Performance characteristics of the tomography setup at the PSI NEUTRA thermal neutron radiography facility

Transcription

1 Performance characteristics of the tomography setup at the PSI NEUTRA thermal neutron radiography facility Vontobel, Peter, Paul Scherrer Institute, Villigen, Switzerland Lehmann, Eberhard, Paul Scherrer Institute, Villigen, Switzerland Frei, Gabriel, Paul Scherrer Institute, Villigen, Switzerland Abstract In 1999 a setup for neutron tomography was installed at the thermal neutron radiography facility NEUTRA of the Paul Scherrer Institute's spallation neutron source SINQ. The performance characteristics of this setup have been gradually improved and many tomographic investigations have been performed for industrial and academic clients. Here we report the actual performance characteristics of the NEUTRA tomography setup. We present results of investigations where neutron tomography was successfully used for the nondestructive inspection of industrial objects. The current limitations of neutron tomography are discussed and some prospects for further improvements will be given. The PSI spallation neutron source SINQ The spallation neutron source SINQ [1] is part of the PSI accelerator complex (see Fig. 1 and providing a continuous high intensity - actually 1.25 ma MeV proton beam driving a solid (lead rods in steel cladding) spallation target. Neutrons are slowed down to thermal energies in a D 2 O moderator tank and extracted through flight tubes looking tangentially in the region of highest thermal neutron flux with ca. 1*10 14 [n/cm 2 /s] ( The SINQ Moderator Tank Shielding (cooled) DO 2 Cold Neutron Port (+1535) (Intermediate Energy) Neutron Port (+1400) Thermal Neutron Port (+1585) Target Central Tube HO-Scatterer 2 Insertion Port (+1375) Neutron Guides (+1535) HO 2 Thermal Neutron Port (+1350) D-Suorce 2 Insertion Port (+1535) D2O Reflector D 2 Cold Moderator (Intermediate Energy) Neutron Port (+1350) Mod-Tank.cdr / /GK87 Fig. 1: The PSI accelerator complex. Fig. 2: D 2 O tank with cold source and beam tubes. 37

2 The contribution of thermal neutron radiography and tomography to non-destructive evaluation The technique of neutron radiography is known since more than 50 years [2]. It s worldwide use is tightly linked to the availability of suitable neutron sources i.e. research reactors or more recently spallation neutron sources, mainly equipped with neutron scattering instruments. Because it is much easier and cheaper to operate X-ray sources for radiography, the use of neutrons for imaging purpose was always restricted to those applications, where traditional X-ray investigations failed or neutrons were able to provide improved imaging contrast. These applications are the non-destructive testing (NDT) of bulky material made from e.g. iron, nickel, lead, etc. or quite small objects containing heavy metal (e.g. UO 2 fuel rods from nuclear power plants), the inspection of pyrotechnic devices used in rockets or more generally the distribution of hydrogenous material in metallic structures or rock : e.g. corrosion in aircraft structures, oil distribution in motors, humidity content of rock material, etc.. The difference of thermal neutron attenuation compared to 150 kv X-ray is shown in the following figures. Fig 3: 150 kv X-ray linear attenuation coefficient scales with atomic number Z, from [3]. Fig. 4: Linear thermal (0.025 ev) neutron attenuation coefficient [cm -1 ], from [4]. 38

3 There is a much higher variation in thermal neutron transmission for neighbouring isotopes in the periodic table than for X-ray attenuation, which scales with atomic number Z. X-ray s of different energy however can be provided much easier than neutrons of a given energy, which allows to x-ray a sample of a given geometry/material composition at optimal X-ray source settings. From the point of view of radiation detector technology, X-ray and thermal neutron imaging detectors have a lot in common. Due to the high thermal neutron cross section of Gadolinium a Gadolinium Oxysulfide scintillator screen can be used for neutrons and X-ray, however a 6 Li based screen 6 LiF/ZnS:Cu,Al,Au from as used at NEUTRA has a three times higher sensitivity for neutrons. The PSI thermal neutron radiography facility NEUTRA The NEUTRA evacuated neutron flight tube features a convergent collimator section leading to a D=2 [cm] diameter pinhole inside the spallation source shielding, then the divergent collimator section opens to the 3 measuring positions shown in Fig. 5. This way a parallel beam projection with 3 different collimation ratios L/D (L distance between measuring position and neutron aperture with diameter D) with neutron flux levels decreasing with 1/R 2 is provided. The geometric unsharpness U g = D/L*s [5] and the range of exposure times needed for a transmission measurement at this 3 positions is thus given. Fig. 5: The NEUTRA beamline and measuring hatch with the CCD camera box. Measuring Position Distance from aperture [mm] Beam diameter [mm] Neutron flux [cm -2 s -1 ma -1 ] Collimation ratio L/D Geometric unsharpness [mm] (s=80mm) Table 1: NEUTRA beamline neutronic characteristics 1 Assumed sample detector distance: s 39

