Backscattering Data Research Articles

High-frequency surface scattering phenomena in high sea states.

The determination of acoustic scattering from a rough sea surface has received considerable attention in recent years. Knowledge of whitecap characteristics [J. Wu, IEEE J. Oceanic Eng. 17, 150–158 (1992)] has led to a better understanding of surface scattering phenomena [D. F. McCammon and S. T. McDaniel, IEEE J. Oceanic Eng. 15, 95–100 (1990)]. For this study a substantial amount of backscattering data was acquired with autonomous underwater vehicles running between 50 and 100 ft of the surface. These vehicles used a short pulse length sonar operating at 215 kHz. The use of a high-frequency sonar allows deep penetration of densely populated bubble plumes. Because of the high repetition rate, good spatial variation of backscattering strength from breaking waves has been obtained. Previous measurements by other scientists of bubble distribution functions in breaking waves allows an approximate conversion of scattering strength at the sonar frequency to most frequencies of interest. The database here consists of measurements made in deep water environments with seas ranging from sea state 0 to 3. The spatial variation of scattering strength due to the sea surface and bubble plumes from breaking waves is discussed as a function of wind speed.

Dredged Material Disposal at the Edge of the Florida Current

Abstract A field data collection project was undertaken to investigate the short-term fate of dredged material discharged in the designated Miami Ocean Dredged Material Disposal Site (ODMDS) before dredging of the Miami River and the Miami Harbor Turning Basin begins. the designated ODMDS is located in relatively deep water for discharge sites with a typical bottom depth of 150 metres and is also located in the western boundary region of the Gulf Stream current off Miami. Acoustical backscattering, current, particulate, temperature and salinity data were gathered over a three day period from April 24, 1990 through April 26, 1990. the major generic features of shallow-water discharge plumes were observed to be present: (a) the presence of a rapid convective descending plume portion; (b) impact of that plume portion with the ocean bottom and concomitant generation of a bottom surge; (c) rapid horizontal width growth of the descending plume through entrainment; and (d) retention of a residual plume portion within the water column. A well-mixed upper water column layer extending to a depth of 40 to 60 metres below the surface of the ocean permitted measurements of the plume entrainment coefficient free from bottom boundary, water column density gradient, and vertical current shear effects which are usually present in relatively shallow, e.g. less than 40 metres bottom depth, coastal ocean discharge studies. Entrainment coefficient estimates obtained in this study were between 0.5 to 0.7. the residual water plume material was tracked over one-half hour during each of eight discharge events and was transported in a north-northeast direction.

Interactive personal-computer data analysis of ion backscattering spectra

A new personal computer program package for ion backscattering data analysis with an interactive graphical interface has been developed. The assumed target composition is modified directly by pointing at the corresponding signals on the screen. Special attention has been paid to the physics details included. All projectile ions may be treated. A data base of non-Rutherford 1H and 4He scattering cross sections of several light elements, readily extendable by the user, is included. The most recent stopping power formulations are used. Contributions from electronic screening, corrected Bohr straggling and nonlinear detector response are taken into account. A procedure for subtracting the low energy tail background is provided. Smooth variations of elemental composition as a function of sample depth, as needed in diffusion or implantation profile analysis, as well as spectra taken in channeled sample orientations may be assumed. A short review of published computer methods for ion backscattering is included. Examples and applications of the present procedure to the analysis of 4He ion non-Rutherford and 12C ion RBS spectra are presented.

