1 Modelling pseudo-strain effects induced in strain measurement using time-of-flight neutron diffraction S.R. Creek, J.R. Santisteban, L. Edwards Department of Materials Engineering, Faculty of Technology, The Open University, Milton Keynes MK7 6AA, UK Abstract The penetrating nature of thermal neutrons makes neutron strain scanning a unique tool for the sub-surface measurement of elastic strain. However, it is also important to be able to measure strain in near surface regions. The gauge volume represents the region of diffracting material over which a measurement is made and is described by the intersection of incident and diffracted beams. In a nearsurface measurement the gauge volume is incompletely filled with material, resulting in a pseudo-strain. Pseudo-strains due to incomplete filling of the gauge volume can be large (1x10-3 (1000µε)) in comparison to the resolution of instruments used to measure such strains (50µε), thus requiring correction. This paper develops a model of a system using a radially collimated diffracted beam and evaluates the pseudo-strain that would be recorded. A Fortran 90 program has been developed allowing real-time correction of such pseudo-strains during the measurement process. 1. Introduction Thermal energy neutrons have wavelengths similar to atomic spacings, allowing diffraction experiments to be performed. Neutron strain scanning determines the vector strain field in a material by measuring the latticeplane spacings, dhkl, within a small volume of the material (gauge volume) using neutron diffraction. These values are then compared to the spacings of the same planes measured in an unstressed sample, do hkl to calculate the strain, ε, as in equation (1): d − d o hkl ∆d hkl ε = hkl = d hkl d hkl Thermal energy neutrons with a velocity v have a momentum p [1], which is classically given by: mL (2) resulting in a de Broglie wavelength, λ: λ= h hτ = mv mL λ = 2d hkl sin θ (3) (4) to be rewritten: hτ 2mL sin θ (1) 1.1 Strain measurement using time-of-flight neutron diffraction τ This wavelength allows Bragg’s law of diffraction: d hkl = As a probe of atomic structure, neutrons have a distinct advantage over X-rays and electrons. In contrast to electrons and X-rays, which interact with the electron cloud, it is the interaction of neutrons with atomic nuclei that results in diffraction. As a consequence, neutrons are only weakly attenuated by materials allowing strain measurements to be performed deep within samples where other techniques would permit only surface measurements. p = mv = where m is neutron mass, v is neutron velocity, L is total flight path length (incident plus scattered), τ is the measured time-of-flight (TOF) and h is Planck’s constant. (5) where 2θ is the Bragg angle – the angle between the incident and scattered beams. It is clear that for constant L and θ, the spacing of a lattice plane, dhkl, is proportional to the TOF, τ. This allows the lattice strain to be evaluated as, ε= ∆τ hkl d hkl − d o hkl τ − τ o hkl = hkl = d hkl τ hkl τ hkl (6) The energy of a neutron is inversely proportional to the square of its wavelength. A spallation source provides a pulsed ‘white’ beam of neutrons that the TOF technique uses for diffraction. This beam consists of a wide range of wavelengths, and hence energies. The high energy (low wavelength) neutrons arrive at the sample first so, for a given angle, the diffraction peaks corresponding to small d-spacings arrive at a detector first, with the larger d-spacings arriving later. Equivalently, considering a constant wavelength, it can be seen that the angle at which Bragg’s law is satisfied sweeps out in the direction of increasing 2θ as time proceeds. Each pulse therefore produces a complete diffraction profile of low intensity in a detector mounted at constant 2θ. To recover a significant profile many detectors are mounted to cover a range of 2θ and then a 2 technique known as electronic time-focussing [2] is employed to merge the profile from each detector into a single d-spacing spectrum. maintain resolution. An xyzω positioner table accurately located the observation point within the sample to within ± 0.1mm. From equation (5), for the ith detector located at 2θi with a neutron flight path length of Li, The ENGINX system currently being installed at ISIS follows the same overall design, but has 4 interchangeable collimators and a redesigned detector bank including an order of magnitude increase in the number of detectors. τi = 2md hkl Li sin θ i h (7) Processing of this measured time can yield a pseudo-time τ*, which is the TOF that would have been measured by a detector located at a reference position Lo, θo, (usually 2θo=90o), according to equation (8): τ* = τi Lo sin θ o Li sin θ i (8) The intensities of these pseudo-time profiles are then summed and a Rietveld refinement [3] is conducted to calculate the lattice cell size. Reference was made above to identifying the observation “point” within the sample at which a strain measurement is made. This concept needs clarification, as it is apparent that the incident beam is of finite width in 2 dimensions and the collimator blades limit the volume of space at the collimator front end from which diffraction events may be observed. The intersection of the incident beam with this volume of space at the collimator front end defines a diffracting volume known as the “gauge volume”. This gauge volume can be considered to consist of many such diffracting points. Consider a coordinate system set up as in Fig. 1.2. 