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Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique
Appl. Radiar.hr. Vol. 41, No. 2, pp. 199-205, 1990 lnt. J. Rodiar.Appl. Insrrum.Part A Printed in Great Britain. All rights reserved
Development Collimators Testing
0883-2889:90 $3.00 + 0.00 Copyright IC 1990 Pergamon Press plc
of High Intended
by the Compton Scattering Tomography Technique
P. DUVAUCHELLE, Laboratoire
Resolution Focusing for Nondestructive
d’Etude
des Matkriaux,
P. GIRIER INSA,
and G. PEIX*
Bit. 303, 69621 Villeurbanne
Cedex,
France
(Received 2 I April 1989)
An original
method for building high resolution focusing collimators with a great number of channels is described. The method consists of filling a mould with tungsten powder and impregnating it with a fluid epoxy resin, by pumping through the powder. The fine strips, used as cores for the channels, are then pulled out. The collimator that was built had 49 fine slots, all converging in one little bar-shaped volume, to convey x-ray photons from this volume to the entrance window of a large scintillation crystal detector. The collimator exhibits a very good precision of the relative positions of slots, allowing the volume V to be very sharply shaped. Use of this collimator for nondestructive testing by the Compton scattering tomography (CST) technique allowed us to detect plane defects down to 0. I mm, inside composite plates.
1. Introduction Compton scattering tomography (CST) is a relatively new technique for industrial nondestructive testing (Stokes et al., 1982; Trippe and Marlowe, 1982; Harding et al., 1983/84; Bridge, 1985; Bossi et al., 1988), making use of y- or x-ray photons scattered inside the component under examination. Figure 1 shows the source (or x-ray tube focus), emitting a finely collimated beam into the material. A finely collimated detector allows measurement of the number of photons that are scattered in the volume V. Compton scattering being roughly proportional to density (Babot et al., 1989), the homogeneity of the materials can be tested by scanning V along three perpendicular directions. Three kinds of industrial applications were investigated in our laboratory:
even those parallel to the plane direction of a sheet (like delaminations in composite plates). Another advantage is that source and detector are located on the same side of the object, allowing testing of very large shells, even when full of a dense material. As can be seen from Fig. 1, the exact shape and dimensions of the volume V is of great importance for the detection and sizing of voids. Fine delaminations give rise to a relative drop in photon counting which is proportional to the percentage of missing material inside V. In this way, the accuracy of the nondestruc-
lncldent beam
material
-three-dimensional void detection and sizing (Babot et al., 1989); -sizing of thicknesses and dimensions (Babot et al., 1989; Le Floc’h et al., 1988); -measurement of density and atomic number (Berodias and Peix, 1988).
air
A main advantage of CST over transmission methods lies in the ability to detect very fine plane defects, Fig. 1. Principle *Author
for correspondence. 199
of CST. A delamination line).
is shown
(dotted
P. DLVAUCHELLF
200
tive evaluation is related to the value of the Compton angle (which determines the shape) and to the width of the collimator holes (which gives the dimensions). High accuracy demands a rather low Compton angle and a small size volume V. A major drawback of CST is the very small photon flux striking the detector. Three-dimensional scanning of a whole object is therefore a rather long process. Development of high efficiency collimators with a high geometrical precision is of major importance. The use, for instance, of several detectors pointing at different locations along the primary beam allows several measurements to be made at the same time, thus reducing test duration. In addition, another improvement can be made by using a big detector coupled with a “focusing” collimator consisting of numerous channels converging in the same volume V. Thus, a better efficiency is achieved, owing to the higher solid angle of acceptance of photons issuing from an elementary volume. With this in view. we built a 49-slot collimator. The photon source. in our device, consists of a 320 kV x-ray tube with a very low ripple. The focus is a square of side length 3.5 mm.
