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Inspection of the metal composite materials usinga combination of X-ray radiography and NeutronImagingTo cite this article D Vavrik et al 2011 JINST 6 C03001

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2011 JINST 6 C03001

PUBLISHED BY IOP PUBLISHING FOR SISSA

RECEIVED November 30 2010REVISED December 15 2010

ACCEPTED December 16 2010PUBLISHED March 1 2011

12th INTERNATIONAL WORKSHOP ON RADIATION IMAGING DETECTORSJULY 11thndash15th 2010ROBINSON COLLEGE CAMBRIDGE UK

Inspection of the metal composite materials using acombination of X-ray radiography and NeutronImaging

D Vavrikab I Jeond E Lehmannc A Kaestnerc and J Vacike

aInstitute of Experimental and Applied Physics Czech Technical University in PragueHorska 3a22 CZ 12800 Prague 2 Czech Republic

bInstitute of Theoretical and Applied Mechanics AS CRProsecka 76 Prague 9 Czech Republic

cPaul Scherrer Institut Neutron Imaging and Activation GroupWBBA110 CH-5232 Villigen PSI Switzerland

dSchool of Mechanical Systems Engineering Chonnam National UniversityGwangju Republic of Korea

eNuclear Physics Institute AS CR amp Research Center RezCZ-25068 Czech Republic

E-mail vavrikitamcascz

ABSTRACT Neutron Imaging and X-ray radiography are complementary methods from the pointof view of the visibility and contrast of the material imaged One potential application is the obser-vation of the behavior of so-called lsquometal composite materialsrsquo in which metallic and light materialparts are combined This material type is commonly used in the aerospace industry Neutron Imag-ing is a suitable tool for the observation of the structure of light materials in an environment ofheavier elements like metals X-ray radiography on other hand is an appropriate tool for the obser-vation of geometry of metal components inside an environment of lighter materials

KEYWORDS Computerized Tomography (CT) and Computed Radiography (CR) Detection ofdefects Neutron radiography

1Corresponding author

ccopy 2011 IOP Publishing Ltd and SISSA doi1010881748-0221603C03001

2011 JINST 6 C03001

Contents

1 Introduction 1

2 Model specimen analyzed 1

3 X-ray CT 2

4 Neutron CT 241 Neutron imaging with a thermal neutron beam 3

411 Neutron source divergence correction 4412 Ring artifacts correction 4413 Results 4

42 Neutron tomography with a cold neutron beam 5

5 Conclusions 5

1 Introduction

Development of new advanced metal composite materials used in aerospace industry requires newnon-destructive techniques for their inspection Nowadays existing non-destructive techniqueshave a number of limitations which in many cases can be successfully overcome utilizing neutronimaging However neutron imaging requires demanding instrumentation like a nuclear reactor ora spallation neutron source Neutron Imaging is an irreplaceable tool for some applications suchas inspection of light materials accompanying large and relatively heavy materials such as liquidsinside porous stones or metals an organic materials surrounded by the inorganic materials Metalcomposite material such as the mixture of metallic components and a resin which is reinforced byfibers or particles is one of these cases Although the detection efficiency for imaging thermal andcold neutrons is relatively low a neutron image can be obtained within a few seconds The spatialresolution is however low in comparison to current micro- or nano-focus X-ray tubes because it isimpossible to attain such a small focusing of the neutron source

If one tries to inspect metal composite materials using X-rays strong artifacts appear in thelight material originating from neighbor metal parts Consequently the morphology of the lightmaterial is difficult to observe as illustrated in the next section A model metal composite specimenwas manufactured to demonstrate these conditions and limits

2 Model specimen analyzed

The model specimens were manufactured from Aluminum wedges which were glued by a resincontaining randomly distributed voids and used for methodological testing see figure 1 Each spec-imen contains a different relative volume of voids to study their influence on the metal composite

ndash 1 ndash

2011 JINST 6 C03001

Figure 1 Model of metal composite specimen The Al wedges are glued with the resin layer

Figure 2 CT slice in the centre of the specimen mm scaleShallow notches are visible on the metal face

Figure 3 3D visualization of the resinlayer

3 X-ray CT

A Hamamatsu micro-focus tube and a Flat panel imager were used The tube has a tungsten anodewith a 5 microm spot size and a divergent beam The flat panel with pixel size 50 microm consists ofa photodiode matrix coupled to a CsITl scintillator X-ray radiograms were recorded with 30seconds exposure each 360 snapshots were taken for full 2 π rotation The specimen was placedas close as possible to the Flat panel sensor The resultant magnification was approximately 11and the radiogram pixel size was consequently 60 microm