1.2 The ENGIN and ENGINX systems An example of a system that operated successfully to measure strain using TOF neutron diffraction is the ENGIN instrument [4,5] at ISIS, the pulsed neutron facility located at the Rutherford Appleton Laboratory. A schematic of the ENGIN instrument can be found in Fig. 1.1. Fig. 1.2. Coordinate system in the ENGIN instrument. The widths of the collimating slits in the incident beam define the dimensions of the gauge volume in the x and z directions, while the y dimension is determined by the field of view of the radial collimator. Fig. 1.1. Schematic of the ENGIN instrument. The ENGIN pulsed neutron strain scanner consisted of an incident neutron beam with small angular divergence, defined by slits placed in the beam, and two large radial collimators centred about a Bragg angle of 2θ = ±90o. The function of the radial collimators was to provide a large surface area over which to detect neutrons while restricting the angular field of view of individual detectors in the large detector banks to To define the gauge volume mathematically the spatial resolution function (SRF) [6] is introduced. The SRF defines the relative detected intensity of scattered neutrons as a function of position within the sample. SRF(r ) = Pi (r ) Ps (r ) Pd (r ) (9) where Pi(r)δV, the incident beam resolution function, is the probability of incidence of a neutron on an infinitesimal volume δV centred at r, Ps(r)δV is the probability that a neutron will be scattered within δV and Pd(r)δV, the detector resolution function, is the probability of detection given that a scattering event has 3 occurred within δV. The dimensions of the gauge volume then follow naturally from the volume in space containing 68% of the probability density of the SRF (equivalent to ± one standard deviation for a onedimensional probability distribution) when the scattering volume is completely immersed in the sample. To determine the performance of the instrument it is useful to set Ps(r) equal to 1 so that the SRF is only affected by instrumental parameters, the attenuation of the beam by the sample material and the location of the sample. It is important to note that the SRF is a strong function of the attenuation coefficient and specimen position as this will cause it to be biased to areas of shorter path length and will return a value of zero for regions of the gauge volume not immersed in the sample. This is a particularly important consideration when making measurements of near surface strains. computation time required to produce such a result would be substantial and prohibitive to real-time measurement correction. The problem can be greatly simplified by noticing that the angular term in the pseudo-strain is independent of the azimuthal angle, Φ, and the change in path length resulting from incorporation of the third dimension will be small. Also it is unnecessary to model each detector element individually due to the time-focusing routine ultimately combining the signals from all detectors. Modelling the pseudo-strains for individual collimator channels will suffice, reducing the number of calculations required by an order of magnitude. It is the purpose of this paper to develop a 2dimensional model to predict and correct for pseudostrains introduced by attenuation and surface scanning with the ENGIN/ENGINX systems. 1.3 Pseudo-strain 2. The Spatial Resolution Function For the purpose of assigning a lattice strain to a point in the sample material, the most representative location of the scattering volume is the neutron-weighted centre of gravity (ncog), which takes into account variations in intensity due to attenuation or absence of sample material in the gauge volume. The ncog is evaluated as the centroid of the SRF. Variation in the position of the ncog due to attenuation effects or the incomplete filling of the gauge volume will cause variations in the values of L and θ for scattering from the ncog to each detector. It is clear from equation (5) that this will cause variation in the measured lattice-spacings, independent of residual stress, and hence will introduce error in the ultimate evaluation of the lattice strain according to: As outlined above the spatial resolution function is obtained as the product of the incident beam and detector resolution functions, including the effect of attenuation where necessary. From this it is possible to identify the gauge volume and calculate the location of the ncog. A Fortran 90 subroutine has been written to evaluate the SRF and locate the ncog for the 2-dimensional region of space over which the gauge volume is defined. ∆τ τ = ∆d d + b ∆ L sin θ L sin θ g Peak shift = lattice strain + pseudo - strain (10) where the pseudo-strain, , can be decomposed into spatial and angular components: b ∆ L sin θ L sin θ g = ∆L + cot θ∆θ = ~ε L (11) Where attenuation effects are very strong or there is partial filling of the gauge volume it is necessary to correct for this pseudo-strain. To do this accurately a 3dimensional model of the system should be constructed in order to evaluate the pseudo-strain recorded by each detector element from each point in a 3-dimensional gauge volume. This would be a very complex task, requiring the evaluation of the incident beam resolution function, the detector resolution function and the attenuation effect in a 3-dimensional sample. The 2.1 The Incident Beam Resolution Function The shape of the gauge volume in the x direction is determined by the divergence of the incident beam. This is assumed to have a gaussian angular profile. According to Withers et al. (2000), upon passing through the final collimating slit this shape is described by the convolution of a top hat function (for the slit) and a gaussian spatial function (to account for the beam divergence). For a slit of width s located at (xs,ys), with a beam of angular divergence σθ propagating in the positive y direction, the incident beam resolution function is given by: Pi ( x , Σ ) = b 1 sΣ 2π z FG H x−x + s2 s exp − x − xs − s 2 IJ K t2 dt 2Σ 2 g b g Σ( y) = y − y s tan σ θ (12) (13) For the purpose of numerical evaluation it is convenient to note that this is equivalent to: b g Pi x , Σ = F F GH GH IJ K FG H x − xs + s 2 x − xs − s 2 1 erf − erf 2s Σ 2 Σ 2 IJ I K JK (14) where Σ is defined as in equation (13). 