Solid–liquid interface reconstructions from ultrasonic time-of-flight projection data

Large differences in the ultrasonic velocity of the solid and liquid phases of semiconductors have stimulated an interest in the use of laser ultrasonic methods for locating and characterizing solid–liquid interfaces during single crystal growth. A previously developed two-dimensional ray tracing analysis has been generalized and used to investigate three-dimensional ultrasonic propagation across solid–liquid interfaces in cylindrical bodies where the receiver is located at an arbitrary position relative to the source. Numerical simulations of ultrasonic ray paths, wavefronts, and time of flight have indicated that ultrasonic sensing in the diametral plane is a preferred sensing configuration since the transmitted, reflected, and refracted rays all propagate in this plane, significantly simplifying analysis of the results. While other sensing configurations can also provide information about solid–liquid interfaces, they require a more complicated analysis because the planes in which reflected and refracted rays propagate are not known a priori, and fewer ray paths are accessible for interface interrogation because of large refractions.

Time-of-flight (TOF) positron emission tomography (PET) scanners offer the potential for significantly improved signal-to-noise ratio (SNR) and lesion detectability in clinical PET. However, fully 3D TOF PET image reconstruction is a challenging task due to the huge data size. One solution to this problem is to rebin TOF data into a lower dimensional format. We have recently developed Fourier rebinning methods for mapping TOF data into non-TOF formats that retain substantial SNR advantages relative to sinograms acquired without TOF information. However, mappings for rebinning into non-TOF formats are not unique and optimization of rebinning methods has not been widely investigated. In this paper we address the question of optimal rebinning in order to make full use of TOF information. We focus on FORET-3D, which approximately rebins 3D TOF data into 3D non-TOF sinogram formats without requiring a Fourier transform in the axial direction. We optimize the weighting for FORET-3D to minimize the variance, resulting in H(2)-weighted FORET-3D, which turns out to be the best linear unbiased estimator (BLUE) under reasonable approximations and furthermore the uniformly minimum variance unbiased (UMVU) estimator under Gaussian noise assumptions. This implies that any information loss due to optimal rebinning is as a result only of the approximations used in deriving the rebinning equation and developing the optimal weighting. We demonstrate using simulated and real phantom TOF data that the optimal rebinning method achieves variance reduction and contrast recovery improvement compared to nonoptimized rebinning weightings. In our preliminary study using a simplified simulation setup, the performance of the optimal rebinning method was comparable to that of fully 3D TOF MAP.

We have irradiated bulk single crystal piezoelectric AlN operating as an ultrasonic transducer up to a fast and thermal neutron fluence of 1.85X1018 n/cm2 and 5.8X1018 n/cm2 respectively along with a gamma dose of 26.8 MGy. Our primary interest was in the radiation effects on the piezoelectric performance of the AlN crystal. Ideally standard procedures [2] for extracting material properties from the impedance spectra would have been applied. However, this procedure was anticipated to be very error prone due the effects of radiation on passive components, such as electrodes and cabling. As a result the transducer was coupled to an aluminum cylinder and operated in pulse echo mode throughout the irradiation. In addition to the pulse echo testing impedance data was obtained. Although, material parameters such as d 33 could not be measured in-situ during irradiation with our procedure, our work is the only publication, to the author's knowledge, showing the operation of an ultrasonic transducer in a reactor core. Further, the piezoelectric coefficient was measured prior to irradiation and found to be 5.5 pC/N which is unchanged form as grown samples, and in fact higher than the measured d33 for many as grown samples. Further proof of the viability of utilizing ultrasonic transducers in a reactor core is provided by means of successful time of flight (TOF) measurements throughout the irradiation. The TOF data was used to measure the gamma heating of an aluminum cylinder. The heating due to short lived isotopes in the Al was also detected via TOF.

