Small Scattering Cross Section Research Articles

Development of sensitivity calculation capability of Legendre moments of scattering angular distributions in NECP-MCX

The capability of sensitivity coefficients calculation of keff and reaction rate ratios to Legendre moment of scattering angular distributions based on a fully continuous method has been developed in the Monte Caro code NECP-MCX. The sensitivity calculation was based on Iterated Fission Probability (IFP) and Contributon-Linked eigenvalue sensitivity/Uncertainty estimation via Track-length importance Characterization (CLUTCH) method for k-eigenvalue sensitivity analysis and Generalized Adjoint Responses in Monte Carlo (GEAR-MC) method for generalized sensitivity analysis. The Jezebel and Flattop problems are analyzed and the sensitivity results calculated by NECP-MCX are compared with those by Serpent and ERANOS to demonstrate the accuracy of NECPMCX. In addition, the CLUTCH-FC (CLUTCH coupled with forced collisions) method is implemented to reduce the variance of sensitivity coefficients of Legendre moment of scattering angular distributions with relatively small scattering cross sections. The numerical results indicate that 1) the sensitivity coefficients calculated by NECP-MCX agree well with those by Serpent and ERANOS, and 2) the CLUTCH-FC method is efficient in reducing the variance of sensitivity coefficients of Legendre moment of scattering angular distributions with relatively small scattering cross sections. The Figure of Merit (FOM) values of sensitivity coefficients calculated by the CLUTCH-FC method have increased by 1.9~306.0 times for Jezebel and Flattop problems compared to those calculated by the CLUTCH method.

Time-resolved photoelectron diffraction imaging of methanol photodissociation involving molecular hydrogen ejection.

Imaging ultrafast atomic and molecular hydrogen motion with femtosecond time resolution is a challenge for ultrafast spectroscopy due to the low mass and small scattering cross section of the moving neutral hydrogen atoms and molecules. Here, we propose time- and momentum-resolved photoelectron diffraction (TMR-PED) as a way to overcome limitations of existing methodologies and illustrate its performance using a prototype molecular dissociation process involving the sequential ejection of a neutral hydrogen molecule and a proton from the methanol dication. By combining state-of-the-art molecular dynamics and electron-scattering methods, we show that TMR-PED allows for direct imaging of hydrogen atoms in action. More specifically, the fingerprint of hydrogen dynamics reflects the time evolution of polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) as would be recorded in X-ray pump/X-ray probe experiments with few-femtosecond resolution. We present the results of two precursor experiments that support the feasibility of this approach.

Analysis of possibility to improve the civil marine radar efficiency during navigation in the Arctic waters

The paper presents the civil marine radar (CMR) efficiency in the polar waters in the case of advanced signal processing based on the optimal filtering of the voltage coming to the intermediate frequency amplifier. The structure of the radar subsystem was modified by adding two complementary filter units that allow to transform the filter frequency response into the almost sinc-function curve providing much better detection of weak radar echoes. The studies have shown that the updated radar matched filter makes it possible for CMR to observe objects with relatively small scattering cross section at the considerably longer distances than up-to-date CMR. Also it is explained that this radar signal processing doesn’t require the increasing of CMR cost which can also be considered as advantage of the method.

Kerker effect, superscattering, and scattering dark states in atomic antennas

We study scattering phenomena such as the Kerker effect, superscattering, and scattering dark states in a subwavelength atomic antenna consisting of atoms with only electric dipole transitions. We show that an atomic antenna can exhibit arbitrarily large or small scattering cross sections depending on the geometry of the structure and the direction of the impinging light. We also demonstrate that atoms with only an electric dipole transition can exhibit a directional radiation pattern with zero backscattering when placed in a certain configuration. This is a special case of a phenomenon known as the Kerker effect, which typically occurs in the presence of both electric and magnetic transitions. Our findings open a pathway to design highly directional emitters, nonradiating sources, and highly scattering objects based on individually controlled atoms.

