Total Scattering Cross Section Research Articles

Direct measurement of the scattering cross sections of liquid ortho-deuterium for ultracold neutrons and comparison with model calculations

Liquid deuterium is a fluid between the quantum and classical regimes. It attracts interest from fundamental research for the verification of quantum calculations but also from neutron physics as it is a widely used neutron moderator medium. We have measured the scattering cross sections of liquid ortho-deuterium in the ultracold-neutron range for four different temperatures (19-23 K) and compared them with calculations from a parameter-free analytical calculation model as well as with previous measurements at 19 K by another research group. All three show remarkable agreement, which establishes the validity of the calculation model and proves it is a reliable basis for the derivation of scattering kernels. The deconvolution of our measured transmission data changed the cross section results noticeably only for neutrons faster than 10 m/s. We found the total scattering cross section of liquid deuterium to be inversely proportional to velocity, as is predicted by theory.

Unidirectional invisibility in a PT-symmetric structure designed by topology optimization.

This study designs a piecewise homogeneous dielectric structure with parity-time (PT) symmetry that realizes the unidirectional invisibility of a perfect electric conductor in two dimensions. We apply topology optimization and design a PT-symmetric material that minimizes the total scattering cross section for a given plane wave to achieve unidirectional invisibility. A rigorous mode-matching finite element method is used to perform all computations. The designed PT-symmetric structure suppressed plane-wave scattering by approximately 99% for the given incident direction, whereas the reversed incident wave experienced 83 times larger scattering intensity. The proposed method provides a novel approach, to the best of our knowledge, to promote various applications of PT symmetry.

Coherent multichannel optical theorem: Quantum control of the total scattering cross section

The optical theorem is a fundamental aspect of quantum scattering theory. Here, we generalize this theorem to the case where the incident scattering state is a superposition of internal states of the collision partners, introducing additional interference contributions and, e.g., providing a route to control the total integral cross section. As in its standard form, forward scattering plays an essential role in the multichannel optical theorem, but with interference terms being related to the inelastic forward scattering amplitudes between states in the initial superposition. Using the resultant control index, we show that extensive control is possible over ultracold collisions of oxygen molecules in their rovibrational ground states, and of $^{85}\mathrm{Rb}\text{\ensuremath{-}}^{85}\mathrm{Rb}$ collisions, promising systems for the experimental demonstration of the quantum interference control of the total scattering cross section.

Improving the Reverberation Chamber Performance Using a Reconfigurable Source Stirring Antenna Array

An antenna array composed of 16 asymmetric distributed monopoles on a circular dielectric substrate is proposed to use as a reconfigurable source stirrer in a reverberation chamber (RC). The distance between antennas is above λ2 to ensure the isolation between antennas below −15 dB. In order to ensure the mutual independence of the stirring modes of the antenna array, the 16 monopoles are asymmetrically distributed on both sides with X and Y as axes. The stirring efficiency of a stirrer is related to its size, and usually a higher stirring efficiency means a larger stirrer. The source stirring technique can achieve high efficiency, but the rotation of a source stirring antenna is limited to the radius of the turntable in an RC. To solve this problem, we propose to use an antenna array as a reconfigurable source stirrer, which can achieve a high efficiency of source stirring. As the antenna array can achieve source stirring and mechanical stirring at the same time, the source stirring section is not limited to the radius of the turntable; the switch between the selected antennas of the antenna array can increase the actual stirring samples of the stirring. Thus, the efficiency can be improved significantly and has been verified by the measurement results. The volume occupied by the conventional mechanical stirrer is reduced, which improves the working volume. The figures of merit, such as the field uniformity (FU), the independent sample numbers, the average K-factor, and the total scattering cross-section (TSCS), are measured and compared to the conventional mechanical stirring, mechanical stirring, and source stirring of the antenna array. Measurement results of the reconfigurable stir are illustrated below. FU has the best convergence and the lowest value, below 0.5 dB. The independent sample number is the largest. K-factor is the lowest, and most ranges are below −15 dB. Additionally, the proposed method has the largest TSCS, above 2 m2. These confirm that the proposed reconfigurable source stirring technique outperforms other three stirring techniques significantly. The antenna array can be easy to disassemble and install in different RC. With a wide range of working band, the proposed method can be applied to 5G OTA testing.