2. Aim of Our Work The need for a CST equipment able to detect very thin delaminations in composites led us to develop a high resolution collimator. making the best use of a 50.8 mm diameter (2”) scintillation crystal detector [NaI(Tl)]. The problem was to determine the best geometrical arrangement for the channels. Particular attention was paid to the exact shape and dimensions of the volume V. As some defects were expected to be plane and parallel to the composite plate, the width of each channel (in the plane of Fig. 1) was fixed at a low value of 0.5 mm. In the direction perpendicular to the figure, delaminations exhibit larger dimensions and, for this reason, the height of the volume V was chosen larger (6mm). We. thus, obtain a high photon flux in the detector. without damaging the collimator resolution. A schematic drawing of the incident beam and of one of the 49 detection pencils is shown in Fig. 2. For reasons
CT
of clarity, the detector and its attached 49-slot collimator are not shown. These two parts form a single block, independent of the primary beam collimator. The Compton angle can thus be set at the desired value by a simple mechanical adjustment. Note that dimensions given in Fig. 2 are in fact those of the channels. and not those of the x-ray pencil which, at the volume V, is bigger, due to beam divergence. The exact dimensions of the beam depend on the relative geometrical arrangement of the collimators, photon source, detector and volume V. In the case of our apparatus, owing to the fact that the x-ray tube focus is 7 times larger than that of the incidence slot, it can be understood that only part of the source can be seen from the volume V, even from a point set on beam axis. A large-scale drawing indicates, for our own device, that the incident beam is I .75 mm wide. at the level of volume V. In the direction perpendicular to Fig. I, the source. on the contrary, does not cover the whole slot. The photon flux is expected to be more uniform in that direction and to fall off more sharply. Beam divergence is also expected to be less important; a dimension of 9 mm is forecast for volume V. An x-ray film, set across the incident beam and exposed until saturation, confirms that the pencil does not exceed I .75 x 9 mm at the abscissa of volume V. The width of the scattered beam was also calculated. A geometrical estimation led us to a value of 3.5 mm, assuming a good convergence of channels. This value. twice the width of the incident beam, is accounted for by the fact that the collimator is shorter (55 mm instead of 70 mm) and that the distance from this collimator to the volume V is higher (1 IO mm instead of 75 mm). Nevertheless, the width of the scattered beam is much more difficult to forecast. because it depends mainly on the precision with which the 49 slots converge. This is a point of major importance. During the construction of such a device. another critical aspect is to ensure the perfect straightness of each channel. even a slight deviation would be sufficient to prevent free travelling of photons. Finally, care must be taken to choose, for the collimator, a high density and high atomic number material.
6mm
To the detector
Fig. 2. Shape of the incident
0.5mm
’
u
beam and of one of the 49 scattered the collimators slots.
beams.
The sizes are in fact those of
High resolution
focusing
collimators
201
3. General Description of the Fabrication Process Building a collimator with a great number of slots, regularly distributed on the circular entrance window of a big detector, is not an easy thing to realize by machining. Hence, in order to be less constrained in the choice of geometrical arrangement, we decided to cast this object. Figure 3 shows a schematic view of the collimator, with its three rows of channels. Note that this scheme is rather an artistic view, since the real number of slots is higher: 19 in the central row and 15 in the lower and upper rows. Given the fact that the 49 cores that correspond to the channels are very thin strips (0.5 mm thick), hot casting of liquid metal was non-trivial, carrying the risk of twisting the strips. We finally chose to construct a mould made of methyl-polymethacrylate. The filling material, of high density and high 2, is composed of tungsten powder impregnated with epoxy resin. Figure 4 shows a schematic view of the mould. The central part 1 holds tungsten powder and the upper part 2 allows the introduction of an epoxy resin on the top of the powder. Parts 3 and 4 are lower and upper plates ensuring correct arrangement of cores. Pumping the mould to make the impregnation faster and eliminate air-bubbles in the epoxy is achieved through part 5.
Tungsten
t
To the
powder
pump
Fig. 4. Schematic view of the mould. during Impregnation with epoxy resin: 1, central part of the mould: 2, upper part; 3 and 4, lower and upper plates ensuring correct arrangement of cores; 5, connection with the pumping system.
