The method of computerized tomography by fast neutrons and gamma-rays are used for inspecting and testing of materials by non-destructive technique. The transmission technique is applied using a narrow collimated beam reactor neutrons and gamma - rays. The neutrons and gamma-rays transmitted through the objects under inspection were measured by means of a neutron gamma detector with NE-213 liquid organic scintillator. The undesired pulses of neutrons or gamma-rays are rejected from the transmitted beam by a discrimination technique based on the difference in the decay pact of light pulse produced by recoil electrons or recoil portons.
The transmitted neutrons and gamma-rays for different projections were used to get the image of the section through the investigated object using the method of Filtered Back Projection ( FBP.)
Computerized tomography ( C-T) by neutrons and gamma rays are a very precise and efficient method to study the two and three dimensional distribution of materials in objects under investigation by a non-destructive manner[1-4]. This technique gives a cross-sectional image of the examined object. The image is displayed as if it is possible to cut and view the tested object over an arbitrarily oriented plane, but usually over a plane perpendicular to its long axis. A full three dimensional picture can be obtained by stacking a sequence of such layers. Therefore, this technique is now considered as the most precise way to have three-dimensional information about the internal structure and elemental distribution within the object under investigation. By this technique it became possible to see directly the cracks inclusions and other inhomogeneties in the examined objects.
The CT-technique is now used for non-destructive assay of reactor materials and components. Also steel welds, concrete technology and other industrial components are precisely examined by this technique  . Further, applying this technique by fast and thermal neutrons has also increased its ability to reveal low density media, e.g. water, oil and plastic within dense materials, e.g. steel, lead and uranium. Moreover, among the main advantages of this technique by neutrons is the high penetrability of neutrons and therefore thick objects can be examined.
The inhomogenity of concrete structure was tested b y imaging two cylindrical probes of magnetite -limonite concrete using the CT-scanner shown in fig. 1. This CT- scanner is composed of a translating and rotating tables with 5 phase step motors, detector table with detector collimator, scintillation detector with NE-213 liquid organic scintillator, radiation measuring instruments, main controller and image reconstruction computers.
Fig 1: Schematic diagram of the experimental layout and measuring process|
The fast neutron and gamma-rays fluxes which have possed through the materials were measured by a neutron-gamma spectrometer with NE-213 liquid organic scintillator. The scintillator was coupled to a 12 stage photomultplier tube, Type RAC 8850. The photomultiplier output signal is fed directly to the signal input of the pulse shape discrimination, type link 5010. Discrimination of undesired pulses of neutrons or gamma-rays was achieved by a method based on pulse shape discrimination technique fig. 2. Shows a block diagram of the neutron-gamma measuring system with the recommended dynode chain for the photomultiplier tube.
Fig 2: Dynode chain and block diagram of the spectrometer|
The cylindrical concrete samples investigation have 15 cm diameter and one of them was heated for 48 hours at 400 Co. The concrete samples are imaged by fast neutrons and gamma-rays emitted from one of the horizontal channels
 of the T-R reactor . A steel collimator of 400 mm length and with inner slit of 2X10 mm. Was placed inside the channel and at the beam exit, also a steel collimator of 40 cm. length and with inner slit of 1X5 mm. was placed before the detector. A special care was paid to adjust both beam and detector collimators axis with the beam geometrical axis.
The scanner spatial and density resolution were tested by imaging a steel probe
(50 mm diameter ) with air holes of diameters vary from 1 to 5 mm.l. The probe was imaged by measuring the transmitted neutrons and gamma-rays for 71 translational steps and 180 rotational scans. In case of imaging the concrete samples, the transmitted neutron and gamma-fluxes were measuring for 171 translational steps each of 1mm. and for 90 rotational angles in steps of 2˚. The measuring time for each translational steps was controlled by a fixed number of counts given by a fixed number of counts given by a neutrons monitor placed in one of the vertical channels in the biological shield of the reactor.
RESULTS AND DISCUSSIONS
The CT-image of the tested object was reconstructed from the projection data by the reconstruction program FBP ( Filtered Back Projection ) method which is based on the convolution technique ( 1.5.6). In this program the projections are first filtered and then backprojected. Also the transmitted data are normalized to 1 for the measurements going only through air. The reconstruction program doses this normalization for the first 5 values of each projection.
