Physical conditions for UO formation in laser-produced uranium plumes.

We investigate the oxidation of uranium (U) species, the physical conditions leading to uranium monoxide (UO) formation and the interplay between plume hydrodynamics and plasma chemistry in a laser-produced U plasma. Plasmas are produced by ablation of metallic U using nanosecond laser pulses. An ambient gas environment with varying oxygen partial pressures in 100 Torr inert Ar gas is used for controlling the plasma oxidation chemistry. Optical emission spectroscopic analysis of U atomic and monoxide species shows a reduction in the emission intensity and persistence with increasing oxygen partial pressure. Spectral modelling is used for identifying the physical conditions in the plasma that favor UO formation. The optimal temperature for UO formation is found to be in the temperature range of ∼1500-5000 K. The spectrally integrated and spectrally filtered (monochromatic) imaging of U atomic and molecular species reveals the evolutionary paths of various species in the plasma. Our results also highlight that oxidation in U plasmas predominantly occurs at the cooler periphery and is delayed with respect to plasma formation, and the dissipation of molecular species strongly depends on oxygen partial pressure.

Background characterization of an ultra-low background liquid scintillation counter.

The Ultra-Low Background Liquid Scintillation Counter developed by Pacific Northwest National Laboratory will expand the application of liquid scintillation counting by enabling lower detection limits and smaller sample volumes. By reducing the overall count rate of the background environment approximately 2 orders of magnitude below that of commercially available systems, backgrounds on the order of tens of counts per day over an energy range of ~3-3600keV can be realized. Initial test results of the ULB LSC show promising results for ultra-low background detection with liquid scintillation counting.

Development of a low background liquid scintillation counter for a shallow underground laboratory.

Pacific Northwest National Laboratory has recently opened a shallow underground laboratory intended for measurement of low-concentration levels of radioactive isotopes in samples collected from the environment. The development of a low-background liquid scintillation counter is currently underway to further augment the measurement capabilities within this underground laboratory. Liquid scintillation counting is especially useful for measuring charged particle (e.g., ß and α) emitting isotopes with no (or very weak) gamma-ray yields. The combination of high-efficiency detection of charged particle emission in a liquid scintillation cocktail coupled with the low-background environment of an appropriately designed shield located in a clean underground laboratory provides the opportunity for increased-sensitivity measurements of a range of isotopes. To take advantage of the 35m-water-equivalent overburden of the underground laboratory, a series of simulations have evaluated the scintillation counter's shield design requirements to assess the possible background rate achievable. This report presents the design and background evaluation for a shallow underground, low background liquid scintillation counter design for sample measurements.

Causes of focus-error feedthrough in optical-disk systems: astigmatic and obscuration methods.

High-density magneto-optical recording systems require sensitive and robust focus position sensors that are immune to transient changes in the amplitude and the phase of the light diffracted from pregrooved media during the seek operation. The false focus-error signal produced by track crossing during seeking is termed feedthrough. Total immunity to feedthrough is never achieved, although some focus-error detection methods, notably the obscuration method, perform better in this regard. The astigmatic focus-error detection method is usually operated with a large astigmatic foci separation distance to facilitate detector alignment and to permit push-pull tracking, which increases pattern noise and contributes to its poor resistance to feedthrough. Pattern noise is caused by the projection of the intensity pattern at the exit pupil of the objective lens onto the detector plane, at which it produces false focus-error signals. The obscuration method, a diffraction-limited method of focus-error sensing, evens out his pattern noise and is therefore more resistant to feedthrough. We present numerical modeling results that compare the feedthrough performance of the astigmatic and the obscuration methods of focus-error detection.

Characterization of a novel focusing/tracking technique with increased feedthrough immunity for optical-disk applications: the double-astigmatic method.

The astigmatic-focusing/push-pull tracking-error detection method is an elegant and sensitive optical servo technique. Unfortunately the formation of error signals far from either line focus of the astigmat (for relaxing alignment tolerances and broadening the servo's acquisition range) gives rise to undesired diffraction effects in the focus servo channel owing to track crossings of the pregrooved disk by the optical stylus, especially if certain aberrations are present. These undesired effects are given several names: pattern noise, optical servo cross talk, and feedthrough. By combining two astigmatic lenses and their associated detectors, one can configure a differential variant of the astigmatic technique. This double-astigmatic method greatly reduces pattern noise caused by the presence of spurious astigmatism oriented with its line foci at ±45° to the disk tracks. In this paper we present numerical modeling and experimental data that demonstrate the effectiveness of this focusing/tracking technique in feedthrough suppression.

Investigation of substrate birefringence effects on optical-disk performance.

Substrates for magneto-optical disks are produced economically and efficiently by injection molding of polycarbonate plastics. Unfortunately, most plastics are birefringent, with different refractive indices in the plane of the disk (lateral birefringence) and perpendicular to the plane of the disk (vertical birefringence). One manifestation of media birefringence is the existence of a focus offset between the two distinct best-focus positions for data detection and tracking. This focus offset degrades overall system performance as the compromise focus position between the two best-focus points reduces the operating margin of the individual data and tracking channels. We present detailed numerical modeling results on the role of substrate birefringence in causing this focus offset.

Alignment-insensitive technique for wideband tuning of an unmodified semiconductor laser.

Using simple optical components and an unmodified commercial semiconductor laser, a frequency-selective self-aligning optical-feedback technique has been devised that allows a semiconductor laser to be tuned to and scanned about any optical frequency within the laser gain curve. This technique employs a graded-index rod lens cat's eye and an intracavity étalon.