3d-computed Tomography over Four Orders of Magnitude of X-ray Energies

In the last twenty years the Federal Institute for Materials Research and Testing (BAM) has developed several tomographs from high resolution computed tomography with synchrotron radiation at the Berlin Electron Storage Ring Company for Synchrotron Radiation (BESSY) in the energy range from 8 to 80 keV, over laboratory equipment using different kind of micro focus X-ray tubes (a 100 kV transmission type X-ray tube, a conventional 225 kV and a 320 kV bi-polar micro focus X-ray tube up to high energy sources like Co and an 12 MeV electron linear accelerator. Different kind of detectors are used either flat panel detectors on the basis of amorphous silicon or cooled 16 Bit CCD-cameras coupled to a coated fiber taper, to an X-ray image intensifier or to a scintillating plate by a lens optic. The limits of detectors are shown together with the applied methods to reduce artifacts. The applications cover a broad spectrum analyzing the structure and assembly of rare small insects, characterizing fiber reinforced materials, metal matrix composites up to the dimensional analysis of castings and flaws in welding seams. Introduction: The study of volume properties as well as of dimensional features with computed tomography requires an optimized selection of source-detector combination depending on material composition (energy dependent linear attenuation coefficient μ), size of the samples and maximum thickness of material d which has to be irradiated. Additionally the manipulator system and the mounting needs to suffice the accuracy required. The maximum S/N ratio of a CT measurement of an homogenous sample is given for μ∗d≅2.2 corresponding to a transmission of about 11 %. Due to limitations especially of flat detectors conditions for optimum image quality differ from this theoretical value. This article concerns the presently used CT equipment at BAM and presents some technical and biological applications performed with the apparatus described in detail before. Experimental Setup: At BAM four tomographs developed for own use in the laboratory are available presently and additionally there is a computed tomography facility at the beam line of BAM at the Berlin Electron Storage Ring Company for Synchrotron Radiation (BESSY). Some of the details of all apparatus are described elsewhere therefore only some newer specifications are described. Universal Tomograph Starting with a multi-detector system for a 380 kV X-ray tube and a Co radionuclide source described as early as 1985 [1], the universal scanner was extended for measurements with a 12 MeV electron linear accelerator (LINAC, Raytech 4000) in combination with a multi-detector system with step-motor controlled collimator slits [2]. Since last year an a-Si flat panel detector (Perkin Elmer, 16 Bit ADC, 256∗256 pixel à (0.8 mm)) was used to study some effects of high-energy cone beam CT with the LINAC and Co. The constant intensity output of Co together with the low angular intensity decay can be used to perform measurements with up to five positions of the detector extending the usable object diameter range from 140 mm up to 500 mm. For the LINAC source it is necessary to monitor always intensity variations by measuring the light intensity, that means without an additional external detector only the conventional two-circle measurement can be applied. 320 kV micro focus tomograph attenuation for Fe with 2 mm Cu filter 0,001 0,01 0,1 1 0 5 10 15 20 25 30 thickness [mm] at te nu at io n 100kV