4. Design of the Mould 4.1. Design and machining of plates 3 and 4 In order to ensure a good collimator efficiency, the 49 thin strips used as cores were held in place with a precision of 0.01 mm. They were slightly wedged in rectangular holes machined in the two plates. A calculation gave the position of each notch, as well as its exact dimensions, which differed from one notch to another. Only the central notch had the exact dimensions of the strips (0.5 x 6 mm). All other sizes were computed in order to take into account the tilting of lamellae along two directions. Machining was performed with a laser beam, in a 3 mm thick sheet of methyl-polymethacrylate. Great attention was paid to the relative orientations and positions of the two plates, at the final state of the mould fitting up. Machining, for this reason, was achieved with part 3 already set on part 1, and part 4 set on part 2. For the purpose of machining, a special device was built, allowing precise positioning of the two plates on the laser-machine table. Figure 5 shows the lower and upper plates with their notches.
4.2. Cores manufacture
No1 (TL I detector
Fig. 3. Artistic view of the constructed collimator. The real one possesses 49 channels. instead of the 31 shown here.
In order to cast collimators up to 80mm long, the total height of the mould approaches 150 mm. Dimensions of the lamellae are 0.5 x 6 x 150 mm. A camber of 0.05 mm is allowed. The only way we found to achieve such a result was to manufacture the lamellae one by one with carbon-epoxy “preimpregnated” sheets. Polymerization was made under a press, at a temperature of 110°C. Lamellae were pressed between two plane plates, the final thickness being ensured by stays. Very good results were obtained by piling up monodirectional carbon-fibre sheets along the length of the strips. Attempts to cross fibres or to use weaved sheets led to twisted lamellae.
102
P. DUVALCHELLE ef ul
Figure 6 shows an upper view of the mould, the lamellae set in place.
with
120 r
4.3. Choice qf tungsten powder Tungsten powder, with grain dimensions around 200nm. was slowly poured into the mould and penetrated between every iamellae. Vibrating the mould. during this process. ensured a higher density of powder.
+
20
In order to ensure complete impregnation of the mould bcforc setting, the desired characteristics of the resin were fluidity and slow polymerization. A slight lowering of temperature, down to 14 C (57 F), was found to slow down hardening without appreciably altering fluidity. Under these conditions, a theoretical time of 5 h was allowed, which was quite sufficient for the epoxy to reach the lowest part of the mould. Removal of lamellae was possible by using an epoxy resin which expanded when heated. A warming of 30 min. at 120 C (250 F). ensured easy removal without damaging the collimator.
5. Characterization
of the Collimator
Our main objective was to image the volume V in such a way as to detect any anomaly at the convergence of the 49 channels. The most comprehensive test we found consisted of scanning a fine tungsten wire (0.25 mm dia) through a large region enclosing the volume V. The sampling step was 0.25 mm. The wire direction was perpendicular to the two beams (i.e. parallel to the height of V), in order to image the plane corresponding to Fig. 1 with the greatest resolution. Figure 7 shows a three-dimensional imaging of a square of 20 mm. The third dimension, that is to say height, represents the intensity of Compton scattering for the tungsten wire set at this place. Incident and scattered beams. defined according to this figure. are shown; their estimated widths, measured up to zero level, are respectively 1.75 and incident beam _ --_-------__
_I_
Fig. 7. Result of the scan of a thin tungsten wire through the region where all the beams are converging. Each division represents 0.25 mm: the total area plotted here corresponds to 20 X 20 mm.
+ FWHM
-+----~-it-
+ A+
13Smm
+ + +
Fig. 8. Intensit) ofCompton scattering corresponding to the scan of a thin copper sheet through volume I’. The sheet is directed along the expected direction for delaminations (see Fig. I) and the displacement is perpendicular to the sheet; each diwsion represents 0.1 mm. 3.5 mm. These results are absolutely equal to our geometrical estimations. described in Section 2. The Compton angle was chosen to be 60 Such a check, very useful with respect to the global description of volume I,‘. was completed with a monodirectional scanning of a thin (0.1 mm) copper sheet, able to take into account the response of the whole apparatus to a plane defect. The sampling step was chosen to be 0.1 mm. The copper sheet was held parallel to the direction of expected delamination defects, as shown in Fig. I. and scanned perpendicularly in such a way as to cross the whole of volume V. The results are plotted in Fig. 8 where the number of counts (ordinate) increases as the sheet passes through the volume 1;. Such a curve represents the impulse response, or line spread function (LSF) of our apparatus. The full width at half maximum (FWHM) is also shown. We then tested the expected life of such an object in an ionizing environment. This collimator, intended to be used in front of the detector. i.e. apart from the primary beam, is not exposed to a strong irradiation. Measurements, made in air in the vicinity of the collimator, indicated an exposure rate of 1.4 x IO-“’ A kg ‘. We then exposed our collimator to a strong direct irradiation, in an x-ray beam of the same mean energy. Af’ter an exposure up to 0.3 C did not exhibit any change in kg ‘. the collimator appearance or in supcrticial hardness. Such an irradiation corresponds to an expected lift of 70 years. working nights and days! The best way to characterize our CST apparatus was to examine a sample with a known defect. We chose an aluminium-rubber interface consisting of 10 mm of rubber stuck on an aluminium plate, with an artificial ungluing 0.1 mm in thickness and 5 mm in length (Fig. 9). A scan. carried out from the rubber side, allowed us to record the scattered photon flux as a function of the abscissa along the plane interface. Hence. v,olume C’ was moving along the 2’ direction of Fig. 9. A sharp local decrease of the count reveals the defect (Fig. IO).