A Jump-U model of echo pattern for a sonar ranging module

This contribution discusses the 3D reconstruction of deformable freeform surfaces with high spatial and temporal resolution. These are conflicting requirements, since high-resolution surface scanners typically cannot achieve high temporal resolution, while high-speed range cameras like the Time-of-Flight (ToF) cameras capture depth at 25 fps but have a limited spatial resolution. We propose to combine a high-resolution surface scan with a ToF-camera and a color camera to achieve both requirements. The 3D surface deformation is modeled by a NURBS surface that approximates the object surface and estimates the 3D object motion and local 3D deformation from the ToF and color camera data. A set of few NURBS control points can faithfully model the motion and deformation and will be estimated from the ToF and color data with high accuracy. The contribution will focus on the estimation of the 3D deformation NURBS from the ToF and color data.

In this article, we analyze the effects of ambient light on Time of Flight (ToF) depth imaging for a plant’s leaf in sunlight, shadow and room conditions. ToF imaging is sensitive to ambient light and we try to find the best possible integration
\ntimes (IT) for each condition. This is important in order to optimize camera calibration. Our analysis is based on several
\nstatistical metrics estimated from the ToF data. We explain the estimation of the metrics and propose a method of predicting the deteriorating behavior of the data in each condition using camera flags. Finally, we also propose a method to improve the quality of a ToF image taken in a mixed condition having different ambient light exposures.

In this report, a measurement method for cross-sectional temperature and wind velocity distribution is discussed. A novel method based on an acoustic CT technique is proposed. Specifically, the temperature and the wind velocity distributions are estimated simultaneously using the time of flight data of several ultrasonic propagation paths. The temperature and the wind velocity effects can be distinguished by considering sum and difference of the time of flight (TOF) data. Two types of projection data, temperature-oriented data and wind-velocity-oriented data, components are obtained. A filtered back projection method and inverse matrix method are applied to reconstruct the temperature and the wind velocity distributions, respectively. An experimental system is designed and fabricated to realize simultaneous temperature and wind velocity distribution measurements. Through this system, the effectiveness of the proposed measurement method is confirmed.

Density of states of a-Se near the valence band

Dynamics of polarization relaxation at the solid–liquid interface is investigated theoretically by using time dependent density functional theory. The time dependence of the building up of solvent polarization next to a newly charged solid surface is calculated by solving both Markovian and non-Markovian equations of motion for density and momentum variables. The initial inhomogeneity of the medium is taken into account and it is calculated by using a weighted density functional theory. Explicit numerical results are obtained for the relaxation of solvent polarization at varying distances from the solid surface and the dynamics of interfacial relaxation is compared with that of the bulk. A slowing down of the rate of relaxation is found for interfacial molecules. However, the relaxation rate changes nonmonotonically with distance from the surface which can be attributed to the layering of solvent molecules in the interfacial region.

We are developing a time-of-flight (TOF) multiprobe imaging system, which is used to determine the distribution of the administered tracer substances labeled with positron emitting nuclides. A prototype system has eight detector probes, each that consists of a BaF/sub 2/ scintillator and a fast photomultiplier tube. The system construction is simple compared to positron emission tomographs by virtue of using TOF information of annihilation. We have studied the time resolution for different shapes of BaF/sub 2/ scintillators by Monte Carlo simulations and experiments, and found that truncated pyramid shaped crystals provided good time resolution with reasonable sensitivity. Coincidence time resolution of 180 to 200 ps were obtained with 16 combinations of the detectors in the system. An iterative algorithm for image reconstruction using TOF data was proposed. By introducing a proper deblurring algorithm into the image reconstruction process, the spatial resolution can be improved by a factor of two without significant amplification of noise, which corresponds to a spatial resolution of 1.5 cm. >

The number of systems exploiting Time-of-Flight (ToF) cameras for gesture recognition has greatly increased in the last years, confirming a very positive trend of this technology within the field of Human-Computer Interaction. In this work we present a new kind of application for the interaction with a virtual keyboard which is based on the use of an ordinary RGB webcam and a ToF camera. Our approach can be subdivided into two steps: firstly a segmentation of the entire body of the user is achieved exploiting only the ToF data; then the extraction of hands and head is obtained applying color information on the retrieved clusters. The final tracking step, based on the Kalman filter, is able to recognize the chosen hand also in presence of a second hand or the head. Tests, carried out with users of different ages, showed interesting results and a quick learning curve.