4 The incident beam resolution function is illustrated in Fig. 2.1 for a slit of width 1mm, located at (0,-60) with an angular divergence of 1o. Note the difference in scales of the x and y axes used for illustration. Fig. 2.1. The incident beam resolution function for a slit with s=1mm, σθ = 1o, (xs,ys) = (0,-60) and a beam propagating in the positive y direction. Considering first a single channel, a neutron scattered from a point at the front end will only be detected if it can pass through the channel without incidence on a blade (assumed to be a perfect neutron absorber). For convenience the channel is oriented such that its axis is co-linear to that of the coordinate axis. Assuming isotropic scattering from all points in front of the channel, the probability of detecting a neutron will be proportional to the angular range through which the neutron may be scattered and detected (acceptance angle, Ψ). The channel geometry is such that from a given point source of scattered neutrons the acceptance angle is always limited by two of four defining points – the two front, and two rear ends of the blades. The acceptance angle is calculated using ray tracing from the scattering point to the two appropriate limiting points. The collimator function, Ψ(x,y), describes analytically the acceptance angle of a channel as a function of scattering position and the four collimator parameters which locate the limiting points – R, l, α and n. The function is continuous, but divided into 4 regions depending on which limiting points are imposing constraint. The situation is illustrated in Fig. 2.3 for a channel with zero blade thickness. 2.2 The Detector Resolution Function A radial collimator consists of radially aligned neutron absorbing blades. These blades define channels of constant angular thickness along which neutrons may pass. Detectors are mounted at the far end of the channels to record the arrival of diffracted neutrons at fixed values of 2θ. A radial collimator can be described by 5 parameters: the radius from the geometrical centre to the end of blades, R; the length of the blades, l; the total acceptance angle, α; the number of channels, n; and the blade thickness, t, as shown in Fig. 2.2, illustrated schematically for a collimator with only 5 channels. Fig. 2.3. Schematic of a collimator channel with zero blade thickness showing the 4 regions of space at the collimator front end that are defined by the 4 limiting points. Consideration of the effect of finite thickness blades on the acceptance angle of a channel reveals several interesting features, illustrated in Fig. 2.4. Fig. 2.2. Schematic of a radial collimator demonstrating the 5 parameters with which it can be completely described. 5 for each channel. The detector resolution function for the point (x,y) is computed according to equation (17), ∑ Ψb x , y g n Pd ( x , y ) = k k (17) k =1 where Ψ(xk,yk) is the value of the collimator function at the rotated scattering point (xk,yk), representing the contribution from the kth channel due to scattering from the point (x,y). Fig. 2.4. Schematic of a collimator channel with finite blade thickness showing how the 4 regions are altered by the reparametrisation of the limiting points in terms of the fifth parameter t. The overall effect of introducing blade thickness is to redefine the location of the limiting points in terms of a fifth parameter, t. The first feature is that the overall acceptance angle is reduced, as the thickness of the blade narrows the channel. Secondly the focal point of a channel (the intersection of all 4 regions) is displaced towards the channel by a distance d: t d= 2 sin FG α IJ H 2n K (15) The final effect is that while the channel has the same angular width, α/n, and blade length, l, the radius of the channel is reduced to give an effective radius, R′: R' = R − t 2 tan FG α IJ H 2n K Fig. 2.5. Equivalent constructions of the collimator. (16) Explicit evaluation of the collimator function is outlined in the appendix §A.1. The detector resolution function describes the probability of detecting a neutron at the far end of the collimator assuming uniform irradiation and isotropic scattering from a non-attenuating sample. It is calculated as the sum of angles through which a neutron scattered from a point in front of the collimator can be detected at the end of the channels, divided by the total angle through which a neutron can be scattered (assumed 2π for isotropic scattering). From the model of a single channel the detector resolution function can be calculated for a point in space either by summing the contributions from n channels rotated about the origin by the angular thickness of a channel, α/n, or by summing the contributions from n rotated observation points in front of the single channel, as in Fig. 2.5. For ease of computation the second method was adopted, requiring a simple rotation of the observation point about the origin One benefit of detecting neutrons over a large surface area with the radial collimator system, as opposed to slit based systems commonly found on reactor neutron sources, is that it substantially reduces the time required to detect statistically significant intensity. It may then seem counter-productive to place neutron-absorbing blades in the collimator that absorb a large proportion of the scattered intensity. However, evaluation of the detector resolution function using the method outlined above for the ENGIN collimator parameters in table 2.1, both with and without blades, clearly demonstrates the blades’ purpose. Radius R (mm) Blade length l (mm) 750 600 Total acceptance angle α (o) 17.5 No. channels n Blade thickness t (mm) 41 0.16 Table 2.1. Parameters for ENGIN collimator 6 µ= ρσ T A (19) where ρ is the material density, σT is the collision cross section and A is the atomic weight. In reality I0 and σT are both wavelength dependent [7], but this fact will initially be ignored. Strongly attenuating materials severely reduce the overall detected intensity and cause the ncog to be shifted towards regions of shorter path length. The shift of the ncog results in an attenuation related pseudostrain. This pseudo-strain is in addition to that which occurs when the gauge volume is partially filled during surface scanning. Fig. 2.6. Detector resolution function for the ENGIN collimator without blades – note the 4 regions defined above visible in the description of a single channel. Fig. 2.7. Detector resolution function for the ENGIN collimator with blades. The blades provide spatial resolution, reducing the full width at half maximum in the y direction at the geometrical centre from 57mm to 1.4mm. This is essential to provide strain resolution as the relative dspacing spread in a Bragg peak for