A thermal neutron image acquiring system is being developed at the Instituto de Engenharia Nuclear-IEN (CNEN). This system would be capable to perform non-destructive assays after 2D images acquired on a real or quasi real time basis as well as 3D tomographic images reconstructed from the two-dimensional ones. The source-detector set is constituted by the Argonauta reactor - furnishing a thermal neutron flux of 4.46 ×10(5) n · cm-2·s-1 - and by a converter-scintillating screen for this kind of radiation. After a 90º deviation from its original path through a dark chamber provided with a properly positioned mirror, the light emitted by this screen is captured by a CCD-based video-camera coupled to a microcomputer which processes the data and supplies the final digital images. As a previous step aiming at the gathering of parameters and features to characterize the system, a conventional photo camera has been used instead of the video-camera being yet purchased. This work presents the qualitative and quantitative results obtained by using this preliminary system, such as 2D images of several objects and the Modulation Transfer Function-MTF respectively. Besides that, a comparison is made between the results achieved by using this system and those arising from a traditional neutrongraphy employing radiographic films.

Since 1985 the Argonauta Reactor at the Instituto de Engenharia Nuclear - IEN/CNEN has been used as a source of thermal neutrons to acquire transmission-generated radiographic images. From that time until the end of the last century, some Universities and Research Institutions have as well used the irradiation facilities of this reactor for that purpose and for technical and scientific training of personnel. Quite recently, a joint work program with the Laboratório de Instrumentação Nuclear - LIN/COPPE-UFRJ has been initiated addressing the research in the field of radiographic images using thermal neutrons, with special emphasis on Computer Aided Tomography. Within this frame, different tomographic systems - each one with their intrinsic features - have been developed aiming at their use in the industry field which requires the inspection of objects exhibiting different sizes and compositions. This work focuses the developed tomographs presenting their constraints and performances such as Spatial Resolution, Modulation Transfer Function and Density Resolution. The acquired images of some objects and test-bodies are as well presented for the sake of qualitative evaluation. They show a fair agreement with the performance of the tomographic system as expected from the measured quantitative parameters.

The utilization of a position sensitive detector in tomographic systems is an attractive possibility because it is capable of furnishing the position where the ionizing event occurs. This feature can reduce significantly the image acquiring time, since a sample translation is no longer required. In this work the performance of a gaseous position sensitive detector equipping a thermal neutron tomographic system has been improved by a stepwise increase of the filling-gas (³He-enriched helium) pressure from 3 to 6 atm. Important quantitative detector parameters such as resolution, linearity and homogeneity have been measured for that pressure range, and compared with the tomographic images of test-samples. Several test-samples have been studied, all of them constituted by an aluminum cylinder containing inserts of different materials. Besides that, the modulation transfer function-MTF for the system has been experimentally obtained and compared with the expected theoretical curve. An improvement of both detector efficiency and resolution has been observed, as theoretically expected from an increase of the filling-gas pressure.

A computer program to simulate tomographic images generated by transmitted radiation was developed. The algorithm uses a deterministic approach to generate the projections, which supply an existing image reconstruction software. A dispersion associated with the counting statistics is also incorporated into the algorithm, in order to simulate the influence of the detector efficiency and counting time on the final image quality. The detector resolution is also included in the algorithm by assuming a gaussian shape for its line spread function - LSF. The program deals with cylindrical objects containing any desired number of cylindrical rods inside and requires their positions, dimensions and attenuation properties as input data. Images of such objects, acquired with a thermal neutron tomograph equipped with a position sensitive detector, have been compared with those simulated by the developed program in order to evaluate its ability to reproduce those images.