of neutron scattering methods are employed and developed. In the present contribution, a short description of the methods and facilities is given, as well as some demonstrative examples of their use in non-destructive materials testing. Keywords: Neutron scattering, neutron diffraction, neutron reflectometry, small angle neutron scattering. 1 Introduction Neutron scattering techniques constitute a powerful tool different neutron scattering power. SANS can be used for for non-destructive testing of materials. Neutron has a predicting failure in nickel-base superalloy turbine high penetration depth into matter and this allows the blades, to study porosity in cements and other porous study of volume effects and bulky components. materials, to reveal the kinetics of precipitation in alloys Furthermore, in-situ time-dependent experiments may be or to study the debonding of fibers in carbon-carbon fiber performed within complex sample environments such as composites. furnaces, cryostats, pressure vessels or chemical reactor Neutron Reflectometry (NREF) is a relatively new vessels. technique that has widespread applications as a powerful Compared to the commonly used X-ray techniques, tool to analyze interfacial structure and composition of neutrons have the additional advantage of distinguishing thin films, multilayers, surfaces, solid/liquid interfaces, among different metals and are sensitive to light membranes and other « two-dimensional » structures. Its elements. Another unique advantage of neutrons relies on application extends in the new emerging research areas the fact that the neutron is scattered differently by the in the fields of Material Science, Biology, Chemistry and various isotopes of an element. This gives the ability to Nanotechnology [2]. vary the contrast of a defect, for example, by changing In this paper the neutron scattering facilities at the Greek the isotopes in the defect or its surrounding matrix. The Research Reactor GRR-1 installed at the National Centre strong interaction of neutrons with hydrogen gives a for Scientific Research « Demokritos » are described in unique ability to investigate through scattering connection with some examples of non-destructive techniques, non-destructively, the concentrations, applications in Materials Science. locations and movements of hydrogen in solids, which is useful in the development of hydrogen storage alloys and 2 Neutron Scattering Facilities at GRR-1 fuell cells, and in studies of hydrogen embrittlement of structural metals. The Greek Research Reactor GRR-1 is an open pool-type Neutron diffraction technique is the most useful method reactor of 5 MW thermal power, cooled and moderated for studies of structural changes of the crystal lattice by light water, and employing beryllium reflectors at two particularly in the presence of heavy elements. The opposing sides of the core. Its neutron scattering structure and volume fraction of minority phase can be facilities consist of a two-axis neutron diffractometer, measured at levels appreciably below that possible by X- which is in operation since 2001, a Time-of-Flight (TOF) ray diffraction. A rapidly growing field is the Reflectometer which is under installation, a SANS and a measurement of internal stresses in engineering USANS instrument which are under development. components through the shifts in lattice spacing, the diagnosis of the modification of irradiated materials, etc [1]. Neutron diffraction is unique in being able to 2.1 Neutron Diffraction measure the full strain tensor from a specified volume within a bulk specimen. Neutron diffraction refers to the phenomenon associated Small Angle Neutron Scattering (SANS) and Ultra Small with the interference processes which occur when Angle Neutron Scattering (USANS) are actually neutrons are scattered by the atoms within solids, liquids, scattering methods for detecting heterogeneities in the and gases. The use of neutron diffraction requires high range from 1 to 2000 nm embedded within a matrix of thermal-neutron fluxes which can be obtained only from nuclear reactors. These diffraction investigations are possible because thermal neutrons have energies with suppression of high order harmonics. equivalent wavelengths near 0.1 nm and are therefore Two Soller collimators placed before the ideally suited for interatomic interference studies. monochromator, made