Antiferromagnetic coupling and magnetoresistance enhancement in Co-Re metallic superlattices (abstract)

Co-Re metallic superlattices were prepared that show antiferromagnetic exchange coupling and enhanced saturation magnetoresistance for particular values of the Re spacer thickness. We report studies on films with the structure glass /150 Å Re/[13 ÅCo/tRe]16/50 Å Re, with tRe ranging from 3 to 40 Å. These structures were grown by magnetron sputtering in a system with a base pressure of 1×10−7 Torr with deposition rates of 0.3 and 0.4 Å/s for Co and Re, respectively. x-ray diffractograms indicate the structure to be highly textured with the c axis perpendicular to the sample plane. The superlattice structure was obtained from high-angle θ-2θ scans. First-, second-, and third-order satellites are observed on both sides of the central [002] peak. Periodicity and bilayer composition are obtained from comparison of the data with a theoretical calculation of the x-ray diffractogram. Thickness calibration was confirmed by Rutherford backscattering and profilometer data. In-plane magnetization and magnetoresistance data (Δρ/ρ) indicate that stronger antiferromagnetic coupling and highest Δρ/ρ occur for tRe≊6 Å. The saturation field (Hs) needed to align contiguous antiferromagnetically coupled Co layers is about 1 T. This corresponds to an exchange coupling between the Co layer J≊−1 erg/cm2. (Δρ/ρ) reaches 2% in samples deposited at 170 °C. This data confirms results obtained by Parkin et al.1 in Co-Ru and Co-Cr superlattices.

Protons, helium and heavy ions in backscattering analysis of high- Tc superconductors

Ion backscattering spectra for 1H, 4He, 7Li, 12C, 16O and 28Si ions incident on thin and thick superconductor samples in the energy range 2–28 MeV have been measured. The spectra are analyzed by using a computer program package for ion backscattering data analysis. The general characteristics of the ion beams for the backscattering analysis of high- T c superconductors both in the Rutherford and non-Rutherford energy regime are discussed. The advantages and disadvantages of using proton and heavy ion beams are compared to those of conventional 4He ion backscattering.

Inverse backscattering in two dimensions

This article extends the authors' previous results (Commun. Math. Phys.124, 169–215 (1989) to inverse scattering in two space dimensions. The new problem in two dimensions is the behavior of the backscattering amplitude near zero energy. Generically, this has the form $$a({\xi \mathord{\left/ {\vphantom {\xi {\left| \xi \right|,}}} \right. \kern-\nulldelimiterspace} {\left| \xi \right|,}} - {\xi \mathord{\left/ {\vphantom {\xi {\left| \xi \right|,\left| \xi \right|}}} \right. \kern-\nulldelimiterspace} {\left| \xi \right|,\left| \xi \right|}}) = 2\pi (2\pi \beta + \ln \left| \xi \right|)^{ - 1} + b(\xi ),$$ whereb(0)=0 andb(ζ) is Holder continuous. In order to work in weighted Holder spaces as before, the constant β and the functionb(ζ) must now be interpreted as “coordinates” on the space of backscattering data. In this setting the mapping to backscattering data is again a local diffeomorphism at a dense open set in the real-valued potentials.

Rutherford backscattering and channeling studies on epitaxial YBa 2Cu 3O 7− x thin films

We have conducted ion-backscattering and channeling experiments on epitaxial c-axis oriented YBa 2Cu 3O 7− x (YBCO) thin films ( T c ∼ 90 K, J c ⩾10 6 Acm −2) grown onto (001)MgO and MgO buffered (1 102) Al 2 O 3 substrates. From our results on films with a range of compositions around 1:2:3 stoichiometry we infer that a film, with metal ratios (Y: Ba:Cu): 1.25:1.78:2.97 (film A), has a minimum channeling yield χ min, Ba of 8% compared with a value of 14% obtained on a film with metal ratios: 1.03:2.22:2.74 (film B). Angular-yield profiles for film A in the 〈001〉 and 〈101〉 MgO substrate directions along the {010}; plane demonstrate that full epitaxy of the YBCO film on (001)MgO is achieved. This film (3300 Å thick) is relaxed as expected, with the 〈103〉 direction in the film being at an angle of 44.6° to the 〈001> film direction. Backscattering and channeling data on a YBCO film ( T c ∼ 88 K, J c ⩾10 6 Acm −2) on MgO buffered Al 2O 3 show a χ min, Ba of 25% the beam becoming about 90% dechanneled in the MgO buffer layer and Al 2O 3/MgO interface. The interlayer reaction between MgO and Al 2O 3 is at a minimum.