a given reflection has a contribution from the relative uncertainty in the total flight path length of a neutron which is largely determined by the collimator resolution in the y direction. A subroutine has been incorporated into the program that returns both the incident beam and diffracted beam path lengths in the material in order that the attenuation effect can be applied to the detected intensity. This subroutine is quite versatile as it can accommodate both convex and concave samples of any size, shape and orientation to the system. At present the sample is described by the coordinates of the vertices of a single simply-connected polygon in the frame of the system. The polygon represents the intersection of the 3dimensional sample with the 2-dimensional plane normal to the z-axis, passing through the centre of the collimator. This defines the extent and position of the sample relative to the system. The order of labelling sample vertices must be in the clockwise direction for calculation to be correct. The sample surface can then be scanned through the gauge volume by adjusting the sample coordinates in accordance to the movement of the sample positioner. A full discussion of this subroutine is presented in the appendix §A.3. 3. Calculation of Pseudo-strains Two methods of calculating pseudo-strain, , have been investigated. The first approximates all scattering to the ncog and calculates pseudo-strain relative to this point, while the second is an integration of the pseudostrains over the entire gauge volume. Both models attempt to imitate the measurement process by calculating ∆L and ∆θ with reference to a gauge volume immersed in the centre of an 11mm diameter silicon cylinder (µ=0.083mm-1), which is the reference material used in the creation of time-focussing routines. 2.3 Attenuation Considering a path through a neutron-attenuating material, the neutron intensity as a function of path length, x, through the material is given by: bg b g I x = I 0 exp − µx (18) 3.1 The centroid method As the ncog is the point most representative of the scattering volume the first model evaluates pseudostrains by calculating the ncog of the silicon run and the appropriate values of L and θ for each channel for the paths from the incident beam slit to the centre of the illuminated area at the end of a channel, via the centroid 7 position, as in Fig. 3.1. The procedure is then repeated for the sample material in the orientation of interest and the pseudo-strain is returned for each channel as in equation (11). The mean pseudo-strain < > is then the channel intensity-weighted sum of the individual channel pseudo-strains, normalised by the total detected intensity, as in equation (20), ~ ε centroid = c c k c i c c k c c i c k =1 x,y F ∑ GGH n k =1 zz k i x, y (21) 4.1 The Collimator n k =1 = I b g b g b g JJ K I Ψ b x , y g P b x , y gdxdyJ JK ~ ε k x , y Ψk x , y Pi x , y dxdy c k =1 ∑ Ψ bx , y g P bx , y g integration zz 4. Results ∑ ~ε b x , y gΨ b x , y g P b x , y g n k ~ ε F ∑ GGH n c (20) where (xc,yc) is the position of the ncog, k(xc,yc) is the pseudo-strain of the kth channel evaluated using the position of the ncog and Ψk(xc,yc)Pi(xc,yc) is the probability of detecting a neutron in the kth channel due to scattering from the ncog, which is proportional to the detected intensity. The collimator model has allowed evaluation and direct comparison of the detector resolution functions for the ENGIN and 4 ENGINX collimators. This is particularly useful in assessing the performance of the new instrument. Figs. 4.1–4.5 show the detector resolution functions for the 5 collimators and Fig. 4.6 shows how the full width at half maximum (FWHM) of each resolution function varies as a function of displacement from the focal point (defocus). A comparison of ENGIN and ENGINX collimator parameters is given in table 4.1. ENGIN ENGINX R (mm) l (mm) α (o) n t (mm) 750 450/510 /660/840 600 350 17.5 32 41 160 0.16 0.05 Table 4.1. Comparison of the ENGIN/ENGINX collimator parameters. Fig. 3.1 Schematic of system illustrating evaluation of L and θ for each channel. 3.2 The integration method The second method, described in equation (21) calculates the mean intensity-weighted pseudo-strain for each channel by integrating the product of the pseudostrain and the channel intensity, both as a function of position, over the gauge volume. The overall mean pseudo-strain is then evaluated as the sum of these values over the number of channels, normalised by the total detected intensity. In practise, this integration must be performed numerically, introducing slightly increased computation time. As before, ∆L and ∆θ are calculated for each channel and at each scattering point with reference to the ncog of a silicon run. Fig. 4.1. Detector resolution function for the ENGIN collimator. 8 Fig. 4.2. Detector resolution function for the ENGINX collimator R=840mm. Fig. 4.5. Detector resolution function for the ENGINX collimator R=450mm. Fig. 4.6. Graph of FWHM against Defocus for the ENGIN and 4 ENGINX collimators. Fig. 4.3. Detector resolution function for the ENGINX collimator R=660mm. Fig. 4.4. Detector resolution function for the ENGINX collimator R=510mm. It is important to consider the general symmetry properties of the detector resolution function and the effect this has on the centroid position regardless of attenuation. The entire collimator has symmetry in the y direction resulting in a zero y-centroid coordinate. Although on initial examination it appears that there is also symmetry in the x-direction, this is not the case. The model of a single collimator channel does not possess this symmetry and consequently on construction of the collimator from a single channel there is slight but definite asymmetry present. This causes a small positive displacement of the x-centroid coordinate of order 104 mm. It is also interesting to note the effect of blade thickness on the transmitted intensity. Fig. 4.7 shows the transmitted intensity of neutrons scattered from the geometrical centre of the collimator as a function of blade thickness, while Fig. 4.8 represents the total transmitted intensity over the region defining the gauge volume using a 