A beam hardening effect and non-homogeneous detector response were corrected using theSignal to Equivalent Thickness method [1] The iterative method of ordered subsets (OSEM) wasapplied for CT reconstruction [2]

It was proven as expected that denser material influences the CT reconstruction inside of thevery light material such as resin This effect manifests by the shadows in the resin layer as shownin figure 2 Similarly even though voids are visible in the 3D visualization the density of theresin is virtually varying in its volume see figure 3 Consequently although voids are visible it ispractically impossible to analyze their relative volume

4 Neutron CT

A thermal neutron beam and a cold neutron beam were used alternatively for neutron imagingFirstly a reactor thermal neutron source was used Several difficulties had to be overcome to obtainreasonable results as described in the next sections Afterwards the neutron spallation source SINQat PSI (CH) was used for high-quality neutron imaging as described below The neutron imagingdata obtained show the primary morphology of the resin while the enclosing metal parts becomealmost invisible as expected

ndash 2 ndash

2011 JINST 6 C03001

Figure 4 Neutron CT device the specimen in theholder with the stepper motor (left) the Medipix-2 neutron imager with the USB-1 readout device(right)

Figure 5 Neutron image after Flat field correction(pixel scale)

41 Neutron imaging with a thermal neutron beam

The thermal neutron beam available at the Nuclear Physics Institute AS CR amp Research Center Rezis primarily designed for the Neutron Depth Profiling technique The related 6 m long neutron guideis equipped with the 15 vertically bent horizontal mirrors The resultant thermal neutron beam hasan intensity of 107 neutronssecmiddotcm2 at the reactor power 6 MW and has a high purity (Cd ratio issim 105) and a Maxwellian distribution The beam is slightly divergent in the horizontal direction(LD = 100) and more divergent in the vertical direction (LD = 50) so it is not symmetricallydivergent The geometrical cross section of the beam at the exit of the neutron guide is 80 mm(width) times 4 mm (height)

It was not possible to use optimal irradiation geometry ie to have specimen as close to thedetector as possible to suppress neutron scattering influence due to the Rez neutron beam geomet-rical cross section The horizontal position of the specimen was chosen to fit the beam height seesetup in figure 4 The relatively small active area of the used neutron imager was another limitationfor our work the central part of the specimen had to be reconstructed for this reason

The pixelated Medipix 2 [3] device (256times256 square pixels with pitch of 55microm total activearea 14times14 mm) was adapted for neutron detection by applying a spin coating 6LiF as a neutronconverter [3] and then used as neutron imaging detector Each pixel is equipped with its owndiscriminator allowing to cut off undesired gamma rays using an appropriate threshold This devicehas approximately 3 efficiency for the thermal neutron detection

The CT neutron data were recorded as projections of 390 seconds each 200 snapshots weretaken for the full 2 π rotation A flat field correction comprising non-homogeneity of the beam aswell as the non-uniform pixel efficiency of the detector was applied A resulting neutron image isshown in figure 5 where the resin layer is clearly visible while the enclosing Al wedges are almostinvisible Although the total measurement time was quite long statistics of the individual neutronimages were quite low (ie not enough neutrons detected) Moreover the imaging properties of thedetector were slightly changed during measurement due to self activation

It was necessary to correct the non-symmetric beam divergence and for rings artifacts appear-

ndash 3 ndash

2011 JINST 6 C03001

Figure 6 Schema of the circular rings filtering

ing in the CT slices in order to render the voids in the resin visible The position of the rotationcentre against the beam-detector axis was determined using a cadmium strip and a set of neutronimages obtained during rotation of the strip

411 Neutron source divergence correction

Voids were not visible in the CT reconstruction when the asymmetric beam divergence was nottaken into account The distance L2 between rotation centre and detector was 130 mm while thedistance L11 between the virtual beam spot and the rotation centre is 76 m in the horizontal planeand the distance L12 between the virtual spot and rotation axis in the vertical plane is 38 m Theseparameters were determined numerically by varying of their values when the behavior of the CT re-construction was checked The following equations were used for the CT reconstruction assumingdifferent divergence in both directions

xd = [x middot cosα+y middot sinα] middot (L11+L2) middot[L11 +y middot cosα-x middot sinα]-shift (41)

zd = [x2d +(L12 +L2)2]05 middot z middot[xp2 +(L12 +yp)2]05 (42)

The point with the CT slice coordinates [xy] is projected onto the detector with coordinates[xd zd] where α is rotation angle and shift is distance between actual rotation axis and the rsquospot-detector centrersquo line The CT slice point coordinates [xpyp] are obtained by the rotation transfor-mation of the [x y] coordinates The variable z is the vertical coordinate of the CT slice The wellknown filtered Radon back projection algorithm was used for the CT reconstruction utilizing theseequations