UC Davis has fabricated an ultrashort pulse reflectometer (USPR) diagnostic instrument for electron density profile measurements on compact, short duration, magnetically-confined fusion-energy concept devices such as spheromaks and FRCs. The USPR system transmits extremely short duration (~few nsec) chirped waveforms that together span 29 to 75 GHz. These chirped waveforms illuminate and reflect from the target plasma, with each frequency component reflecting from a different density layer (higher frequencies probe deeper into the plasma before reflecting). The reflected waveforms are split into roughly 42 different frequencies; time-of-flight (TOF) measurements made at each frequency with high resolution (~25 psec measurement resolution which corresponds to ~5 mm). These TOF data may then be inverted via software to generate electron density profiles with high time resolution (~10 μsec). At the heart of the system is a field programmable gate array (FPGA) based controller which collects and processes all of the USPR data in addition to generating all of the control signals required for maximum flexibility. The FPGA controller has the software flexibility to be easily reconfigured for different plasma devices, and the entire system sufficiently compact to be easily and quickly transported between devices. A high speed impulse generator was transformed into a set of three ultrashort pulse transmitter chirps using a combination of dispersive waveguide, frequency doublers and high-pass filters. A mm-wave controller was fabricated to sequentially switch between the three chirps, directing the chirps one-by-one to three different mm-wave assemblies spanning 29-75 GHz. Each mm-wave assembly consists of a high power active multiplier chain which converts the transmitter chirp to higher frequencies, and a broadband mixer which downconverts the reflected waveform to the 2-18 GHz range of the UPSR receiver. The 16-channel receiver (shared by all 3 mm-wave assemblies) was fabricated employing custom TOF modules capable of operating at a high 1 MHz sampling rate. Laboratory testing of the full system revealed the presence of unwanted harmonics from the multiplication process, with interference observed in the downconverted reflections at selected frequency channels that could not be completely filtered out. Additional interference effects arising from internal reflections within the mm-wave assemblies were minimized using a high-speed switch which served to “gate out” much of these reflections. The USPR diagnostic was transported and installed onto the HIT-SIU plasma device, becoming operational on 11/08/2022. Although designed to span 3 distinct mm-wave bands, the HIT-SIU plasmas at this time were sufficiently low density such that only the lowest of the three bands was likely to have strong plasma reflections. The system was then set to operate on only the lowest band (assembly #1), with data collected every 1 μsec rather than 3 μsec which would have been the case when cycling through all three bands. Connected to HIT-SIU, time-varying plasma reflections were observed on 9 of 16 possible frequency channels. Close examination of the data collected revealed issues previously unobserved in laboratory testing, associated with (a) reflections from the small aperture horns required for operation within the HIT-SIU device, and (b) a dependence of the recorded TOF with the threshold voltage of a given channel. Plans were made to address each of these issues before undertaking any future campaigns.

