Near-surface Density Research Articles

X-ray reflectivity investigation of near-surface density changes induced in Al–Au multilayers by high-current ion beam bombardment

Alternating Al–Au multilayers (typical thickness of each layer 150 nm) were deposited on polished glassy carbon substrates by evaporation under high-vacuum conditions at 278 K and subsequently interdiffused with high-current 2.0 MeV 4He + ions. After ion beam bombardment, non-destructive X-ray reflectometry measurements reveal a significant decrease of the density in the near-surface region from 19 to 12 g/cm 3. This change in density is caused by the Al–Au interdiffusion during ion beam bombardment, as measured with RBS and X-ray diffraction. Based on the advantage of X-ray reflectometry of no specific sample preparation, detailed integral information of the surface roughness is achieved, additionally. For example, the surface roughness of the interdiffused Al–Au layers increases from 3.1 to 4.1 nm accompanied by the appearance of a gradient layer at the surface that even increases in thickness after irradiation. In addition, the density of this gradient layer decreased from the as-deposited to the irradiated state.

Molecular-beam epitaxial growth of HgCdTe infrared focal-plane arrays on silicon substrates for midwave infrared applications

Molecular beam epitaxy has been employed to deposit HgCdTe infrared detector structures on Si(112) substrates with performance at 125K that is equivalent to detectors grown on conventional CdZnTe substrates. The detector structures are grown on Si via CdTe(112)B buffer layers, whose structural properties include x-ray rocking curve full width at half maximum of 63 arc-sec and near-surface etch pit density of 3–5 × 105 cm−2 for 9 µm thick CdTe films. HgCdTe p+-on-n device structures were grown by molecular beam epitaxy (MBE) on both bulk CdZnTe and Si with 125K cutoff wavelengths ranging from 3.5 to 5 µm. External quantum efficiencies of 70%, limited only by reflection loss at the uncoated Si-vacuum interface, were achieved for detectors on Si. The current-voltage (I-V) characteristics of MBE-grown detectors on CdZnTe and Si were found to be equivalent, with reverse breakdown voltages well in excess of 700 mV. The temperature dependences of the I-V characteristics of MBE-grown diodes on CdZnTe and Si were found to be essentially identical and in agreement with a diffusion-limited current model for temperatures down to 110K. The performance of MBE-grown diodes on Si is also equivalent to that of typical liquid phase epitaxy-grown devices on CdZnTe with R0A products in the 106–107 Θ-cm2 range for 3.6 µm cutoff at 125K and R0A products in the 104–105 Θ-cm2 range for 4.7 µm cutoff at 125K.

Champollion

At the time of the 1996 COSPAR meeting, the US NASA and the French CNES were planning to develop a comet lander as part of the International Rosetta Mission. This lander, named Champollion after the translator of the Rosetta stone, was to be released from the orbiter and landed on Comet P/Wirtanen to perform in situ analyses of the cometary nucleus. The investigations on Champollion focused on objectives that could not be performed from the Rosetta orbiter. Unfortunately, in September 1996, NASA withdrew its support for Champollion, forcing its cancellation. The description provided in this paper is therefore principally of historical interest, but might prove useful to the development of the remaining Rosetta Lander. The instruments on Champollion would have determined the elemental, molecular, mineralogical, and isotopic compositions of material down to depths approaching 1 m below the surface of the nucleus. The physical structure of the nucleus would have been determined through measurements of the near-surface strength, density, texture, porosity, ice phases, and thermal properties. Images were to have been obtained at resolutions ranging from 1 m to 5 μm. Some key features of the lander were its actively controlled descent from the orbiter to the comet nucleus, its 2.5-m spike which served as an anchor, and the high degree of integration of the instruments with the lander structure and subsystems. The lander was powered by primary batteries. The nominal lifetime of the lander on the surface of the comet was 84 hours, during which measurement sequences could be repeated if necessary and subsurface samples could be obtained from at least three different depths.

