Backscattering Effects Research Articles

A multislice theory of electron scattering in crystals including backscattering and inelastic effects

In the framework of the slice transition operator technique, a general multislice theory for electron scattering in crystals is developed. To achieve this generalization, we combine the approaches for inelastic scattering derived by Yoshioka [J. Phys. Soc. Jpn. 12, 6 (1957)] and backscattering based on the formalism of Chen and Van Dyck [Ultramicroscopy 70, 29–44 (1997)]. A computational realization of the obtained equations is suggested. The proposed computational scheme is tested on elastic backscattering of electrons, where we consider single backscattering in analogy to the computational scheme proposed by Chen and Van Dyck.

몬테카를로 시뮬레이션을 이용한 선형가속기 구성요소 변화에 따른 후방산란에 관한 연구

It should be accurate dose calculation to increase the efficiency of radiation therapy, and it is priority to figure out the beam characteristics for this purpose. The target and primary collimator in head components of the linear accelerator have the greatest influence on determining the beam characteristics which is caused by backscatter and it is the factor to consider the shielding structures and equipment management. In this study, we made modeling of the linear accelerator through the Geant4 Monte Carlo simulation and investigated backscatter according to the change of the size and shape in head components. For the scattered electrons, it showed the greatest number of distributions inside of the inner radius at primary collimator. But, for the scattered photons which have the high energy, it was mostly located outside of the inner radius at primary collimator. Scattered positrons showed a small occurrence in about 0.03%. According to the change of the inner radius at primary collimator, it was great changes in the inside of inner radius for all three scattered particles. According to the change of the outer radius at primary collimator, it was shown some considerable effects from more than 60 mm outer radius. It was no significant effect according to the change of target thickness. In this study, we found that backscatter should be considered, and figured out that geometric size and shape of the peripheral components are the factors that influences the backscatter effect. Keyword : Medical Linear Accelerator, Monte Carlo Simulation, backscatter

Large Reynolds number self-similar states of unstably stratified homogeneous turbulence

We study the influence of the large scale energy distribution on the long term dynamics of unstably stratified homogeneous turbulence at high Reynolds number Re = 106, using a statistical two-point spectral model based on the eddy-damped quasi-normal closure. We consider several initial spectral scalings ks in the infrared range with s ∈ [1; 5] and we establish that the resulting kinetic energy growth rates are controlled by s, with the appearance of backscatter effects for s ≳ 3.5. We then assess that only for s ≤ 4 do we observe self-similarity in the infrared and in the inertial ranges, but not in the dissipative range. Compensated energy and buoyancy spectra exhibit the expected Kolmogorov-Obukhov k−5/3 scaling at long time, and a trend to the theoretically predicted k−7/3 scaling for velocity-buoyancy cross-correlation spectrum thanks to the very large Reynolds number. We also show a direct link between the late-time anisotropy of the flows and the infrared spectrum, thus demonstrating long-lasting effect of initial conditions on unstably stratified turbulence. We show that, in addition to the Kolmogorov k−5/3 scaling, the kinetic energy spectrum inertial range includes a k−3 zone due to polarization anisotropy, and we confirm the clear sin2θ dependence of the velocity-buoyancy spectrum in the inertial range, where θ is the orientation of the wave vector to the axis of gravity. However, an unexpected quick return to isotropy of the scalar spectra has been identified, which cannot be explained by a standard dimensional analysis.

A few Canadian contributions to underwater acoustics

An historical perspective on four Canadian initiatives in underwater acoustics will be presented: Towed Variable Depth Sonar (TVDS); the Chapman-Harris model for reverberation; seabed-interaction in shallow water; and Arctic acoustics. The poor performance of hull-mounted sonars in Canadian waters during WWII prompted TVDS development in 1947. The resulting prototype (CAST/1X) was tested against a UK developmental system in 1958. Subsequently, the UK purchased the Canadian system and the US re-packaged their sonars according to the Canadian design. An understanding of the effects of back-scattering from the sea surface and bottom was required to model sonar system performance. Experiments began circa 1960 leading to a new model for sea surface back-scattering, and revealing the ocean volume as an additional factor. The conditions governing propagation and noise in shallow waters remained mysterious in the 1970s, until the significant role of the seabed emerged through theoretical developments (e.g., loss due to shear waves) and experiments conducted over a variety of seabed types. Political, sovereignty and energy issues prompted Canada to investigate under-ice acoustics in the Arctic beginning circa 1958. The efforts ultimately resulted in the ability to undertake under-ice acoustic and geophysical surveys using long endurance Autonomous Underwater Vehicles (AUVs).

