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- Computed tomography (2)
- Limited angle tomography (2)
- Mobile Radiographie (2)
- Neutronentomographie (2)
- Radiographie (2)
- 3D-Rekonstruktion (1)
- Annual rings (1)
- Archäologischer Nassholzfund (1)
- Computer-Simulation (1)
- Delaminations (1)
Abweichend von der gängigen Vorgehensweise, mit einem Bleistiftstrahl
die Lage eines Bildpunktes festzulegen und die rückgestreute Strahlung großflächig aufzunehmen, bietet die neu entwickelte Kamera Unabhängigkeit von Einstrahl- und Blickrichtung. Ausgestattet mit einer 'Optik' für hochenergetische Strahlung, wie die bereits beschriebene Schlitzblendenkamera, ist die variable Gestaltung von Einstrahl- und Blickrichtung möglich. Damit eröffnet sich die
Chance, Strahlung absorbierende Details in einem Objekt sichtbar werden zu lassen, die sonst von einer streuenden Umgebung überstrahlt würden. Zudem wird gezeigt, wie sich das Erscheinungsbild von Objekten umgebungsbedingt im Rückstreubild ändern kann.
Verleimte Holzproben wurden je nach Größe mit thermischen (ANTARES) bzw. schnellen Neutronen (NECTAR) an der Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) der Technischen Universität München in Garching untersucht. Entweder ließen die durchstrahlbare Schichtdicke oder die eingeschränkte Strahlgeometrie es nicht zu, das Probenteil allseitig zu durchstrahlen.
Um dennoch innere Strukturen wie Leimschichten zumindest mit einem eingeschränkten Winkelbereich tomographisch darzustellen, wurde mit Hilfe des Slice-Theorems erfolgreich rekonstruiert. Ein Brett von 2 cm Dicke wurde mit thermischen Neutronen erfolgreich untersucht, ein größeres mit schnellen.
Neutrons are absorbed particularly by hydrogen containing materials so
they can be used as a tool for visualising heterogeneous density distributions of organic materials. Penetration limits are set by the layer thicknesses and the neutron energies applied. In case of specimens with a flat shape the situation may be encountered that the object only could be penetrated in selected directions. In addition, the overall size may exceed the beam diameter and thus the viewing window if a certain region of interest should be studied by tomography without destroying the integrity of the specimen.
This study shows the capabilities and limits of thermal and fast neutrons to investigate flat wooden specimens such as boards and girders by neutron tomography under such circumstances. Taking projections was impaired either by the limits of penetrability or by the total size of the object. As a consequence, projections were included for reconstruction only from a limited angular range of 90°. It could be shown that an approach based on the slice theorem was capable to visualise structural features along the beam directions while simply omitting the perpendicular ones without causing additional artefacts.
Small samples with a thickness of up to 2 cm but several times broader could be studied with the ANTARES facility of the FRM II neutron source in Garching providing thermal neutrons while larger objects required a beam of higher energy as available in the NECTAR facility of the same institution. The fast (fission) neutrons (1.5 – 2 MeV) of this site allowed investigating an area of interest inside a girder with a cross section of 23.5 x 49 cm². Internal features such as inclusions could be detected as well as a heterogeneous density distribution in glue layers.
X-ray computed tomography (CT) is a volumetric (3D) Imaging diagnostic method, well established in the medical field, and in industrial NDE as well. Developments in industrial CT aim to extent the applicability to complex structures, which do not allow the access of all directions. This are e.g. limited view, data and angle CT applications. New reconstruction algorithms are required on one
side, and the accuracy has to be improved on the other side. Numerical Simulation can support such developments by providing well defined data sets for the testing of reconstruction algorithms. This approach of virtual CT is realized within the radiographic simulator aRTist, developed by BAM. The poster shows the possibilities of this tool to consider complex scan paths. Simulated data sets have been reconstructed by an versatile backprojection algorithm.