Investigation of Electron Scattering from Vinyl Ether and Its Isomers

This article reports a comprehensive theoretical study of electron scattering from vinyl ether and its isomers. The electron–molecule quantum collision problem is solved through a complex optical potential approach. From the solution of the Schrödinger equation corresponding to this scattering problem, various cross sections were obtained for energies from ionization threshold of target to 5 keV. To deal with the non-spherical and complex structure, a multi-center group additivity approach is used. Furthermore, geometrical screening correction is applied to compensate for the overestimation of results due to electron charge density overlap. We found an interesting correlation between maximum ionization cross section with polarizability and ionization energy of the target molecule. The fitting of the total cross section as a function of the incident electron energy is reported in this article. The correlation between the effective diameter of the target and the projectile wavelength at maximum ionization energy is also reported for vinyl ether and its isomers. The data presented here will be useful to biomedical field, mass spectrometry, and chemical database for military range applications. The cross sections are also important to model Mars’s atmosphere due to their presence in its atmosphere. The gas-kinetic radius and the van der Waals coefficients are estimated from the electron-impact total scattering cross sections. In addition, the current study predicts the presence of isomeric effects in the cross section.

Acoustic backscatter and attenuation of a flocculated cohesive sediment suspension

The acoustic return from a flocculated suspension recorded with a mono-static transducer was computed considering that the small distances among the primary particles that form the floc increased the backscatter coherent component, which otherwise would be neglected. Using this approach the floc backscatter form function was computed and it was found not only to depend on floc size and fractal dimension, but also on size distribution of the primary particles. Sound attenuation was found to be dominated by the primary particle total scattering cross section, with no deviations that could be clearly associated with the occurrence of flocculation. The proposed theory accurately represented the detailed experimental data of MacDonald et al. (2013).

Acoustic metamaterial design using Conditional Wasserstein Generative Adversarial Networks

This talk presents a method for generating planar configurations of scatterers with a reduced total scattering cross section (TSCS) by means of generative modeling and deep learning. The TSCS minimization via repeated forward modeling techniques, trial-error methods, and traditional optimization methods requires considerable computer resources and time. However, similar or better results can be achieved more efficiently by training a deep learning model to generate such optimized configurations producing low scattering effect. In this work, the Conditional Wasserstein Generative Adversarial Networks (cWGAN) is combined with Convolutional Neural Networks (CNN) to create the generative modeling architecture [1]. The generative model is implemented with a conditional proponent to allow the TSCS targeted design generation and is enhanced with the coordinate convolution (CordConv) layer to improve the model’s spatial recognition of cylinder configurations. The cWGAN model [1] is capable of generating images of 2D configurations of scatterers that exhibit low scattering. The method is demonstrated by giving examples of generating 2-cylinder and 4-cylinder planar configurations with minimal TSCS. [1]. P. Lai, F. Amirkulova, and P. Gerstoft. “Conditional Wasserstein generative adversarial networks applied to acoustic metamaterial design,” J. Acoust. Soc. Am. 150(6), 4362–4374 (2021).

The correction of Inelastic Neutron Scattering data of organic samples using the Average Functional Group Approximation

The use of the Average Functional Group Approximation for self-shielding corrections at inelastic neutron spectrometers is discussed. By taking triptindane as a case study, we use the above-mentioned approximation to simulate a synthetic dynamic structure factor as measured on an indirect-geometry spectrometer, as well as the related total scattering cross section as a function of incident neutron energy and sample temperature, and the transmission spectra depending on the sample thickness. These quantities, obtained in a consistent way from the Average Functional Group Approximation, are used to calculate the energy-dependent self-shielding correction affecting the sample under investigation. The impact on the intensities of low-energy vibrational modes is discussed, showing that at typical experimental conditions the sample-dependent attenuation factor is about 15% higher compared to the correction at higher energies.