Transport Properties of the Simple Lennard-Jones/Spline Fluid I: Binary Scattering and High-Accuracy Low-Density Transport Coefficients

Inspired by recent work on the thermodynamic properties of the Lennard-Jones/spline (LJ/s) fluid, we have considered the transport properties of the simple LJ/s fluid. The binary scattering problem for LJ/s particles was solved numerically, and results were compared to the untruncated LJ fluid. The scattering dynamics are affected both by the restricted range of the LJ/s potential, and the stronger attraction between LJ/s particles at distances between the inflection point of the potential and the cutoff range. At small relative kinetic energies, it was found that the scattering cross section of the LJ/s particles is much smaller than that of the LJ particles. The shear viscosity, thermal conductivity, and the self-diffusion coefficient were calculated from the scattering cross sections by the Chapman-Enskog method, and a six-parameter equation with a worst case accuracy of roughly 1 % over the temperature interval [0.1,1000] in LJ units is provided. The smaller scattering cross section at low kinetic energies leads to transport coefficients of the LJ/s fluid to be greater than those of the LJ fluid at low temperatures, and were all found to be roughly 50 % greater at T = 0.1, which is the lowest temperature considered.

Mitigation of self-focusing in Thomson scattering experiments

A fundamental challenge associated with measuring Thomson scattering comes from the small scattering cross section associated with the interaction. To improve photon statistics, a powerful Thomson-scattering probe laser is required. Ponderomotive self-focusing limits the maximum power in the Thomson-scattering probe and was shown to limit the maximum achievable Thomson-scattering signal-to-noise ratio. Operating the laser at powers above the self-focusing critical power was shown to cause beam degradation, which reduced the amount of collected Thomson-scattered light. Using a phase plate was shown to improve laser beam propagation and consequently improve the signal-to-noise ratio in the measured spectrum.

The Influence of Material Composition and Geometry of Heterogeneous Medium with Resonance Scattering on the Density of Fast Neutrons Remote from the Source

An option of high-density neutron flux generation locally, at a distance from the source, is considered in this paper. The energy dependence of the cross sections of neutron scattering by nuclei of some isotopes displays resonance behavior. Below the resonance level by energy, there is a deep drop of the scattering cross section dictated by the effect of resonance and potential scattering interference. When a slowing-down neutron gets the energy corresponding to a very small scattering cross section, it is capable of flying for several meters without interaction with nuclei of the medium. The possibilities of creating conditions for selection of high-energy neutrons emitted by a reactor or other source and moving in the required direction are investigated in the paper. The research described in the paper is focused on various factors (material composition, geometry) influencing the neutron radiation density and achievable neutron flux limits.

Nanoarchitecture's Influence on Surface-Enhanced Spectroscopy: The Use of Surface-Enhanced Raman Scattering Substrates

The rapid and reliable detection of a diverse set of biomolecules, such as metabolites, pharmaceuticals, nucleic acids, amino acids, proteins, and peptides, requires analytical techniques capable of label-free chemical identification. Inelastic light scattered by a molecule, a quantized vibrational signature, is the physical phenomenon behind Raman spectroscopy [1]. Despite the rich information offered by regular Raman spectroscopy, it has not been the primary choice as a handy analytical tool comparable to Fourier-transform infrared and ultraviolet-visible spectroscopy due to its inherently small scattering cross section. The signal is weak for most molecules except in some special cases of resonating molecular structures. For example, the benzene molecule, which is relatively strong for Raman scattering, exhibits a scattering cross section of 2.6 x 10-29 cm2?molecule-1?sr-1. This value is 12-14 orders of magnitude lower than a typical fluorescence cross section [2].