The effective high neutron scattering absorption coefficient of hydrogen (48.5 cm²/g) due to the scattering allows neutrons to reveal hydrocarbon structures with more contrast than X-rays, but at the same time limits the sample size and thickness that can be investigated. Many planar shaped objects, particularly wood samples, are sufficiently thin to allow thermal neutrons to transmit through the sample in a direction perpendicular to the planar face but not in a parallel direction, due to increased thickness. Often, this is an obstacle that prevents some tomographic reconstruction algorithms from obtaining desired results because of inadequate information or presence of distracting artifacts due to missing projections. This can be true for samples such as the distribution of glue in glulam (boards of wooden layers glued together), or the course of partially visible annual rings in trees where the features of interest are parallel to the planar surface of the sample. However, it should be possible to study these features by rotating the specimen within a limited angular range. In principle, this approach has been shown previously in a study with fast neutrons [2]. A study of this kind was performed at the Antares facility of FRM II in Garching with a 2.6×107/cm² s thermal neutron beam. The limit of penetration was determined for a wooden step wedge carved from a 2 cm×4 cm block of wood in comparison to other materials such as heavy metals and Lucite as specimens rich in hydrogen. The depth of the steps was 1 cm, the height 0.5 cm. The annual ring structures were clearly detectable up to 2 cm thickness. Wooden specimens, i.e. shivers, from a sunken old ship have been subjected to tomography. Not visible from the outside, clear radial structures have been found that are typical for certain kinds of wood. This insight was impaired in a case where the specimen had been soaked with ethylene glycol. In another large sample study, a planar board made of glulam has been studied to show the glued layers. This study shows not only the limits of penetration in wood but also demonstrates access to structures perpendicular to the surface in larger planar objects by tomography with fast neutrons, even with incomplete sets of projection data that covers an angular range of only 90° or even 60°.
Computed THz - tomography
(2012)
Available THz Time Domain Spectroscopy (THz-TDS) systems can be applied in a pulse echo mode for
tomographic application. In contrast to common transmission methods, a TDS reflection geometry was used to
inspect cylindrical polyethylene samples with internal voids of known shape. The geometry of the voids was
simulated and its shape was reconstructed. B-scan pulse echo images were acquired in the time domain, similar
to UT pulse echo techniques on basis of the numeric model. The results were used to verify the simulation model
and the reconstruction algorithm, based on a modified Synthetic Aperture Focusing Technique (SAFT). The
model was used to conclude recommendations on an optimized measurement setup in the reflection mode. Based
on that, the quality of the reconstructed tomograms was improved.
Die radiologische Untersuchung von Holz ist hinsichtlich der Durchführung
und Auswertung komplex. Im Gegensatz zu künstlich hergestellten Materialien zeichnet
sich Holz besonders durch seine Heterogenität und Individualität aus, kein Stück ist wie das
andere. Dies erschwert bei der radiologischen Inspektion die Fehlerdetektion, da diese teilweise
schwer von natürlichen Wachstumsmerkmalen zu unterscheiden sind. Insektenfraßgänge
und Hohlräume durch biologischen Abbau sind deutlich erkennbar, schwieriger
wird es bei feinen Rissen und Anzeichen für Fäulnis. Metallische Einbauten wie Nägel und
Bolzen sind natürlich gut zu erkennen, großflächigere Platten können jedoch den Einblick
in die benachbarten Gebiete versperren. Klebstoffschichten sind im Röntgenbild nur dann
sichtbar, wenn sie exakt parallel zum Strahlengang getroffen werden. Verschiedene
Strukturmerkmale treten also radiologisch recht unterschiedlich in Erscheinung, deshalb
liegt es nahe, zu deren Erkennung mehr als nur eine Durchstrahlungstechnologie
anzuwenden. Während bei immobilen Objekten, d.h. Bauwerken und Bäumen, mobile
Durchstrahlungstechniken mit Röntgen- oder Gammastrahlen (Se-75) in Frage kommen,
stehen im Labor auch (schnelle) Neutronen alternativ zur Verfügung. Die verschiedenen
Techniken, einschließlich der Tomographie mit Neutronen, werden an zwei größeren Prüfteilen
demonstriert, einer Brettschichtholzprobe und einem Knotenpunkt aus einer Holzbrücke
mit eingebauter Stahlplatte. Neutronen lassen Materialunterschiede differenzierter hervortreten
und durchdringen sperrige Metallteile besser, trotz verringerter Bildqualität. An
Verbesserungen wird derzeit gearbeitet.