Spectrally Efficient Enhanced-Performance Bidirectional Coherent PON With Laserless 10 Gb/s ONU [Invited

We present and experimentally characterize a spectrally efficient, bidirectional, ultradense wavelength-division multiplexing passive optical network (PON) (2 × 120 Gb/s at 50 GHz) over 30 km standard single-mode fiber. This is achieved using a 4 GHz grid with partial spectrum overlapping, Nyquist-shaped 16-ary quadrature amplitude modulation, and digital frequency shifting. This scheme is based on a laserless, self-homodyne PON with centralized wavelength control. We achieve reduced impact of the Rayleigh back-scattering effect due to Nyquist-pulse shaping and frequency upshifting. Self-homodyne detection relaxes the laser linewidth requirement and digital signal processing complexity at the optical network unit. To demonstrate the extra benefit of the proposed PON system, we also experimentally and theoretically evaluate the impact of nonlinear Raman crosstalk in video overlay. Compared to a baseband Nyquist signal and baud rate non-return-to-zero equivalent, the proposed system improves the carrier-to-Raman crosstalk ratio of video overlay by 4 and 6 dB, respectively.

Photon beam dose distributions for patients with implanted temporary tissue expanders

This study examines the effects of temporary tissue expanders (TTEs) on the dose distributions of photon beams in breast cancer radiotherapy treatments. EBT2 radiochromic film and ion chamber measurements were taken to quantify the attenuation and backscatter effects of the inhomogeneity. Results illustrate that the internal magnetic port present in a tissue expander causes a dose reduction of approximately 25% in photon tangent fields immediately downstream of the implant. It was also shown that the silicone elastomer shell of the tissue expander reduced the dose to the target volume by as much as 8%. This work demonstrates the importance for an accurately modelled high-density implant in the treatment planning system for post-mastectomy breast cancer patients.

Diversity Gain Measurement in a Reverberation Chamber Without Extra Antennas

This study aims to simplify the diversity gain measurement in a reverberation chamber since the conventional method requires two extra antennas: a transmitting antenna and a reference antenna with known radiation efficiency. By incorporating the one- or two-antenna method for the radiation efficiency measurement in a reverberation chamber and taking advantage of the enhanced backscattering effect, the proposed method in this letter does not need extra antennas, and only the antenna under test is enough to complete the measurement. A virtual antenna is introduced to help understand the proposed method. The measurement results obtained from the proposed method show a good match with the results obtained from the conventional method; preconditions and limitations of the proposed method are also identified and discussed.

Multicore-Fiber-Enabled WSDM Optical Access Network With Centralized Carrier Delivery and RSOA-Based Adaptive Modulation