4 The tomography setup at NEUTRA Tomography image sequences can be measured at all 3 positions shown in Fig. 5. The basic setup consists of a Peltier cooled slow-scan CCD camera inside a dark box looking across a mirror on to a neutron scintillator screen, a rotary desk fixed on top of a tilting table and an xyz positioning table. Fig. 7: Tomography detector setup at NEUTRA measuring position 3 Dark boxes with size appropriate to sample dimension and the neutron beam diameters at positions 1-3 are used together with different camera lenses. They provide a large range of fields of view and allow tomographic investigations of objects with lateral dimension of 1 27 [cm]. Object heights up to 50 [cm] are practicable, which are measured in a scanning mode. Additional neutron apertures of varying size, put into the neutron collimator at position 1, restrict the neutron beam to its useful area for a given sample dimension, hereby minimizing neutron background and keeping the CCD chip outside the direct neutron beam. 6 Li based neutron scintillator screens with thickness ranging from [mm] are imaged with a 1024 X 1024 pixel CCD camera (DV434) from Andor technology ( and 16bit images are read out at 2µsec/pixel ( ~ 2.2 sec readout time / frame ) with a low dark current < 0.05 [elecs/pix/sec] (cooling temperature C). Exposure times from a few seconds up to 40 seconds are needed for sufficient dynamic range ( ~14bit or higher) in the projection data. Exposure times are imposed by the neutron flux levels (see Table 1), lens properties, scintillator thickness and the material composition of the sample. In order to reduce image noise and total acquisition time the shortest possible exposure time giving sufficient dynamic range is chosen. The following table lists the actual available fields of view (FOV) for three camera objectives, a 50 mm normal and 2 macro objectives. 50mm F mm F mm F2.8 FOV [mm] Pixel size [mm] FOV Pixel size FOV Pixel size Table 2: Fields of view (FOV) and nominal pixel size [mm] for 3 camera objectives. 40

5 Before starting the automatic tomography image acquisition sequence, i.e. rotating the sample in small angular steps from 0 o to 180 o, the setup has to be calibrated carefully. The lens settings are checked by measuring the edge response function of a thin Gadolinium foil in close contact with the aluminium holding plate of the scintillator (see Fig. 8). Then the vertical alignment of the rotary table axis with the vertical symmetry axis of the CCD image is verified by taking projection images at 0 o and 180 o of a special test sample and comparing the horizontally flipped 180 o with the 0 o projection image. Finally a series of at least 3 flatfield images and background images is taken with an area of interest setting large enough to show the whole sample under investigation with some remaining open beam region outside the object s projection (used for exposure normalization). The number of angular sampling intervals is chosen depending on the number of neutron ray s transmitting the sample: e.g. angular increments from 1 o down to 0.3 o for large objects or high spatial resolution of small objects. The automatic sequence of rotating the sample and taking a transmission image is controlled by a LabView interface, which incorporates a neutron beam monitor. A neutron flux threshold together with a time integral threshold for neutron fluence has to be fulfilled in order to accept a projection with sufficient neutron exposure (due to neutron flux fluctuation caused by the proton accelerator). Fig. 8: Gd-edge response measurement with 50mm objective and 0.25 mm thick scintillator. Tomography data processing After neutron transmission image acquisition (total acquisition times ranging from 20 minutes to 4 hours) the 16bit unsigned integer data is put into sinogram [6] form. The following image preprocessing steps are used: white spot speckle noise elimination, exposure normalization, flatfield- and background-correction. The sinogram data is stored as scaled 16bit unsigned integer volume. A filtered backprojection algorithm using a fast C based FFT implementation is called from an IDL [7] package provided by M. Rivers for synchrotron micro-tomography reconstruction [8] and adapted for neutron tomography at PSI. Typical reconstruction times on a 1.7 GHz / 1.5 Gbytes RAM PC range from 0.5 to 2 seconds per slice. We use VG Studio software from Volume-Graphics [9] for the 3D rendering and segmentation of the reconstructed volume data. 41