Ion scattering studies of modified metal surfaces

The technique of ion backscattering and channeling provides valuable information about the structure of solid surfaces, both clean and as modified by adsorbed atoms. Analysis of the backscattering data is based on the concepts of shadowing and blocking. Because the theory of Rutherford scattering is well understood, it is possible, using comparisons between experiment and computer simulations for model structures, to extract information about the geometry and dynamics of atoms on solid surfaces. Studies of the structure of clean W(110) and Nb(110) surfaces, and the modification of these surfaces with H adsorption/absorption will be presented. Ion scattering also provides structural information about solid-solid interfaces. Results for Pd deposited on A1(111), and for Ni on W(110) will be discussed.

Thresholds of non-Rutherford nuclear cross sections for ion beam analysis

A theoretical model of classical scattering in the presence of combined Coulomb and weakly perturbing nuclear fields predicts the incident ion energies at which elastic backscattering cross sections begin to deviate from their Rutherford values [M. Bozoian et al., Nucl. Instr. and Meth. B51 (1990) 311]. The model, parameterized with two nuclear constants, is in good agreement with experimental data for proton and helium-ion beams. This classical model was revised by including the nature of the Z = 1 and 2 projectiles. In this paper the revised model is applied to Z = 3 projectiles. Although little backscattering data exists, there is consistency among the optical model calculations of departures from Rutherford backscattering for the lithium-target systems considered. The revised model is in good agreement with these calculations. New experimental results for 6Li and 7Li ion beams compared with the revised model are presented at this conference [K.M. Hubbard et al., Bull. Am. Phys. Soc. 35 (1990) 1785].

Effect of lattice damage on impurity depth profiles in BF 2+-implanted silicon

BF + 2 ions were implanted on n-type Si(100) wafers which were annealed in a rapid thermal annealing furnace and in a conventional furnace. The crystal defects arising from the lattice damage caused by BF + 2 ion implantation and subsequent annealing were investigated using transmission electron microscopy and Rutherford backscattering ion beam channelling. The effects of these defects on the depth profiles of boron and fluorine were analysed by secondary ion mass spectrometry. The depth profiles of both boron and fluorine in the annealed specimens show two peaks and higher peak concentrations are observed for fluorine than in the as-implanted specimen. The two peaks in the fluorine depth profile are observed for all annealed specimens, whereas for boron the two peaks are only observed in specimens that are annealed above 1100 °C. These results indicate that BF + 2 ion implantation produces two types of defect where the boron and fluorine can be gettered and that boron is gettered at a higher temperature due to a lower diffusion coefficient. Transmission electron micrographs reveal only one type of defect, whereas Rutherford backscattering data indicate the possible presence of another type of defect closer to the wafer surface.

Acoustics and hydrodynamics of bubble clouds

In the last few years, it has been realized that the near‐surface clouds of bubbles produced by breaking waves may play an important role in the generation of oceanic ambient noise. In addition, the clouds are potentially very strong scatterers of sound and may therefore be responsible for the anomalous backscattering data reported by several investigators. In the first part of this paper, some theoretical results are reviewed concerning the active and passive acoustic behavior of bubble clouds and are compared with experiment. In the second part of the paper, the results obtained with a simple model of the evolution of the clouds in space and time are described. The model is based on two‐fluid‐averaged equations and describes the motion of the bubbles under the action of buoyancy and drag.

Shallow junctions by out-diffusion from Arsenic implanted polycrystalline silicon