Design of electromagnetic cloak with sequentially connected rectangular split ring resonators for S-band applications

<abstract> <p>An electromagnetic (EM) invisible cloak is designed and analyzed with serially interconnected split ring resonators (SRRs). The cloak consists of an array of a network of split ring resonators which operates at a 3 GHz resonating frequency. The split ring resonators are connected with transmission line and are wrapped around the cylindrical object. Cloak coupled with EM waves gets transferred around the cylindrical object and received to the other side of transmission. Scattering cross section (SCS) is analyzed for both cases, which results in the effect of resonance. The total scattering cross section of the cloaked object is reduced by using SRRs. The simulated and measured results are in great agreement with each other. The transmission-line-connected SRR cloak is useful for S-band applications specifically at 3 GHz resonance.</p> </abstract>

Dispersed media: scattering of sound waves, stratification in swirling flows and sedimentation processes

A brief review on three problems from the multiphase media mechanics area related to the study of the dynamic behavior of disperse systems under various conditions and methods of interaction with external fields or the environment is presented. The problems are an integral part of the scientific project “Hydrodynamic effects in multiphase and thermoviscous media under wave and thermal effects”. The review is carried out in the context of the vector of development of modern research in the relevant areas. As a part of solving the problem of acoustic wave scattering by a set of spherical bubbles or drops, taking into account their sound permeability, the fast multipole method was developed to expand the possibilities of its application in the considered cases. On the basis of addition theorems for spherical wave functions, a new formula for the total scattering cross section for a set of interacting sound-permeable spheres arbitrarily located in space is obtained. An important aspect of the research was the estimation of the region in the parameter space of the problem, in which the effects of multiple scattering are significant. The second problem is related to mathematical modeling of a swirling turbulent flow containing particles of a dispersed phase. For numerical studies of temperature stratification in a vortex tube, an algorithm and a computer code were created using an orthogonalized finite-volume mesh with separation of the near-wall layer. A number of parametric studies have been carried out, in particular, the dependence of the temperature of the outgoing air in the cold diaphragm channel depending on the diameter of the diaphragm has been considered. In order to increase the efficiency of geological exploration in solving the third problem for describing the process of magmatic ore formation, a system of equations based on the methods of mechanics of multiphase media and thermohydrodynamics is proposed. The mathematical model provides for the heat exchange of the magmatic melt flow with the surrounding host rocks, as well as the release of heavy and light fractions from basaltic magma during its cooling. The results obtained in the course of the computational experiment indicate the possibility of a periodically inhomogeneous nature of the distribution of ore-forming fractions.

Conditional Wasserstein generative adversarial networks applied to acoustic metamaterial design.

This work presents a method for the reduction of the total scattering cross section (TSCS) for a planar configuration of cylinders by means of generative modeling and deep learning. Currently, the minimization of TSCS requires repeated forward modelling at considerable computer resources, whereas deep learning can do this more efficiently. The conditional Wasserstein generative adversarial networks (cWGANs) model is proposed for minimization of TSCS in two dimensions by combining Wasserstein generative adversarial networks with convolutional neural networks to simulate TSCS of configuration of rigid scatterers. The proposed cWGAN model is enhanced by adding to it a coordinate convolution (CoordConv) layer. For a given number of cylinders, the cWGAN model generates images of 2D configurations of cylinders that minimize the TSCS. The proposed generative model is illustrated with examples for planar uniform configurations of rigid cylinders.

Total Cross Sections for Electron and Positron Scattering on Molecules: In Search of the Dispersion Relation

More than one hundred years of experimental and theoretical investigations of electron scattering in gases delivered cross-sections in a wide energy range, from few meV to keV. An analogy in optics, characterizing different materials, comes under the name of the dispersion relation, i.e., of the dependence of the refraction index on the light wavelength. The dispersion relation for electron (and positron) scattering was hypothesized in the 1970s, but without clear results. Here, we review experimental, theoretical, and semi-empirical cross-sections for N2, CO2, CH4, and CF4 in search of any hint for such a relation—unfortunately, without satisfactory conclusions.

