Backscattering Data Research Articles

High-depth-resolution Rutherford backscattering data and error analysis of SiGe systems using the simulated annealing and Markov chain Monte Carlo algorithms

The thickness and composition of SiGe/Si quantum wells has been studied by high-depth-resolution Rutherford backscattering (RBS) analysis. While a depth resolution at the surface of 8 nm is achieved, the degradation of resolution with depth must be taken into account for a correct analysis of the data. Fully automated analysis incorporating the depth resolution as a function of depth was performed using the simulated annealing algorithm. Bayesian inference using the Markov chain Monte Carlo method was also employed, and confidence limits on the SiGe depth profiles obtained from the RBS data are established. The results obtained are used to show that the interface sharpness in SiGe/Si multilayers depends on the temperature of growth. Finally, it is shown that standard RBS using a normal angle of incidence is sufficient to obtain narrow limits of confidence in the thickness and stoichiometry of single quantum wells, but not of multiple quantum wells, for which high-depth-resolution experiments are necessary.

Analysis of thickness profiles of pulsed laser deposited metal films

Practically all theoretical approaches to pulsed laser deposition start on a condition which is extremely hard to realize in practice: that the target surface is smooth and plane. When using liquid (molten) targets, the problem of surface deterioration upon repetitive ablation can completely be solved, allowing for fair comparison of experiment and theory. In this paper measured thickness profiles of metal films deposited in vacuum from molten In, Sn, Bi and Sn–Bi alloy targets are compared with calculated distribution functions. The strictly symmetrical thickness profiles of tin and indium films, derived from two-dimensional optical density maps and Rutherford backscattering data are analysed in terms of Lorentzian-like functions, originating from the so-called shifted Maxwellian velocity distribution. The bismuth profiles show a characteristic deviation from this shape.

Coincident detection approach based on scale-invariancecharacteristic of scattering locationsin wavelet-based range-scale domain

A coincident detection approach based on the scale-invariance characteristic of scattering locations in the range-scale domain of the wavelet transform for stepped-frequency backscattered data is presented. It is an efficient and robust approach for the extraction of scattering centres in multi-resolution range profiles.

In situ controlled reactions and phase formation of thin films on GaAs

In situ sequential depositions and reactions are used to control a reaction sequence first to consume GaAs and then to release GaAs from the reaction product. The first reaction involves annealing in situ deposited Ni on molecular beam epitaxially grown GaAs (100) at 300 °C to form a Ni3GaAs/GaAs structure. Exposure of this structure to As4 results in a reaction which consumes the Ni3GaAs by the layer-by-layer formation of NiAs at the surface and epitaxially regrown GaAs at the Ni3GaAs/GaAs interface. The NiAs formation and GaAs regrowth are controlled by the As4 flux. Ni diffusion dominates both the Ni3GaAs formation and decomposition mechanisms. Reflection high-energy electron diffraction, Rutherford backscattering, x-ray diffraction, and transmission electron microscopy data are used to confirm the phase formation and reaction sequences.

Assessing the distribution and abundance of zooplankton: a comparison of acoustic and net-sampling methods with D-BAD MOCNESS

Results are described from the first field study with the D-BAD MOCNESS (Dual-Beam Acoustics Deployed on a Multiple Opening/Closing Net and Environmental Sensing System), an instrument designed to collect acoustic data and net samples simultaneously from the same portion of the water column. Our primary objective was to evaluate the advantages and disadvantages of integrating in a single instrument these two very distinctive methods for assessing the distribution and abundance of zooplankton. In this context, we required a means of comparison that would enable us to groundtruth acoustic remote-sensing data with net sample data. The approach chosen, referred to as the forward-problem approach, compares the acoustic volume backscattering coefficients observed in situ with those predicted from net sample data and acoustic scattering models. The results from this study show that the observed acoustic volume backscattering data are generally consistent with the forward-problem predictions. This consistency is true in terms of both total acoustic volume backscattering as well as that portion of the volume backscattering contributed by each of the dominant sound scatterer types. The results also provide two examples of situations in which inconsistencies between the observed and predicted acoustic volume backscattering can be used to detect potential methodological problems. D-BAD MOCNESS appears to be a useful instrument for groundtruthing acoustic data; however, due to its slow towing speed, it is not a suitable instrument for large-scale, acoustic survey work.

