Betsy A. Dowd

High-resolution microtomography for density and spatial information about wood structures

Microtomography has successfully been used to characterize loss of structural integrity of wood. Tomographic images were generated with the newly developed third generation x-ray computed microtomography (XCMT) instrument at the X27A beamline at the national Synchrotron Light source (NSLS). The beamline is equipped with high-flux x-ray monochromator based on multilayer optics developed for this application. The sample is mounted on a translation stage with which to center the sample rotation, a rotation stage to perform the rotation during data collection and a motorized goniometer head for small alignment motions. The absorption image is recorded by a single-crystal scintillator, an optical microscope and a cooled CCD array detector. Data reconstruction has provided three-dimensional geometry of the heterogeneous wood matrix in microtomographic images. Wood is a heterogeneous material composed of long lignocellulose vessels. Although wood is a strong natural product, fungi have evolved chemical systems that weaken the strength properties of wood by degrading structural vessels. Tomographic images with a resolution of three microns were obtained nonintrusively to characterize the compromised structural integrity of wood. Computational tools developed by Lindquist et al (1996) applied to characterize the microstructure of the tomographic volumes.

Microtomography with 3-D visualization

A facility has been developed for producing high quality tomographs of order one micrometer resolution. Three dimensional volumes derived from groups of adjacent tomographic slices are then viewed and navigated in a stereographic viewing facility. This facility is being applied to problems in geological evaluation of oil reservoir rock, medical imaging, protein chemistry, and CADCAM.

Synchrotron x-ray microtomography and solid state NMR of environmental wastes in cement

Synchrotron X-ray microtomography shows vesicular structures for toluene/cement mixtures prepared with 1.22 to 3.58 wt% toluene. Three-dimensional imaging of the cured samples shows spherical vesicles with diameters ranging from 20 to 250 microns; a search with electron microscopy for vesicles in the range of 1 - 20 microns proved negative. However, the total vesicle volume, as computed from the microtomography images, accounts for less than 10% of initial toluene. Evidence for toluene in the cement matrix comes from 29Si MAS NMR spectroscopy, which shows a reduction in chain silicates with added toluene. Also, 2H NMR of d8-toluene/cement samples shows high mobility for all toluene and thus no toluene/cement binding. A model that accounts for all observations follows: For loadings below about 3 wt%, most toluene is dispersed in the cement matrix, with a small fraction of the initial toluene phase separating from the cement paste and forming vesicular structures that are preserved in the cured cement. Furthermore, at loadings above 3 wt%, the abundance of vesicles formed during toluene/cement paste mixing leads to macroscopic phase separation.

Synchrotron tomography of a boreal forest bark beetle

The tomography beamline X27A at the National Synchrotron Light Source at Brookhaven National Laboratory was used to study the destructive spruce bark beetle, Dendroctonus rufipennis (Kirby). The x-ray computed microtomography (CMT) instrument is equipped with filtered white x-ray beam with energy of around 18 keV and, alternatively, a monochromatic beam with energy of around 4 to 14 keV and a 1% band pass. The instrument records microtomographic volumes with 108 to 109 voxels and spatial resolution down to about 3- micron voxels. Three-dimensional image reconstruction provides density and spatial information about solid heterogeneous forms. We have demonstrated that CMT images can be used to nondestructively characterize the internal structure of the beetle - symbiont fungal complex as part of an effort to understand the role of these organisms in the devastation of spruce forests throughout south-central Alaska.