Inverse metacluster design using generative modeling for minimal scattering response

Metamaterials are subwavelength-sized artificial structures with the ability to manipulate incident waves in such a way that affects how the energy propagates throughout the medium. In acoustics, particularly placed scattering elements can reduce the total scattering cross section (TSCS) response. We propose a method to inversely design acoustic metamaterial configurations using deep learning and generative modeling. Using our proprietary multiple scattering solver with MATLAB optimization toolbox, we generate a dataset of optimal configurations with minimized TSCS within a discrete range of wavenumbers. We use this dataset to train a Conditional Wasserstein Generative Adversarial Network (cWGAN) to generate similar metacluster designs corresponding to specified input TSCS. To improve the coordinate recognition ability of the cWGAN, we include the novel CoordConv layer in the generator and critic. After training, the cWGAN can produce a variety of optimal configurations given an expected TSCS. Evaluating TSCS of generated configurations shows that the model is capable of proposing scatterer configurations that are comparable or better than the dataset within the optimized range.

Design of Acoustic Cloak Using Generative Modeling and Gradient-Based Optimization

Metamaterials are engineered composites that can achieved electromagnetic and mechanical properties that do not exist in natural materials by rearranging their structures. Due to the complexity of the objective functions, it is difficult to find the globally optimized solutions in metameterial design. This talk outlines a gradient-based optimization with generative networks that can search for the globally optimized cloaking devices over a wide range of parameters. The GLO-Net[1] model was developed originally for one-dimensional nano-photonic metagratings is generalized in this work to design two-dimensional broadband acoustic cloaking devices by perturbing positions of each scatterer in planar configuration of cylindrical scatterers. Such optimized cloaking devices can efficiently suppress the total scattering cross section to the minimum at certain parameters over range of wavenumbers. During training each iteration, a generative model generates a batch of metamaterials and compute the total scattering cross section and its gradients using an in-house built multiple scattering MATLAB solver. To evaluate our approach, we compare our obtained results with fmincon in MATLAB. Reference: [1] Jiaqi Jiang and Jonathan A. Fan. Simulator-based training of generative neural networks for the inverse design of metasurfaces. Nanophotonics, 9(5):1059-1069, nov 2019.

Reinforcement learning applied to metamaterial design.

This paper presents a semi-analytical method of suppressing acoustic scattering using reinforcement learning (RL) algorithms. We give a RL agent control over design parameters of a planar configuration of cylindrical scatterers in water. These design parameters control the position and radius of the scatterers. As these cylinders encounter an incident acoustic wave, the scattering pattern is described by a function called total scattering cross section (TSCS). Through evaluating the gradients of TSCS and other information about the state of the configuration, the RL agent perturbatively adjusts design parameters, considering multiple scattering between the scatterers. As each adjustment is made, the RL agent receives a reward negatively proportional to the root mean square of the TSCS across a range of wavenumbers. Through maximizing its reward per episode, the agent discovers designs with low scattering. Specifically, the double deep Q-learning network and the deep deterministic policy gradient algorithms are employed in our models. Designs discovered by the RL algorithms performed well when compared to a state-of-the-art optimization algorithm using fmincon.

Design and simulation of broadband multiphase pentamode metamaterials

Pentamode metamaterial (PM) has great potential in controlling underwater waves due to its broadband and solid-state merits. The reported underwater PM devices were all designed with single metallic materials, which largely limited the realizable properties range of PM. Pentamode metamaterials for simulating water with dual phase and triple phase were proposed and investigated in this article. It was revealed that the acoustic scattering of the proposed PM in water is very weak within the simulation frequency range except at some resonance frequencies. The mean total scattering cross section of the proposed pentamode metamaterials is about two orders smaller than that of the titanium alloy block, while the far-field sound pressure of pentamode microstructures was only about 1/3 of that of titanium alloy block. It also illustrates that the second and third phase of the unit cell could not only broaden the realizable properties range of PM but also suppress the bending and rotational vibration modes, demonstrating the efficacy and broadband characteristic of the multiphase pentamode metamaterial.