Confocal-cavity-enhanced Raman scattering of ambient air

Raman spectroscopy is a powerful diagnostic method for gas analysis due to its advantages like non-invasiveness and fast speed. However, its applications are greatly restricted because of the weak signal level caused by small scattering cross section. In order to enhance the Raman signal level and improve the detection sensitivity, a sample cell of confocal cavity is designed and the enhanced Raman signal of ambient air based on this cavity is demonstrated experimentally. The confocal cavity is constructed with a pair of plano-concave reflectors with a curvature radius of 150 mm and reflectivity of 92%. This low reflectivity design not only allows for bandwidth matching with the line-width of excitation laser but also makes the resonant condition satisfied easily. The measured output power of the confocal cavity is over 42 mW in resonant condition, which gives a coupling efficiency of 87.5% when divided with the input power 48 mW. The high coupling efficiency enables the output power efficiently to reach 11 times that for the intra-cavity laser power in one direction. Raman scattering of ambient air is tested to verify the performance of the confocal cavity. In our experiments, the Raman signals are collected in a forward scattering configuration by an imaging Raman spectrometer which is connected to a CCD camera. Strong Raman signals of O2 and N2, even H2O are observed with 1 s exposure time in resonant condition, and rotational lines (O-branch and S-branch) of O2 and N2 are also clearly detected when exposure time is set to be 10 s. Compared with the results obtained without confocal cavity, the Raman signal level is enhanced 17 times and the signal-to-noise ratio is improved twice. In addition, a limit of detection (3) at a magnitude of 200 ppm for CO2 in ambient air is achieved for the resonant confocal cavity. These results indicate that the system can significantly enhance the spontaneous Raman scattering signal level and improve the detection sensitivity. Furthermore, the confocal cavity is applicable to the Raman analyses of other gas samples.

Near-field-assisted localization: effect of size and filling factor of randomly distributed zinc oxide nanoneedles on multiple scattering and localization of light

We investigate the influence of the diameter and the filling factor of randomly arranged ZnO nanoneedles on the multiple scattering and localization of light in disordered dielectrics. Coherent, ultra-broadband second-harmonic (SH) microscopy is used to probe the spatial localization of light in representative nm-sized ZnO arrays of needles. We observe strong fluctuations of the SH intensity inside different ZnO needle geometries. Comparison of the SH intensity distributions with predictions based on a one-parameter scaling model indicate that SH fluctuations can be taken as a quantitative measure for the degree of localization. Interestingly, the strongest localization signatures are found for densely packed arrays of thin needles with diameters in the range of only 30 nm range, despite the small scattering cross section of these needles. FDTD simulations indicate that in this case coupling of electric near-fields between neighbouring needles governs the localization.

COMPREHENSIVE MULTISPECTRAL APPROACH TO DETECTION OF GROUND, SURFACE AND AIR TARGETS

System approach has been investigated with respect to detection of ground, surface and air targets with small scattering cross section (from 0,01 m2), on the basis of which modern security systems can be developed, that are able to detect people, vehicles and high-tech robots. The main advantage of this approach is its high performance, easy scalability, technological characteristics when producing elements of the system

Detecting Stealth Dark Matter Directly through Electromagnetic Polarizability.

We calculate the spin-independent scattering cross section for direct detection that results from the electromagnetic polarizability of a composite scalar "stealth baryon" dark matter candidate, arising from a dark SU(4) confining gauge theory-"stealth dark matter." In the nonrelativistic limit, electromagnetic polarizability proceeds through a dimension-7 interaction leading to a very small scattering cross section for dark matter with weak-scale masses. This represents a lower bound on the scattering cross section for composite dark matter theories with electromagnetically charged constituents. We carry out lattice calculations of the polarizability for the lightest "baryon" states in SU(3) and SU(4) gauge theories using the background field method on quenched configurations. We find the polarizabilities of SU(3) and SU(4) to be comparable (within about 50%) normalized to the stealth baryon mass, which is suggestive for extensions to larger SU(N) groups. The resulting scattering cross sections with a xenon target are shown to be potentially detectable in the dark matter mass range of about 200-700 GeV, where the lower bound is from the existing LUX constraint while the upper bound is the coherent neutrino background. Significant uncertainties in the cross section remain due to the more complicated interaction of the polarizablity operator with nuclear structure; however, the steep dependence on the dark matter mass, 1/m(B)(6), suggests the observable dark matter mass range is not appreciably modified. We briefly highlight collider searches for the mesons in the theory as well as the indirect astrophysical effects that may also provide excellent probes of stealth dark matter.

