Strong Scatterers Research Articles

Improved joint X-ray and neutron refinement procedure in Phenix.

Neutron diffraction is one of the three crystallographic techniques (X-ray, neutron and electron diffraction) used to determine the atomic structures of molecules. Its particular strengths derive from the fact that H (and D) atoms are strong neutron scatterers, meaning that their positions, and thus protonation states, can be derived from crystallographic maps. However, because of technical limitations and experimental obstacles, the quality of neutron diffraction data is typically much poorer (completeness, resolution and signal to noise) than that of X-ray diffraction data for the same sample. Further, refinement is more complex as it usually requires additional parameters to describe the H (and D) atoms. The increase in the number of parameters may be mitigated by using the `riding hydrogen' refinement strategy, in which the positions of H atoms without a rotational degree of freedom are inferred from their neighboring heavy atoms. However, this does not address the issues related to poor data quality. Therefore, neutron structure determination often relies on the presence of an X-ray data set for joint X-ray and neutron (XN) refinement. In this approach, the X-ray data serve to compensate for the deficiencies of the neutron diffraction data by refining one model simultaneously against the X-ray and neutron data sets. To be applicable, it is assumed that both data sets are highly isomorphous, and preferably collected from the same crystals and at the same temperature. However, the approach has a number of limitations that are discussed in this work by comparing four separately re-refined neutron models. To address the limitations, a new method for joint XN refinement is introduced that optimizes two different models against the different data sets. This approach is tested using neutron models and data deposited in the Protein Data Bank. The efficacy of refining models with H atoms as riding or as individual atoms is also investigated.

Strong Clutter Suppression Using Spatial and Signal Similarity for Multi-Channel SAR Ground-Moving-Target Indication

This paper presents a new two-stage approach for suppressing strong clutter and detecting moving targets using scatterers’ spatial structure and signal similarity. Compared with the traditional strong clutter suppression methods, the proposed method considers both the spatial similarity and the channel correlation of the scatterers, effectively alleviating the false alarm probability and avoiding the missed detection problem caused via identifying strong moving targets as strong stationary clutter. Additionally, a detector is presented based on the linear degree of the radial velocity interferometric phase (LDRVP) to eliminate false alarms from isolated strong scatter points and the edges of strong scatterers. The experimental results of the X-band radar indicate the presented approach’s lower false alarm probability and superior robustness.

Direct measurements of gas flux at seep sites using a broadband split-beam echosounder

Measurement of methane gas flux from oceanic seep sites is challenging, given the ephemeral nature and the heterogeneous spatial distribution of seeps. Acoustic systems offer a solution, as they make synoptic measurements of the water column, and the gas bubbles are strong acoustic scatterers. Here, we present a method to directly estimate gas flux using a calibrated broadband split-beam echosounder. The vertical range resolution of the broadband system facilitates discrimination of individual bubbles in the acoustic record. By comparing measurements of bubble target strength to an analytical scattering model, bubble radii can be directly measured from the acoustic data. Concurrently, split-aperture processing allows for the precise tracking of bubbles as they rise from the seafloor for measurement of bubble rise velocity. Together, the observations of bubble radius and rise velocity offer a measure of gas flux, requiring nothing more than a vessel transiting over a seep site. Application of this method to seep data collected on the East Siberian Arctic Shelf shows good agreement between resulting measurements of bubble radii (0.68–8.40 mm) with those made using optical sensors, and bubble rise velocities (4–36 cm/s) are consistent with published measurements and models. Extrapolating from single bubbles measurements, our estimates of regional methane flux (2.9 × 104 kg/year) suggest that carbon emissions in this region may be lower than previously believed.

Effects of scattering parameters on seismic observables of PKP precursors and apparent D″ reflections – A parameter study