412 Ring artifacts correction

Ring artifacts appear in the CT slices due to the non accurate Flat Field correction These artifactswere successfully removed by filtering the CT slices after transforming them to polar coordinatesThe CT slice is projected onto the rotation axis direction in the polar coordinate space A high-pass filter is applied to this projection The obtained function is projected back in the rotationaxis direction and transformed back into Cartesian coordinates The final disk-like 2D function issubtracted from the filtered CT slice as depicted in figure 6

413 Results

CT slices were calculated and filtered to obtain information about the voids distribution inside theresin layer It is possible to detect large voids only thanks to statistics and neutron scatteringThe visualization of such void distribution in the central plane vertically crossing the resin layer is

ndash 4 ndash

2011 JINST 6 C03001

Figure 7 Visualization of the voids distribution in the resin layer scale in mm The same void in bothslices is labeled by the red circle

Figure 8 Visualization of the voids distribution using a neutron CT model

shown on the left in figure 7 The vertical CT slice crossing void (labeled by the white line left) ison the right in figure 7

42 Neutron tomography with a cold neutron beam

A cold neutron beam at the ICON facility [5] at the Paul Scherrer Institut was alternatively usedfor high-resolution neutron CT The parameter of divergence LD is equal to 1000

The 20 microm GdOx scintillator coupled with the 13 microm pixel size camera was used as neutronimager Using this device an efficient spatial resolution of about 50 microm measured with a Gdedge was reached Optimal irradiation geometry was chosen in which analyzed specimens wereplaced as close to the detector as possible Only one half of each specimen was analyzed due togeometrical limitations (field of view 27 mm) A high quality CT model was obtained where theshape and volume of the void distribution are well visible see figure 8 (different CT slices as in thefigure 7) Even the shape of the resin boundaries is well defined where shallow contact protrusionsare visible

5 Conclusions

It was proven that although neutron CT is a highly demanding technique it is an appropriate toolfor structural analysis of metal composite materials

It was shown that it is necessary to take into account the asymmetric beam divergence whenthe neutronograms are not acquired in the close detector geometry Ring artefacts can be effectively

ndash 5 ndash

2011 JINST 6 C03001

removed by the mathematical filtering in the polar coordinate space A high quality neutron beamand imaging technique is required when all information about a void distribution and morphologyare demanded

Acknowledgments

This work was carried out in the frame of the Medipix Collaboration and has been supported by theProject LC06041 AV0Z20710524 and 6840770040 of the Ministry of Education Youth and Sportsof the Czech Republic and in part by the Research Grant No 103092101 of the Grant Agency ofthe Czech Republic and KAN 400480701 of the Academy of Sciences of the Czech Republic

References

[1] D Vavrik CT artefact reduction by signal to thickness calibration function shaping Nucl InstrumMeth A in press

[2] HM Hudson and RS Larkin Accelerated image reconstruction using ordered subsets of projectiondata IEEE Trans Med Imag 13 (1994) 601

[3] Medipix collaboration home page httpmedipixwebcernchMEDIPIX

[4] J Jakubek T Holy E Lehmann S Pospısil J Uher J Vacık D Vavoslashık Neutron Imaging withMEDIPIX2 Chip and a Coated Sensor Nucl Instrum Meth A 560 (2006) 143

[5] httpneutrawebpsichfacilityicon indexhtml

ndash 6 ndash

2011 JINST 6 C03001

Figure 1 Model of metal composite specimen The Al wedges are glued with the resin layer

Figure 2 CT slice in the centre of the specimen mm scaleShallow notches are visible on the metal face

Figure 3 3D visualization of the resinlayer

3 X-ray CT

A Hamamatsu micro-focus tube and a Flat panel imager were used The tube has a tungsten anodewith a 5 microm spot size and a divergent beam The flat panel with pixel size 50 microm consists ofa photodiode matrix coupled to a CsITl scintillator X-ray radiograms were recorded with 30seconds exposure each 360 snapshots were taken for full 2 π rotation The specimen was placedas close as possible to the Flat panel sensor The resultant magnification was approximately 11and the radiogram pixel size was consequently 60 microm

A beam hardening effect and non-homogeneous detector response were corrected using theSignal to Equivalent Thickness method [1] The iterative method of ordered subsets (OSEM) wasapplied for CT reconstruction [2]

It was proven as expected that denser material influences the CT reconstruction inside of thevery light material such as resin This effect manifests by the shadows in the resin layer as shownin figure 2 Similarly even though voids are visible in the 3D visualization the density of theresin is virtually varying in its volume see figure 3 Consequently although voids are visible it ispractically impossible to analyze their relative volume