203
Fig. 6. Upper
view of the complete
mould
wth
the lamellae
set in place
High resolution
Fig.
9. Examination
along an interface artificial defect.
comprising
‘ocusing collimators
an
It can also be noticed that the photon flux varies slowly along the scan. This corresponds to the fact that the studied interface was not strictly parallel to displacement of volume V. Thus, measurements correspond to a slightly evolving depth, leading to a change in the photon attenuation. The speed of the scan was 2 mm s-l; each division in the figure corresponds to 1.36mm.
6. Conclusion A collimator was built using a “cold casting” method. Tungsten powder, impregnated in a mould with epoxy resin, forms the basic material. It reaches a density of 10.5, which is close to that of lead. The constructed collimator, which has a length of 55 mm, has 49 channels converging at the same point. These channels, each with a cross-section of 0.5 x 6 mm, are regularly distributed inside a circle of about 48 mm dia. Due to the high precision in the convergence of the channels, this collimator, used in a CST apparatus, defines a sharply shaped volume for the evaluation of materials. It allows detection of plane defects (ungluing) of 0.1 mm thickness inside composite materials. Intended to be used slightly apart from the primary beam, it withstands an irradiation corresponding to 70 years of use. Owing to the great number of channels converging in a well-defined volume, this collimator offers a high efficiency. In the field of industrial CST, it allows an appreciable increase in scanning speed, and saving a proportional amount of time. It can also be very useful in the medical field, where a great number of radiometric methods are being developed nowadays. High efficiency of detection is of major importance, allowing in this case a reduction in the delivered dose.
Abscissa
Fig. 10. Detection of a thin (0.1 mm) ungluing in an aluminium-rubber plane interface. The photon flux (in arbitrary units) is recorded when volume V is displaced at a constant speed along the interface. Each division on the abscissa represents I .36 mm. The length of the bars indicates a 95% reliability.
References Babot D.. Berodias G., Malo P. and Peix G. (1989) Contr8le. cara&risation et dimensionnement par diffusion Compton de rayons X ou gamma. C’omposires. 2, 20. Berodias G. and Peix G. (1988) Nondestructive measurement of density and effective atomic number by photon scattering. Mater. Eual. 46, 1209. Bossi R. H., Friddell K. D. and Nelson J. M. (1988) Backscatter X-ray imaging. Mu&r. Eoal. 46, 1462. Bridge B. (1985) A theoretical feasibility study of the use of Compton backscatter gamma-ray tomography for underwater offshore NDT. &it. J. NDT Nov., 357. Harding G., Strecker H. and Tischler R. (1983/84) X-ray imaging with Compton-scatter radiation. Philips Tech. RetI. 41, 46. Le Floc’h C., Mazeron B., Babot D. and Peix G. (1988) Pro&d& et dispositif pour determiner la masse volumique d’un volume bltmentaire de matitre. Brevet d’lnvention nC88/04545, Socii.tC Anonyme dite Aerospatiale. Stokes J. A., Alvar K. R., Corey R. L., Costello D. G., John J., Kocimski S., Lurie N. A., Thayer D. D., Trippe A. P. and Young J. C. (1982) Some new applications of collimated photon scattering for nondestructive examination. Nucl. Instrum. Methods 193, 261. Trippe A. P. and Marlowe M. B. (1982) AIDECS-second generation system. Perspectives 5, 2.