We proposed and experimentally demonstrated a wavelength-space division multiplexing (WSDM) optical access network architecture with centralized optical carrier delivery utilizing multicore fibers (MCFs) and adaptive modulation based on reflective semiconductor amplifier (RSOA). In our experiment, five of the outer cores are used for undirectional downstream (DS) transmission only, whereas the remaining outer core is utilized as a dedicated channel to transmit upstream (US) signals. Optical carriers for US are delivered from the optical line terminal (OLT) to the optical network unit (ONU) via the inner core and then transmitted back to the OLT after amplification and modulation by the RSOA in the colorless ONU side. The mobile backhaul (MB) service is also supported by the inner core. Wavelengths used in US transmission should be different from that of the MB in order to avoid the Rayleigh backscattering effect in bidirectional transmission. With quadrature phase-shift keying--orthogonal frequency-division multiplexing (QPSK-OFDM) modulation format, the aggregation DS capacity reaches 250 Gb/s using five outer cores and ten wavelengths, and it can be further scaled to 1 Tb/s using 20 wavelengths modulated with 16 QAM-OFDM. For US transmission, 2.5 Gb/s QPSK-OFDM transmission can be achieved just using a low-bandwidth RSOA, and adaptive modulation is applied to the RSOA to further enhance the US data rate to 3.12 Gb/s. As an emulation of high-speed MB transmission, 48 Gb/s in-phase and quadrature (IQ) modulated popularization division multiplexing (PDM)-QPSK signal is transmitted in the inner core of MCF and coherently detected in the OLT side. Both DS and US optical signals exhibit acceptable performance with sufficient power budget.

A novel platform to study the effect of small-scale turbulent density fluctuations on underwater imaging in the ocean

Optical signal transmission is an important component of numerous underwater applications, including visibility and electro-optical (EO) communication. In addition to the well-studied effect of particle backscatter, underwater optical signal transmission can be limited by changes in the index of refraction (IOR) due to small-scale variations in temperature and salinity, sometimes called “optical turbulence”. These variations in IOR, which are associated with oceanic turbulence, can lead to the blurring of an underwater optical target, particularly at high spatial frequencies, thus reducing target detail. The 2011 Bahamas Optical Turbulence Experiment (BOTEX) was conducted to investigate this impact of turbulence on underwater optical signal transmission. Investigating naturally occurring “optical turbulence” requires a platform held at depth, capable of concurrent measurements of optical impairment by turbulence, which requires a significant optical path length, as well as associated physical and optical background conditions of the ambient environment. Our novel platform consisted of a high-speed camera and optical target mounted on a 5m-long frame, along with several Nortek Vector Acoustic Doppler Velocimeter (ADV) and PME Conductivity–Temperature (CT) probes, to estimate turbulent kinetic energy and temperature variance dissipation rates experienced by the frame. Data on the background turbulence was collected with a Rockland Oceanographic Vertical Microstructure Profiler, to aid in analysis and guide error estimates of the ADV/CT measurements. This study was the first effort attempting to collect turbulence measurements on a frame designed for the investigation of the effect of density microstructure variations on optical signal transmission in the open ocean. Our results highlight the numerous challenges associated with studying this phenomenon in the dynamic oceanic environment. Here, we present the interpretation of the high-resolution velocity and temperature measurements collected on the frame and discuss the associated difficulties. Despite the numerous challenges, the investigation of the effect of microstructure on underwater optics is needed for efforts aimed at mitigating the impact of “optical turbulence” on underwater EO signal transmission and may help advance optical methods to quantify oceanic microstructure.

Measurements of the relative backscatter contribution to the monitor chamber for modern medical linear accelerators; a multi-center study

Conversion to absolute dose in Monte Carlo (MC) simulations of MV radiotherapy beams needs correct modeling of backscatter (BS) to the linear accelerator (linac) monitor chamber. For some linacs the BS depends largely on jaw settings. The backscattered fraction (BSF) of radiation can be determined experimentally by measuring ratios of target charge for a given number of monitor units as a function of jaw settings. This was done using the in-house developed ME40 dosimetry system, which is able to determine the target charge for each radiation pulse from the linac. The BSF measurements were performed for different linac models at five Danish radiotherapy clinics. The investigated linacs were four Varian TrueBeams (TB), one Varian iX (iX) and one Elekta Synergy (ES). BSF measurements were performed for square field side lengths ranging from 1 to 40 cm, using the 10 × 10 cm2 field as reference. The impact of the flattening filter on the BSF was investigated through measurements in flattened as well as flattening filter free (FFF) beams. Furthermore, to investigate the contribution from the upper and lower jaws separately, measurements at one of the clinics also included asymmetric fields. For the iX, the obtained BSF measurements were compared with MC simulations performed using the BEAMnrc user code. For flattened beams on the Varian linacs, the measured BSF exhibited a clear linear correlation with square jaw settings (correlation coefficient r > 0.9 with p < 0.001 in all cases). For the ES, however, no correlation between BSF and jaw settings was found (r = 0.04, p = 0.92). The change in BSF with jaw settings was also found to be negligible for FFF beams on the TB linacs, indicating that the flattening filter has a substantial influence on the BSF. Furthermore, the backscatter effect was found to be more pronounced (up to a factor of 7) for the iX compared to the TB. MC simulations on the iX agreed within 0.4% with BSF measurements, indicating that the target charge measurement method used for determination of BSF is accurate. Furthermore, the similar BSF observed for the four TB linacs included in the study also indicates that the method used for target charge measurements is reproducible. The reproducibility lies mainly in the fact that the method basically has no set-up errors and therefore is user independent.