4 Neutron CT

A thermal neutron beam and a cold neutron beam were used alternatively for neutron imagingFirstly a reactor thermal neutron source was used Several difficulties had to be overcome to obtainreasonable results as described in the next sections Afterwards the neutron spallation source SINQat PSI (CH) was used for high-quality neutron imaging as described below The neutron imagingdata obtained show the primary morphology of the resin while the enclosing metal parts becomealmost invisible as expected

ndash 2 ndash

2011 JINST 6 C03001

Figure 4 Neutron CT device the specimen in theholder with the stepper motor (left) the Medipix-2 neutron imager with the USB-1 readout device(right)

Figure 5 Neutron image after Flat field correction(pixel scale)

41 Neutron imaging with a thermal neutron beam

The thermal neutron beam available at the Nuclear Physics Institute AS CR amp Research Center Rezis primarily designed for the Neutron Depth Profiling technique The related 6 m long neutron guideis equipped with the 15 vertically bent horizontal mirrors The resultant thermal neutron beam hasan intensity of 107 neutronssecmiddotcm2 at the reactor power 6 MW and has a high purity (Cd ratio issim 105) and a Maxwellian distribution The beam is slightly divergent in the horizontal direction(LD = 100) and more divergent in the vertical direction (LD = 50) so it is not symmetricallydivergent The geometrical cross section of the beam at the exit of the neutron guide is 80 mm(width) times 4 mm (height)

It was not possible to use optimal irradiation geometry ie to have specimen as close to thedetector as possible to suppress neutron scattering influence due to the Rez neutron beam geomet-rical cross section The horizontal position of the specimen was chosen to fit the beam height seesetup in figure 4 The relatively small active area of the used neutron imager was another limitationfor our work the central part of the specimen had to be reconstructed for this reason

The pixelated Medipix 2 [3] device (256times256 square pixels with pitch of 55microm total activearea 14times14 mm) was adapted for neutron detection by applying a spin coating 6LiF as a neutronconverter [3] and then used as neutron imaging detector Each pixel is equipped with its owndiscriminator allowing to cut off undesired gamma rays using an appropriate threshold This devicehas approximately 3 efficiency for the thermal neutron detection

The CT neutron data were recorded as projections of 390 seconds each 200 snapshots weretaken for the full 2 π rotation A flat field correction comprising non-homogeneity of the beam aswell as the non-uniform pixel efficiency of the detector was applied A resulting neutron image isshown in figure 5 where the resin layer is clearly visible while the enclosing Al wedges are almostinvisible Although the total measurement time was quite long statistics of the individual neutronimages were quite low (ie not enough neutrons detected) Moreover the imaging properties of thedetector were slightly changed during measurement due to self activation

It was necessary to correct the non-symmetric beam divergence and for rings artifacts appear-

ndash 3 ndash

2011 JINST 6 C03001

Figure 6 Schema of the circular rings filtering

ing in the CT slices in order to render the voids in the resin visible The position of the rotationcentre against the beam-detector axis was determined using a cadmium strip and a set of neutronimages obtained during rotation of the strip

411 Neutron source divergence correction

Voids were not visible in the CT reconstruction when the asymmetric beam divergence was nottaken into account The distance L2 between rotation centre and detector was 130 mm while thedistance L11 between the virtual beam spot and the rotation centre is 76 m in the horizontal planeand the distance L12 between the virtual spot and rotation axis in the vertical plane is 38 m Theseparameters were determined numerically by varying of their values when the behavior of the CT re-construction was checked The following equations were used for the CT reconstruction assumingdifferent divergence in both directions

xd = [x middot cosα+y middot sinα] middot (L11+L2) middot[L11 +y middot cosα-x middot sinα]-shift (41)

zd = [x2d +(L12 +L2)2]05 middot z middot[xp2 +(L12 +yp)2]05 (42)

The point with the CT slice coordinates [xy] is projected onto the detector with coordinates[xd zd] where α is rotation angle and shift is distance between actual rotation axis and the rsquospot-detector centrersquo line The CT slice point coordinates [xpyp] are obtained by the rotation transfor-mation of the [x y] coordinates The variable z is the vertical coordinate of the CT slice The wellknown filtered Radon back projection algorithm was used for the CT reconstruction utilizing theseequations

412 Ring artifacts correction

Ring artifacts appear in the CT slices due to the non accurate Flat Field correction These artifactswere successfully removed by filtering the CT slices after transforming them to polar coordinatesThe CT slice is projected onto the rotation axis direction in the polar coordinate space A high-pass filter is applied to this projection The obtained function is projected back in the rotationaxis direction and transformed back into Cartesian coordinates The final disk-like 2D function issubtracted from the filtered CT slice as depicted in figure 6