2mm wide incident beam slit. 9 suitable approximation for the overall transmitted intensity when considering collimator design. Fig. 4.7. Effect of increasing blade thickness on the detected intensity from neutrons scattered at the geometrical centre of the collimator. A secondary consequence of blade thickness is that the focal point (point of greatest intensity) is displaced towards the collimator. This is an interesting effect as it is in direct contradiction to the Monte Carlo analysis previously mentioned. However there would seem adequate physical justification for the phenomenon according to the analysis already presented in §2.2. The Monte Carlo analysis predicted an almost complete translation of the resolution function away from the collimator and hence a corresponding shift in the centroid position of order 0.5mm in that direction. The analytic model presented here predicts a significant shift of the focal point and its immediate surroundings towards the collimator of order 0.5mm for the ENGIN and 0.1mm for the ENGINX collimators, but without translation of the majority of the detector resolution function. This results in a much smaller shift of the centroid towards the collimator of order10-4mm. 4.2 The Gauge volume and shift of the centroid position Fig. 4.8. Effect of increasing blade thickness on the detected intensity from neutrons scattered over the entire gauge volume using a 2mm wide incident beam slit. It is apparent that increasing blade thickness decreases the width of channels, so reduces detected intensity from all points. What is particularly notable is the strong linearity between the blade thickness and the transmitted intensity from the geometrical centre. This has been previously observed by performing a Monte Carlo simulation of the collimator [8]. Attention was drawn at that time to the similarity between the transmitted intensity from the geometrical centre and the ratio between the front end width of a channel to the blade thickness. Analysis of the collimator presented here has shown this to be no coincidence and a proof that the relative transmission from the geometrical centre as a function of blade thickness obeys the equation, I (t ) = 1 − Fig. 4.9. Gauge volume for the ENGINX collimator R=450mm with a 2mm wide incident slit. The highlighted area is the common rectangular approximation. t b R − lg αn The SRF is described by the intersection of the incident beam and detector resolution functions and is illustrated in Fig. 4.9 for the R=450mm ENGINX collimator with a 2mm wide incident beam slit. From this the shape of the gauge volume can be obtained as the area containing 68% of the probability density of the SRF. Although the shape of the SRF appears quite complex in Fig. 4.9, the gauge volume, as defined, is excellently approximated to the rectangular area superimposed on the contour plot as most of the probability density is located within this region. It should be noted that the dimensions of the gauge volume are highly dependent on the collimator used. For the R=840mm collimator the width in the y direction at x=0 is approximately 8 times greater than that of the R=450mm collimator. (22) where t is the blade thickness and (R-l)α/n is the width of a channel at the front end, is presented in the appendix §A.2. Fig. 4.8 demonstrates that this linearity is only true for the geometrical centre, although it may suffice as a When near surface measurements are made so that the gauge volume is incompletely filled, or attenuation effects are strong, the zero position of the sample positioner table does not correspond to the location of the ncog. It is necessary to be able to relate the positioner 10 table reading to the location of the ncog in order to determine the point at which the strain field is sampled. Fig. 4.10 shows the effect attenuation has on the relationship between the distance of the ncog from the surface of the sample and the positioner table reading for the ENGIN collimator (0 is taken as the sample surface passing through the centre of the gauge volume). Fig. 4.11 shows how this phenomenon varies between ENGINX collimators. 4.3 Pseudo-strain calculation The pseudo-strain as a function of the distance of the ncog from the surface of the sample for both the centroid and integration methods is plotted in Fig. 4.12 for the ENGIN instrument. The graph shows 3 levels of attenuation, µ=0mm-1 (a good approximation to Al: µ=0.087mm-1), µ=0.118mm-1 (Fe), and µ=0.5mm-1. This plot is a constructive test of the model as it can be compared against the experimental data obtained while the ENGIN instrument was in operation. This test is particularly important as no data is yet available for the ENGINX instrument. Fig. 4.13 shows a comparison between ENGINX collimators assuming an attenuation level equivalent to Steel (µ=0.118mm-1) for both methods of calculation. Both graphs are plotted using a 2mm wide incident beam with negligible angular divergence. Fig. 4.10. The relationship between the distance of the ncog from the sample surface to the positioner table reading, illustrated for the ENGIN collimator with a 2mm wide collimating slit, showing 3 levels of attenuation. Fig. 4.12. Pseudo-strain as a function of distance of ncog from sample surface for the ENGIN collimator. Fig. 4.11. Comparison between the distance of the ncog from the sample surface to the positioner table reading for the ENGINX collimators. Values shown are for Steel (µ=0.118mm-1). It can be seen that even where attenuation is zero, if the gauge volume is only partially filled then there is significant deviation of the location of the ncog from the positioner reading. Attenuation reinforces this fact, introducing a systematic error of 0.06mm for steel using the R=450mm ENGINX collimator and 0.5mm using the R=840mm collimator when the gauge volume is completely filled. Fig. 4.13. Comparison of the pseudo-strain as a function of distance of ncog from sample surface for the ENGINX collimators. It is clear that the magnitude of the pseudo-strain is strongly dependent on the dimensions of the gauge volume and hence collimator used. For the ENGIN instrument strain resolution was ±50x10-6 (±50µε) and needed correction when the gauge volume was incompletely filled. Considering the anticipated 11 improvement in resolution of ENGINX then from Fig. 4.13 it is obvious that pseudo-strain correction is essential, particularly for the collimators with larger front end radii. 