Modern neutron imaging: Radiography, Tomography, dynamic and phase contrast imaging with neutrons

Transcription

1 Solid State Phenomena Vol. 112 (2006) pp Online available since 2006/May/15 at (2006) Trans Tech Publications, Switzerland doi: / Modern neutron imaging: Radiography, Tomography, dynamic and phase contrast imaging with neutrons Burkhard Schillinger 1, a, Elbio Calzada 1, Klaus Lorenz 1 1 TU Muenchen - FRM-II and Physics E21, Lichtenbergstr.1, D Garching, Germany Burkhard.Schillinger@frm2.tum.de, Elbio.Calzada@frm2.tum.de, Klaus.Lorenz@frm2.tum.de Keywords: neutron radiography, neutron tomography, dynamic imaging, stroboscopic imaging, scintillation screen, neutron detector, CCD camera. Abstract. This paper gives a review about the current state of the art in neutron imaging like neutron radiography, neutron tomography, stroboscopic imaging and phase contrast imaging. The different techniques are described and compared to X-rays. Introduction Neutron radiography with film has been around since the first research reactors became available in the 1950s, but never really gained importance due to difficult handling, low image definition and limited dynamic range. With the appearance of electronic detectors such as cooled CCD cameras, neutron radiography has experienced a renaissance since the 1990s. The rapid technological development in detectors, but also in next generation neutron sources like the FRM-II reactor at Technische Universitaet Muenchen has since then enabled for advanced techniques like computed tomography, stroboscopic short-time imaging and even phase contrast imaging. A little physics X-rays and gamma rays interact with the electron shell of the atomic nuclei mainly by photo effect, Compton effect and pair production. The interaction probability increases with the number of electrons in an atomic electron shell and thus with the atomic number in the table of elements. Neutrons interact with the atomic nuclei mainly by nuclear reactions, elastic scattering and inelastic scattering. Fast neutrons interact mainly by incoherent scattering. The probability decreases with increasing atomic number. Thermal neutrons can interact by all three mechanisms, including coherent scattering on crystal lattices. The probabilities depend on the inner structure of the nuclei and do not follow a simple rule. Fig. 1 shows the mass attenuation coefficient for the elements for X-rays, gamma rays, fast and thermal neutrons. While the curve for X-rays increases with the atomic number, and the one for fast neutrons decreases with the increasing mass of the scattering nuclei, there is no obvious regularity for thermal neutrons. As most important difference compared to X-rays, the neutron attenuation of most metals like Al, Fe is much lower than for 120 kev X-rays, while the attenuation for hydrogen is very high. Lead is very transparent for neutrons. Fig.2 illustrates the relative attenuation of 1 cm and 4 cm of material for the different kinds of radiation. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-15/06/14,02:46:02)

2 62 Materials in Transition Fig 1: The mass attenuation coefficients for the elements for X-rays, fast and thermal neutrons. Fig.2: Relative attenuation of 1cm and 4 cm material thickness as grey values 1 cm of water is nearly impenetrable for thermal neutrons, while metals like Mg, Al, Cr, Mn, Fe, Cu, Zn and especially heavy ones like Pb and Bi are rather transparent. This reveals the most important application of neutron radiography, which is the penetration of metals and the sensitive detection of hydrogen and thus organic substances like plastics, adhesives, sealants and lubricants, so neutron radiography is a complementary method to X-rays. Beam geometries for radiography and tomography X-ray tubes generate a small focal spot of the electron beam on the anode. This spot acts like a point source that generates a cone beam (Fig.3). A sample placed between the focal spot and a

