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| Home > Publications database > Entwicklung eines Mikrotomographiesystems als Zusatz für Rasterelektronenmikroskope |
| Book/Report | FZJ-2018-04113 |
1993
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/19293
Report No.: Juel-2718
Abstract: Microtomography as a nondestructive imaging procedure has evolved during the last few years into a valuable tool for various applications in research and developement. Microtomography systems published until now, mainly use synchrotron X-radiation. Although this X-ray source offers several advantages, it turned out, that its use is severely limited. Especially the lack of measurement time at synchrotron beamlines prevented microtomography to become a standard method in nondestructive analysis. Therefore it was the aim of this work to make microtomography more easily accessible for a multitude of applications by the design of a laboratory microtomography system. In order to reach this goal, it was necessary to take all essential parts of the whole imaging process under consideration. According to the two major phases during the process of computertomography, this work deals with problems concerning the physical and electronic detector system as well as with those of reconstructing a threedimensional object function from its cone beam projections. This results in three focal points of this thesis: - the discussion of useful reconstruction algorithms and the presentation of a new reconstruction method for cone beam projections; - the evaluation of conventional X-ray sources in respect to their suitability for the designed microtomography system and the description of how to use a scanning electron microscope as a microfocus X-ray source thus enabling the additional operation mode "microtomography "; - the developement of a new twodimensional X-ray detector on the basis of a charge coupled device, which uses a fibrecoupled scintiltator and does not require a light amplifier. In general the reconstruction of a threedimensional function from its central projections can not be solved with the help of known twodimensional reconstruction methods. Special reconstruction algorithms are needed, if the projections are generated by a cone beam geometry. Basing on the work of Grangeat [Grangeat 1991] a reconstruction method is proposed, which operates in the frequency domain and allows for an abitrary source path. The implementation of this algorithm is on the way. For the reconstruction of the shown example tomographies the method of Feldkamp [Feldkamp 1984] was used, which is based on a circular trajectory of the source. The analysis of the quanta flux of a conventional X-ray source shows, that also with these 'types of sources, microtomograhy measurements can be performed within acceptable time, if the distance between source and detector is short. Two practical proposals for the extension of a scanning electron microscope (SEM) into a microfocus X-ray source show, how a suitable laboratory X-ray source for microtomography can be realized. In this context it is essential, that the normal operation modes of the SEM are retained. That means, that microtomography becomes an additional feature of a SEM system. The use of a SEM as main part of a laboratory microtomography system offers five advantages:- There are no additional costs for a special microfocus X-ray tube. - The SEM allready comprise a precise mechanical system for the manipulation of the specimen.- In addition to the microtomographical evaluation it is possible to image the surface of the specimen by means of the electron microscope's normal operation mode.- The electron beam can be positioned very precisely by electronic means. Thereby it is possible to use different targetmaterials for the generation of different characteristic radiation wavelenghts or to realize complicated source pathes. - The shielding of the X-radiation by the walls of the specimen chamber ensures the radiation protection of the operator. Known X-ray detectors are not well suited for the use in the designed microtomography system. Therefore a special two dimensional detector system has been developed, which uses a charge coupled device. A special feature of this detector is, that photons generated in the scintillator layer by the entering X-ray quanta, are directly guided onto the lightsensitive semiconductor by a radiation hardened fibreoptic faceplate. Due to such an efficientuse of the generated amount of light it is possible to omit additional light amplification. First results of tomographical reconstructions showing objects from the fieldof biology, material science, chemistry and geology, demonstrate the potential of the described laboratory microtomography system.
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