5. Discussion zz SRF( x , y )dxdy Gauge Volume Fortran 90 subroutines have been written allowing evaluation of the incident beam and detector resolution functions for slits and radial collimators respectively. This then permits calculation of the spatial resolution function as the product of the above functions. The spatial resolution function is of great importance when undertaking strain measurement using neutron diffraction for three reasons: 1. When designing collimators it allows a direct comparison of performance both in terms of spatial resolution and total transmitted intensity. Spatial resolution contributes to ultimate d-spacing resolution and hence the strain resolution of the instrument, while total transmitted intensity dictates experimental duration. The detector resolution function program permits the study and optimisation of collimator design. 2. neutrons. The 2-dimensional equivalent, relevant to the model presented above, is defined by equation (25). (25) Fig. 5.1 shows the relative total detected intensity as a function of rear end radius, R, with data points corresponding to the 4 ENGINX collimators. It is interesting to note that this relationship is almost perfectly quadratic in nature, intersecting the x axis at the value corresponding to the blade length, 350mm, such that the front end radius would be zero. The relationship would suggest that the time required to perform an experiment, as a function of R, is inversely proportional to this quadratic expression. When planning an experiment, the SRF enables prediction of the d-spacing resolution and minimum experiment duration. The resolution of a diffractometer, ∆d/d, can be calculated from equation (5), assuming all variables act independently [9], ∆d = d LMFG ∆τ IJ + bcot θ∆θ g + FG ∆L IJ OP H L K PQ MNH τ K 2 2 2 Fig. 5.1. The relationship between rear end radius, R, and the total detected intensity from the gauge volume defined using a 2mm wide slit and each of the ENGINX collimators. 1 2 (23) where the contribution to ∆τ is from the moderation time of the neutron and the contribution to ∆θ is from the angular width of a detector as observed from the scattering point, but ∆L is determined by the dimensions of the gauge volume for a given detector. As SRF(r)δV represents the probability of an incident neutron being diffracted and detected from the volume δV located at r (as in equation (9)) then equation (24) represents the total probability that an incident neutron is scattered from the gauge volume and detected at the far end of the radial collimator. zzz SRF(r )dV Gauge Volume (24) This is clearly proportional to the total detected intensity, which will be inversely related to the time required to detect a statistically significant number of The study of total transmitted intensity brings to attention the need for further consideration of the collimator parameters. It is apparent that there are three geometrical variables that directly influence the acceptance angle of a channel – the front and rear radii, Rf & R respectively, and the blade length, l. It is also apparent that only two of these variables are independent due to the relationship, Rf = R − l (26) It is then necessary to determine which two of the three variables best characterise the response of a channel. It would seem most appropriate to use Rf and R as it is these values which directly locate the limiting points of a channel, defining the four collimator function domains. Although a full analysis of the effect of varying collimator parameters on the spatial resolution function is not presented here, were it to be performed it would seem most appropriate to proceed in terms of R, Rf, n, α and t. 12 3. On completion of a measurement the SRF allows the evaluation and correction of pseudo-strain effects due to attenuation or partial filling of the gauge volume. Two methods of evaluating the pseudo-strain have been investigated and tested against experimental data obtained for the ENGIN instrument. While not perfectly consistent, both methods have been shown to produce results that are accurate to within the instrumental resolution and the difference in calculation time has been found to be slight, both allowing real-time correction. No data is yet available for the ENGINX instrument, but it is essential that the model is tested against new experimental data once the instrument is operational. The improved resolution of ENGINX may determine which method of evaluating pseudo-strain most accurately represents the phenomenon. would substantially increase computation time, but comparison of 3-dimensional theoretical data to experiment would prove conclusively whether or not a 2-dimensional model is really sufficient for realtime correction. 6. Conclusions In this paper a model has been developed of the ENGIN/ENGINX strain scanning systems. This model permits evaluation of the spatial resolution function using a Fortran 90 program. The spatial resolution function has three direct applications to neutron strain scanning: • There are several logical extensions to this project that should be considered: • • • • The first is to develop the computer program to allow for other than simply-connected polygons so that hollow specimens may be accommodated. This should be straightforwardly achieved (see appendix §A.3). The second development is to account for pseudostrain introduced by using a white beam of neutrons. Both the incident intensity and the attenuation coefficients are a function of wavelength. As the pseudo-strain contribution from a channel is weighted by the received intensity, variations in detected intensity due to wavelength will affect the pseudo-strain calculated. A study of this for the ENGIN instrument revealed that any pseudo-strains introduced were smaller than the experimental error, but the improved resolution of ENGINX may make this correction necessary. A third logical development would be to conduct