We describe reverse bias diode leakage and physical analysis (secondary ion mass spectroscopy, Rutherford backscattering and transmission electron microscopy) data from shallow n+/p junctions made by implanting various doses of arsenic into polycrystalline Si and out diffusing at various temperature/time conditions, into the underlying (100) Si substrate. The polycrystalline Si is cobalt disilicided to provide the first level of metallization. The minimum process specifications giving good junction quality (reverse bias diode leakage current density ≤10 nA/cm2 at 10 V) are 7×1015 cm−2 As out diffused at 950 °C, 30 min with a junction depth 1200 Å below the polycrystalline Si. At these conditions, most of the As (≳90%) uniformly redistributes in the polycrystalline Si. There is a segregation to the polycrystalline Si/Si interface which is characterized by a porous, <50 Å thin silicon oxide interface. The polycrystalline Si/Si interface is also characterized by a laterally non-uniform large grain(∼2000 Å) or epitaxial regrowth at the 950 °C process temperature. These interface conditions may be used to explain the ∼20% high leakage population which is independent of out-diffusion condition at high As dose. Such a high sport population is not observed when P is implanted into polycrystalline Si and out diffused at 900 °C, 30 min to obtain ∼2500-Å-deep junctions below the polycrystalline Si/Si interface.

Shallow junctions by out-diffusion from BF2 implanted polycrystalline silicon

We describe reverse bias diode leakage and physical analysis (secondary ion mass spectroscopy, Rutherford backscattering and transmission electron microscopy) data from shallow p+/n junctions made by implanting various doses of BF2 into polycrystalline Si and out-diffusing at various temperature/time conditions, into the underlying (100) Si substrate. The polycrystalline Si is cobalt disilicided to provide the first level of metallization. Subsequent metallization consists of electroless plated Co followed by sputtered Al. The minimum process specifications giving good junction quality (reverse bias diode leakage current density ≤10 nA/cm2 at 10 V) are 5×1015 cm−2 BF2 outdiffused at 900 °C, 30 min with a junction depth 1200 Å below the polycrystalline Si. At these conditions, most of the BF2 (>90%) uniformly redistributes in the polycrystalline Si. There is a slight increase in B concentration towards the polycrystalline Si/Si interface which is characterized by a porous, <50-Å thin silicon oxide interface. The polycrystalline Si/Si interface is also characterized by a peaked fluorine concentration. The lower boron dose diffusing into the Si substrate may be used to explain the observation that these p+/n diodes are ∼10 times leakier than similar n+/p diodes made by outdiffusing As or P implanted from the polycrystalline Si diffusion source.

Finescale biological patchiness of 70 kHz acoustic scattering at the edge of the Gulf Stream—EchoFront 85

The spatial distribution of biological scatterers within the Gulf Stream front is inferred from an analysis of patch statistics obtained from digitally recorded backscattering data. Acoustic data were collected along 10 transects perpendicular to the front using a downward-looking 70 kHz echosounder. Patches were defined using an algorithm selected to search for finescale patches from within 200 × 900 element (approx. 200 m depth × 24 km length) integrated echo data. Based on principal component analyses of 17 patch parameters, we identified the third most important component as a measure of “acoustic roughness” (containing the coefficient of variance and coefficient of roughness of the integrated echo independent of echo intensity). This third component was a good descriptor of differences among patches within scattering layers and between water masses. It is independent of echo strength and patch size which constitute the first two components. We interpret higher acoustic roughness within patches to indicate a more contagious (clumped) distribution of animals within those patches. Classification of patches on acoustic roughness showed that patches were often acoustically different from distant neighbors but more similar to neighbors within the same scattering layer or region. We infer that finescale layers are made up of small patches of like animals exhibiting a similar spatial arrangement throughout the layer. Cross-stream differences in acoustic roughness indicate a greater number of solitary scatterers occur within the slope water than within the Gulf Stream. Acoustic roughness is also reduced at night when compared with day, indicating that the ascent of vertical migrators into the near-surface waters (<200 m) results in a more dispersed spatial pattern.