Fish’n krill: A 38/120-kHz acoustic separation

Volume backscattering data (Sv) at 38 and 120 kHz were collected in the Gulf of St. Lawrence, during daylight, in regions where krill scattering layers (SL) and pelagic fish (mainly capelin) were present. The SLs and fish aggregations were sampled with the Bioness zooplankton samples and with small-mesh pelagic trawls equipped with acoustic monitoring systems. Echointegration was performed on small bins of ∼5 m vertically and ∼50 m horizontally. Comparisons of Sv at the two frequencies indicated that krill SLs had stronger backscattering at 120 than at 38 kHz, as expected. The ratio was relatively constant within the same SL, but varied from one SL to the other in parallel with the size of krill. The backscattering from fish schools or fish aggregations or fish layers was stronger at 38 kHz than at 120 kHz. This permitted the separation of fish from krill on the echograms, even for complex cases where fish were embedded in krill SL or had structural patterns that mimic the signature of krill SL. The 38/120-kHz backscattering ratio appears therefore useful to separate krill from fish, or vice-versa, for better interpretation and estimation of these taxa individually.

Inverse Backscattering Problem for the Acoustic Equation in Even Dimensions

We show that the sound speedc(x) of the acoustic wave equation in any even dimension can be uniquely determined by the backscattering data provided that it is close to a constant. In the three-dimensional case, P. Stefanov and G. Uhlmann (SIAM J. Math. Anal.28,1997, 1191–1204) have proved a similar result. Their method takes advantage of the inversion formula for the Radon transform in odd dimensions being a local operator. This is not true in even dimensions. Moreover, the odd-dimensional Lax and Phillips modified Radon transform fails to work in even dimensions. In this paper, we overcome these difficulties and prove an even-dimensional version of Stefanov and Uhlmann's result.

The RBS data furnace: Simulated annealing

A computer program was written which carries out an automatic analysis of Rutherford Backscattering (RBS) data with minimal human involvement. The inputs which are required are the system parameters (e.g. experimental geometry, energy calibration), and the elements present in the sample. Parameters such as the number of layers, layer thickness and layer composition, are determined automatically during the procedure. The global optimisation simulated annealing (SA) algorithm was used, due to its two main features: First, the solution is independent of the initial guess chosen, and therefore a human-input initial layer structure is not needed. Second, it tends asymptotically to the absolute global minimum rather than a local minimum as in conventional minimisation algorithms, and hence high quality solutions can be achieved.

Stability estimate for an inverse acoustic backscattering problem

We prove a stability estimate for an inverse acoustic backscattering problem. This inverse problem consists of the determination of the sound speed, which is in the principal part of the equation, by the backscattering data. Previously, the stability estimates for inverse problems dealing with the unknown coefficient in the principal part of the equation were of logarithmic or Hölder type. Here we are able to obtain a Lipschitz-type estimate for this inverse backscattering problem.

Recovering the total singularity of a conormal potential from backscattering data

On étudie le problème de la restitution de singularités d’un potentiel de la rétrodiffusion. Soit Ω un domaine précompact, convexe et C ∞ . Soit V i =v+w i avec v∈C c ∞ (ℝ n ) et w i conormale au bord de Ω et avec support dans Ω ¯; si les données de la rétrodiffusion de V 1 et V 2 sont égaux, alors V 1 -V 2 ∈C ∞ .