the study of the effect of collimator parameters on the SRF that has been made possible by the creation of the Fortran subroutine. The final development would be to extend the model presented above to accurately represent the system in 3 dimensions. Incorporation of the z coordinate • • Equipment design – the direct comparison of the performance of radial collimator designs, both in terms of resolution and transmitted intensity, and the study of the effect of incident beam properties in terms of beam width and angular divergence. Experimental planning – the prediction of measurement resolution and minimum experimental duration. Correction of pseudo-strains – the evaluation and correction of pseudo-strain effects due to attenuation and incomplete filling of the gauge volume. Two methods of pseudo-strain calculation have been presented and compared to available experimental data, both proving to be accurate to within experimental error. The need to compare both methods to data obtained from the ENGINX instrument, once operational, has been emphasised. Whichever method is determined to most accurately represent the phenomenon of pseudo-strain, the speed of calculation is such that the Fortran 90 subroutines will be fully integrated into ENGINX analysis software allowing real-time pseudo-strain correction. Acknowledgements The authors wish to thank M.R. Daymond for his time and helpful suggestions in the development of the models and description of pseudo-strain effects. 13 Appendix A.1 Collimator function N .B . R E G IO N 1: d = t F α IJ 2 s in G H 2n K & t R'= R − 2 ta n FG α IJ H 2n K D o m a in : g FGH 2αn IJK < y < b x − d g ta n FGH 2αn IJK F α IJ + d d < x ≤ b R '− l g c o sG H 2n K F R ' s in FG α IJ − y I F R ' s in FG α IJ + y I G J G JJ H H 2n K 2n K Ψ = a ta n G + a ta n G J α α GG R ' c o s FGH IJK − x + d JJ GG R ' c o s GFH IJK − x + d JJ 2n 2n H K H K b − x − d ta n (A 1 ) R E G IO N 2: D o m a in : F α IJ ta n G α I F b x − d g ta n GH 2 n JK ≤ y ≤ Hl 2 n K FGH − x b 2 R ' − l g + 2 R ' b R ' − l g c o s FGH 2αn IJK + d b 2 R ' − l gIJK F α IJ + d d ≤ x ≤ b R '− l g c o sG H 2n K A ls o : F α IJ ≤ − b x − d g ta n G H 2n K ta n y ≤ FG α IJ H 2 n K FG − x b 2 R ' − l g + 2 R ' b R ' − l g c o s FG α IJ + d b 2 R ' − l g IJ H 2n K l H K x < d Ψ F b R ' − l g s in FG α IJ − y I F R ' s in FG α IJ + y I G G J JJ H H 2n K 2n K + a ta n G = a ta n G J GG b R ' − l g c o s FGH α IJK − x + d JJ GG R ' c o s FGH α IJK − x + d JJ 2n 2n H H K K (A 2 ) R E G IO N 3: D o m a in : FG α IJ H 2 n K FG x b 2 R ' − l g + 2 R ' b R ' − l g c o s FG α IJ − d b 2 R ' − l gIJ ≤ H 2n K l H K F α IJ + d d ≤ x ≤ b R '− l g c o sG H 2n K ta n b g FGH 2αn IJK y ≤ − x − d ta n A ls o : ta n FG α IJ H 2 n K FG x b 2 R ' − l g + 2 R ' b R ' − l g c o s FG α IJ − d b 2 R ' − l gIJ ≤ H 2n K l H K y ≤ b x − d g ta n FGH 2αn IJK x < d Ψ F R ' s in FG α IJ − y I F b R ' − l g s in FG α IJ + y I G J G JJ H H 2n K 2n K + a ta n G = a ta n G J α α F I F I GG R ' c o s GH JK − x + d JJ GG b R ' − l g c o s GH JK − x + d JJ 2n 2n H K H K R E G IO N 4: D o m a in : b x − d g ta n FGH 2αn IJK < b (A 3 ) g FGH 2αn IJK y < − x − d ta n x < d Ψ F b R ' − l g s in FG α IJ − y I F b R ' − l g s in FG α IJ + y I G G J JJ H H 2n K 2n K + a ta n G = a ta n G J GG b R ' − l g c o s FGH α IJK − x + d JJ GG b R ' − l g c o s FGH α IJK − x + d JJ 2n 2n H H K K (A 4 ) 14 A.2 The effect of blade thickness on the transmitted intensity from the geometrical centre. The geometrical centre is unique as it is the point of rotation about which the collimator is constructed. It is therefore the only point invariant under rotation making it identical for all channels. This allows the detector resolution function evaluated at (0,0) to be expressed, b g Pd 0,0 = nΨ(0,0) (A5) It is also apparent from Fig. 2.4 in §2.2 that the effect of increasing blade thickness is to translate region 4 towards the collimator. As a consequence Ψ(0,0) is always described by the collimator function for region 4, F b R'−lg sinFG α IJ I G H 2n K JJ P b0,0g = 2n atanG GG b R'−lg cosFGH α IJK + d JJ 2n H K d (A6) Substituting, d= t FαI 2 sinG J H 2n K & R' = R − t 2 tan FG α IJ H 2n K F F I I GG G R − t − lJ sinF α I JJ GG GGG 2 tanFG α IJ JJJ GH 2nJK JJ H 2nK K H P b0,0g = 2n atanG JJ I GG FG GG GG R − tF α I − lJJJ cosFGH 2αnIJK + Ft α I JJJ 2 sinG J J GH GH 2 tanGH 2nJK JK H 2nK K d (A7) The typical angular width of a channel is (for ENGINX) 32/160= 0.2o, so using the small angle approximation, F F I I GG G R − t − lJ F α I JJ GG GGG 2FG α IJ JJJ GH 2n JK JJ H H 2n K K P b0,0g = 2nG JJ I GG FG GG GG R − F tα I − lJJJ + F tα I JJJ GH GH 2GH 2n JK JK 2GH 2n JK JK d which on simplifying becomes, (A8) b g FGH b R −ntlgα IJK Pd 0,0 = α 1 − (A9) After normalisation by the acceptance angle of the empty collimator viewed from the origin (α) and slight rearrangement, this is expressed by, b g Pd 0,0 = 1 − t b R − lg αn (A10) where the second term is the ratio of the blade thickness to the channel width at the front end. A.3 The Path Length Problem In applying the effect of attenuation of neutrons on the detected intensity it is essential to know the path length of a neutron through the material both in the source to scattering point and scattering point to detection point path segments. The source to scattering point path length, ps, will obviously vary from point to point in the gauge volume and the scattering point to detector path length, pd, will vary, not only with location in the gauge volume, but also with location of the detection point. This will be dependent on the channel through which the neutron passes. Most importantly the path lengths depend on the location of the sample with respect to the gauge volume. This would initially seem quite a complex problem to solve, but separate consideration of the two path segments for a given detector reveals that each path segment can effectively be regarded as the same problem. The problem is defined by the following: given the coordinates of the vertices of a polygon representing a sample Ri = (xi,yi), {i=1,…,nvertices}, a source point outside, Rs, and an observation point inside the polygon, Ro, to calculate p, the length of the