3 Solid State Phenomena Vol detector is projected onto that detector with a magnification dependent on its position. Due to this magnification, details much smaller than the actual pixel size of the detector can be resolved if the focal spot is smaller than the details. A neutron point source cannot be realized with sufficient intensity, so a parallel beam has to be approximated at reactor sources by using a small aperture and a large distance. The achievable resolution depends both on the beam collimation and the detector resolution. Fig.3: Fan beam (for line detectors), cone beam and parallel beam The beam collimation that determines the image quality is defined as the ratio of the source-tosample-distance L to the diameter D of the source. It describes the angle under which a point on the sample sees the edges of the source (Fig.4). The point on the sample will be projected onto a disk on the detector according to that angle, which defines the image unsharpness. For neutrons, the source diameter is the smallest diameter of a collimator or diaphragm in the beam path. Note that the collimator walls define the maximum opening angle or spread of the total beam. Fig. 4: Beam geometry: Definition of the L/D ratio If radiography is performed at the end of a neutron guide, the divergence of the beam is given by the critical angle of reflection c ( ) of the neutron guide. The divergence is constant within the cross section of a straight neutron guide, it acts like a divergent area source (Fig.5) [1]. Fig.5: Beam geometry at a neutron guide For the average wavelength, an effective L/D can be estimated as L/D = 1/tan(2 c ). A neutron guide is not a good choice for a neutron radiography facility, as can be seen in Fig.6, which shows radiographs of a 3,5" floppy drive in 0 cm, 10 cm and 20 cm distance from a film + Gd sandwich taken at a cold neutron guide with L/D=71. Fig.7 shows the same toy plane engine at different L/D ratios.

4 64 Materials in Transition Fig.6: a 3,5" floppy drive in 0 cm, 10 cm and 20 cm distance from a film + Gd sandwich taken at a cold neutron guide with L/D=71 L/D=71 L/D=115 L/D=320 L/D>500. Fig.7: Radiographs of a small motor taken at different beam positions with different L/D ratios. The ANTARES facility The Antares facility (Fig.8) is situated at beam line SR-4b at the research reactor FRM-II of Technische Universitaet Muenchen, which faces the cold source and delivers a thermal/cold spectrum [2]. It was built as a flight tube system with interchangeable collimators for L/D=400 at a neutron flux of 10 8 n/cm 2 s and L/D=800 at a flux of 2.6 x 10 7 n/cm 2 s. An hydraulically driven vertical shutter can position the two different collimators into the beam path. A pneumatic fast shutter is used to suppress the thermal neutron flux in times of detector readout or sample re-positioning in order to keep the activation as low as possible. A selector wheel can rotate different pinhole collimators for phase contrast imaging into the beam path. A 300kV X-ray tube can be moved into the beam path in order to record X-ray images in exactly the same beam geometry as for neutrons, so that the images can be overlaid (see below) without any adjustment (Fig.9). After the 12m evacuated flight tube, a beam size limiter can reduce the maximum available beam cross section of 40 cm x 40 cm to the required amount to reduce the background in the blockhouse. A position for a neutron velocity selector is foreseen, but not occupied yet due to lack of funding. The sample manipulator is designed to carry samples up to 500 kg of weight and 1 m diameter (Fig.10). All inside wall surfaces of the blockhouse are covered with mats of borated rubber in order

5 Solid State Phenomena Vol to prevent activation of the steel casings of the heavy concrete wall elements by scattered neutrons. A beam catcher of powdered 6 LiF on the back wall captures the thermal/cold flux with a radiationfree capture reaction in order to keep the gamma background low. Fig.8: The ANTARES facility Fig.9: Fast shutter, selector wheel, X-ray tube and optional velocity selector Fig.10: The sample manipulator