Development Collimators Testing
0883-2889:90 $3.00 + 0.00 Copyright IC 1990 Pergamon Press plc
of High Intended
by the Compton Scattering Tomography Technique
P. DUVAUCHELLE, Laboratoire
Resolution Focusing for Nondestructive
d’Etude
des Matkriaux,
P. GIRIER INSA,
and G. PEIX*
Bit. 303, 69621 Villeurbanne
Cedex,
France
(Received 2 I April 1989)
An original
method for building high resolution focusing collimators with a great number of channels is described. The method consists of filling a mould with tungsten powder and impregnating it with a fluid epoxy resin, by pumping through the powder. The fine strips, used as cores for the channels, are then pulled out. The collimator that was built had 49 fine slots, all converging in one little bar-shaped volume, to convey x-ray photons from this volume to the entrance window of a large scintillation crystal detector. The collimator exhibits a very good precision of the relative positions of slots, allowing the volume V to be very sharply shaped. Use of this collimator for nondestructive testing by the Compton scattering tomography (CST) technique allowed us to detect plane defects down to 0. I mm, inside composite plates.
1. Introduction Compton scattering tomography (CST) is a relatively new technique for industrial nondestructive testing (Stokes et al., 1982; Trippe and Marlowe, 1982; Harding et al., 1983/84; Bridge, 1985; Bossi et al., 1988), making use of y- or x-ray photons scattered inside the component under examination. Figure 1 shows the source (or x-ray tube focus), emitting a finely collimated beam into the material. A finely collimated detector allows measurement of the number of photons that are scattered in the volume V. Compton scattering being roughly proportional to density (Babot et al., 1989), the homogeneity of the materials can be tested by scanning V along three perpendicular directions. Three kinds of industrial applications were investigated in our laboratory:
even those parallel to the plane direction of a sheet (like delaminations in composite plates). Another advantage is that source and detector are located on the same side of the object, allowing testing of very large shells, even when full of a dense material. As can be seen from Fig. 1, the exact shape and dimensions of the volume V is of great importance for the detection and sizing of voids. Fine delaminations give rise to a relative drop in photon counting which is proportional to the percentage of missing material inside V. In this way, the accuracy of the nondestruc-
lncldent beam
material
-three-dimensional void detection and sizing (Babot et al., 1989); -sizing of thicknesses and dimensions (Babot et al., 1989; Le Floc’h et al., 1988); -measurement of density and atomic number (Berodias and Peix, 1988).
air
A main advantage of CST over transmission methods lies in the ability to detect very fine plane defects, Fig. 1. Principle *Author
for correspondence. 199
of CST. A delamination line).
is shown
(dotted
P. DLVAUCHELLF
200
tive evaluation is related to the value of the Compton angle (which determines the shape) and to the width of the collimator holes (which gives the dimensions). High accuracy demands a rather low Compton angle and a small size volume V. A major drawback of CST is the very small photon flux striking the detector. Three-dimensional scanning of a whole object is therefore a rather long process. Development of high efficiency collimators with a high geometrical precision is of major importance. The use, for instance, of several detectors pointing at different locations along the primary beam allows several measurements to be made at the same time, thus reducing test duration. In addition, another improvement can be made by using a big detector coupled with a “focusing” collimator consisting of numerous channels converging in the same volume V. Thus, a better efficiency is achieved, owing to the higher solid angle of acceptance of photons issuing from an elementary volume. With this in view. we built a 49-slot collimator. The photon source. in our device, consists of a 320 kV x-ray tube with a very low ripple. The focus is a square of side length 3.5 mm.