segment of the straight line joining Rs and Ro located inside the polygon. In order to achieve this the order in which the vertices of the polygon are connected must be known so that the sample surfaces are formed correctly. It was decided to label the vertices with increasing subscript in the clockwise direction. An edge of the polygon will only contribute to the path length if the line joining Rs and Ro intersects it. Criteria must be developed in order to determine whether intersection occurs for each edge. Defining ΦR, Φn and Φn-1 in the region [–π, π] relative to the line joining Rs to the origin, as in Fig. A.1, it can be seen that intersection only occurs between the line and the nth edge if: 15 φ n ≤ φ R < φ n −1 (A11) & y − y n −1 y x − y n x n −1 y0 > n x 0 + n −1 n x n − x n −1 x n − x n −1 FG H IJ FG K H = mn x0 + cn IJ K (A12) where yn =mnx + cn is the equation of the nth edge and the “equal to” part of the ≤ sign in the angular criterion is to accommodate intersection through a vertex. Else if y s > cn then intersection occurs providing y0 < mn x0 + cn (A15) If intersection occurs then the intersection point can be obtained simply by solution of simultaneous equations and the path length calculated using Pythagoras’ theorem. At this point it is important to notice a limitation to the model imposed by introducing the angular criterion. The angles Φi and ΦR must be defined over some interval relative to an axis. For convenience the axis has been chosen to be the y axis in the positive direction from Rs to ∞ (after rotation), and the interval to be [–π, π]. This introduces a 2π discontinuity in Φ along the y axis in the negative direction from ys to -∞. If a sample edge traverses this discontinuity then the angular criterion as defined above will not correctly apply. The physical consequence of this limitation is that the path length will not be correctly calculated for any line joining Rs and R0 for which there is no unobstructed line of sight from a point at infinity. This effect is illustrated in Fig. A.2. In application of this model to the problem of calculating ps and pd it can be seen that this limitation will not affect the desired results. The position of Rs represents a point on the collimating slit and a point on a detector in the calculation of ps and pd respectively, both of which are distant points in comparison to the dimensions of the sample, ensuring that there is always an unobstructed line of sight from infinity. Fig. A.1. Demonstrating the criteria for intersection of an edge by the path. However this is only the case in Fig. A.1 as ys < yn(xs) for all n which may be intersected. It is obvious that if Rs had been located above the specimen, i.e. ys > yn(xs) for all n which may be intersected, then the second condition for intersection would have been, y0 < mn x0 + cn (A13) Thus the second intersection criterion is dependent on the relative positions of Rs and the two vertices forming an edge. The second criterion (and evaluation of the first) can be somewhat simplified by initially rotating the entire system about the origin by the amount required to bring the line joining Rs to the origin collinear with the negative y axis. From this point it is assumed that Ri, Rs and R0 refer to these rotated coordinates. The second criterion becomes: If y s < cn then intersection occurs providing y0 > mn x0 + cn (A14) Fig. A.2. Illustration of the limitations of the solution to the path length problem. This limitation would not be an issue if all samples could be represented by simply connected convex polygons, as in that case no edges would be able to traverse the discontinuity. However, this is not the case as samples often contain concave regions and may be doubly or triply connected to accommodate holes. At present the model outlined here accommodates only simply connected convex and concave polygons. 16 When a polygon is entirely convex then the line joining Rs and Ro may intersect only one edge or vertex and the nature of that intersection must be in the positive sense (i.e. into the sample). If concave regions are present then it is possible that the line may intersect multiple edges in both the positive (entering sample) and negative senses (exiting sample), depending on the orientation of the sample and the location of Rs and Ro. It is necessary to adjust the criteria that determine whether intersection occurs to also determine the sense of the intersection. This is simply achieved by reconsidering the angular criterion. It can be seen that for the case in Fig. A.3. the criteria: φ n −1 ≤ φ R < φ n (A16) & y0 > mn x0 + cn (A17) achieved by repeating the path length algorithm for additional polygons, representing the holes, that have vertices labelled with increasing subscript in the anticlockwise direction. The total path will then be the sum of the outputs from the iterations due to each polygon. determine a negative contribution to the path length. Fig. A.4. Flow diagram for determining the contribution of a sample edge to the path length. References [1] [2] [3] [4] Fig. A.3. Demonstrating the criteria for negative intersection of an edge by the path. A complete set of outcomes of the two criteria for different locations of Rs and R0 in relation to the nth edge described by the straight line, yn =mnx + cn , can be summarised by the flow diagram in Fig. A.4 (although slight mathematical adjustment is required to accommodate vertical edges), and it is this flow diagram, along with evaluation of the intersection points and then path segments contributed by an intersected edge, which provides a solution to the path length problem. At present the Fortran program that evaluates path lengths can only accommodate simply connected polygons. However, it would not be dificult to extend this to accommodate holes in the sample. This could be [5] [6] [7] [8] [9] S. W. Lovesey, Theory of Neutron Scattering from Condensed Matter, Clarendon Press, Oxford, 1984. J.D. Jorgensen, J. Faber Jr., J.M. Carpenter, R.K. Crawford, J.R. Haumann, R.L. Hitterman, R. Kleb, G.E. Ostrowski, F.J. Rotell, T.G. Worlton, J. Appl. Crystallogr. 22 (1989) 321. H.M. 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