6 66 Materials in Transition The detector systems The standard detector for neutron radiography and tomography is a cooled Andor CCD camera with 2048 x 2048 pixel and 16 Bit resolution [3] in combination with a surface mirror and a ZnS + LiF(AG,Au,CU) scintillation screen. A full dynamics 16 bit image can be recorded in 1.5 seconds with the L/D=400 collimator, in 7 seconds with the L/D=800 collimator. Fig.11 shows one of the first neutron radiographies taken at FRM-II depicting a diesel injection pump. Different rubber o- rings and sealants are clearly visible, even some oil remains in the thread beyond the large horizontal screw can be recognized. Fig. 11: Neutron radiography of a diesel injection pump The same detector system can be used in combination with a Gadoliniumoxisulfide screen and the X-ray tube mentioned above. As the X-ray tube is mounted in 12 m distance from the detector and sample, 99% of its intensity are wasted. This setup is clearly not optimized for X-rays, but delivers the big advantage of directly overlaying X-ray and neutron images. Fig. 12 shows a photo, X-ray radiography and neutron radiography of a toy plane engine. The X-rays easily penetrate the plastic propeller, but have some difficulties with the metal body of the engine. The neutrons cannot penetrate the propeller at all, but show a clear image of the metal engine, while showing the plastic fuel tube and even some oil remains in the muffler on the exhaust tube. Fig.13 shows a printed circuit board. The X-ray shows mainly the metal parts like the pins of the components and the solder pads, while the neutron image reveals the plastic casings of the integrated circuits and the memory banks. Both scintillation screens in combination with the camera can be used for computed tomography by simply rotating the sample with a rotation table. For stroboscopic neutron imaging, we have a cooled 1024 x 1024 pixel Andor CCD camera [4] with a gateable image intensifier, which can be used as a very fast shutter down to a few ns gating time. Both tomography and stroboscopic imaging are described below.

7 Solid State Phenomena Vol Fig. 12: Photo, X-ray and neutron radiography of a toy plane engine Fig. 13: Photo, X-ray and neutron radiography of a printed circuit board Computed tomography Since we have an approximation to parallel beam geometry, it is sufficient to rotate the sample by only 180 degrees, since the second 180 degree data set would be identical safe for a mirror inversion. In contrast, for X-ray cone beam geometry, projections would not be redundant due to the inherent magnification of the projection. Samples are rotated by a stepper motor driven rotation table. The control computer for the turntable also delivers the synchronization signal for the camera. By mathematical theory, one would need Pi/2 times as many angular projections as there are pixels in a camera line, i. e. more than 2800 projections. In practice, we take only 400 projections as the additional gain in image quality does not justify the extra amount of data. In parallel beam geometry, the calculation of a tomographic reconstruction can be split up in the reconstruction of horizontal slices made from equally numbered lines of the projection image. The projections are re-sorted into sinograms, which contain all equally numbered lines over the whole angular range. The slices are then individually reconstructed, then stacked together to form a 3D volume data set. Visualisation is then performed with the software VgStudio, which has become a quasi-standard at many neutron and X-ray facilities in the world. Fig.14 shows one of the first tomographies performed at ANTARES showing a carburetor, with different attenuation values set to different levels of transparency and colour. Fig.15 shows the segmentation of data into colours according to their attenuation values and geometric coherence.

8 68 Materials in Transition Fig.14: Neutron tomography of a carburetor, with different transparency levels. Fig.15: Segmentation of the data according to attenuation and geometric coherence Stroboscopic Imaging Even at FRM-II, the required exposure time for one neutron radiography image with high counting statistics is in the order of one second. Continuous time-resolved imaging of objects in motion is thus very limited in time resolution and signal dynamics. However, repetitive motions can be recorded with a stroboscopic technique: A trigger-able accumulating detector, the cooled CCD camera with image intensifier described above, is triggered for many identical time windows of the cyclic motion until sufficient fluence is accumulated for one image. The image is read out, the delay for the time window is shifted and the recording repeated until a complete movie of the cyclic motion can be put together. The first experiments were carried out at the beam NEUTROGRAPH at ILL Grenoble, which is the most intense neutron radiography beam in the world, with a flux of 3*10 9 n/cm 2 s and a collimation of L/D=140. Several institutes collaborated on the measurement of an electrically driven four-piston BMW engine [5]. The engine was driven by a 2kW electric motor,

