David W. Camp

Depth profiling of polishing-induced contamination on fused silica surfaces

Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. This is particularly the case for UV optics. In the present study, secondary ion mass spectroscopy (SIMS) was used to measure depth profiles of finishing-process contamination on fused silica surfaces. Contaminating detected include the major polishing compound components, Al present largely because of the use of Al2O3 in the final cleaning process, and other metals incorporated during the polishing step or earlier grinding steps. Depth profile data typically showed an exponential decay of contaminant concentration to a depth of 100-200 nm. This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. Peak contaminant levels are typically in the 10-10 pm range, except for Al which often exceeds 1000 ppm.

Effects of polishing, etching, cleaving, and water leaching on the UV laser damage of fused silica

A damage morphology study was performed with a 355 nm, 8-ns Nd:YAG laser on synthetic UV-grade fused silica to determine the effects of post-polish chemical etching on laser-induced damage, compare damage morphologies of cleaved and polished surfaces, and understand the effects of the hydrolyzed surface layer and water-crack interactions. The samples were polished, then chemically etched in a buffered HF solution to remove 45, 90, 135, and 180 nm of surface material. Another set of sample was cleaved and soaked in boiling distilled water for 1 second and 1 hour. All the samples were irradiated at damaging fluences and characterized by Normarski optical microscopy and scanning electron microscope. Damage was initiated at micro-pits on both input and output surface of the polished fused silica sample. At higher fluences, the micro-pits generated cracks on the surface. Laser damage of the etched fused silica surface shoed that the real density of micro-pits decreased with etched thickness. SIMS analysis of the polished surface showed significant trace contamination levels within a 50 nm surface layer. Micro-pits formation also appeared after irradiating cleaved fused silica surfaces at damaging fluences. Linear damage tracks corresponding cleaving cracks were often observed on cleaved surfaces. Soaking cleaved samples in water produced wide laser damage tracks.

Imaging with a Small Ultra Pure Germanium Gamma-Camera

Semiconductor detector gamma-cameras promise marked improvement in spatial resolution, compared to NaI based system. In addition, solid state systems offer the potential of simultaneous imaging of multiple isotopes, or polychromatic nuclides. Beciuse semiconductor systems offer improved resolution, and expand the spectrum of radionuclides applicable to diagnostic imaging, their impact on nuclear medicine will be significant.

Application of total internal reflection microscopy for laser damage studies on fused silica

Damage studies show that the majority of damage on UV grade fused silica initiates at the front or rear surface. The grinding and polishing processes used to produce the optical surfaces of transparent optics play a key role in the development of defects which can ultimately initiate damage. These defects can be on or breaking through the surface or can be sub-surface and surface defects in transparent materials. Images taken which compare both total internal reflection microscopy and atomic force microscopy show that the observed defects can be less than one micron in size. Total internal reflection microscopy has the added benefit of being able to observe large areas with sub-micron detection. Both off-line and in- situ systems have been applied in the Lawrence Livermore National Laboratorys damage laboratory in order to understand defects in the surface and subsurface of polished fused silica. There is a preliminary indication that TIRM quality can be related to the damage resistance. The in-situ microscope is coupled into a 355 nm, 7.5 ns, 10 Hz Nd:YAG laser system in order to study damage occurring at localized scatter sites revealed with the total internal reflection microscopy method. The tests indicate damage initiating at observed artifacts which have many different morphologies and damage behaviors. Some of the scatter sites and damage morphologies revealed have been related back to the finishing process.

Large High Purity Germanium Well Detector for Biomedical Application

Cardiovascular research uses the microsphere technique as a tool to measure regional blood flow. Although the technique is extremely powerful, the instruments used to assay the microspheres limit the number of different tracers that can be used in one animal to 5 or 6. As is often the case, many more measurements per animal are needed, and the limitation necessitates using many more animals per experiment. This in turn increases costs and decreases the quality of the data, since no animal is its own control for the full set of experiments. A large germanium well counter designed for microsphere work has been evaluated. The detector is 9.8cm long, 5.7cm in diameter, with a 2.1cm well bore. The useable well has a 1.5cm diameter and reaches to within 2cm from the far end of the detector. Energy resolution ranges from 3 keV FWHM at 100 keV to 5 keV FWHM at 500 keV. Detector performance matches well that predicted from Monte Carlo calculations. The counter will increase the research utility of the microsphere technique in a cost-effective manner.

A Thermoplasticity Model for Oil Shale

Several regions of the world have abundant oil shale resources, but accessing this energy supply poses a number of challenges. One particular difficulty is the thermomechanical behavior of the material. When heated to sufficient temperatures, thermal conversion of kerogen to oil, gas, and other products takes place. This alteration of microstructure leads to a complex geomechanical response. In this work, we develop a thermoplasticity model for oil shale. The model is based on critical state plasticity, a framework often used for modeling clays and soft rocks. The model described here allows for both hardening due to mechanical deformation and softening due to thermal processes. In particular, the preconsolidation pressure—defining the onset of plastic volumetric compaction—is controlled by a state variable representing the kerogen content of the material. As kerogen is converted to other phases, the material weakens and plastic compaction begins. We calibrate and compare the proposed model to a suite of high-temperature uniaxial and triaxial experiments on core samples from a pilot in situ processing operation in the Green River Formation. We also describe avenues for future work to improve understanding and prediction of the geomechanical behavior of oil shale operations.

Results of mathematical modeling of an oil shale retort having a fluidized-bed pyrolyzer and a lift-pipe combustor

Abstract A commercial-size, hot-solids recycle retorting process, consisting of a staged fluidized-bed pyrolyzer and a lift-pipe combustor, was simulated using a previously developed mathematical model. The focus of this work at Lawrence Livermore National Laboratory (LLNL) is to show the effects of varying key process parameters. A design study shows the effects of certain design variables (such as pyrolyzer temperature, combustor pressure, etc.) on items that would affect the capital or operating costs of the plant (such as reactor sizes, utility requirements, yield, etc.). An operation study, in which the reactor dimensions were fixed, shows how the plant would respond to changes imposed on it. We evaluated the plants sensitivity to some unavoidable changes and to the effectiveness of several strategies for controlling process conditions. Tabulated and graphical results from 22 individual parameter studies are presented. Conclusions drawn from these results include the following. Costs can be shifted from the combustor side of the process to the pyrolyzer side by decreasing pyrolysis temperature and/or increasing recycle ratio. Higher combustor pressure or oxygen/air enrichment would reduce the number and height of lift-pipes at the expense of large blower capacity or oxygen costs. Very large changes in shale grade or rate would not significantly upset plant operation. Plant operation would be extremely sensitive to the fraction of fines in the raw shale unless the fines are pyrolyzed completely before being fed to the combustor. Ranked from best to worst, the control options considered are: manipulation of the solids recycle ratio, oxygen/air fraction, lift-pipe pressure, raw-shale preheat temperature, and extent of fines pyrolysis.

Large germanium well counter for blood flow measurements by the microsphere technique

Abstract Cardiovascular research uses the microsphere technique as a tool to measure regional blood flow. Although the technique is extremely powerful, the instruments used to assay the microspheres limit the number of different tracers that can be used in one animal to 5 or 6. As is often the case, many more measurements per animal are needed, and the limitation necessitates using many more animals per experiment. This in turn increases costs and decreases the quality of the data, since no animal is its own control for the full set of experiments. A large germanium well counter designed for microsphere work has been evaluated. Detector performance matches well that predicted from Monte Carlo calculations. The counter will increase the research utility of the microsphere technique in a cost-effective manner.