High quality large-area CdTe(211)B on Si(211) grown by molecular beam epitaxy

We describe the growth of high quality CdTe(211)B layers by molecular beam epitaxy on nominal Si(211) substrates. Prior to CdTe deposition, thin ZnTe(211)B buffer layers were grown to preserve the homo-orientation. Large-area CdTe(211)B layers were routinely obtained by optimizing the growth parameters. From x-ray diffraction, we observed the presence of twin-free CdTe(211)B layers. One 8 μm thick CdTe epilayer had a near-surface etch pit density of 1.5×105 cm−2, which surpassed the best value reported for CdTe(211)B grown on GaAs(211)B, GaAs/Si(211), or Si(211) substrates.

Intercontinental bistatic radar observations of 6489 Golevka (1991 JX)

Unlike the major planets and main belt asteroids, some near-Earth objects (NEOs) make their closest approaches to Earth at high northern declinations and therefore are visible simultaneously from North America, Europe and/or Asia, providing occasional opportunities for intercontinental radar experiments. The first celestial target of intercontinental radar was the Earth-crossing asteroid 6489 Golevka (1991 JX), which in June 1995 passed 0.034AU from Earth at a declination of ∼40deg. High power (∼0.5MW), continuous-wave signals at 3.5cm wavelength were transmitted toward the asteroid from the 70m antenna at the JPL/NASA Goldstone Deep Space Communication Complex (DSCC) in California on 13, 14 and 15 June 1995 during several hours on each date. This illumination of the asteroid created an artificial radio source for astronomers anywhere on the asteroid-facing side of Earth. Five astronomical groups tried to detect the radar echoes and two succeeded. Detections were obtained on each of the three days by the 70m antenna at Evpatoria DSCC in Crimea and on June 15 by the 34m antenna at Kashima Space Research Center in Japan. The Goldstone 34m antenna monitored echoes throughout all the observations. From the results of the Goldstone-Evpatoria experiment it can be inferred that the asteroid is about 0.5km across, is not very elongated, possesses considerable surface irregularity and is very reflective presumably due to a large near-surface bulk density. This first intercontinental radar astronomy experiment can be considered as an initial step toward a global radar network for routine NEO investigations.

Heteroepitaxy of HgCdTe(112) infrared detector structures on Si(112) substrates by molecular-beam epitaxy

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 106 cm−2 for 8 µm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 106 cm−2. These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R0A = 1.64 × 104 Ωm2 (0 FOV) for devices with 7.8 µm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.

Radar Observations of Asteroid 1620 Geographos

Goldstone radar observations of Geographos from August 28 through September 2, 1994 yield over 400 delay-Doppler images whose linear spatial resolutions range from ∼75 to ∼151 m, and 138 pairs of dual-polarization (OC, SC) spectra with one-dimensional resolution of 103 m. Each data type provides thorough rotational coverage. The images contain an intrinsic north/south ambiguity, but the equatorial view allows accurate determination of the shape of the radar-facing part of the asteroid's pole-on silhouette at any rotation phase. Sums of co-registered images that cover nearly a full rotation have defined the extremely elongated shape of that silhouette (S. J. Ostro, K. D. Rosema, R. S. Hudson, R. F. Jurgens, J. D. Giorgini, R. Winkler, D. K. Yeomans, D. Choate, R. Rose, M. A. Slade, S. D. Howard, and D. L. Mitchell 1995,Nature375, 474–477). Here we present individual images and co-registered sums over ∼30° of rotation phase that show the silhouette's structural characteristics in finer detail and also reveal numerous contrast features “inside” the silhouette. Those features include several candidate craters as well as indications of other sorts of large-scale topographic relief, including a prominent central indentation. Protuberances at the asteroid's ends may be related to the pattern of ejecta removal and deposition caused by the asteroid's gravity field. The asteroid's surface is homogeneous and displays only modest roughness at centimeter-to-meter scales. Our estimates of radar cross section and the currently available constraints on the asteroid's dimensions are consistent with a near-surface bulk density between 2 and 3 g cm−3. The delay-Doppler trajectory of Geographos' center of mass has been determined to about 200 m on August 28 and to about 100 m on August 31, an improvement of two orders of magnitude over pre-observation ephemerides.