A new time delay estimation in subbands for resolving multiple specular reflections

A new time delay estimation procedure is proposed using the multiresolution analysis framework through a discrete wavelet transform (DWT). Once the signals are decomposed, the time delays are estimated iteratively in each subband using two different adaptation mechanisms that minimize the mean squared error (MSE) between the reference and primary signals in the corresponding subband and level. The localization of the minima of the MSE curves at different levels and subbands is used in order to arrive at the time delay estimates. The proposed scheme is then applied to a real-life problem of underwater target detection from acoustic backscattered data.

Sound scattering by several zooplankton groups. II. Scattering models.

Mathematical scattering models are derived and compared with data from zooplankton from several gross anatomical groups--fluidlike, elastic shelled, and gas bearing. The models are based upon the acoustically inferred boundary conditions determined from laboratory backscattering data presented in part I of this series [Stanton et al., J. Acoust. Soc. Am. 103, 225-235 (1998)]. The models use a combination of ray theory, modal-series solution, and distorted wave Born approximation (DWBA). The formulations, which are inherently approximate, are designed to include only the dominant scattering mechanisms as determined from the experiments. The models for the fluidlike animals (euphausiids in this case) ranged from the simplest case involving two rays, which could qualitatively describe the structure of target strength versus frequency for single pings, to the most complex case involving a rough inhomogeneous asymmetrically tapered bent cylinder using the DWBA-based formulation which could predict echo levels over all angles of incidence (including the difficult region of end-on incidence). The model for the elastic shelled body (gastropods in this case) involved development of an analytical model which takes into account irregularities and discontinuities of the shell. The model for gas-bearing animals (siphonophores) is a hybrid model which is composed of the summation of the exact solution to the gas sphere and the approximate DWBA-based formulation for arbitrarily shaped fluidlike bodies. There is also a simplified ray-based model for the siphonophore. The models are applied to data involving single pings, ping-to-ping variability, and echoes averaged over many pings. There is reasonable qualitative agreement between the predictions and single ping data, and reasonable quantitative agreement between the predictions and variability and averages of echo data.

Bistatic bottom scattering: Model, experiments, and model/ data comparison

A model is presented for bistatic scattering from ocean sediments. It treats scattering due to both roughness of the seabed and volume inhomogeneities within the sediment. Accordingly, the scattered intensity is assumed to be a sum of two terms, one proportional to the roughness-scattering cross section and the other proportional to the volume-scattering cross section. The model is tested against data acquired as part of the Coastal Benthic Boundary Layer (CBBL) research program. As part of that program, an autonomous, circularly scanning sonar system was deployed in well-characterized regions. This sonar operated at 40 kHz, had a 5° horizontal beam, and acquired backscattering data over a 50-m radius. During part of the deployment, it operated in conjunction with a mobile receiving array so as to acquire bistatic data. The experimental apparatus and procedures are presented, and results are compared with model predictions.

High-frequency bistatic scattering by sub-bottom gas bubbles

A previous study of high-frequency acoustic backscattering data collected at Eckernfoerde Bay, Germany revealed that scattering is mainly due to methane gas bubbles buried about a meter beneath the seafloor [Tang et al., J. Acoust. Soc. Am. 96, 2930–2936 (1994)]. A backscattering model was developed [Tang, Geo-Marine Lett. 16, 161–169 (1996)] where the gas bubbles were approximated by oblate spheroids. In this paper, a bistatic scattering model is proposed as an extension of the previously developed backscattering model. In this model, gas bubbles are again assumed to be oblate spheroids with varying aspect ratios and a single-scattering approximation is used. The model is compared to bistatic data acquired in Eckernfoerde Bay, Germany. In particular, the azimuthal dependence of the bistatic scattering strength predicted by the model is tested against experimental data and it is found that both the model and the bistatic scattering strength data exhibit a mild azimuthal dependence. Best agreement between model and data requires a 35% reduction in areal bubble density relative to that used in the backscattering model/data comparison. Possible reasons for this are discussed including multiple scattering effects.