from mylar-foil covered with Diffraction line positions and intensities give structural gadolinium oxide, offer a collimation of 15′ and 30′ with information on the material under investigation. Line transmission of 93% and 96%, respectively. widths give information on the size, strain and defect Table 1. Characteristics of the 2-axis neutron nature of the crystallites responsible for the diffraction. diffractometer NEDI at the GRR-1 reactor. A perfectly ordered crystalline material gives only Bragg scattering together with a small contribution from Collimators Automatic exchange 1=15’, 30’ temperature dependent thermal diffuse scattering. Any (Soller) disorder gives diffuse scattering between Bragg peaks. Monochromator Automatic exchange of germanium The magnitude of diffuse scattering may therefore be (Ge/hkk) and pyrolytic graphite used as a test of the perfection of crystalline materials. (PG/002) with vertical focusing on The neutron powder diffractometer NEDI, installed at the sample GRR-1, was designed so as to enhance the quality of Graphite Filter Elimination of 2nd order Bragg diffraction data which can be obtained from a reactor of reflections modest thermal power. Its applications are mainly Take-off angles 2 M=22°, 45°, 60°, 90° and 120° focused on the study of crystalline and magnetic Wavelength range Ge:0.41 – 2.95 Å structures, phase transitions induced, e.g., by temperature PG: 1.28– 5.81 Å or pressure changes, residual stress measurements and Angular range 5° <= 2θ <=120° kinetics of chemical reactions. A schematic layout of the Neutron flux 5 108 n/cm2/s (at the instrument is given in figure 1. Table 1 summarizes the monochromator) most important instrument parameters. Sample-detector 0.5 to 1.6 m distance Sample 10 – 300 K (closed cycle environment refrigerator) Position sensitive 300 – 1800 K (furnace) detectors placed vertically The collimator and monochromator units are housed within the main instrument shield, which is constructed as to allow easy access to both units. The main sample monochromator shield provides take-off angles at 22, 45, aperture 60, 90 and 120°. A sample stage sustained on air cushion pads allows easy Collimators movements to the different beam exit tubes. Different sample environments can be easily assembled allowing convenient sample mounting. The incident to the sample neutron monochromator neutron beam is defined by an adjustable in height and beam width aperture. The incident beam may be focused on a minimum sample height of 2 cm giving an intensity gain Reactor face of a factor of about three. The detection system consists of seven linear position Figure 1 : Schematic Layout of the Neutron sensitive 3He detectors placed horizontally one on top of Diffractometer at GRR-1. the other. The whole detector assembly can be moved on rails in and out towards the sample position and the An in-pile main beam collimator of 1.87 m length is acceptance angle of the detectors system can vary from installed in the beam tube ending at a rotating drum 18° to 56° in Bragg angle 2θ. This offers the flexibility to which enables opening and closing of the beam. A choose between high resolution or high counting rate sapphire filter of 75 mm thickness is used in the primary needed for a dynamic experiment. beam to reduce the fast neutron flux [3]. The neutron More details about the instrument design can be found in diffractometer radiation shielding was optimized by Ref. [5]. Monte Carlo neutron and photon transport code An example of the application of neutron diffraction calculations (MCNP) [4]. Figure 1 shows the schematic technique refers to the structural characterization of layout of the instrument. Two alternative multi-crystal irradiated SiCf/SiC composite materials with fusion focusing monochromators are available: germanium applications [6]. Composite specimens of Nicalon-S (Ge/hkk) and pyrolytic graphite (PG/002), having fibres in a SiC matrix, types N3 and N4, were subjected mosaicities of 15 and 30′, respectively. The Ge to neutron irradiation at GRR-1. Different dose levels monochromator provides wavelengths in the range 0.41 – were studied with irradiation times ranging from 200 to 2.95 Å. The pyrolytic graphite uses the 002 reflection 1600 hours at a neutron flux of 7.5×1013 n/cm2/s and a and can provide wavelengths from 1.28 – 5.81 Å. It is temperature