Mark L. Rivers

In situ Chemical Speciation of Uranium in Soils and Sediments by Micro X-ray Absorption Spectroscopy.

There has been substantial interest recently in chemical speciation and species transformations of uranium in contaminated soils, sediments, and nuclear wastes, both from the standpoint of predicting its mobility to and within subsurface environments and for developing effective strategies for remediating contaminated sites. We exploited the microanalytical capabilities of beam line X-26A at Brookhaven National Laboratory to collect XANES and SXRF spectra on localized (50-300-[mu]m) regions within a number of U-contaminated soils and sediments. This provided specific information on U oxidation states, qualitative information on U-bonding environments, and information on associated elemental distributions. 26 refs., 4 figs.

Evidence for Extreme Partitioning of Copper into a Magmatic Vapor Phase

The discovery of copper sulfides in carbon dioxide- and chlorine-bearing bubbles in phenocryst-hosted melt inclusions shows that copper resides in a vapor phase in some shallow magma chambers. Copper is several hundred times more concentrated in magmatic vapor than in coexisting pantellerite melt. The volatile behavior of copper should be considered when modeling the volcanogenic contribution of metals to the atmosphere and may be important in the formation of copper porphyry ore deposits.

THE SCANNING TRANSMISSION MICROSCOPE AT THE NSLS

Abstract The scanning transmission soft X-ray microscope (STXM) that has been under development at the National Synchrotron Light Source [H. Rarback et al., Rev. Sci. Instr. 59 (1988) 52] has been substantially ungraded for operation with the X1 undulator [C. Buckley et al., Rev. Sci. Instr. 60 (1989) 2444]. The principal new features are: optical prefocusing, using a visible light interferometer; a dedicated VAXstation 3200 with a more user friendly and flexible software system for image acquisition and analysis; a flow cell that makes it possible not only to keep the specimen wet during exposure, but to change the fluid around the specimen as well; and a more compact proportional counter that is capable of counting rates of several MHz. In conjunction with new zone plates of better resolution and higher efficiency [E.H. Anderson, SPIE 1160 (1989) 2], the microscope is ready for a period of extended use in biological imaging.

Monochromatic computed tomography of the human brain using synchrotron x rays: Technical feasibility

Abstract A monochromatic computed tomography (CT) scanner is being developed at the X17 superconducting wiggler beamline at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, to image the human head and neck. The system configuration is one of a horizontal fan beam and an upright seated rotating subject. The purposes of the project are to demonstrate improvement in the image contrast and in the image quantitative accuracy that can be obtained in monochromatic CT and to apply the system to specific clinical research programs in neuroradiology. This paper describes the first phantom studies carried out with a prototype system, using the dual photon absorptiometry (DPA) method at energies of 20 and 38 keV. The results show that improvements in image contrast and quantitative accuracy are possible with monochromatic DPA CT. Estimates of the clinical performance of the planned CT system are made on the basis of these initial results.

Soft X-ray Microscopy in Biology and Medicine: Status and Prospects

There are two central motivations for developing new scientific methods. One is, of couse, to accomplish what established methods cannot. A second is for comparison: To verify the conclusions of established methods. That is, are results obtained by one method congruent with those obtained by another independent means of measurement? In regard to microscopic imaging in biology, this means that we seek to ground our view of microscopic structure on more than a single methodological standard, with whatever particular uncertainties that standard presents. These are the motivations that underlie the current impetus for the development of x-ray microimaging methods. Our knowledge of the internal structures of biological cells has been shaped in great part by 40 years of study applying and developing the methods of electron microscopy. This has led to the evolution of a model of the cell that contains defined structures with established details and known spatial relationships. Belief in the fidelity of this model to the natural cell rests in great part on the understanding that the preparative procedures commonly used in electron microscopy, procedures that greatly modify the natural object, do not alter or distort intracellular structure as to form, location or high resolution detail. Even though the cell as seen in the electron microscope most certainly resembles the natural object, important questions of the faithfulness of the image often remain.

X-ray Microscopy with the NSLS Soft X-ray Undulator

Soft x-rays are attractive for microscopy because they tend to be less damaging to specimens than charged probes. In addition their interactions with specimens can result in element-specific information and the penetration depth is adjustable by choice of the beam energy. Resolution on the order of 50 nm has been demonstrated, and further improvements are anticipated. The experimental program at the NSLS X1A beamline is dedicated to soft x-ray microscopy. We are developing two types of instruments, both dependent on the remarkable brightness of the undulator source. One of these uses a Fresnel zone plate to focus the beam to a small size. This microprobe is used either to study biological specimens, or, in a different apparatus, to study surfaces by photoelectron microscopy. The other type of instrument makes use of x-rays diffracted by the specimen. In this category we are involved in the development of Gabor holography, Fourier transform holography, and in imaging by soft x-ray diffraction.

CT imaging of small animals using monochromatized synchrotron X-rays

Rats and chicken embryos were imaged in vivo with a prototype multiple energy computed tomography (MECT) system using monochromatized X-rays from the X17 superconducting wiggler at the National Synchrotron Light Source (NSLS). The CT configuration consisted of a horizontal, low-divergence, fan-shaped beam, 70-mm wide and 0.5-mm high, and a subject rotating about a vertical axis. A linear-array high-purity Ge detector with 140 elements, each 0.5-mm wide and 6-mm thick, was used with a data acquisition system that provides a linear response over almost six orders of magnitude of detector current. The dual photon absorptiometry (DPA) algorithm was applied to images of the rat head acquired at 20 and 45 keV to obtain two new images, one representing the low-Z and the other the intermediate-Z element group. The results indicate that the contrast resolution and the quantification accuracy of the images improve stepwise; first, with the monochromatic beam and, second, with the DPA method. The system is a prototype for a clinical brain scanner.<<ETX>>

Multiple energy computed tomography with monochromatic X-rays from the NSLS

Monochromatic X-rays from the X17 superconducting wiggler beamline at the National Synchrotron Light Source (NSLS) were used for dual-energy quantitative computed tomography (CT) of a 27-ram-diameter phantom containing solutions of different KOH concentrations in cylindrical holes of 5-mm diameter. The CT configuration was a fixed horizontal fan-shaped beam of 1.5-mm height and 30-mm width, and a subject rotating around a vertical axis. The transmitted X-rays were detected by a linear-array Si(Li) detector with 120 elements of 0.25-mm width each. A two-crystal Bragg-Bragg fixed-exit monochromator with Si<220> crystals was used. Dual photon absorptiometry CT data were taken at 20 and 38 keV. The reconstructed phantom images show the potential of the system for quantitative CT.<<ETX>>