The CT-image were reconstructed with air image size which is equal to the square of the number of the transmission values in one projection. The CT-matrix was transformed to a Tiff image file with the programme CTIFF and was visulized with the programme Graphic WorkShop GWS.
The evaluated CT-images given by fast neutrons and gamma-rays are shown in fig. 3 and 4. Both images show clearly the air holes with diameters down to 2mm., but, the 1mm. diameter hole is not observed in both images. The CT-image given by fast neutrons shows nearly the same linear attenuation coefficients for the steel probe while the imae given by gamma photons shows some differences in the attenuation coefficients.
The CT-images obtained by fast neutrons and gamma-rays for unheated and heated samples of Magnetite-Limonite concrete are given in figs. 5, 6, 7, 8. All images show clearly the inhomogenity in concrete structure and positions of Limonite coarse aggregated and moritor positions of air cavities are more-clearly obtained in images obtained by fast neutrons than those obtained by gamma-rays. All images for both unheated and heated samples show clearly the differences in attenuation coefficients and density variation of concrete construction. The combined evaluation of these images shows that the variation of the attenuation coefficients can be attributed to differences in density and inhomogentity in concrete structure. The images also indicate that both unheated and heated samples show no remarkable change in the density distribution of water content. This proves that Magnetite-Limonite concrete can keep on its water content without any appreciable loss when exposed to heating at temperatures up to 300°
Fig 3: Gamma CT-Image of steel test sample with air holes
Fig 4: Neutron CT-Image of steel test sample with air holes|
Fig 5: Neutron CT-Image Mag-Lim concrete probe unheated sample.
Fig 6: Gamma CT-Image of Mag-Lim concrete unheated sample
Fig 7: Neutron CT-Image of Mag-Lim Concrete probe heated at 300 C
Fig 8: Gamma CT-Image of Mag-Lim Concrete probe heated at 300 C
The obtained CT-images for investigated concrete samples have proved the feasibility of using the CT-technique by transmission method to study the pattern of the attenuation coefficients for neutrons and gamma-rays in shielding materials. It is a versatile technique which can be used to study inhomogenity in concrete structure due to different components, air cavities and local variation of water content.
The CT-image for unheated and heated Magnetite-Limonite concrete samples indicate that exposing Magnetite-Limonite concrete to temperatures up to 300 degs. C. do not cause any remarkable change in its water content. This ensures the validity of using this concrete as heat resistant concrete for constructing the inner part of the primary shield around the core of nuclear power.
- Gilboy W.B. and Pfisher G., Industrial Application of Computerized Tomography with X and Gamma - Radiation, Research Technique in Non-destructive Testing . Vol.6. R. S. Sharp. Ed, Academic Press. New York (1982).
- Maier p., Pfister G., stier G., Kegreiss W., and Hehn G., progress in Neutron Tomography for Nondestructive Testing proc. 2nd world conf. On Neutron Radiography, Paris, France, June, 16-20 (1986).
- Maier p. , Pfister G., in Neutron Tomography for Nondestructive Testing proc. 2nd world conf. On Neutron Radiography, Paris, France, June, 16-20(1986).
- Pfisher G., Schatz A. K. and Siegel C., Non-destructive Testing of Materials and components by Computerized Tomograpy with Fast and Thermal Neutrons Reactor. Nud. Sci. Eng. 110, pp 303 - 315 (1992) .
- Levaif., Computed Tomographic Methods for Nuclear Fuel Characteriztion and Safeguard, Periodical Polytech. Elect. Eng. Vol. 26, Nos. 1-2, Budapest (1982).
- Herman G. T., Images Reconstruction from projections, the Fundamental of Computerized Tomography. Academic Press, New York (1980).
- Siegel C., Neues Energieselektives Verfahren zur Spektral Korrekur beider com0puter Tomography mit Schnellen Neutron IKE report 6-182, (1993)
- Tug, Image File Format Specification Revision 5.0. Aldus Corporation, 44 First 16011 NE 26th. Way, Box 97017, Redmond, WA 98073-9717 (USA).