2. Aim of Our Work The need for a CST equipment able to detect very thin delaminations in composites led us to develop a high resolution collimator. making the best use of a 50.8 mm diameter (2”) scintillation crystal detector [NaI(Tl)]. The problem was to determine the best geometrical arrangement for the channels. Particular attention was paid to the exact shape and dimensions of the volume V. As some defects were expected to be plane and parallel to the composite plate, the width of each channel (in the plane of Fig. 1) was fixed at a low value of 0.5 mm. In the direction perpendicular to the figure, delaminations exhibit larger dimensions and, for this reason, the height of the volume V was chosen larger (6mm). We. thus, obtain a high photon flux in the detector. without damaging the collimator resolution. A schematic drawing of the incident beam and of one of the 49 detection pencils is shown in Fig. 2. For reasons
CT
of clarity, the detector and its attached 49-slot collimator are not shown. These two parts form a single block, independent of the primary beam collimator. The Compton angle can thus be set at the desired value by a simple mechanical adjustment. Note that dimensions given in Fig. 2 are in fact those of the channels. and not those of the x-ray pencil which, at the volume V, is bigger, due to beam divergence. The exact dimensions of the beam depend on the relative geometrical arrangement of the collimators, photon source, detector and volume V. In the case of our apparatus, owing to the fact that the x-ray tube focus is 7 times larger than that of the incidence slot, it can be understood that only part of the source can be seen from the volume V, even from a point set on beam axis. A large-scale drawing indicates, for our own device, that the incident beam is I .75 mm wide. at the level of volume V. In the direction perpendicular to Fig. I, the source. on the contrary, does not cover the whole slot. The photon flux is expected to be more uniform in that direction and to fall off more sharply. Beam divergence is also expected to be less important; a dimension of 9 mm is forecast for volume V. An x-ray film, set across the incident beam and exposed until saturation, confirms that the pencil does not exceed I .75 x 9 mm at the abscissa of volume V. The width of the scattered beam was also calculated. A geometrical estimation led us to a value of 3.5 mm, assuming a good convergence of channels. This value. twice the width of the incident beam, is accounted for by the fact that the collimator is shorter (55 mm instead of 70 mm) and that the distance from this collimator to the volume V is higher (1 IO mm instead of 75 mm). Nevertheless, the width of the scattered beam is much more difficult to forecast. because it depends mainly on the precision with which the 49 slots converge. This is a point of major importance. During the construction of such a device. another critical aspect is to ensure the perfect straightness of each channel. even a slight deviation would be sufficient to prevent free travelling of photons. Finally, care must be taken to choose, for the collimator, a high density and high atomic number material.
6mm
To the detector
Fig. 2. Shape of the incident
0.5mm
’
u
beam and of one of the 49 scattered the collimators slots.
beams.
The sizes are in fact those of
High resolution
focusing
collimators
201
3. General Description of the Fabrication Process Building a collimator with a great number of slots, regularly distributed on the circular entrance window of a big detector, is not an easy thing to realize by machining. Hence, in order to be less constrained in the choice of geometrical arrangement, we decided to cast this object. Figure 3 shows a schematic view of the collimator, with its three rows of channels. Note that this scheme is rather an artistic view, since the real number of slots is higher: 19 in the central row and 15 in the lower and upper rows. Given the fact that the 49 cores that correspond to the channels are very thin strips (0.5 mm thick), hot casting of liquid metal was non-trivial, carrying the risk of twisting the strips. We finally chose to construct a mould made of methyl-polymethacrylate. The filling material, of high density and high 2, is composed of tungsten powder impregnated with epoxy resin. Figure 4 shows a schematic view of the mould. The central part 1 holds tungsten powder and the upper part 2 allows the introduction of an epoxy resin on the top of the powder. Parts 3 and 4 are lower and upper plates ensuring correct arrangement of cores. Pumping the mould to make the impregnation faster and eliminate air-bubbles in the epoxy is achieved through part 5.
Tungsten
t
To the
powder
pump
Fig. 4. Schematic view of the mould. during Impregnation with epoxy resin: 1, central part of the mould: 2, upper part; 3 and 4, lower and upper plates ensuring correct arrangement of cores; 5, connection with the pumping system.
4. Design of the Mould 4.1. Design and machining of plates 3 and 4 In order to ensure a good collimator efficiency, the 49 thin strips used as cores were held in place with a precision of 0.01 mm. They were slightly wedged in rectangular holes machined in the two plates. A calculation gave the position of each notch, as well as its exact dimensions, which differed from one notch to another. Only the central notch had the exact dimensions of the strips (0.5 x 6 mm). All other sizes were computed in order to take into account the tilting of lamellae along two directions. Machining was performed with a laser beam, in a 3 mm thick sheet of methyl-polymethacrylate. Great attention was paid to the relative orientations and positions of the two plates, at the final state of the mould fitting up. Machining, for this reason, was achieved with part 3 already set on part 1, and part 4 set on part 2. For the purpose of machining, a special device was built, allowing precise positioning of the two plates on the laser-machine table. Figure 5 shows the lower and upper plates with their notches.