of 40ºC. The maximum fast neutron fluence used in combination with a graphite filter for the was up to 4.3 × 1024 n/m2 which corresponds to about 0.43 dpa (displacement per atom). sample surface at grazing angles of incidence. The The irradiated samples were measured by neutron reflected neutron intensity is recorded as a function of the diffraction at the NEDI neutron diffractometer of GRR-1. scattering wave-vector, Q 4 sin / , where is the Structural changes are observed to occur mainly after 800 angle of incidence and the neutron wavelength. hours of irradiation. These show lattice contraction and indicate the existence of defect accumulation. There is no clear indication of amorphization caused by fast neutron Up to an angle of incidence c, called the critical angle, displacements up to 0.43 dpa. neutrons of a given wavelength undergo total external reflection. Beyond c the beam penetrates the 1500 stratification and gets reflected at the interfaces. The t=1600 h information that can be obtained through the use of this 1000 technique are the thickness, density and roughness of each layer of the thin film sample, as well as phenomena Intensity 500 20 40 60 80 100 like inter-diffusion can be revealed. If one measures 3000 reflectivity outside the specular condition, i.e. angle of 2000 t=237 h reflection different than that of incidence, then the reflected beam provides information on the in-plane 1000 height-height correlation function at various interfaces in 0 20 40 60 80 100 the thin film and on lateral structures. 2 Figure 2 : Neutron diffraction spectra of irradiated SiCf/SiC-N3 for 237 and 1600 hours. 0.437 Figure 4 : Schematic layout of the neutron reflectometer. N4 0.436 N3 The reflectometer at GRR-1 has been designed so as to Lattice constant (nm) 0.435 satisfy different experimental requirements such a) the 0.434 ability to measure both solid and liquid samples, b) 0.433 utilization of the short neutron wavelengths, c) availability of different wavelength ranges, d) ability to 0.432 vary the resolution, e) easily maintainable detection system and electronics, f) reliable and easy alignment of 0.431 samples and g) ability for the extension of the instrument 0.430 24 24 24 24 24 for polarized neutrons. Taking into account the absence 0 1x10 2x10 3x10 4x10 5x10 2 of a cold neutron source at the GRR-1 reactor, the Neutron Fluence (n/m ) reflectometer has been designed to operate in Time-Of- Flight (TOF) mode. Figure 3 : The lattice constant of irradiated N3 and N4 type of SiCf/SiC versus the neutron fluence. Table 2. Characteristics of the time-of-flight reflectometer at the GRR-1 reactor. In Figure 2 it is shown the neutron diffraction spectra Scattering Geometry Vertical from SiCf/SiC-N3 composites for irradiation times of 237 Incidence Angle and 1600 hours. Lattice contraction takes place for both Liquids 0 – 1.5° N3 and N4 types after irradiation at a neutron fluence of Solids 0 – 5° 1.6×1024 n/cm2 (about 0.16 dpa) and 0.6×1024 n/cm2 for N4 and N3 correspondingly. The lattice contraction is Beam Size Max. more intense (of about 1%) for N4 type than for N3 type. 10mm(V)×50mm(H) In Figure 3 it is shown the lattice constant for both types Angular Resolution 0.05° – 0.5° N3 and N4 of SiCf/SiC composites versus the fast Pulse repetition rate 20 – 200 Hz neutron fluence. Chopper–Detector distance 6 – 10 m Wavelength Range 1 – 10 Å Wavelength Resolution 0.1 – 0.5 Å 2.2 Neutron Reflectometry Neutron reflectometry is an established technique for the In the TOF technique a white neutron beam is pulsed by study of thin films, multilayers, and surfaces. In its means of a chopper, a rotating disk of neutron absorbing simplest form, neutron reflectometry consists of material with suitable cut-out windows for shaping the measuring the specular reflection of neutrons from the neutron pulse. Neutrons of different wavelengths are discriminated based on the time they need to reach the properties of SiC coatings. SiC films of 128 nm nominal detector. The TOF mode has the advantage that a large Q thickness were grown on Si wafers by the rf sputtering range can be covered in one geometrical setting and a technique. The samples were subsequently oxidized in air wide wavelength range can be utilized. In addition, the at different temperatures and for various oxidation times. complexity and the time lost in a continuous