413 Results

CT slices were calculated and filtered to obtain information about the voids distribution inside theresin layer It is possible to detect large voids only thanks to statistics and neutron scatteringThe visualization of such void distribution in the central plane vertically crossing the resin layer is

ndash 4 ndash

2011 JINST 6 C03001

Figure 7 Visualization of the voids distribution in the resin layer scale in mm The same void in bothslices is labeled by the red circle

Figure 8 Visualization of the voids distribution using a neutron CT model

shown on the left in figure 7 The vertical CT slice crossing void (labeled by the white line left) ison the right in figure 7

42 Neutron tomography with a cold neutron beam

A cold neutron beam at the ICON facility [5] at the Paul Scherrer Institut was alternatively usedfor high-resolution neutron CT The parameter of divergence LD is equal to 1000

The 20 microm GdOx scintillator coupled with the 13 microm pixel size camera was used as neutronimager Using this device an efficient spatial resolution of about 50 microm measured with a Gdedge was reached Optimal irradiation geometry was chosen in which analyzed specimens wereplaced as close to the detector as possible Only one half of each specimen was analyzed due togeometrical limitations (field of view 27 mm) A high quality CT model was obtained where theshape and volume of the void distribution are well visible see figure 8 (different CT slices as in thefigure 7) Even the shape of the resin boundaries is well defined where shallow contact protrusionsare visible

5 Conclusions

It was proven that although neutron CT is a highly demanding technique it is an appropriate toolfor structural analysis of metal composite materials

It was shown that it is necessary to take into account the asymmetric beam divergence whenthe neutronograms are not acquired in the close detector geometry Ring artefacts can be effectively

ndash 5 ndash

2011 JINST 6 C03001

removed by the mathematical filtering in the polar coordinate space A high quality neutron beamand imaging technique is required when all information about a void distribution and morphologyare demanded

Acknowledgments

This work was carried out in the frame of the Medipix Collaboration and has been supported by theProject LC06041 AV0Z20710524 and 6840770040 of the Ministry of Education Youth and Sportsof the Czech Republic and in part by the Research Grant No 103092101 of the Grant Agency ofthe Czech Republic and KAN 400480701 of the Academy of Sciences of the Czech Republic

References

[1] D Vavrik CT artefact reduction by signal to thickness calibration function shaping Nucl InstrumMeth A in press

[2] HM Hudson and RS Larkin Accelerated image reconstruction using ordered subsets of projectiondata IEEE Trans Med Imag 13 (1994) 601

[3] Medipix collaboration home page httpmedipixwebcernchMEDIPIX

[4] J Jakubek T Holy E Lehmann S Pospısil J Uher J Vacık D Vavoslashık Neutron Imaging withMEDIPIX2 Chip and a Coated Sensor Nucl Instrum Meth A 560 (2006) 143

[5] httpneutrawebpsichfacilityicon indexhtml

ndash 6 ndash

2011 JINST 6 C03001

Figure 4 Neutron CT device the specimen in theholder with the stepper motor (left) the Medipix-2 neutron imager with the USB-1 readout device(right)

Figure 5 Neutron image after Flat field correction(pixel scale)

41 Neutron imaging with a thermal neutron beam

The thermal neutron beam available at the Nuclear Physics Institute AS CR amp Research Center Rezis primarily designed for the Neutron Depth Profiling technique The related 6 m long neutron guideis equipped with the 15 vertically bent horizontal mirrors The resultant thermal neutron beam hasan intensity of 107 neutronssecmiddotcm2 at the reactor power 6 MW and has a high purity (Cd ratio issim 105) and a Maxwellian distribution The beam is slightly divergent in the horizontal direction(LD = 100) and more divergent in the vertical direction (LD = 50) so it is not symmetricallydivergent The geometrical cross section of the beam at the exit of the neutron guide is 80 mm(width) times 4 mm (height)

It was not possible to use optimal irradiation geometry ie to have specimen as close to thedetector as possible to suppress neutron scattering influence due to the Rez neutron beam geomet-rical cross section The horizontal position of the specimen was chosen to fit the beam height seesetup in figure 4 The relatively small active area of the used neutron imager was another limitationfor our work the central part of the specimen had to be reconstructed for this reason

The pixelated Medipix 2 [3] device (256times256 square pixels with pitch of 55microm total activearea 14times14 mm) was adapted for neutron detection by applying a spin coating 6LiF as a neutronconverter [3] and then used as neutron imaging detector Each pixel is equipped with its owndiscriminator allowing to cut off undesired gamma rays using an appropriate threshold This devicehas approximately 3 efficiency for the thermal neutron detection