9 Solid State Phenomena Vol mounted on a vertical translation stage. Since water cooling was not possible, the spark plugs were removed to reduce drag and heat production. The detection system was a MCP intensified CCD camera PImax owned by PSI with a Peltier cooled chip (1300 * 1024 pixels) with 16 bit digitization [6]. The full cycle of this four-stroke engine running at 1000 rpm was split into 120 individual frames over 2 rotations, 150 individual images were recorded as an on-chip accumulation of a 200 microseconds exposure each. Fig. 16 shows one frame of the recorded movie, showing valves, pistons, piston rods, piston pins and piston rings. For the first time, the oil cooling of the piston bottoms was visualised. Since the pistons are only connected to the engine body via the piston rings with very low heat dissipation, a continuous oil jet is directed from below at the piston bottoms, lowering the piston temperature by more than 200 C. In the movie, the oil jet of 1-2 mm diameter is clearly visible. Around the upper turning point of the piston, the dome-like spread of the oil at the underside of the piston can be observed. Fig.16: stroboscopic image of a BMW engine driven at 1000 rpm New measurements were recently performed at FRM-II with the described Andor camera. The engine was driven at only 600 rpm, so the oil pump did not reach its nominal pressure. The oil jet was not continuous but produced occasional blobs (Fig.17). The different field of view shows also the oil filled pressure tubes for the oil ejection nozzle as well as a filled backflow tube from the lubrication of the camshaft on top. Fig.17: stroboscopic image of a BMW engine driven at only 600 rpm at FRM-II

10 70 Materials in Transition Phase Contrast Imaging Only a short glimpse on phase contrast imaging can be given here. Phase effects become visible if a very high lateral coherence of the neutron beam can be achieved. This is done by using a very small pinhole in the order of one half to a millimeter diameter in several meters distance from the sample. The detector again must be placed in about two meter distance to see fringe effects caused by dispersion of the neutron waves at edges. Depending on the sign of the refractive index, the neutron waves are bent to or away from edges, enhancing them either with less or more intensity. The second effect will be caused by the combination of identical to no absorption, but pure phase shift. For more details, please refer to [7,8]. Fig. 18 shows a conventional and a phase contrast radiography recorded at NEUTRA at Paul-Scherrer-Institute in about three hours. Fig. 19 shows an phase contrast image of Aluminium foam. Due to the higher source intensity, similar images can now be recorded at FRM-II in about ten minutes. Fig.18: Conventional and phase contrast radiography with edge enhancement of a cast component Fig.18: Phase contrast radiography of Aluminium foam

11 Solid State Phenomena Vol Summary Recent advances in neutron sources and in computer and detector technology have open new fields of applications for neutron imaging. All methods described are available both at Paul-Scherrer- Institute and at FRM-II. Both institutions invite users with new ideas for free beam time for scientific experiments. Acknowledgments Thanks and acknowledgements go to the colleagues and friends who have developed the described technologies to the reported level at PSI (E. Lehmann, G. Frei and P. Vontobel) and FRM-II (J. Brunner, F. Grünauer, N. Kardjilov, M. Schulz). References [1] B. Schillinger, Estimation and Measurement of L/D on a Cold and Thermal Neutron Guide, Nondestructive Testing and Evaluation Vol.16 No.2-6, pp , Gordon & Breach Publishers, 2001 [2] E. Calzada, B. Schillinger, F. Grünauer, Construction and assembly of the neutron radiography and tomography facility ANTARES at FRM-II, Nucl. Inst. & Meth. A 542 (2005) [3] classic [4] istar [5] B. Schillinger et al., Detection systems for short-time stroboscopic neutron imaging and measurements on a rotating engine, Nucl. Inst. Meth. A542 (2005), [6] [7] B.E. Allman, P.J. McMahon et al., Nature 408 (2000) 158 [8] E. Lehmann, K. Lorenz et al., Non-destructive testing with neutron phase contrast imaging, Nucl. Inst. Meth. A542 (2005),95-99

12 Materials in Transition / Modern Neutron Imaging: Radiography, Tomography, Dynamic and Phase Contrast Imaging with Neutrons / DOI References [2] E. Calzada, B. Schillinger, F. Grünauer, Construction and assembly of the neutron radiography nd tomography facility ANTARES at FRM-II, Nucl. Inst. & Meth. A 542 (2005) doi: /j.nima [7] B.E. Allman, P.J. McMahon et al., Nature 408 (2000) 158 doi: /