4.2. Cores manufacture
No1 (TL I detector
Fig. 3. Artistic view of the constructed collimator. The real one possesses 49 channels. instead of the 31 shown here.
In order to cast collimators up to 80mm long, the total height of the mould approaches 150 mm. Dimensions of the lamellae are 0.5 x 6 x 150 mm. A camber of 0.05 mm is allowed. The only way we found to achieve such a result was to manufacture the lamellae one by one with carbon-epoxy “preimpregnated” sheets. Polymerization was made under a press, at a temperature of 110°C. Lamellae were pressed between two plane plates, the final thickness being ensured by stays. Very good results were obtained by piling up monodirectional carbon-fibre sheets along the length of the strips. Attempts to cross fibres or to use weaved sheets led to twisted lamellae.
102
P. DUVALCHELLE ef ul
Figure 6 shows an upper view of the mould, the lamellae set in place.
with
120 r
4.3. Choice qf tungsten powder Tungsten powder, with grain dimensions around 200nm. was slowly poured into the mould and penetrated between every iamellae. Vibrating the mould. during this process. ensured a higher density of powder.
+
20
In order to ensure complete impregnation of the mould bcforc setting, the desired characteristics of the resin were fluidity and slow polymerization. A slight lowering of temperature, down to 14 C (57 F), was found to slow down hardening without appreciably altering fluidity. Under these conditions, a theoretical time of 5 h was allowed, which was quite sufficient for the epoxy to reach the lowest part of the mould. Removal of lamellae was possible by using an epoxy resin which expanded when heated. A warming of 30 min. at 120 C (250 F). ensured easy removal without damaging the collimator.
5. Characterization
of the Collimator
Our main objective was to image the volume V in such a way as to detect any anomaly at the convergence of the 49 channels. The most comprehensive test we found consisted of scanning a fine tungsten wire (0.25 mm dia) through a large region enclosing the volume V. The sampling step was 0.25 mm. The wire direction was perpendicular to the two beams (i.e. parallel to the height of V), in order to image the plane corresponding to Fig. 1 with the greatest resolution. Figure 7 shows a three-dimensional imaging of a square of 20 mm. The third dimension, that is to say height, represents the intensity of Compton scattering for the tungsten wire set at this place. Incident and scattered beams. defined according to this figure. are shown; their estimated widths, measured up to zero level, are respectively 1.75 and incident beam _ --_-------__
_I_
Fig. 7. Result of the scan of a thin tungsten wire through the region where all the beams are converging. Each division represents 0.25 mm: the total area plotted here corresponds to 20 X 20 mm.
+ FWHM
-+----~-it-
+ A+
13Smm
+ + +
Fig. 8. Intensit) ofCompton scattering corresponding to the scan of a thin copper sheet through volume I’. The sheet is directed along the expected direction for delaminations (see Fig. I) and the displacement is perpendicular to the sheet; each diwsion represents 0.1 mm. 3.5 mm. These results are absolutely equal to our geometrical estimations. described in Section 2. The Compton angle was chosen to be 60 Such a check, very useful with respect to the global description of volume I,‘. was completed with a monodirectional scanning of a thin (0.1 mm) copper sheet, able to take into account the response of the whole apparatus to a plane defect. The sampling step was chosen to be 0.1 mm. The copper sheet was held parallel to the direction of expected delamination defects, as shown in Fig. I. and scanned perpendicularly in such a way as to cross the whole of volume V. The results are plotted in Fig. 8 where the number of counts (ordinate) increases as the sheet passes through the volume 1;. Such a curve represents the impulse response, or line spread function (LSF) of our apparatus. The full width at half maximum (FWHM) is also shown. We then tested the expected life of such an object in an ionizing environment. This collimator, intended to be used in front of the detector. i.e. apart from the primary beam, is not exposed to a strong irradiation. Measurements, made in air in the vicinity of the collimator, indicated an exposure rate of 1.4 x IO-“’ A kg ‘. We then exposed our collimator to a strong direct irradiation, in an x-ray beam of the same mean energy. Af’ter an exposure up to 0.3 C did not exhibit any change in kg ‘. the collimator appearance or in supcrticial hardness. Such an irradiation corresponds to an expected lift of 70 years. working nights and days! The best way to characterize our CST apparatus was to examine a sample with a known defect. We chose an aluminium-rubber interface consisting of 10 mm of rubber stuck on an aluminium plate, with an artificial ungluing 0.1 mm in thickness and 5 mm in length (Fig. 9). A scan. carried out from the rubber side, allowed us to record the scattered photon flux as a function of the abscissa along the plane interface. Hence. v,olume C’ was moving along the 2’ direction of Fig. 9. A sharp local decrease of the count reveals the defect (Fig. IO).