The CT neutron data were recorded as projections of 390 seconds each 200 snapshots weretaken for the full 2 π rotation A flat field correction comprising non-homogeneity of the beam aswell as the non-uniform pixel efficiency of the detector was applied A resulting neutron image isshown in figure 5 where the resin layer is clearly visible while the enclosing Al wedges are almostinvisible Although the total measurement time was quite long statistics of the individual neutronimages were quite low (ie not enough neutrons detected) Moreover the imaging properties of thedetector were slightly changed during measurement due to self activation

It was necessary to correct the non-symmetric beam divergence and for rings artifacts appear-

ndash 3 ndash

2011 JINST 6 C03001

Figure 6 Schema of the circular rings filtering

ing in the CT slices in order to render the voids in the resin visible The position of the rotationcentre against the beam-detector axis was determined using a cadmium strip and a set of neutronimages obtained during rotation of the strip

411 Neutron source divergence correction

Voids were not visible in the CT reconstruction when the asymmetric beam divergence was nottaken into account The distance L2 between rotation centre and detector was 130 mm while thedistance L11 between the virtual beam spot and the rotation centre is 76 m in the horizontal planeand the distance L12 between the virtual spot and rotation axis in the vertical plane is 38 m Theseparameters were determined numerically by varying of their values when the behavior of the CT re-construction was checked The following equations were used for the CT reconstruction assumingdifferent divergence in both directions

xd = [x middot cosα+y middot sinα] middot (L11+L2) middot[L11 +y middot cosα-x middot sinα]-shift (41)

zd = [x2d +(L12 +L2)2]05 middot z middot[xp2 +(L12 +yp)2]05 (42)

The point with the CT slice coordinates [xy] is projected onto the detector with coordinates[xd zd] where α is rotation angle and shift is distance between actual rotation axis and the rsquospot-detector centrersquo line The CT slice point coordinates [xpyp] are obtained by the rotation transfor-mation of the [x y] coordinates The variable z is the vertical coordinate of the CT slice The wellknown filtered Radon back projection algorithm was used for the CT reconstruction utilizing theseequations

412 Ring artifacts correction

Ring artifacts appear in the CT slices due to the non accurate Flat Field correction These artifactswere successfully removed by filtering the CT slices after transforming them to polar coordinatesThe CT slice is projected onto the rotation axis direction in the polar coordinate space A high-pass filter is applied to this projection The obtained function is projected back in the rotationaxis direction and transformed back into Cartesian coordinates The final disk-like 2D function issubtracted from the filtered CT slice as depicted in figure 6

413 Results

CT slices were calculated and filtered to obtain information about the voids distribution inside theresin layer It is possible to detect large voids only thanks to statistics and neutron scatteringThe visualization of such void distribution in the central plane vertically crossing the resin layer is

ndash 4 ndash

2011 JINST 6 C03001

Figure 7 Visualization of the voids distribution in the resin layer scale in mm The same void in bothslices is labeled by the red circle

Figure 8 Visualization of the voids distribution using a neutron CT model

shown on the left in figure 7 The vertical CT slice crossing void (labeled by the white line left) ison the right in figure 7

42 Neutron tomography with a cold neutron beam

A cold neutron beam at the ICON facility [5] at the Paul Scherrer Institut was alternatively usedfor high-resolution neutron CT The parameter of divergence LD is equal to 1000

The 20 microm GdOx scintillator coupled with the 13 microm pixel size camera was used as neutronimager Using this device an efficient spatial resolution of about 50 microm measured with a Gdedge was reached Optimal irradiation geometry was chosen in which analyzed specimens wereplaced as close to the detector as possible Only one half of each specimen was analyzed due togeometrical limitations (field of view 27 mm) A high quality CT model was obtained where theshape and volume of the void distribution are well visible see figure 8 (different CT slices as in thefigure 7) Even the shape of the resin boundaries is well defined where shallow contact protrusionsare visible

5 Conclusions

It was proven that although neutron CT is a highly demanding technique it is an appropriate toolfor structural analysis of metal composite materials

It was shown that it is necessary to take into account the asymmetric beam divergence whenthe neutronograms are not acquired in the close detector geometry Ring artefacts can be effectively

ndash 5 ndash

2011 JINST 6 C03001

removed by the mathematical filtering in the polar coordinate space A high quality neutron beamand imaging technique is required when all information about a void distribution and morphologyare demanded

Acknowledgments

This work was carried out in the frame of the Medipix Collaboration and has been supported by theProject LC06041 AV0Z20710524 and 6840770040 of the Ministry of Education Youth and Sportsof the Czech Republic and in part by the Research Grant No 103092101 of the Grant Agency ofthe Czech Republic and KAN 400480701 of the Academy of Sciences of the Czech Republic