The lowermost mantle has been studied with a variety of seismic waves which show that different structures on many length-scales exist there but it is possible that several seismic phases could be generated by one structure. Specifically, we use a randomly distributed scattering layer that is able to generate precursors to the core phase PKPdf as well as waves that resemble reflections off the D″ layer (PdP wave) and here we present a parameter study to understand how scattering parameters in this layer affect the two tested seismic waves and whether it is possible to discriminate between a reflector or a scattering layer as cause for apparent D″-reflected waves. We compute synthetic data for several source-receiver combinations and vary the parameters of the heterogeneity model above the core-mantle boundary: correlation length, velocity (and density) perturbation, taper to soften the transition, thickness, location of the scattering layer and the randomness of the models. We measure the effects on the amplitudes and arrival times of the seismic waves, and in addition for the apparent D″ reflection, we also record the number of reflected waves, their polarity and slowness. We find that the precursor and reflection amplitudes are influenced by all scattering parameters and that trade-offs exist. One conclusion therefore is that it is difficult to match local scattering parameters to fit observations with a unique model since several combinations of scattering parameters result in similar amplitudes and travel times. Interestingly, amplitudes for different correlation lengths are affected differently for larger and smaller epicentral distances, therefore testing the amplitudes of precursors at different distance ranges could potentially help to distinguish between correlation length and perturbation effects. We were not able to further narrow down the scattering parameters of the layer when using a combination of PKP precursors and PdP-like reflections due to the non-uniqueness. Analyzing the effects on PdP arrivals that arrive between P and PcP, we find influences on all seismic observables albeit without clear trends. But based on our modeling it seems prudent to assume that the waves are caused by a scattering layer if 1) the observations show strong scatter in arrival time, slowness and amplitude and 2) if more than one arrival is present between P and PcP.

High-energy X-ray spectrum reconstruction: solving the inverse problem from optimized multi-material transmission measurements

Reconstructing the unknown spectrum of a given X-ray source is a common problem in a wide range of X-ray imaging tasks. For high-energy sources, transmission measurements are mostly used to recover the X-ray spectrum, as a solution to an inverse problem. While this inverse problem is usually under-determined, ill-posedness can be reduced by improving the choice of transmission measurements. A recently proposed approach optimizes custom thicknesses of calibration materials used to generate transmission measurements, employing a genetic algorithm to minimize the condition number of the system matrix before inversion. In this paper, we generalize the proposed approach to multiple calibration materials and show a much larger decrease of the condition number of the system matrix than thickness-only optimization. Additionally, the spectrum reconstruction pipeline is tested in a simulation study with a challenging high-energy Bremsstrahlung X-ray source encountered in Linear Induction Accelerators with strong scatter noise. Using this approach, a realistic noise level is obtained on measurements. A generic anti-scatter grid is designed to reduce noise to an acceptable -yet still high-noise range. A novel noise-robust reconstruction method is then presented, which shows much less sensitive to initialization than common expectation-maximization approaches, enables a precise choice of spectrum resolution and a controlled injection of prior knowledge of the X-ray spectrum.

Statistical Analysis of the Critical Frequency foF2 Dependence on Various Solar Activity Indices

A description of the ionospheric F2-layer critical frequency foF2 dependence on solar activity byvarious indices (proxies) has been analyzed. The results of the vertical ionospheric sounding at the Juliusruhstation during 2 winter months (January and February), 2 equinox months (March and October), and a summermonth (June) are considered. Five solar proxies: Ly-α, MgII, Rz, F30, and F10.7 have been analyzed.The changes in foF2 are compared to the corresponding changes in 1957–1980. The determination coefficientR2 according to the Fisher F-test is used as a measure of the quality of the description of the foF2 dependenceon SA by each of the proxies. It has been found that a well-pronounced diurnal variation in the R2 valueis observed in winter months: this is higher at the near-noon hours than at night. In other words, all indicesdescribe the foF2 behavior better in the daytime than at night. A well-pronounced diurnal variation in R2 isalso observed for four proxies in the equinoctial months, whereas that variation is pronounced much less forRz: a strong scatter of the R2 values is seen. A diurnal variation in the R2 is absent completely in June: jumpsfrom one hour to another are observed. This analysis allows us to conclude that the most reliable SA proxiesfor description of the foF2 dependence on SA at all LT hours are MgII, F30, and Ly-α.

Strong Scattering Effects in the Emission of Soft Gamma-Ray Bursts

When a light beam enters a scattering-dominated medium, the radiation is isotropized. Part of the radiation goes backwards, leading to non-monotonicity in the radiation energy density profile inside this medium. There arises a local maximum at which the energy density at a scattering albedo 1 is severalfold greater than that without scattering at the same extinction. This effect is studied numerically in one-dimensional and two-dimensional simulations. It is demonstrated that a local maximum of the radiation energy density arises in the medium, whose value depends on the optical depth of the region. This effect can manifest itself, for example, when the radiation from a gamma-ray burst (GRB) enters heated regions in the interstellar medium. The presence of scattering in the GRB radiation generation region, near the front of strong shocks, affects the radiation pattern. The structure of such shocks is remarkable for the presence of a preshock preheating tail. Strong scattering in this region leads to the escape of a significant fraction of the radiation sideways and backwards in the shock reference frame, forming additional tails in the angular distribution of GRB radiation after the relativistic transformation to the laboratory frame. This effect is also studied numerically in three-dimensional simulations.