Diffusing magnetic Tb impurities and magnetotransport in strongly spin-polarized Bi films

As a contribution to electronic transport within strongly spin-polarized surface states and its modification by adsorption of magnetic impurities we studied the adsorption of Tb (atomic magnetic moment 10 ${\ensuremath{\mu}}_{B}$) on epitaxial Bi(111) films by means of surface sensitive (magneto)conductance and low-energy electron diffraction. Surface diffusion turned out to be non-negligible even at substrate temperatures of 10 K. The Tb adatoms finally nucleate at intrinsic defects of the Bi(111) surface, where the Tb impurities act as dopants but not as scatterers. Nevertheless, time-dependent measurements allowed to determine also single-particle Tb scattering properties, as also supported by simulations of adsorption kinetics and time-dependent conductance. The magnetoconductance properties are characterized by small charge transfer (0.05 $e$/atom) and strong spin-orbit scattering, which in this case results only in strong reduction of the weak antilocalization effect but not a reversal to weak localization as for Fe and Co [see L\"ukermann, Sologub, Pfn\"ur, Klein, Horn-von Hoegen, and Tegenkamp, Phys. Rev. B 86, 195432 (2012)]. Although Tb has a magnetic moment, which is by far higher than for adsorbed Fe and Co, it turns out that the $f$ electrons of Tb play essentially no role in scattering of the conduction electrons, yielding an even smaller scattering cross section than that for Fe and Co. The adatom coordination (interstitial or on the surface) may also play an important role.

Three-dimensional structure determination protocol for noncrystalline biomolecules using x-ray free-electron laser diffraction imaging

Coherent and intense x-ray pulses generated by x-ray free-electron laser (XFEL) sources are paving the way for structural determination of noncrystalline biomolecules. However, due to the small scattering cross section of electrons for x rays, the available incident x-ray intensity of XFEL sources, which is currently in the range of 10(12)-10(13) photons/μm(2)/pulse, is lower than that necessary to perform single-molecule diffraction experiments for noncrystalline biomolecules even with the molecular masses of megadalton and submicrometer dimensions. Here, we propose an experimental protocol and analysis method for visualizing the structure of those biomolecules by the combined application of coherent x-ray diffraction imaging and three-dimensional reconstruction methods. To compensate the small scattering cross section of biomolecules, in our protocol, a thin vitreous ice plate containing several hundred biomolecules/μm(2) is used as sample, a setup similar to that utilized by single-molecule cryoelectron microscopy. The scattering cross section of such an ice plate is far larger than that of a single particle. The images of biomolecules contained within irradiated areas are then retrieved from each diffraction pattern, and finally provide the three-dimensional electron density model. A realistic atomic simulation using large-scale computations proposed that the three-dimensional structure determination of the 50S ribosomal subunit embedded in a vitreous ice plate is possible at a resolution of 0.8 nm when an x-ray beam of 10(16) photons/500×500 nm(2)/pulse is available.

Plasmonic Nanorice Antenna on Triangle Nanoarray for Surface-Enhanced Raman Scattering Detection of Hepatitis B Virus DNA

The sensitivity and the limit of detection of Raman sensors are limited by the extremely small scattering cross section of Raman labels. Silver nanorice antennae are coupled with a patterned gold triangle nanoarray chip to create spatially broadened plasmonic "hot spots", which enables a large density of Raman labels to experience strong local electromagnetic field. Finite difference time domain simulations have confirmed that the quasi-periodic structure increases the intensity and the area of the surface plasmon resonance (SPR), which enhances the surface-enhanced Raman scattering (SERS) signal significantly. The SERS signal of the nanorice/DNA/nanoarray chip is compared with that of the nanorice/DNA/film chip. The SERS signal is greatly enhanced when the Ag nanorices are coupled to the periodic Au nanoarray instead of the planar film chip. The resulting spatially broadened SPR field enables the SERS biosensor with a limit of detection of 50 aM toward hepatitis B virus DNA with the capability of discriminating a single-base mutant of DNA. This sensing platform can be extended to detect other chemical species and biomolecules such as proteins and small molecules.