References

[1] D Vavrik CT artefact reduction by signal to thickness calibration function shaping Nucl InstrumMeth A in press

[2] HM Hudson and RS Larkin Accelerated image reconstruction using ordered subsets of projectiondata IEEE Trans Med Imag 13 (1994) 601

[3] Medipix collaboration home page httpmedipixwebcernchMEDIPIX

[4] J Jakubek T Holy E Lehmann S Pospısil J Uher J Vacık D Vavoslashık Neutron Imaging withMEDIPIX2 Chip and a Coated Sensor Nucl Instrum Meth A 560 (2006) 143

[5] httpneutrawebpsichfacilityicon indexhtml

ndash 6 ndash

2011 JINST 6 C03001

Figure 6 Schema of the circular rings filtering

ing in the CT slices in order to render the voids in the resin visible The position of the rotationcentre against the beam-detector axis was determined using a cadmium strip and a set of neutronimages obtained during rotation of the strip

411 Neutron source divergence correction

Voids were not visible in the CT reconstruction when the asymmetric beam divergence was nottaken into account The distance L2 between rotation centre and detector was 130 mm while thedistance L11 between the virtual beam spot and the rotation centre is 76 m in the horizontal planeand the distance L12 between the virtual spot and rotation axis in the vertical plane is 38 m Theseparameters were determined numerically by varying of their values when the behavior of the CT re-construction was checked The following equations were used for the CT reconstruction assumingdifferent divergence in both directions

xd = [x middot cosα+y middot sinα] middot (L11+L2) middot[L11 +y middot cosα-x middot sinα]-shift (41)

zd = [x2d +(L12 +L2)2]05 middot z middot[xp2 +(L12 +yp)2]05 (42)

The point with the CT slice coordinates [xy] is projected onto the detector with coordinates[xd zd] where α is rotation angle and shift is distance between actual rotation axis and the rsquospot-detector centrersquo line The CT slice point coordinates [xpyp] are obtained by the rotation transfor-mation of the [x y] coordinates The variable z is the vertical coordinate of the CT slice The wellknown filtered Radon back projection algorithm was used for the CT reconstruction utilizing theseequations

412 Ring artifacts correction

Ring artifacts appear in the CT slices due to the non accurate Flat Field correction These artifactswere successfully removed by filtering the CT slices after transforming them to polar coordinatesThe CT slice is projected onto the rotation axis direction in the polar coordinate space A high-pass filter is applied to this projection The obtained function is projected back in the rotationaxis direction and transformed back into Cartesian coordinates The final disk-like 2D function issubtracted from the filtered CT slice as depicted in figure 6

413 Results

CT slices were calculated and filtered to obtain information about the voids distribution inside theresin layer It is possible to detect large voids only thanks to statistics and neutron scatteringThe visualization of such void distribution in the central plane vertically crossing the resin layer is

ndash 4 ndash

2011 JINST 6 C03001

Figure 7 Visualization of the voids distribution in the resin layer scale in mm The same void in bothslices is labeled by the red circle

Figure 8 Visualization of the voids distribution using a neutron CT model

shown on the left in figure 7 The vertical CT slice crossing void (labeled by the white line left) ison the right in figure 7

42 Neutron tomography with a cold neutron beam

A cold neutron beam at the ICON facility [5] at the Paul Scherrer Institut was alternatively usedfor high-resolution neutron CT The parameter of divergence LD is equal to 1000

The 20 microm GdOx scintillator coupled with the 13 microm pixel size camera was used as neutronimager Using this device an efficient spatial resolution of about 50 microm measured with a Gdedge was reached Optimal irradiation geometry was chosen in which analyzed specimens wereplaced as close to the detector as possible Only one half of each specimen was analyzed due togeometrical limitations (field of view 27 mm) A high quality CT model was obtained where theshape and volume of the void distribution are well visible see figure 8 (different CT slices as in thefigure 7) Even the shape of the resin boundaries is well defined where shallow contact protrusionsare visible

5 Conclusions

It was proven that although neutron CT is a highly demanding technique it is an appropriate toolfor structural analysis of metal composite materials

It was shown that it is necessary to take into account the asymmetric beam divergence whenthe neutronograms are not acquired in the close detector geometry Ring artefacts can be effectively

ndash 5 ndash

2011 JINST 6 C03001

removed by the mathematical filtering in the polar coordinate space A high quality neutron beamand imaging technique is required when all information about a void distribution and morphologyare demanded