Strong Scattering Effects in the Emission of Soft Gamma-Ray Bursts

Strong Scattering Effects in the Emission of Soft Gamma-Ray Bursts

Thermal Transport in Poly(p-phenylene): Anomalous Dimensionality Dependence and Role of π-π Stacking.

For heat conduction along polymer chains, a decrease in the axial thermal conductivity often occurs when the polymer structure changes from one-dimensional (1D) to three-dimensional (3D). For example, a single extended aliphatic chain (e.g., polyethylene or poly(dimethylsiloxane)) usually has a higher axial thermal conductivity than its double-chain or crystal counterparts because coupling between chains induces strong interchain anharmonic scatterings. Intuitively, for chains with an aromatic backbone, the even stronger π-π stacking, once formed between chains, should enhance thermal transport across chains and suppress the thermal conductivity along the chains. However, we show that this trend is the opposite in poly(p-phenylene) (PPP), a typical chain with an aromatic backbone. Using molecular dynamics simulations, we found that the axial thermal conductivity of PPP chains shows an anomalous dimensionality dependence where the thermal conductivity of double-chain and 3D crystal structures is higher than that of a 1D single chain. We analyzed the probability distribution of dihedral angles and found that π-π stacking between phenyl rings restricts the free rotation of phenyl rings and forms a long-range order along the chain, thus enhancing thermal transport along the chain direction. Though possessing a stronger bonding strength and stabilizing the multiple-chain structure, π-π stacking does not lead to a higher interchain thermal conductance between phenyl rings compared with that between aliphatic chains. Our simulation results on the effects of π-π stacking provide insights to engineer thermal transport in polymers at the molecular level.

Enhancement of the upper critical field in disordered nanocrystalline MgB2 bulks

Nanocrystalline superconducting MgB2 bulks were prepared by high pressure sintering of ball milled MgB2 powders. The disorder produced in these nanocrystalline materials shows a significant improvement in the upper critical field Hc2(0) of 38 T without severe suppression the superconducting transition temperature Tc. The band gap Δσ(0) decreases from 6.3 ± 0.1 meV to 5.4 ± 0.1 meV, while Δπ(0) keeps nearly fixed with the decrease in grain size to 20 nm. The ratios of the mean free path to the BCS coherence lengths li/ξ0,i(i = σ, π) turn out to be far less than 1, confirming that the two bands of the nanocrystalline bulks are both in the dirty limit. Strong intraband scatterings are responsible for the enhancement of Hc2(0).

Statistical Uncertainty in the Frequency Dependence of the Intensity Scintillation Index (S4)

AbstractThe intensity scintillation index (S4) exhibits a linear frequency dependence derived from a power law phase screen model, with the exponent determined by the phase spectral index (p). However, it is well known that a departure from linearity occurs as S4 increases and saturates under strong scatter conditions. Additionally, statistical errors over a finite time window can also cause the scintillation statistic to deviate from the power law dependence on frequency. This paper presents a parametric analysis on the deviation in the S4 frequency dependence from the power law form. This investigation focuses on the contribution of statistical uncertainty in S4 calculations to the deviation in the frequency dependence according to the selected parameters: S4 and Fresnel frequency (fF). The deviation is determined by comparing p estimates calculated from the power law relationship of S4 at different L‐band frequencies with those inferred from a model‐fitting method using a multi‐frequency global navigation satellite system scintillation data set. The results show the well‐known departure from linearity as S4 increases, and reveal increasing deviations with decreasing fF. The spectral analysis indicates that the variability at the lowest frequencies of intensity spectra contributes to the statistical errors in S4 calculations due to the finite time window, particularly when scintillations are dominated by low‐frequency contributions at low fF. Simulation results of scintillation realizations show much lower deviations with a 30‐min compared to 1‐min window. This suggests a 1‐min window can be too short for reliable S4 calculations due to statistical uncertainty, especially at low fF.

Ultraviolet communication with a large scattering angle via artificial agglomerate fog.

Considering strong signal attenuation of the large-angle non-line-of-sight (NLOS) link achieved due to the ultraviolet (UV) scattering properties, we propose to increase the UV communication link gain under a large scattering angle via generating agglomerate fog within a certain range as a secondary light source. In this study, a channel model with locally strong scatterers from agglomerate fog is proposed based on Monte Carlo ray-tracing approaches. Mie theory is adopted to calculate the atmospheric channel parameters, to further evaluate the link gain of a channel under non-uniform atmosphere. The performance of scattering system in the presence of fog conditions depends on the relative positions of the light source to the fog and to the receiver. The link gain results reveal the transmission capabilities for different scattering channel geometries, and give the optimal spray point location within a 10 m communication range. We further establish a foggy NLOS system using an agglomerate fog generator to verify our research in the real environment. The results show that the received signal strength of the NLOS link can be effectively enhanced by constructing scatterers in the atmospheric channel, which significantly decreases the bit-error rate (BER).