change of Figure 5 presents the reflectivity data from the SiC , 2 angles can be avoided and lower Q values can coatings oxidized at 700°C for 1, 4 and 16 hours. The be obtained. solid lines are least square fits to the data. The information that was 10 0 o 120 400 oC SiC -1 10 EX84 700 C/16h SiO2 10 -2 fit 100 10 -3 80 -4 10 -5 60 10 0 0.1 0.2 0.3 0.4 0.5 0.6 40 10 Reflectivity 10 -1 EX81 700 C/4h o 20 -2 fit 10 0 -3 10 10 -4 120 700 oC -5 10 100 thickness (nm) 0.1 0.2 0.3 0.4 0.5 0.6 0 10 -1 o 80 SiC 10 EX75 700 C/1h -2 fit 60 SiO2 10 -3 10 -4 40 10 10 -5 20 0.1 0.2 0.3 0.4 0.5 0.6 0 -1 Q (nm ) 120 900 oC 100 SiC Figure 5 : Neutron reflectivity data from SiC films 80 SiO2 oxidised at 700°C for 1, 4 and 16 hours. The solid line is 60 a least square fit to the data. 40 For measuring liquid samples the in-pile collimation 20 system has been designed so that the neutron beam 0 emerging from the reactor is inclined with respect to the 0 2 4 6 8 10 12 14 16 18 horizontal axis. Thus the desired reflection angle θ is oxidation time (hours) achieved by simply moving the apertures and the sample vertically. With this method a maximum value of θ = 1.6º may be achieved for liquid samples. Different wavelength ranges are available and all of them have the same minimum wavelength of 0.15 nm utilizing, thus, the short wavelengths where the GRR-1 reactor offers the Figure 6 : The thickness of the SiC (● solid circles) and maximum flux [7][7]. The instrument resolution is also a SiO2 (▲ solid triangles) layer versus the oxidation time variable parameter, determined by the chopper rotation at 400, 700 and 900°C. The dotted lines are guide to the speed and the chopper-detector distance. Thus, the user eye. may choose between high resolution and high counting rate settings, according to the experimental needs. Pulse obtained from the oxidized films includes the thickness overlap is avoided by the employment of a supermirror. of a SiO2 layer that forms during oxidation, the thickness Since there are no guides and it is essential to reduce the of the remanent SiC layer, as well as the density and background, a sapphire filter is used for the removal of interfacial roughness of both layers. The evolution of the the fast neutron component. For achieving an efficient thickness as a function of oxidation time, both for the removal of the fast neutrons without considerable SiO2 and the SiC layer, is depicted in Figure 6. The reduction of the neutron flux in the wavelength range dotted lines in Figure 6 are guides to the eye. from 0.15 to 1.0 nm the optimum sapphire thickness has been calculated by MCNP code. Further, towards a low 2.3 Small Angle Neutron Scattering (SANS) background instrument, optimization of the shielding has been made by MCNP calculations. SANS allows the study of inhomogeneities in a matrix. The layout of the neutron reflectometer is presented in In a typical SANS experiment the measured quantity is Figure 4 while the instrument parameters are given in the macroscopic scattering cross-section of the Table 2. inhomogeneities, An example of the application of the neutron reflectometry technique refers to the oxidization  d F C (r) 2 n p Vp2 2 S 2 (QR) I QD (1) d The SANS technique can be used in obtaining information about the precipitation in an alloy. as a function of the scattering wave-vector Q. In the Isothermal ageing of AlLi alloys results in the formation above equation F{C (r)} represents the Fourier of a L12 ordered metastable δ´ phase of stoichiometric transform of the neutron scattering length density C(r), composition Al3Li The growth of δ´ precipitates in an Al- n p is the number density of the precipitates, V p is the 8.9at%Li alloy was studied by SANS experiments in the temperature range from 90 to 210°C [8]. In Figure 8 is volume of one precipitate, and is the contrast, which presented the scattering cross section versus the depends generally on the atomic volume in the matrix scattering vector Q from an AlLi alloy aged at 210°C for and in the precipitates and on the neutron scattering 0.17, 0.50 and 1 hours. lengths of the atoms involved. S (QR) is the Fourier transform of the shape of the precipitate and R is the size of the precipitate. I (QD) describes the interparticle 120 scattering, where D is a characteristic distance between t = 0.17 h t = 0.50 h the precipitates. t=1h d /d / (barn/str/atom) At