Femtosecond laser ablation of ZnO nanorods for two-photon-pumped random lasing and optical data storage

Femtosecond laser ablation based on two-photon absorption was employed to cut ZnO nanorods into uniform ZnO nanoparticles of deep subwavelength size. The fabricated ZnO nanoparticles possess a shorter mean transport length for photons at the emission wavelength and a much smaller scattering cross section at the pump wavelength, leading to highly efficient two-photon-pumped random lasing with a low threshold of ∼8 mJ/cm2. It was demonstrated that the significant enhancement in two-photon luminescence after the irradiation of femtosecond laser pulses could also be utilized for realizing optical data storage.

Diagnosis of Multiple Gases Separated from Transformer Oil Using Cavity-Enhanced Raman Spectroscopy

The Raman signal of gas molecules is very weak due to its small scattering cross section. Here, a near-confocal cavity-enhanced Raman detection system is demonstrated. In the cavity, a high power light of 9W is achieved by using a cw 200 mW 532 nm laser, which greatly enhances the detection sensitivity of gas species. A photomultiplier tube connected to a spectrometer is used as the detection system. The Raman spectra of the mixed gases separated from transformer oil has been observed. The relationship of absolute Raman intensity and gas pressure is also obtained. To our knowledge, this is the first Raman system to detect the gases separated from transformer oil.

The Study of Nanovolumes of Amorphous Materials Using Electron Scattering

Many of the structural elements of importance in materials applications (e.g., thin films, barrier layers, intergranular films in ceramics) are small in volume and amorphous. Although the characterization of the structure of amorphous materials by X-ray and neutron diffraction methods is well established, these techniques are not suitable for studies of nanovolumes of materials because of the relatively small scattering cross sections. This chapter reviews recent developments in electron techniques, and particularly electron diffraction, for overcoming this problem.

Small-angle neutron scattering data on C60 clusters in weakly polar solutions of fullerenes

Solutions of fullerence C60 in carbon disulfide CS2 have been investigated by small-angle neutron scattering. Combination of solubility, contrast, and incoherent scattering make it possible to measure and analyze the relatively small scattering cross section of this system. Along with single fullerene molecules, a small amount of large fullerene clusters (more than 100 A in size) is found in these solutions. The formation of these clusters depends on the procedure of solution preparation. The size distribution functions of clusters are compared with the results of the phenomenological cluster model of fullerene solubility.

High resolution general purpose D-layer experiment for EISCAT incoherent scatter radars using selected set of random codes

Abstract. The ionospheric D-layer is a narrow bandwidth radar target often with a very small scattering cross section. The target autocorrelation function can be obtained by transmitting a series of relatively short coded pulses and computing the correlation between data obtained from different pulses. The spatial resolution should be as high as possible and the spatial side lobes of the codes used should be as small as possible. However, due to the short pulse repetition period (in the order of milliseconds) at any instant, the radar receives detectable scattered signals not only from the pulse illuminating the D-region but also from 3–5 ambiguous-range pulses, which makes it difficult to produce a reliable estimate near zero lag of the autocorrelation function. A new experimental solution to this measurement problem, using a selected set of 40-bit random codes with 4 µs elements giving 600 m spatial resolution is presented. The zero lag is approximated by dividing the pulse into two 20-bit codes and computing the correlation between those two pulses. The lowest altitudes of the E-layer are measured by dividing the pulse into 5 pieces of 8 bits, which allows for computation of 4 lags. In addition, coherent integration of data from four pulses is used for obtaining separately the autocorrelation function estimate for the lowest altitudes and in cases when the target contains structures with a long coherence time. Design details and responses of the experiment are given, and analysed test data are shown.Key words. Radio science (signal processing); Ionosphere (plasma temperature and density; instruments and techniques)