Simulated annealing analysis of Rutherford backscattering data

The combinatorial optimization simulated annealing algorithm is applied to the analysis of Rutherford backscattering data. The analysis is fully automatic, i.e., it does not require time-consuming human intervention. The algorithm is tested on a complex iron-cobalt silicide spectrum, and all the relevant features are successfully determined. The total analysis time using a PC 486 processor running at 100 MHz is comparable to the data collection time, which opens the way for on-line automatic analysis.

Coherent dynamic structure factor of orthoterphenyl around the mode coupling crossover temperatureTc

The coherent dynamic structure factor of supercooled orthoterphenyl has been measured by time-of-flight neutron scattering. The data extend previous backscattering and spin-echo data towards shorter times. In the $\ensuremath{\beta}$ relaxation regime the two-step decay of density correlations can be described by the same master function as found before for self-correlations. We show that the mode-coupling crossover temperature ${T}_{c}$ can be determined from $\ensuremath{\beta}$ relaxation data alone. The wave-number dependence of the relaxation amplitude follows, to some extent, that of the static structure factor.

Multivariate Fourier and wavelet analysis of acoustic backscattering and environmental data from the acoustic surface reverberation experiment

Data from the acoustic surface reverberation experiment (ASREX) [Williams et al., J. Acoust. Soc. Am. 97, 3404 (1995)], is analyzed using multivariate Fourier and wavelet techniques to investigate relationships between environmental conditions and observed acoustic surface backscatter at 200, 400, and 800-Hz frequencies. Environmental variables observed included wind speed and direction, wave height, air and sea temperatures, current speed and direction, ambient noise, and various measurements of bubble activity characterized by depth-integrated air volume and e-folding depth. The relative contribution of the environmental parameters to observed backscattering strengths at the various frequencies will be discussed. The analysis techniques will be described, compared, and contrasted. [Work supported by ONR.]

Imaging using limited-angle backscattered data from real targets

We discussed a spectral-estimation method, referred to as the PDFT, which was used to overcome the problems of limited k-space data being measured from a monostatic or backscatter radar system. We have also shown how this estimation method can be used to estimate the size of a target, and also how the method permits target recognition to be achieved with only a small amount of backscatter data. That amount of required data points decreases as the information about possible target shapes, expressed by the prior estimate, gets more and more specific.

Rainfall rate profiling with attenuating-frequency radar using nonlinear LMS technique under a constraint on path-integrated rainfall rate*

This paper deals with a new algorithm applicable to rainfall rate profiling with a rain-attenuating-frequency radar for vertical incidence from an air or space platform. The proposed algorithm is independent from external radar calibration which is especially difficult to perform in airborne or space-borne applications. In addition to backscattering data from raindrops, path-integrated rainfall rate is used to estimate a rainfall rate profile. The path-integrated rainfall rate may be estimated from surface echo attenuation or from brightness temperature measured with an auxiliary microwave radiometer. The performance of the proposed algorithm is evaluated by a computer simulation experiment assuming backscattering and attenuation models of the radar wave due to rain. Then, airborne radar data are analysed with the algorithm to demonstrate its validity.

Optical‐microwave synergy for estimating surface sensible heat flux over a semi‐arid rangeland

This study reports the first results of the Walnut Gulch’ 92 experiment concerning the combined use of radar backscattering (ERS‐1) and thermal infrared (Landsat TM) data to estimate surface sensible heat flux. The purpose is to use the radar/thermal synergy to retrieve both vegetation and soil temperatures required by a two‐layer type model. The first step investigates the potential use of ERS‐1 SAR images for surface soil moisture monitoring of the watershed using five calibrated images acquired during the year 1992 (dry to wet conditions). Results show that despite the typical low level of biomass of semi‐arid rangeland, an attenuation of the soil backscatter (up to 2 dB) can occur during the rainy season mainly due to the vegetation characteristics. A statistical relationship is then used to retrieve the volumetric surface soil moisture from ERS‐1 backscattering (sensitivity of 0.23 dB/% moisture) with a resulting root mean square error (RMSE) of 1.3% of soil moisture. In a second step a semi‐empirica...