A novel method to obtain integral parameters of the orientation distribution function of textured polycrystals from wavelength-resolved neutron transmission spectra

A novel method to estimate integral parameters of the orientation distribution function (ODF) in textured polycrystals from the wavelength-resolved neutron transmission is presented. It is based on the expression of the total coherent elastic cross section as a function of the Fourier coefficients of the ODF. This method is broken down in detail for obtaining Kearns factors in hexagonal crystals, and other material properties that depend on the average of second- and fourth-rank tensors. The robustness of the method against three situations was analyzed: effects of sample misalignment, of cutoff value l max of the series expansion and of experimental standard deviation. While sample misalignment is shown not to be critical for the determination of Kearns factors and second-order-rank properties, it can be critical for fourth-rank and higher-order tensor properties. The effect of the cutoff value on the method robustness is correlated to the standard deviation of the experimental data. In order to achieve a good estimation of the Fourier coefficients, it is recommended that the experimental standard deviation be around 3–5% of the total scattering cross section of the material for the method to be stable. The method was applied for the determination of Kearns factors from transmission measurements performed at the instrument ENGIN-X (ISIS) on a Zr–2.5 Nb pressure tube along two sample directions and was shown to be able to estimate Kearns factors with an error below 5%.

Compton Scattering Total Cross Section at Next-to-Leading Order.

An analytic formula is given for the total scattering cross section of an electron and a photon at order α^{3} in QED. This includes both the double-Compton scattering real-emission contribution as well as the virtual Compton scattering part. When combined with the recent analytic result for the pair-production cross section, the complete α^{3} cross section is now known. Both the next-to-leading order calculation as well as the pair-production cross section are computed using modern multiloop calculation techniques, where cut diagrams are decomposed into a set of master integrals that are then computed using differential equations.

MENP: an open-source MATLAB implementation of multipole expansion for nanophotonics

In modern nanophotonics, multipolar interference plays an indispensable role to realize novel optical devices represented by metasurfaces with unprecedented functionalities. Not only to engineer sub-wavelength structures that constitute such devices but also to realize and interpret unnatural phenomena in nanophotonics, a program that efficiently carries out multipole expansion is highly demanded. MENP is a MATLAB program for computation of multipole contributions to light scattering from current density distributions induced in nanophotonic resonators. The main purpose of MENP is to carry out post-processing of a rigid multipole expansion for full-field simulations that in principle provide the information of all near- and far-field interactions (e.g., as a total scattering cross section). MENP decomposes total scattering cross sections into partial ones due to electric and magnetic dipolar and quadrupolar terms based on recently developed exact multipole expansion formulas. We validate the program by comparing results for ideal and realistic nanospheres with those obtained with the Mie theory. We also demonstrate the potential of MENP for analysis of anapole states by calculating the multipole expansion under the long-wavelength approximation, which enables us to introduce toroidal dipole moments.

Excitation of guided waves of grounded dielectric slab by a THz plane wave scattered from finite number of embedded graphene strips: Singular integral equation analysis

The scattering and absorption of the H-polarised wave by a finite number of graphene strips embedded into a grounded dielectric slab is considered in the terahertz frequency range. The rigorous and efficient method of singular integral equations is used here. The discretisation is based on the convergent meshless Nystrom-type algorithm. Graphene is modelled as a zero thickness layer with a complex conductivity obtained from the Kubo formalism. Dependences of the total scattering cross section, including the part that is associated with dielectric slab guided waves, and absorption cross section as well as near field patterns are studied. The behaviour of the scattering and absorption characteristics is explained in terms of resonances on the natural modes supported by the studied structure. Special attention is paid to the excitation of the dielectric waveguide eigenwaves near the grating-mode resonances.