6 Selected neutron tomography investigations Here we present results from 2 selected industrial neutron tomography investigations. The first object shown in Fig. 9 is a cast iron exhaust pipe from an old tractor: height 42 [cm], outer pipe diameter at lower end 7.5 [cm], nominal pixel size [mm]. The customer asked for a surface model from the pipe. This request could only be partly satisfied, because we were not able to extract the contour in the region of the pipe bend (lower part on the right hand side of Fig. 9). No sufficient image contrast was left in this region of low neutron transmission and high scattering background. The high scattering background, detrimental to image contrast, could be minimized by increasing the sample detector distance, thereby increasing the geometrical unsharpness, or by the use of a linear detector array with a neutron collimator between sample and detector. The reduction of scattering background is a major issue when measuring thick or strongly scattering material. Fig. 9: Radiography (left) and view of 3D volume from a cast iron exhaust pipe. The second example of an industrial neutron tomography investigation is the analysis of small amounts of oil entering the interface between the two material components of a crank shaft casing. The whole casing was cut into pieces in order to fit the available field of view. A vertical slice cut out from the reconstructed volume is shown on the left in Fig. 10. The two material components and the dark oil inclusions are clearly visible. An animation, created by the VG Studio Max version of the 3D rendering software, allows to visualize the 3D distribution of the oil inclusions. Due to the high sensitivity of thermal neutrons for small amount of hydrogenous material, oil inclusions can be located easily. This object of industrial 42

7 neutron tomography investigation is typical. The advantage of high sensitivity for small amount of hydrogenous material within a metallic casing favours neutron imaging of a component in the early stage of development. There are almost no neutron tomographic inspections of series production components, because there is not enough beam time available on short notice at the few existing neutron tomography facilities in Europe. It is expected that this situation will improve, when the research reactor FRM-II, in Munich will start normal operation in Fig. 10: Cut out from Al/Mg crank shaft casing with oil inclusions. Conclusions and outlook The NEUTRA thermal neutron tomography setup allows the investigation of objects with the size of a few up to 27 centimetres. The spatial resolution provided depends on the measuring position needed for the object dimension and the inherent geometric unsharpness given at that position listed in Tab. 1. The use of macro lenses allows the tomographic inspection of small objects (width a few centimetres) at nominal pixel size of tens of µm. Here the limiting factor for spatial resolution is mainly given by the scintillator thickness. Improvements can be expected when using very thin scintillators at a new high intensity cold neutron radiography beamline (CNR) planned at SINQ. Although large size industrial samples are transmitted by neutrons, the available dynamic range may be severely limited by a large scattered neutron fraction reaching the area detector. This leads to blurring of object contours and reduced sensitivity for different materials. Here the use of an additional collimator between sample and detector can reduce the fraction of scattered neutrons hitting the detector. This is usually accomplished by the use of a linear detector array, leading to scanning mode investigations. Neutron tomography is a useful tool for non-destructive testing of industrial objects with material composition and properties suitable for neutron transmission. Due to complementary attenuation characteristics to X-ray, neutron tomography can give additional information about sample composition or location of defects. Neutron radiography or tomography investigations fail for sample compositions mainly containing hydrogenous material, but are highly sensitive to small amounts of hydrogen within another metallic or rock structure. 43

8 References [1] Bauer GS, Operation and development of the new spallation neutron source SINQ at the PSI, Nucl. Instr. Meth. B 139(1-4):65-71, Apr [2] Hawesworth MR, Radiography with neutrons, British Nuclear Energy society, London ISBN , [3] National Institute of Standards, Gaithersburg, Physical Reference Data, [4] Sears VF, Neutron scattering lengths and cross sections, Neutron News, Vol 3, No 3, [5] Domanus JC, Editor, Practical Neutron Radiography, Kluwer, Dordrecht, [6] Kak AC, Slaney M, Principles of computerized tomographic imaging, IEEE press NewYork, 1988, [7] Research Systems Inc, Boulder Co, IDL Data visualization language [8] Rivers M, An IDL based tomography reconstruction package. [9] 3D volume rendering software VGStudio by Volume Graphics, Heidelberg, Germany, 44