Self-Consistent Field Analysis of Grafted Star Polymers

A self-consistent mean-field treatment of star polymers grafted to an impenetrable surface is developed and used to calculate the near-surface equilibrium segment density profiles. The effects of a...

Modelling of near-surface seismic structure beneath Hawaii using reverberations

SUMMARY The reverberation in near-surface layers contains valuable information about the near-surface velocity and density structure. However, previously the reverberation was often regarded as ‘noise’,to be eliminated. This paper presents an approach for modelling the shallow velocity structure with the reverberation in the near-surface layers. The method was applied to three-component waveform data recorded by seismic arrays during a passive seismic experiment, Arrays for Lithosphere Observations in HAwaii (ALOHA). The near-surface multiple reflections were enhanced by stacking the seismograms of local upper mantle earthquakes with near-vertical incidence angles. By matching synthetics and observed P and S multiple reflections, we have successfully modelled the impedance structure for the top 2-3km of the crust beneath seven stations of ALOHA arrays in southern Hawaii. Reverberative site effects are similar for stations in an array, but vary significantly from one array to another, reflecting the lateral heterogeneity of the near-surface structure. Two short profiles of near-surface P-wave velocity structure were constructed across the Ainapo and South Flank areas in southern Hawaii, based on interpretation of the multiple reflections recorded by two of the ALOHA arrays. The profiles reveal the detailed and variable shallow structure near two active volcanoes, Kilauea and Mauna Loa, and provide basic information about site effects for further investigations of seismic hazard assessment, earthquake source properties, and deeper structure.

Temperature and heat-flow density along European transcontinental profiles

The project “Geothermal Atlas of Europe” for the first time gives a compilation of geothermal data for the whole of Europe. Along two transcontinental profiles temperatures at different depths and heat-flow density are studied. The geothermal field is characterized by large-scale and small-scale anomalies which are connected with the main tectonics of Europe and possibly also with convective heat transport. The relationship between the near-surface heat-flow density and the depth of the crust-mantle boundary is different for the various tectonic units. Extrapolation of temperature for greater depths should start from the deepest possible level for which the temperature distribution is known. As the temperature at a depth of 3 km is quite well established for the most of Europe, these data can be used as a basis for estimating the temperature-depth distribution within the Earth's crust.

Variations of near-surface firn density in the lower accumulation area of the Greenland ice sheet, Pâkitsoq, West Greenland

AbstractFirn-density variations have been studied in the lower accumulation area of the Greenland ice sheet (1440-1620 m a.s.l.) near Pâkitsoq, West Greenland. The main control on density in the near-surface firn layer (of 5-10 m thickness) is the formation of ice layers by the refreezing of meltwater that reaches depths of 2-4 m below the surface. The density variations are described by the ratio of annual surface meltMto the annual accumulationC. The ratioM/Cis about 0.6 at the run-off limit (at about 1400 m a.s.l. in the study area) where refreezing of meltwater transforms snow into impermeable ice. The mean density of near-surface firn decreases with elevation, reflecting a decrease in melt with elevation. There is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface, which reflects lower melt rates and/or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer. The formation of such low-density firn may have partially contributed to the 1978-85 thickening of the ice sheet observed by satellite-radar altimetry. Near-surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring.

Variations of near-surface firn density in the lower accumulation area of the Greenland ice sheet, Pâkitsoq, West Greenland

AbstractFirn-density variations have been studied in the lower accumulation area of the Greenland ice sheet (1440-1620 m a.s.l.) near Pâkitsoq, West Greenland. The main control on density in the near-surface firn layer (of 5-10 m thickness) is the formation of ice layers by the refreezing of meltwater that reaches depths of 2-4 m below the surface. The density variations are described by the ratio of annual surface melt M to the annual accumulation C. The ratio M/C is about 0.6 at the run-off limit (at about 1400 m a.s.l. in the study area) where refreezing of meltwater transforms snow into impermeable ice. The mean density of near-surface firn decreases with elevation, reflecting a decrease in melt with elevation. There is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface, which reflects lower melt rates and/or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer. The formation of such low-density firn may have partially contributed to the 1978-85 thickening of the ice sheet observed by satellite-radar altimetry. Near-surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring.