Analysis of backscattering data in the Tyrrhenian Sea

Research has been conducted in the Tyrrhenian Sea basin to measure low-frequency basin reverberation and relate the measurements to backscattered strength. The receivers were a towed horizontal array and a suspended vertical array. Standard Italian Navy depth (74 kg) and SUS (0.818 kg) charges were used as sources. The unique factor in this broadband basin reverberation experiment was that data were collected simultaneously from the horizontal and vertical array. Towed-array data were taken on multiple headings; combining different headings permitted removal of the ambiguous directionality information from the towed array. Frequency-domain beamformed data are presented and reveal that, in the area examined, basin features are the strongest backscatterers. Receiver beam level data are presented primarily for 300 Hz, but some data are given for 100, 450, and 730 Hz as well. Estimates of backscattering strengths of different features are provided and are in the range quoted in other publications. A comparison of the horizontal and vertical array scattering strengths estimated for some selected basin features is also included. Statistics for the dominant backscatterers are given, and the results from one basin feature are presented showing detailed backscatter structure.

Time-of-flight scattering and recoiling spectrometry. II. The structure of oxygen on the W(211) surface.

The technique of time-of-flight scattering and recoiling spectrometry (TOF-SARS) with detection of both neutrals and ions is applied to structural analysis of oxygen adsorbed on a W(211) surface. The site position for oxygen in the high-dose [\ensuremath{\Theta}=1.5 monolayers (ML), saturation coverage] p(1\ifmmode\times\else\texttimes\fi{}2) low energy electron diffraction (LEED) pattern is determined; a preliminary study of oxygen in the low-dose (\ensuremath{\Theta}=0.5 ML) p(2\ifmmode\times\else\texttimes\fi{}1) LEED pattern is also presented. Both Ar and Ne backscattering (BS) and oxygen recoiling spectra, induced by pulsed 4-keV ${\mathrm{Ar}}^{+}$ and 5-keV ${\mathrm{Ne}}^{+}$ primary-ion beams, are monitored as a function of polar beam incident angle \ensuremath{\alpha}, surface azimuthal angle \ensuremath{\delta}, scattering angle \ensuremath{\theta}, and recoiling angle \ensuremath{\varphi}. Plots of BS (or recoil) intensities in (\ensuremath{\alpha},\ensuremath{\delta}) space provide scattering (or recoiling) structural contour maps and three-dimensional scattering (or recoiling) structural plots which are representative of the adsorption sites of oxygen on the W(211) surface; the symmetry of the adsorption sites is determined from these plots.Measurements of BS and recoil intensities as a function of \ensuremath{\alpha} along different azimuths \ensuremath{\delta} provide experimental values of the critical incident angles ${\ensuremath{\alpha}}_{c}$,sh${}^{\mathrm{i}}$ for shadowing and the critical ejection angles ${\ensuremath{\beta}}_{c}$ (or ${\ensuremath{\alpha}}_{c}$,bl) for blocking by neighboring atoms. Trajectory simulations and calculations of the shadowing and blocking cones obtained from potentials calibrated for this system are used to determine the oxygen-adsorption-site coordinates. The results show that oxygen is dissociatively adsorbed within the troughs along the [1 1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{}] direction. For the saturation p(1\ifmmode\times\else\texttimes\fi{}2) structure, the oxygen is in threefold sites formed by two first- and one second-layer W atoms; these are the only sites which are consistent with all of the experimental data. The coordinates of these sites relative to the W coordinates are determined to an accuracy of \ifmmode\pm\else\textpm\fi{}0.1 A\r{}. The oxygen-tungsten chemisorption bond length is determined as 1.83 A\r{} to the two first-layer W atoms and 2.17 \AA{} to the second-layer W atom; this reflects the unsaturated and fully saturated valencies of the first- and second-layer W atoms, respectively. These trough sites are occupied for coverages up to more than one monolayer of oxygen. For the low-dose p(2\ifmmode\times\else\texttimes\fi{}1) structure, the data indicate occupancy of the same threefold sites, but with different coordinates. The data are consistent with shifting of the relaxed clean W surface structure to the bulk truncated structure upon ${\mathrm{O}}_{2}$ chemisorption. The sensitivity of TOF-SARS to details of adsorbate structure, the use of both BS and recoil data in a complementary manner, and the ability to extract adsorbate structural information with only simple calculations are presented.

