10 research outputs found
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Understanding gas-surface interactions from direct force measurements using a specialized torsion balance
The first comprehensive measurements of the magnitude and direction of the forces exerted on surfaces by molecular beams are discussed and used to obtain information about the microscopic properties of the gas-surface interactions. This unique approach is not based on microscopic measurements of the scattered molecules. The reduced force coefficients are introduced as a new set of parameters that completely describe the macroscopic average momentum transfer to a surface by an incident molecular beam. By using a specialized torsion balance and molecular beams of N{sub 2}, CO, CO{sub 2}, and H{sub 2}, the reduced force coefficients are determined from direct measurements of the force components exerted on surface of a solar panel array material, Kapton, SiO{sub 2}-coated Kapton, and Z-93 as a function of the angle of incidence ranging from 0{degrees} to 85{degrees}. The absolute flux densities of the molecular beams were measured using a different torsion balance with a beam-stop that nullified the force of the scattered molecules. Standard time-of-flight techniques were used to determine the flux-weighted average velocities of the various molecular beams ranging from 1600 m/s to 4600 m/s. The reduced force coefficients can be used to directly obtain macroscopic average properties of the scattered molecules, such as the flux-weighted average velocity and translational energy, that can then be used to determine microscopic details concerning gas-surface interactions without the complications associated with averaging microscopic measurements
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Measurements of momentum transfer coefficients for H{sub 2}, N{sub 2}, CO and CO{sub 2} incident upon spacecraft surfaces
Measurements of momentum transfer coefficients were made for gas-surface interactions between the Space Shuttle reaction control jet plume gases and the solar panel array materials to be used on the International Space Station. Actual conditions were simulated using a supersonic nozzle source to produce beams of the gases with approximately the same average velocities as the gases have in the Shuttle plumes. Samples of the actual solar panel materials were mounted on a torsion balance that was used to measure the force exerted on the surfaces by the molecular beams. Measurements were made with H{sub 2}, N{sub 2}, CO, and CO{sub 2} incident upon the solar array material, Kapton, SiO{sub 2}-coated Kapton, and Z93-coated Al. The measurements showed that molecules scatter from the surfaces more specularly as the angle of incidence increases and that scattering behavior has a strong dependence upon both the incident gas and velocity. These results show that for some technical surfaces the simple assumption of diffuse scattering with complete thermal accommodation is entirely inadequate. It is clear that additional measurements are required to produce models that more accurately describe the gas-surface interactions encountered in rarefied flow regimes
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Performance of CdZnTe detectors passivated with energetic oxygen atoms
Noise caused by surface-leakage current can degrade the performance of CdZnTe spectrometers, particularly devices with closely spaced contacts such as coplanar grid detectors. In order to reduce surface leakage, the authors are treating CdZnTe detector surfaces with energetic, neutral oxygen atoms. Energetic oxygen atoms react with the surface to form a resistive oxide layer. Because the reaction is effective at room temperature, deleterious heating of the substrate is avoided. In most cases, leakage current and noise are shown to decrease significantly after treatment. The effect of the treatment on the performance of coplanar grid detectors is presented
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Synthesis and characterization of a covalently bound self-assembled macrocycle monolayer thin film for nonlinear optical materials
Synthesis and characterization of covalently bound self-assembled, 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine monolayer superlattices on various oxide surfaces such as fused silica, glass or silicon are described. The mono-molecular thin film structures are characterized by UV-visible ({lambda}{sub max} = 447 nm) and FTIR-ATR spectroscopy. In addition, X-ray photoelectron spectroscopy and secondary ion mass spectroscopy studies are used to confirm the formation of a self-assembled monolayer on the fused-silica surface. Attractive features in these complexes are their second-order nonlinear optical properties, enhanced by the completely delocalized {pi}-electrons. Second harmonic generation measurements have been used to determine the uniformity of the film, the relative and absolute magnitudes of nonzero elements of the nonlinear susceptibility, {chi}{sup (2)}, and the average molecular orientation of the chromophores
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High intensity 5 eV cw laser substained O-atom exposure facility for material degradation studies
An atomic oxygen exposure facility has been developed for studies of material degradation. The goal of these studies is to provide design criteria and information for the manufacture of long life (20 to 30 years) construction material for use in low earth orbit. The studies that are being undertaken using the facility will provide (1) absolute reaction cross sections for use in engineering design problems, (2) formulations of reaction mechanisms for use in selection of suitable existing materials and design of new more resistant ones, and (3) calibration of flight hardware (mass spectrometers, etc.) in order to directly relate experiments performed in low earth orbit to ground based investigations. The facility consists of (1) a cw laser sustained discharge source of O-atoms having a variable energy up to 5 eV and an intensity of between 10/sup 15/-10/sup 17/ O-atoms s/sup -1/ cm/sup -2/, (2) an atomic beam formation and diagnostics system consisting of various stages of differential pumping, mass spectrometer detector and time-of-flight analysis, (3) a spinning rotor viscometer for absolute O-atom flux measurements, and (4) provision for using the system for calibration of flight instruments. 15 refs., 10 figs
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Growth and stability of Ag layers on Cu(110)
Combined surface science and high energy ion beam techniques have been used to characterize the composition, structure and thermal stability of Ag layers < 900A thick on Cu(110). A uniform Ag/Cu surface composite is formed for one monolayer coverage. Analysis of its growth with LEED shows that the Ag is initially confined to the (110) troughs and that further deposition results in a Ag(111) layer exhibiting c(2 x 4) symmetry. This surface science determination of Ag coverage is consistent with absolute coverages measured with Rutherford backscattering. Beyond one monolayer coverage, the deposition of Ag at 300K produces clustering (Stranski-Krastanov mechanism) in contrast to a poorly ordered but more uniform layering mode when deposited at 130K. Following nucleation for a deposition between one and two monolayers at 300K, Ag clusters approx.20 A thick grow laterally across the surface up to 5 or 6 monolayers deposition. Continuous films grown cold at thicknesses less than or equal to 115A are found to agglomerate above 500K exposing a tenacious Ag-Cu interface like that formed by one monolayer Ag deposition
Status of the UCNÏ„ experiment
The neutron is the simplest nuclear system that can be used to probe the structure of the weak interaction and search for physics beyond the standard model. Measurements of neutron lifetime and β-decay correlation coefficients with precisions of 0.02% and 0.1%, respectively, would allow for stringent constraints on new physics. The UCNτ experiment uses an asymmetric magneto-gravitational UCN trap with in situ counting of surviving neutrons to measure the neutron lifetime, τn = 877.7s (0.7s)stat (+0.4/−0.2s)sys. We discuss the recent result from UCNτ, the status of ongoing data collection and analysis, and the path toward a 0.25 s measurement of the neutron lifetime with UCNτ
Status of the UCN experiment
International audienceThe neutron is the simplest nuclear system that can be used to probe the structure of the weak interaction and search for physics beyond the standard model. Measurements of neutron lifetime and β-decay correlation coefficients with precisions of 0.02% and 0.1%, respectively, would allow for stringent constraints on new physics. The UCNτ experiment uses an asymmetric magneto-gravitational UCN trap with in situ counting of surviving neutrons to measure the neutron lifetime, τn = 877.7s (0.7s)stat (+0.4/−0.2s)sys. We discuss the recent result from UCNτ, the status of ongoing data collection and analysis, and the path toward a 0.25 s measurement of the neutron lifetime with UCNτ