Planetary radar astronomy

Radar is a powerful technique that has furnished otherwise unavailable information about solar system bodies for three decades. The advantages of radar in planetary astronomy result from (1) the observer's control of all the attributes of the coherent signal used to illuminate the target, especially the wave form's time/frequency modulation and polarization; (2) the ability of radar to resolve objects spatially via measurements of the distribution of echo power in time delay and Doppler frequency; (3) the pronounced degree to which delay-Doppler measurements constrain orbits and spin vectors; and (4) centimeter-to-meter wavelengths, which easily penetrate optically opaque planetary clouds and cometary comae, permit investigation of near-surface macrostructure and bulk density, and are sensitive to high concentrations of metal or, in certain situations, ice. Planetary radar astronomy has primarily involved observations with Earth-based radar telescopes, but also includes some experiments with a spaceborne transmitter or receiver. In addition to providing a wealth of information about the geological and dynamical properties of asteroids, comets, the inner planets, and natural satellites, radar experiments have established the scale of the solar system, have contributed significantly to the accuracy of planetary ephemerides, and have helped to constrain theories of gravitation. This review outlines radar astronomical techniques and describes principal observational results.

Phytoplankton Chl a, primary production and nutrient distributions across a coastal frontal zone off Georgia, U.S.A.

Using results obtained by ship and aircraft surveys of near-shore waters off Georgia, U.S.A. we describe the gradients in major plant nutrients, near-surface cholorphyll (Chl a) and near-surface primary production in relation to near-surface density gradients of a coastal frontal zone. The results show that a relatively narrow band (∼10 km wide) of high phytoplankton biomass (Chl a) is a ubiquitous feature of the shoreward side of the coastal frontal zone. Chl a decreases across the frontal zone as salinity increases and near surface Chl a concentrations are approximately 10-fold greater in the high Chl a band than on the seaward side of the frontal zone. Near-surface phosphate and silicate concentrations are distributed similarly to Chl a. In contrast to silicate and phosphate, we did not detect significant cross-shelf gradients of inorganic nitrogen (N) concentrations. The distributions imply high rates of primary production in the coastal band throughout the year, total consumption of available inorganic N brought in by rivers or other sources and high rates of N recycling.

Density Reduction: A Mechanism For Amortization at High Ion Doses

ABSTRACTAt cryogenic temperatures, the accumulation of vacancy-interstitial pairs in Al2O3 from atomic displacements associated with ion implantation produces amorphization. At room temperature, these pairs recombine, and amorphization occurs only at high doses. X-ray reflectivity measurements show that amorphization of the surface of Al2O3 implanted at room temperature with 160 keV Cr+ ions is preceded by a progressive reduction in near-surface density. Monte Carlo simulations show that this density reduction can be accounted for by high-energy-transfer collisions which knock atoms deep into the target, leaving widely separated vacancies and interstitials, which do not recombine. Electron Microscopy shows that at least some of these vacancies condense into voids. We propose that this reduction in near-surface density can lead to amorphization at high doses. We present simple approximations for the density reduction expected for different ions and targets.

Density Reduction: A Mechanism for Amorphization at High Ion Doses

ABSTRACTAt cryogenic temperatures, the accumulation of vacancy-interstitial pairs in Al2O3 from atomic displacements associated with ion implantation produces amorphization. At room temperature, these pairs recombine, and amorphization occurs only at high doses. X-ray reflectivity measurements show that amorphization of the surface of Al2O3 implanted at room temperature with 160 keV Cr+ ions is preceded by a progressive reduction in near-surface density. Monte Carlo simulations show that this density reduction can be accounted for by high-energy-transfer collisions which knock atoms deep into the target, leaving widely separated vacancies and interstitials, which do not recombine. Electron microscopy shows that at least some of these vacancies condense into voids. We propose that this reduction in near-surface density can lead to amorphization at high doses. We present simple approximations for the density reduction expected for different ions and targets.