Statistical analysis of the critical frequency foF2 dependence on various solar activity indices

A detailed analysis of the description of the F2-layer critical frequency foF2 dependence on solar activity indices (proxies) is analyzed. To do that, the results of the vertical ionospheric sounding at Juliusruh station during two winter months (January and February), two equinox months (March and October), and a summer month (June) are considered. Five solar proxies: Ly-α, MgII, Rz, F30, and F10.7 are analyzed. The determination coefficient R2 according to Fisher F-test is used as a measure of the quality of the description of the foF2 dependence on SA by each of the proxies It is found that a well pronounced diurnal variation in the R2 is observed: this value is higher at the near-noon hours than at night. In other words, all indices describe better the foF2 behavior in the daytime than at night. A well pronounced diurnal variation in R2 is observed also for four proxies in the equinoctial months, whereas that variation is pronounced much worse for Rz: a strong scatter of the R2 values is seen. A diurnal variation in the R2 is absent completely in June: jumps of that value from one hour to another are observed. A few examples of the same analysis are presented also based on the Boulder stations data. The aforementioned analysis allows us to conclude that the most reliable SA proxies for description of the foF2 dependence on SA are (in the declining order) F30, MgII, and Ly- α.

Experimental analysis of lithium-ion cell procurement: Quality differences, correlations, and importance of cell characterization

Since every production process exhibits slight variances, lithium-ion cells are usually tested for their function and quality by the manufacturer during end-of-line testing, whereby specific tolerance limits are generally not communicated to end users. In addition, commercial cells are usually sold through various distribution channels and intermediaries, creating a black box regarding the actual cell properties delivered, unless a clear requirement specification supplier commitment has been defined. In this work, 250 cylindrical 18650 cells from the same manufacturer and model were obtained via two distribution channels and quality grading categories and extensively characterized using established methods of cell characterization. The relationship between the various measurements and cell characteristics was evaluated and visualized to reveal interrelated factors which remain hidden in aggregate observations. The results show that the choice of vendor can have a significant impact on the performance characteristics obtained for the supposedly same product, with strong scatter being evident both between and within the three batches. In addition, the given grading classes do not correlate with the measured cell performance in terms of capacity, internal resistance, cycling performance or a combination of the aforementioned criteria. This could be caused by an imperfect classification by the manufacturer or a mixing of quality classes at the vendor. Hence, the investigation encourages the implementation of appropriate cell quality categorization depending on the application and a profound exchange of data between vendors and customers, in order to prevent the utilization of under-performing cells.

Nonlinearity-Compensated Short-Range FMCW Radar for Weak Target Imaging

An easily implementable, low-cost, and portable broadband frequency-modulated continuous wave (FMCW) synthetic aperture radar (SAR) system is presented for short-range imaging applications. Design considerations and radar performance metrics are discussed in detail while investigating systematic and nonsystematic performance-limiting factors. This article introduces a signal processing procedure based on a closed-form comprehensive mathematical model to characterize the impact of the nonlinear frequency sweep on radar performance. Based on the proposed model, the nonlinearity is compensated using the time-resampling technique and without needing a reference response. A single-zone calibration does not provide enough accuracy when multiple targets are widely distributed in the cross-range direction. A range-and angle-dependent calibration scheme is proposed to mitigate the second-order effects more precisely, which are otherwise difficult to model mathematically. Detecting less reflective targets in the presence of a strong scatterer is challenging. A range-gating method based on a tunable active bandpass filter (BPF) is proposed to improve the radar system’s sensitivity by suppressing the dominant reflection and enhancing the weaker scatterer. The results are verified by developing SAR images of targets in free space and through a residential wall.

Seamount Clutter Modeling for Long Range Seabed Reverberation in Deep Sea

Reverberation clutter induced by discrete strong scatterers is an important reason which will increase the false alarm probability of active detection, which seriously affects the performance of active sonar detection. In this paper, taking seamount as an example, the theoretical modeling and simulation of the clutter in the deep-sea induced by seamount are carried out by using the ray-mode theory. Through ray-mode approximation, the acoustic rays in surface and reverse channels are ignored, a theoretical model is proposed to calculate the seabed reverberation clutter induced by seamounts, and the physical mechanism of how the seabed reverberation clutter is induced by seamounts is analyzed. The results show that reverberation intensity will increase in the location of seamount and induce discrete reverberation clutter. The greater the fluctuation of seamount, the greater the reverberation clutter intensity.