GRR-1 a SANS and a USANS instrument are under 90 design. The two instruments will be installed at the same beam tube. SANS will be used for the investigation of 60 large size inhomogeneities (0.05 – 5 μm) and USANS for smaller size inhomogeneities (0.002 – 0.1 μm). The double crystal USANS instrument will consist of two 30 elastically bent Si crystals, the monochromator and the analyzer (Figure 7). The instrument wave-vector 0 resolution δQ may be tuned in the range from 10-4 to 10-3 0.02 0.04 0.06 0.08 0.10 Å-1 by adjusting the curvatures of the two crystals, -1 Q / nm according to the expected size of investigated inhomogeneities. The fully asymmetric diffraction Figure 8: SANS curves from AlLi alloy aged at 210°C geometry on the elastically bent Si analyzer is employed for 0.17, 0.50 and 1 hours. The solid lines are guide to to transfer the angular distribution of the scattered the eye. neutrons to the spatial distribution and to analyze the whole scattering curve by a one-dimensional position The radius, number density and volume fraction of the sensitive detector. precipitates and the concentration of the solute in the The SANS instrument (Figure 7) will utilize the beam matrix were determined as a function of ageing transmitted through the Si(111) bent crystal. A beryllium temperature and time. crystal cooled at cryogenic temperature will act as a The coarsening kinetics predict that the growth of the filter, cutting out wavelengths smaller than 3.95 Å. A cube of the size of the precipiates is proportional to mirror after the sample may be used to deflect the long ageing time, while the number density of the precipitates wavelength neutrons (for example above 5.5 Å). The varies inversely with time [9]. The proportionality scattered intensity will be monitored on an image plate. constants are the kinetic constants which depend on the diffusion coefficient and the surface energy. The size of the precipitates obtained from the SANS data agrees very well, for long ageing times, with the predictions of the coarsening theory, as can be seen in Figure 9. Bent Si (111) analyser Sample Si (111) bent crystal asymmetric cut Be filter SANS Sample USANS Mirror Position sensitive detector Image plate detector Figure 7: Schematic layout of the SANS and USANS instruments.  350, 162-165, 2004 [6] Fusion RTD Activities, Association EURATOM – T =363 K Hellenic Republic, Annual Report 2004 8 T =393 K T =423 K [7] K. Mergia and G. Apostolopoulos, Neutron T =458 K Reflectometry : A Probe for Materials Surfaces, T =483 K International Atomic Energy Agency, Vienna, 2006 6 [8] K. Mergia K., S. Messoloras, F. Al-Hazmi and R. J. ln[( R (t)- R0 ) / nm ] 3 Stewart, Phil. Mag A 80, 2609, 2000 [9] I. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. 3 4 Solids, 19, 35, 1961 3 2 0 0 2 4 6 8 3 ln[ k1(t-t0) / nm ] Figure 9: Universal behaviour of the sizes of the precipitates in AlLi alloys according to the coarsening theory. 3 Conclusions Neutron scattering techniques offer significant advantages for non-destructive testing applications due to the high penetration depth of neutron beams and the unique sensitivity of neutrons to different isotopes and light elements as hydrogen. At the Greek Research Reactor GRR-1 a variety of neutron scattering methods and facilities are employed and developed, offering a wide range of possibilities for non-destructive testing applications. In the present contribution a description of the GRR-1 facilities has been given and some representative application examples have been described. Neutron diffraction has been employed for the study of lattice distortion in SiCf/SiC composite materials after irradiation. The oxidization properties of SiC coatings has been investigated by means of neutron reflectometry. Finally, small angle neutron scattering has been implemented to determine the kinetics of δ´ phase precipitation in Al-Li alloys. 4 References [1] Neutron in Research and Industry, Vol 2867, Published by SPIE - The International Society for Optical Engineering, 1996 [2] Neutron Reflectometry : A Probe for Materials Surfaces, Published by the International Atomic Energy Agency, Vienna, 2006 [3] I.E. Stamatelatos and S. Messoloras, Rev. Sci. Instrum. 71, 70, 2000 [4] I.E. Stamatelatos, A. Salevris and S. Messoloras, Demo Reports, DEMO 98/7 (1998) [5] K. Mergia, A. Salevris, S. Messoloras, Physica B.