High-precision two-way optic-fiber time transfer using an improved time code.

We present a novel high-precision two-way optic-fiber time transfer scheme. The Inter-Range Instrumentation Group (IRIG-B) time code is modified by increasing bit rate and defining new fields. The modified time code can be transmitted directly using commercial optical transceivers and is able to efficiently suppress the effect of the Rayleigh backscattering in the optical fiber. A dedicated codec (encoder and decoder) with low delay fluctuation is developed. The synchronization issue is addressed by adopting a mask technique and combinational logic circuit. Its delay fluctuation is less than 27 ps in terms of the standard deviation. The two-way optic-fiber time transfer using the improved codec scheme is verified experimentally over 2 m to100 km fiber links. The results show that the stability over 100 km fiber link is always less than 35 ps with the minimum value of about 2 ps at the averaging time around 1000 s. The uncertainty of time difference induced by the chromatic dispersion over 100 km is less than 22 ps.

Hydrogen silsesquioxane on SOI proximity and microloading effects correction from a single 1D characterization sample

Designs patterned by electron beam lithography without applying process effect correction exhibit overexposed dense features and underexposed sparse features for most practical exposure scenarios. This is typified by the limited exposure latitude of hydrogen silsesquioxane resist on silicon-on-insulator substrates used for silicon photonics, which commonly display very high density features (vertical grating couplers, ring resonators) mixed with very sparse features (inverse tapered waveguides, lone waveguides) in a single pattern. The authors have optimized a proximity effect correction (PEC) based on our analysis of a single 1D process control monitor characterization sample. Our PEC verification sample, which includes electron backscatter and process-related microloading effects, achieved linewidths with an RMS error of ±5.0 nm for features with pattern densities spanning 1%–67%. Ignoring the pattern density-dependent microloading effect limits the resolvable pattern density span to a smaller range and degrades the linewidth error.

Electrically continuous graphene from single crystal copper verified by terahertz conductance spectroscopy and micro four-point probe.

The electrical performance of graphene synthesized by chemical vapor deposition and transferred to insulating surfaces may be compromised by extended defects, including for instance grain boundaries, cracks, wrinkles, and tears. In this study, we experimentally investigate and compare the nano- and microscale electrical continuity of single layer graphene grown on centimeter-sized single crystal copper with that of previously studied graphene films, grown on commercially available copper foil, after transfer to SiO2 surfaces. The electrical continuity of the graphene films is analyzed using two noninvasive conductance characterization methods: ultrabroadband terahertz time-domain spectroscopy and micro four-point probe, which probe the electrical properties of the graphene film on different length scales, 100 nm and 10 μm, respectively. Ultrabroadband terahertz time-domain spectroscopy allows for measurement of the complex conductance response in the frequency range 1-15 terahertz, covering the entire intraband conductance spectrum, and reveals that the conductance response for the graphene grown on single crystalline copper intimately follows the Drude model for a barrier-free conductor. In contrast, the graphene grown on commercial copper foil shows a distinctly non-Drude conductance spectrum that is better described by the Drude-Smith model, which incorporates the effect of preferential carrier backscattering associated with extended, electronic barriers with a typical separation on the order of 100 nm. Micro four-point probe resistance values measured on graphene grown on single crystalline copper in two different voltage-current configurations show close agreement with the expected distributions for a continuous 2D conductor, in contrast with previous observations on graphene grown on commercial copper foil. The terahertz and micro four-point probe conductance values of the graphene grown on single crystalline copper shows a close to unity correlation, in contrast with those of the graphene grown on commercial copper foil, which we explain by the absence of extended defects on the microscale in CVD graphene grown on single crystalline copper. The presented results demonstrate that the graphene grown on single crystal copper is electrically continuous on the nanoscopic, microscopic, as well as intermediate length scales.