Compositing effects for high thermoelectric performance of Cu2Se-based materials

Thermoelectric materials can realize direct conversion between heat and electricity, showing excellent potential for waste heat recovery. Cu2Se is a typical superionic conductor thermoelectric material having extraordinary ZT values, but its superionic feature causes poor service stability and low mobility. Here, we reported a fast preparation method of self-propagating high-temperature synthesis to realize in situ compositing of BiCuSeO and Cu2Se to optimize the service stability. Additionally, using the interface design by introducing graphene in these composites, the carrier mobility could be obviously enhanced, and the strong phonon scatterings could lead to lower lattice thermal conductivity. Ultimately, the Cu2Se-BiCuSeO-graphene composites presented excellent thermoelectric properties with a ZTmax value of ~2.82 at 1000 K and a ZTave value of ~1.73 from 473 K to 1000 K. This work provides a facile and effective strategy to largely improve the performance of Cu2Se-based thermoelectric materials, which could be further adopted in other thermoelectric systems.

Simulating superconducting properties of overdoped cuprates: The role of inhomogeneity

Theoretical studies of disordered $d$-wave superconductors have focused, with a few exceptions, on optimally doped models with strong scatterers. Addressing recent controversies about the nature of the overdoped cuprates, however, requires studies of the weaker scattering associated with dopant atoms. Here we study simple models of such systems in the self-consistent Bogoliubov-de Gennes (BdG) framework, and compare to disorder-averaged results using the self-consistent-T-matrix-approximation (SCTMA). Despite surprisingly linear in energy behavior of the low-energy density of states even for quite disordered systems, the superfluid density in such cases retains a quadratic low-temperature variation of the penetration depth, unlike other BdG results reported recently. We trace the discrepancy to smaller effective system size employed in that work. Overall, the SCTMA performs remarkably well, with the exception of highly disordered systems with strongly suppressed superfluid density. We explore this interesting region where gap inhomogeneity dominates measured superconducting properties, and compare with overdoped cuprates.

Reducing patch-like Errors in SAR offset tracking displacements using logarithmic transformation and a weighted NCC algorithm

ABSTRACT Pixel offset tracking (OT) algorithm is a useful tool for measuring large surface displacements by matching amplitudes in master and slave synthetic aperture radar (SAR) images. However, strong backscatters can cause homogeneous errors within a matching window (referred to as patch-like errors) in traditional OT processing, thereby misleading the interpretation of displacement events, especially over a small area. In this letter, we proposed an improved SAR OT algorithm to reduce patch-like errors. In which, a logarithmic transformation was firstly utilized to narrow the SAR amplitude range between strong and weak back scatterers. Strong backscatters causing patch-like errors were then statistically detected with an indicator of median absolute deviation. Finally, those strong backscatters were excluded from SAR OT processing using a weighted normalized cross-correlation scheme, in order to reduce the caused patch-like errors. Two real data tests over the Shuozhou and Yulin coal mining areas, China, suggest that the mean accuracy of the displacements estimated by the presented method improved about 30%, with respect to that estimated by the traditional OT algorithm. The proposed SAR OT algorithm offers a robust option to measure large displacements, especially over a small area, associated with anthropologic or geophysical activities.

Strong Scattering from Low-Frequency Rattling Modes Results in Low Thermal Conductivity in Antimonide Clathrate Compounds

Recent discoveries of materials with ultralow thermal conductivity open a pathway to significant developments in the field of thermoelectricity. Here, we conduct a comparative study of three chemically similar antimonides to establish the root causes of their extraordinarily low thermal conductivity (0.4–0.6 W m–1 K–1 at 525 K). The materials of interest are the unconventional type-XI clathrate K58Zn122Sb207, the tunnel compound K6.9Zn21Sb16, and the type-I clathrate K8Zn15.5Cu2.5Sb28 discovered herein. Calculations of the phonon dispersions show that the type-XI compound exhibits localized (i.e., rattling) phonon modes with unusually low frequencies that span the entire acoustic regime. In contrast, rattling in type I clathrate is observed only at higher frequencies, and no rattling modes are present in the tunnel structure. Modeling reveals that low-frequency rattling modes profoundly limit the acoustic scattering time; the scattering time of the type-XI clathrate is half that of the type-I clathrate and a quarter of that of the tunnel compound. For all three materials, the thermal conductivities are additionally suppressed by soft framework bonding that lowers the acoustic group velocities and structural complexity that leads to diffusonic character of the optical modes. Understanding the details of thermal transport in structurally complex materials will be crucial for developing the next generation of thermoelectrics.