203
Fig. 6. Upper
view of the complete
mould
wth
the lamellae
set in place
High resolution
Fig.
9. Examination
along an interface artificial defect.
comprising
‘ocusing collimators
an
It can also be noticed that the photon flux varies slowly along the scan. This corresponds to the fact that the studied interface was not strictly parallel to displacement of volume V. Thus, measurements correspond to a slightly evolving depth, leading to a change in the photon attenuation. The speed of the scan was 2 mm s-l; each division in the figure corresponds to 1.36mm.
6. Conclusion A collimator was built using a “cold casting” method. Tungsten powder, impregnated in a mould with epoxy resin, forms the basic material. It reaches a density of 10.5, which is close to that of lead. The constructed collimator, which has a length of 55 mm, has 49 channels converging at the same point. These channels, each with a cross-section of 0.5 x 6 mm, are regularly distributed inside a circle of about 48 mm dia. Due to the high precision in the convergence of the channels, this collimator, used in a CST apparatus, defines a sharply shaped volume for the evaluation of materials. It allows detection of plane defects (ungluing) of 0.1 mm thickness inside composite materials. Intended to be used slightly apart from the primary beam, it withstands an irradiation corresponding to 70 years of use. Owing to the great number of channels converging in a well-defined volume, this collimator offers a high efficiency. In the field of industrial CST, it allows an appreciable increase in scanning speed, and saving a proportional amount of time. It can also be very useful in the medical field, where a great number of radiometric methods are being developed nowadays. High efficiency of detection is of major importance, allowing in this case a reduction in the delivered dose.
Abscissa
Fig. 10. Detection of a thin (0.1 mm) ungluing in an aluminium-rubber plane interface. The photon flux (in arbitrary units) is recorded when volume V is displaced at a constant speed along the interface. Each division on the abscissa represents I .36 mm. The length of the bars indicates a 95% reliability.
References Babot D.. Berodias G., Malo P. and Peix G. (1989) Contr8le. cara&risation et dimensionnement par diffusion Compton de rayons X ou gamma. C’omposires. 2, 20. Berodias G. and Peix G. (1988) Nondestructive measurement of density and effective atomic number by photon scattering. Mater. Eual. 46, 1209. Bossi R. H., Friddell K. D. and Nelson J. M. (1988) Backscatter X-ray imaging. Mu&r. Eoal. 46, 1462. Bridge B. (1985) A theoretical feasibility study of the use of Compton backscatter gamma-ray tomography for underwater offshore NDT. &it. J. NDT Nov., 357. Harding G., Strecker H. and Tischler R. (1983/84) X-ray imaging with Compton-scatter radiation. Philips Tech. RetI. 41, 46. Le Floc’h C., Mazeron B., Babot D. and Peix G. (1988) Pro&d& et dispositif pour determiner la masse volumique d’un volume bltmentaire de matitre. Brevet d’lnvention nC88/04545, Socii.tC Anonyme dite Aerospatiale. Stokes J. A., Alvar K. R., Corey R. L., Costello D. G., John J., Kocimski S., Lurie N. A., Thayer D. D., Trippe A. P. and Young J. C. (1982) Some new applications of collimated photon scattering for nondestructive examination. Nucl. Instrum. Methods 193, 261. Trippe A. P. and Marlowe M. B. (1982) AIDECS-second generation system. Perspectives 5, 2.