Estimation of mean scatterer size from sparse, random distributions of polystyrene spheres in solution via Mie scattering and cepstral signal processing: Theory and experimental results

Four solutions of narrowly distributed polystyrene spheres of one diameter, either 41, 136, 271, or 381 μm, sparsely suspended in degassed water, were imaged using multiple 5‐ and 7.5‐MHz medical ultrasonic transducers. Radio‐frequency backscattering data from the 136‐, 271‐, and 381‐μm solutions were processed using the data from the 41‐μm solution as a volumetric reference to correct for transducer diffraction effects and acquisition system gain vibrations. The power cepstrum of the mean log difference spectra averaged over all overlapping data segments was computed and corrected for the effects of attenuation and Rayleigh scattering in the reference. Abcissas of the cepstral peaks were plotted versus scatterer diameter. The zero‐intercept, linear regression line for the longer‐lag peaks yielded a slope of 3.4 μs/mm, r2 = 0.997, corresponding to a Mie‐region periodicity in backscattered cross section of ka = 0.6. This value is in excellent agreement with calculations based on the scattering theory of Faran, which includes shear waves and yields a periodicity of ka = 0.66.

Silicon oxide deposition from tetraethoxysilane in a radio frequency downstream reactor: Mechanisms and step coverage

Isotopic labeling and step coverage studies of silicon oxide deposited from tetraethoxysilane (TEOS) have been carried out by introducing TEOS(16O) downstream from an 18O2 discharge. Rutherford backscattering (RBS) data on films deposited near 440 °C show that, on average, one Si–O bond in the original TEOS molecule is preserved in the process, while mass spectrometric results indicate only H216O and C18O16O as gaseous products of the cleavage of the remaining three Si–O bonds. Infrared analyses of films deposited at room temperature show large amounts of Si–OH in a gel-like material, and the presence of a C■O species. The results suggest a mechanism dominated by diffusion and condensation of Si–OH species that form extensive chains and preserve an Si–16O bond from the original precursor. This is followed by cross-linking to form the final silicate network; however, Si–O bond cleavage is apparently occurring at potential cross-linking sites via a carbonate intermediate that promotes isotopic scrambling. Step coverage is shown to be better under conditions (high temperature, low TEOS flow) that favor a low concentration of surface precursors during film growth. This is consistent with the proposed mechanism since second-order condensation reactions of surface species under these conditions would be kinetically limited allowing effective mean free paths to increase dramatically. Experimental parameters and techniques, such as modulated chemical feeds to enhance the role of surface migration also lead to improved step coverage. However, we also conclude that perfectly conformal coverage based on similar mechanisms will inevitably lead to void formation in high-aspect-ratio topography because of transport limitations.

Determination of the uniformity of uranium fission deposits using Rutherford backscattering spectrometry and alpha-particle scanning

The uniformity of the areal density of uranium deposits used in neutron induced fission cross-section measurements has been measured using Rutherford backscattering spectrometry of 1-MeV He + ions as well as by scanning the natural α-particle decay of the uranium. The measurements used the 3-MV positive-ion accelerator at the National Institute of Standards and Technology along with a versatile scattering chamber with numerous ports, five-axis goniometer, target ladder, and solid state detector. The variation in areal density of a 265- μg/cm 3 UO 2 deposit with a diameter of 89 mm was measured using the 1-MeV He + beam. The results are in excellent agreement with those obtained from α-particle activity measurements. However, the Rutherford backscattering data provide better definition of the uniformity near the edge of the deposits. Our experience indicates that the backscatter technique is useful for measuring variations of 1% in areal densities but is less sensitive to the absolute areal density. The stoichiometry of the deposit was additionally measured with X-ray diffraction.

The use of superconducting bolometers for detecting nanosecond heat pulses

The authors discuss the use of superconducting edge bolometers for detecting heat pulses in the regime in which pulse lengths are shorter than the bolometer response time. An algorithm is described which enables deconvolution of the observed bolometer signal to give the incident phonon flux. Backscattering data taken on 2 mm sapphire crystals suggest that the method will also be successful in the study of much thinner samples, for example, the 0.5 mm thick wafers used in MBE technology.