Solid-liquid interfaces are ubiquitous. Despite the unquestioned relevance, many scientific research fields rely on simplified or macroscopic descriptions, discarding the molecular nature of the constituents. However, it is the specific molecular interactions and the complex chemistry in the interfacial region that are fundamental in phenomena such as heterogeneous catalysis, electrochemistry, crystal growth, membrane transport and body-antibody recognition. In particular, the models used to describe the so-called electrical double layer of ionic distributions at charged surfaces in liquid, are based on continuum assumptions. For example, the Gouy-Chapman-Stern model assumes that a dense, homogenously distributed layer with a thickness of a hydrated ion (referred to as the layer), is adsorbed onto the charged surface, proceeded by an exponentially decaying diffuse ionic cloud, in which ions are point charges in a continuous dielectric media. In this thesis, I will first illustrate how the assumption of a homogeneous ion lateral density in the Stern Layer fails to describe the actual solid-water interface of several relevant surfaces. Then I will show how the lateral organization of the ions indeed plays a role as important as the vertical modulation of the charge distribution. This is because the molecular nature of the ions, the surface, and the water molecules themselves each contribute to the precise lateral organization of the constituents. As result, the ions can reside on discrete surface sites, defined both laterally and vertically in space. Moreover, the water-mediated interaction can impart a correlation in the reciprocal lateral distribution of the ions. To achieve this level of description, high-resolution atomic force microscopy (AFM) is used. Unlike most of other experimental techniques, AFM is able to characterize interfaces locally at the atomic/molecular level. In fact, in the AFM small-amplitude regime, detection of even single ions adsorbed at the surface of the solid is possible by measuring the perturbation they induce on the local solvation environment. This capability represents an invaluable tool for the exploration of phenomena occurring at the Stern layer of solid-liquid interfaces. Following, I will investigate the properties of several solid-aqueous interfaces in the presence of different ionic concentrations and species, ranging from hard (mica) to soft systems such as organic self-assembled monolayers, lipid bilayers, and the dynamic surface restructuring of calcite when covered with fatty acid molecules. The experimental study, combined with molecular dynamic simulations, reveals a water-induced ion-ion attractive interaction for some ionic species. These results show that water alone is the driving force to induce order within the Stern layer, creating hydration-correlation effects on mica, self-assembled monolayers and possibly lipid bilayers. The local modulation of the hydration properties of the surface is fundamental to highly dynamic systems such as calcite, where steps act as nucleating points for the processes of dissolution and growth. The specific interaction of foreign ions and fatty acids modify these processes as well as the equilibrium between the crystal and the solution. This thesis provides clear experimental evidence of new mechanisms developing at solid-liquid interfaces paving the way for a deeper and more conscious understanding of the colloidal properties of materials.

Insight into the aggregation kinetics of gold nanoparticles (GNPs) is critical for developing a colorimetric assay extensively used in chemical and biomolecular sensing. The aggregation of NPs plays a significant role in many natural and industrial processes, demanding comprehensive perceptions of the aggregation kinetics at a solid-liquid interface. However, the direct observation of the melamine-induced aggregation process of GNPs in the time-domain still remains a challenge. There is little to no information on the fundamental mechanisms of such kinetics using evanescent waves. Total internal reflection (TIR) has been applied to generate the evanescent field (EF), exploring aggregation kinetics near the solid-liquid interface. Here, we utilized a precise optical cavity-based method, an evanescent-wave coupled cavity ring-down spectroscopy (EW-CRDS), that can probe the melamine-induced aggregation kinetics of GNPs. The key feature of the present method is that the evanescent field generated by TIR illumination harnesses the power of CRDS to study 2D fractals via the collision and attachment of the GNPs and their melamine-induced aggregates at the interfacial region in real-time. This kinetic study reveals a critical point for diffusion-limited aggregation and provides insights into the design and optimization of colorimetric sensors that exploit the aggregation of GNPs. Furthermore, the EW-CRDS is a unique analytical approach that helps to deepen our understanding in probing the real-time aggregation process, detecting the presence of aggregator as compared to UV-vis and dynamic light scattering (DLS) spectroscopy.

We have studied hole transport in amorphous As2Te3 (a-As2Te3) films by time-of-flight (TOF) and transient field-effect (TFE) measurements. Since the mobility gap of this material is small, the TOF experiments could be carried out only at temperatures below 200 K. Transport appears to be dispersive, due to multiple trapping in the valence-band tail. In order to extend the observations to higher temperature and longer times, we have developed a new technique, the TFE. A careful analysis shows that TFE data can be dispersive even if transport is nondispersive, because a long thin-film transistor is equivalent to a transmission line. Nevertheless, the resulting data can be used to characterize the density of states beyond the mobility edge. We find that multiple trapping indeed predominates, and the valence-band tail is exponential with a characteristic temperature of about 290 K. This is somewhat lower than indicated by TOF data, probably because hopping transport predominates at low temperatures. The long-time transients can be associated with either neutral defect interconversion and a continuous distribution of decay times or deep traps near the semiconductor-oxide interface. However, the ultimate disappearance of the response reflects the effects of Fermi-level pinning by valence alternation pairs with a negative effective correlation energy, as expected.