Acknowledgments

This work was carried out in the frame of the Medipix Collaboration and has been supported by theProject LC06041 AV0Z20710524 and 6840770040 of the Ministry of Education Youth and Sportsof the Czech Republic and in part by the Research Grant No 103092101 of the Grant Agency ofthe Czech Republic and KAN 400480701 of the Academy of Sciences of the Czech Republic

References

[1] D Vavrik CT artefact reduction by signal to thickness calibration function shaping Nucl InstrumMeth A in press

[2] HM Hudson and RS Larkin Accelerated image reconstruction using ordered subsets of projectiondata IEEE Trans Med Imag 13 (1994) 601

[3] Medipix collaboration home page httpmedipixwebcernchMEDIPIX

[4] J Jakubek T Holy E Lehmann S Pospısil J Uher J Vacık D Vavoslashık Neutron Imaging withMEDIPIX2 Chip and a Coated Sensor Nucl Instrum Meth A 560 (2006) 143

[5] httpneutrawebpsichfacilityicon indexhtml

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2011 JINST 6 C03001

Figure 7 Visualization of the voids distribution in the resin layer scale in mm The same void in bothslices is labeled by the red circle

Figure 8 Visualization of the voids distribution using a neutron CT model

shown on the left in figure 7 The vertical CT slice crossing void (labeled by the white line left) ison the right in figure 7

42 Neutron tomography with a cold neutron beam

A cold neutron beam at the ICON facility [5] at the Paul Scherrer Institut was alternatively usedfor high-resolution neutron CT The parameter of divergence LD is equal to 1000

The 20 microm GdOx scintillator coupled with the 13 microm pixel size camera was used as neutronimager Using this device an efficient spatial resolution of about 50 microm measured with a Gdedge was reached Optimal irradiation geometry was chosen in which analyzed specimens wereplaced as close to the detector as possible Only one half of each specimen was analyzed due togeometrical limitations (field of view 27 mm) A high quality CT model was obtained where theshape and volume of the void distribution are well visible see figure 8 (different CT slices as in thefigure 7) Even the shape of the resin boundaries is well defined where shallow contact protrusionsare visible

5 Conclusions

It was proven that although neutron CT is a highly demanding technique it is an appropriate toolfor structural analysis of metal composite materials

It was shown that it is necessary to take into account the asymmetric beam divergence whenthe neutronograms are not acquired in the close detector geometry Ring artefacts can be effectively

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2011 JINST 6 C03001

removed by the mathematical filtering in the polar coordinate space A high quality neutron beamand imaging technique is required when all information about a void distribution and morphologyare demanded

Acknowledgments

This work was carried out in the frame of the Medipix Collaboration and has been supported by theProject LC06041 AV0Z20710524 and 6840770040 of the Ministry of Education Youth and Sportsof the Czech Republic and in part by the Research Grant No 103092101 of the Grant Agency ofthe Czech Republic and KAN 400480701 of the Academy of Sciences of the Czech Republic

References

[1] D Vavrik CT artefact reduction by signal to thickness calibration function shaping Nucl InstrumMeth A in press

[2] HM Hudson and RS Larkin Accelerated image reconstruction using ordered subsets of projectiondata IEEE Trans Med Imag 13 (1994) 601

[3] Medipix collaboration home page httpmedipixwebcernchMEDIPIX

[4] J Jakubek T Holy E Lehmann S Pospısil J Uher J Vacık D Vavoslashık Neutron Imaging withMEDIPIX2 Chip and a Coated Sensor Nucl Instrum Meth A 560 (2006) 143

[5] httpneutrawebpsichfacilityicon indexhtml

ndash 6 ndash

2011 JINST 6 C03001

removed by the mathematical filtering in the polar coordinate space A high quality neutron beamand imaging technique is required when all information about a void distribution and morphologyare demanded

Acknowledgments

This work was carried out in the frame of the Medipix Collaboration and has been supported by theProject LC06041 AV0Z20710524 and 6840770040 of the Ministry of Education Youth and Sportsof the Czech Republic and in part by the Research Grant No 103092101 of the Grant Agency ofthe Czech Republic and KAN 400480701 of the Academy of Sciences of the Czech Republic

References

[1] D Vavrik CT artefact reduction by signal to thickness calibration function shaping Nucl InstrumMeth A in press

[2] HM Hudson and RS Larkin Accelerated image reconstruction using ordered subsets of projectiondata IEEE Trans Med Imag 13 (1994) 601

[3] Medipix collaboration home page httpmedipixwebcernchMEDIPIX

[4] J Jakubek T Holy E Lehmann S Pospısil J Uher J Vacık D Vavoslashık Neutron Imaging withMEDIPIX2 Chip and a Coated Sensor Nucl Instrum Meth A 560 (2006) 143

[5] httpneutrawebpsichfacilityicon indexhtml

ndash 6 ndash