Abnormal anti-Stokes Raman scattering and coherent backscattering as manifestation of Anderson localization of light in nonlinear mesoscopic materials

We demonstrate that under continuous single-beam excitation, one can generate an abnormal anti-Stokes Raman emission (AASRE) whose properties are similar to a coherent anti-Stokes Raman scattering. The effect has been observed in mesoscopic materials, which possess intrinsically nonlinear properties [lithium niobate (LiNbO3), bismuth germanium oxide (Bi12GeO20; BGO), bismuth silicon oxide (Bi12GeO20; BSO)], which have a nonzero electric susceptibility of the third order, χ(3)≠0. Corroborated Raman and coherent backscattering studies demonstrate that the occurrence of both effects is conditioned by the existence of a mesoscopic morphology of the sample, which is able to ensure efficient transport of the light inside the sample through a multiple light scattering mechanism. In this context, both AASRE and coherent backscattering effects are because of the Anderson localization of the light.

The opposition effect of the asteroid 4 Vesta

Abstract We present the results of photometric observations carried out with four small telescopes of the asteroid 4 Vesta in the B, RC, and z ′ bands at a minimum phase angle of 0$_{.}^{\circ}$1. The magnitudes, reduced to unit distance and zero phase angle, were MB(1, 1, 0) = 3.83 ± 0.01, $M_{R_{\rm C}}$(1, 1, 0) = 2.67 ± 0.01, and $M_{z^{\,\prime }}(1, 1, 0) = 3.03\,\pm \,0.01\:$mag. The absolute magnitude obtained from the IAU H − G function is ∼ 0.1 mag darker than the magnitude at a phase angle of 0° determined from the Shevchenko function and Hapke models with the coherent backscattering effect term. Our photometric measurements allowed us to derive geometric albedos of 0.35 in the B band, 0.41 in the RC band, and 0.31 in the z′ bands by using the Hapke model with the coherent backscattering effect term. Using the Hapke model, the porosity of the optically active regolith on Vesta was estimated to be ρ = 0.4–0.7, yielding a bulk density of 0.9–2.0 × 103 kg m−3. It is evident that the opposition effect for Vesta makes a contribution not only to the shadow-hiding effect, but also the coherent backscattering effect that appears from ∼ 1°. The amplitude of the coherent backscatter opposition effect for Vesta increases with a brightening of reflectance. By comparison with other solar system bodies, we suggest that multiple-scattering on an optically active scale may contribute to the amplitude of the coherent backscatter opposition effect (BC0).

A facile route for CdSe nanoparticles: synthesis and structural characterisation

Semiconducting CdSe nanoparticles of a few nanometers in size were synthesised by solvothermal decomposition of novel, single-source molecular precursor Cd(II) complex of bis(aminoethyl)selenide ((NH2CH2CH2)2Se.CdCl2)) using quinoline as coordinating agent at 200°C. Particle size, structural phase, elemental compositions, morphology and optical bandgap of as-synthesised nanoparticles were characterised using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDAX), transmission electron microscopy (TEM) and UV–Vis spectroscopy. The XRD pattern revealed that hexagonal CdSe nanocrystallites of ≈7 nm average size can be obtained by the solvothermal decomposition of ((NH2CH2CH2)2Se.CdCl2)) in quinoline. Elemental composition ratio of Cd:Se reveals non-stochiometric phase as confirmed standard less ZAF quantification, where Z, A and F are the matrix correction parameters, describing the atomic number effect (stopping power and backscatter effect), the absorption effect, and the fluorescence effect, respectively, in EDAX. Absorption spectra show blue shift in the absorption in nano particles with respect to their bulk counterpart and the calculated optical bandgap is estimated at 3·94 eV. This method has several advantages such as non-toxic and non-pyroforic single-source precursor, relatively low temperature and avoids the need for inert environment. However, quinoline used as coordinating agent has proved ineffective in capping and dispersing the nanoparticles and need to be further investigated.

