The ALS Core Environmental Beamlines
The microtomography beamline at the ALS began operating earlier this year, however it is still in the commissioning stage. As such, this line is not yet heavily subscribed, and is available for NABIR, EMSP, and other MES research. Synchrotron-based microtomography yields high spatial resolution (in the micron and submicron range). The ALS beamline 8.3.2 with 3-60 keV photon energy range is well suited to penetrate large geological samples, in the centimeters range. This capability allows imaging of the 3D interior structure and negates the need for difficult and destructive autopsy techniques. Due to the high intensity flux of the x-ray beam, 3D data can be acquired in minutes, which permits the imaging of flow phenomena and dynamically evolving processes in soils and rocks in three dimensions at the micron scale in real time. In contrast with the lower resolution medical CT scanners, in which a human patient is stationary and the X-ray tube/detector assembly revolves around the patient, in synchrotron tomography the sample is rotated while the x-ray beam and the detectors are stationary. To construct the needed data set for the 3D imaging of the sample, a number of projections of the sample are recorded (depending on the resolution desired) as the sample is rotated 180 degrees. The sample can then be moved up or down, and the process repeated until the entire specimen has been imaged.
This line uses a unique combination of the high-brilliance x-rays from the ALS using selectable monochromators (W/B4C multilayer or Si(111)), a high-resolution x-ray detector system; a high-speed data network; a computer system and software for acquisition, processing/reconstruction and display. The ALS tomography beamline currently uses absorption tomography, which readily distinguishes matrix material from pore fluids. The use of doping components enables different pore fluids to be distinguished from one another. The line was also designed for phase contrast imaging, which would be helpful in distinguishing biomass from water, although this capability has not yet been fully developed. Additionally, by varying the photon energy, this line may be useful for identifying different elements in the pore matrix or pore fluids. While the expected resolution of the 8.3.2 beamline is 0.5 microns, already resolution of 2 microns has been achieved with the limited experiments that have been performed on this line during commissioning activities earlier this year. The use of Bragg magnifiers is being considered for achieving resolutions in the 0.1 micron range at this beamline following Stampanoni et al. (2002). For natural materials, the microtomography beamline holds potential for assisting with the following key applications of interest for environmental research:
- Obtaining high resolution, 3D images of natural geologic material pore structure networks, fractures, aggregates, and particles;
- Using density contrasts in geologic materials, such as iron and olivine, to determine how minerals are distributed within a matrix;
- Identifying multiple fluids within porous media in 3D;
- Imaging and investigating dynamic processes, such as water infiltration, pore-clogging due to precipitate or microbial aggregate formation, the influence of pore restrictions on flow phenomena, and how soils retain moisture and nutrients;
- Using 3D pore structure or flow 'images' to estimate permeability and other microstructure properties;
- Use of high resolution, 3D images to test and verify pore-scale simulation of single and multi-phase processes.
We envision that synchrotron microtomography at ALS will be extremely useful for environmental scientists who desire to investigate environmental processes and heterogeneity at the pore scale and under conditions similar to those used during column-scale and field-scale conditions. The figure above (from Silin et al., 2003) illustrates how information from microtomography can be used to obtain pore-scale hydrological properties, investigate pore-scale physics, and test pore-scale simulation models. Software has been developed at LBNL to extract pore information from 3D images and to compute flow parameters for single and multi-phase processes (Silin et al., 2003). The ALS environmental team will encourage cooperation with this group in interpreting the synchrotron generated data and in developing theoretical models for the applications studied.
Additional x-ray (Medical CT scanner modified for earth science application, a portable CT scanner build specifically for core studies), a low field NMR relaxometer, and core processing lab facilities are located within the Earth Science Division of LBNL. The team will also develop a link between the ALS facilities and these imaging facilities, which have a lower resolution but can process larger samples (such as cores). These imaging techniques will serve as a bridge to link pore-scale information about flow and transport phenomena to the mesoscale. The LBNL ALS 8.3.2 mtomography beamline complements U.S. DOE microtomography beamlines at APS and NSLS. Of particular importance is that the ALS microtomography beamline has a high flux, which can yield high spatial resolution at a high image acquisition speed. Another advantage of the ALS beamline is that it is dedicated to microtomography, and thus the experiment set-up time is minimal. Significantly, because this line has only become active this year, it is not yet heavily subscribed, and is currently available for environmental science use.
Dr. Liviu Tomutsa of the Earth Science Division of LBNL will be available to assist NABIR and EMSP users on this beamline.