Characterization of a hoar-development episode using SSM/I brightness temperatures in the vicinity of the GISP2 site, Greenland

Formation of a surface-hoar/depth-hoar complex at the GISP2 site in central Greenland was correlated with large changes in Special Sensor Microwave/Imager (SSM/I) brightness-temperature data. Pass-averaged SSM/I brightness-temperature data over a 1/2° latitude by 1° longitude cell for the 19 and 37 GHz, vertically (V) and horizontally (Η) polarized bands were manipulated to yield differential (V-Η) trends which clearly show a gradual decline as the hoar formation caused a progressively rougher surface with progressively lower density. The hoar episode ended as snowfall, and high winds buried and destroyed the surface-hoar layer and caused rapid V-Η increases in ≈ 1 day. Comparison of the different trends with changes in the field-monitored variables and theoretical values suggest that the V-Η trends are sensitive primarily to changes in surface roughness, and secondarily to near-surface density changes. Consistent expression of trends in microwave brightness temperature over 35 adjacent study cells indicates that this technique may provide a remote-sensing signature capable of defining the timing and spatial extent of surface- and depth-hoar formation in central Greenland.

Characterization of a hoar-development episode using SSM/I brightness temperatures in the vicinity of the GISP2 site, Greenland

Formation of a surface-hoar/depth-hoar complex at the GISP2 site in central Greenland was correlated with large changes in Special Sensor Microwave/Imager (SSM/I) brightness-temperature data. Pass-averaged SSM/I brightness-temperature data over a 1/2° latitude by 1° longitude cell for the 19 and 37 GHz, vertically (V) and horizontally (Η) polarized bands were manipulated to yield differential (V-Η) trends which clearly show a gradual decline as the hoar formation caused a progressively rougher surface with progressively lower density. The hoar episode ended as snowfall, and high winds buried and destroyed the surface-hoar layer and caused rapid V-Η increases in ≈ 1 day. Comparison of the different trends with changes in the field-monitored variables and theoretical values suggest that the V-Η trends are sensitive primarily to changes in surface roughness, and secondarily to near-surface density changes. Consistent expression of trends in microwave brightness temperature over 35 adjacent study cells indicates that this technique may provide a remote-sensing signature capable of defining the timing and spatial extent of surface- and depth-hoar formation in central Greenland.

Determining material properties of metal-matrix composites by NDE

Nondestructive evaluation (NDE) is a promising means of studying silicon carbide particulate (SiCp)-reinforced aluminum metal-matrix composite (MMC) products at various processing stages. Eddy current techniques are effective in characterizing alloy powders and in evaluating the percentage of reinforcement in Al/SiCp powder mixtures. Ultrasonic methods can be used to identify SiCp clusters in large-scale, powder metallurgy processed MMC billets, while eddy current techniques can detect near-surface density variations. Ultrasonic techniques can also be used to determine the anisotropic stiffness constants of composite extrusions; the measured moduli are in good agreement with those determined by tensile testing. These results suggest that NDE can be used to provide on-line, closed-loop control of MMC manufacturing.

Nondestructive evaluation for large-scale metal-matrix composite billet processing

Nondestructive evaluation (NDE) has been conducted on a large-scale, silicon carbide paniculate (SiCp)-reinforced metal matrix composite (MMC) billet. Using powder metallurgy techniques, a 335-mm long × 349-mm diameter MMC billet was specially fabricated to contain implanted discontinuities, such as clusters of SiCp and aluminum powders, and was utilized for nondestructive characterization. Ultrasonic techniques were capable of identifying SiCp clusters in the billet, while eddy current methods could be used to determine near-surface density variation in the billets. Multiple NDE techniques could be employed to evaluate microstructural characteristics of composite billets. These results indicate that NDE techniques can be integrated into manufacturing processes to provide on-line, process-interactive control of fabrication parameters.