Coulomb impurity scattering in topological insulator thin films

Inter-surface coupling in thin-film topological insulators can reduce the surface state mobility by an order of magnitude in low-temperature transport measurements. The reduction is caused by a reduction in the group velocity and an increased sz component of the surface-state spin which weakens the selection rule against large-angle scattering. An intersurface potential splits the degenerate bands into a Rashba-like bandstructure. This reduces the intersurface coupling, it largely restores the selection rule against large angle scattering, and the ring-shaped valence band further reduces backscattering by requiring, on average, larger momentum transfer for backscattering events. The effects of temperature, Fermi level, and intersurface potential on the Coulomb impurity scattering limited mobility are analyzed and discussed.

Out-of-plane light-scattering polarimetric imaging of a thread surface

A novel polarimetric measurement based on hemispherical light-scattering for the assessment of superficial screw is presented. The optical system is capable of capturing polarized light images. The effect of the thread surface backscattering is estimated with the use of in-plane and out-of-plane illumination. The angular distributions of polarized light scattered by the ISO metric screws are measured for light incident from a green laser. A partial Stokes vector imaging detector is mounted on a motorized rotating arm at an oblique angle to the sample normal and consists of a 10-bit scientific camera, an object lens, and a polarizer. The partial Stokes vector images of light scattered towards the camera are generated for each direction and a useful decomposition of the partial Stokes vector is presented. The thread surface effects can be minimized using out-of-plane polarized illumination in conjunction with polarized images. The experimental result may provide a new polarized imaging technique for using visible light to inspect the key features of a screw in automated optical inspection system.

Anisotropic Fabry-Pérot resonant states confined within nano-steps on the topological insulator surface.

The peculiar nature of topological surface states, such as absence of backscattering, weak anti-localization, and quantum anomalous Hall effect, has been demonstrated mainly in bulk and film of topological insulator (TI), using surface sensitive probes and bulk transport probes. However, it is equally important and experimentally challenging to confine massless Dirac fermions with nano-steps on TI surfaces. This potential structure has similar ground with linearly-dispersed photons in Fabry-Pérot resonators, while reserving fundamental differences from well-studied Fabry-Pérot resonators and quantum corrals on noble metal surfaces. In this paper, we study the massless Dirac fermions confined within steps along the x (Γ–K) or y (Γ–M) direction on the TI surface, and the Fabry-Pérot-like resonances in the electronic local density of states (LDOS) between the steps are found. Due to the remarkable warping effect in the topological surface states, the LDOS confined in the step-well running along Γ-M direction exhibit anisotropic resonance patterns as compared to those in the step-well along Γ-K direction. The transmittance properties and spin orientation of Dirac fermion in both cases are also anisotropic in the presence of warping effect.

Effects of Rayleigh backscattering on the stability of distributed feedback fiber laser sensors

Rayleigh backscattering induces mode hopping of distributed feedback (DFB) fiber lasers in the sensor array, and the critical length related to Rayleigh backscattering limits the size of DFB fiber laser sensor networking. Based on a three-mirror cavity model, the critical length for a DFB fiber laser is derived. It increases nearly exponentially with the coupling coefficient for the ideal π-shifted DFB fiber lasers. The reflectivity of the sub-fiber Bragg grating at the lasing wavelength is the main factor to resist Rayleigh backscattering for a nonideal DFB fiber laser. The corresponding experiments have been carried out, and the critical length of larger than 150 m was achieved for 42-mm-long DFB fiber lasers.