Flaw identification from time and frequency features of ultrasonicwaveforms

Time and frequency features have been used with classification algorithms to distinguish between ultrasonic echoes from flaws in pipes and ultrasonic echoes from various geometric configurations of weld root and counterbore. Waveforms containing reflections from known geometries and from flaws were obtained and sets of features were defined using a k-nearest neighbor approach to separate waveforms into classes. Two independent databases containing various flaws and pipe geometries were used to determine these feature sets. From these databases, optimal feature sets were found to separate counterbore waveforms from crack waveforms. Optimal feature sets were also found to distinguish between waveforms from counterbore, waveforms containing both counterbore and root echoes, and waveforms from flaws. The best feature sets used with the classifier algorithms could separate waveforms from the same database with accuracy in the 92-97% range and with high confidence., Another database was obtained from pipe structures in a nuclear power plant to provide a field test of the method. When applied to this database, the same classifier algorithms and feature sets used with the other databases either resulted in a comparable percentage of correct decisions, but with low confidence, or could not classify anywhere from 79 to 88% of the waveforms. Spatial parameters based on averaging feature vectors in axial and circumferential directions were also defined and used for classification. These classifications had higher accuracy but lower confidence levels than the classifications based on individual waveforms     

Metal-organic vapour-phase epitaxy of gallium nitride nanostructures for optoelectronic applications

The two main topics with respect to better gallium nitride (GaN)-based optoelectronic device performance are the improvement of crystal quality and of light extraction. Concerning the first topic, the epitaxial growth of GaN-related materials has to be improved. Since native substrates with good quality are still expensive, foreign substrates like sapphire or silicon which are much cheaper are used, but they introduce a lattice as well as a thermal mismatch leading to detrimental crystal defects. Several approaches to reduce the defect density exist, but in most cases they are coupled with a much higher technological effort.We present an alternative solution to decrease the defect density by the growth of GaN nanostructures, where the small footprint and high aspect ratio leads to a much lower influence of the substrate. Metal-organic vapour-phase epitaxy of GaN nanostructures and InGaN/GaN multi-quantum wells was carried out in a vertical reactor with close-coupled showerhead. Studies on the morphologic properties have been carried out by scanning electron microscopy and energy-dispersive X-ray spectroscopy to reveal the influence of growth parameters and material composition. Cathodoluminescence measurements show the good optical properties of the GaN as well as of the InGaN/GaN structures.     

Sex differences in monitoring performance.

220 male and 220 female undergraduates monitored a visual display for 1 hr. Although the results indicate females were poorer monitors, detecting 10% fewer signals and committing more false alarms, these sex differences accounted for only 4% of the variance of detection performance and less than 1% of the variance of the false alarm measure. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Measurement of Ultrasonic Attenuation Within Regions Selected from B-Scan Images

This paper describes the calculation of absolute ultrasonic attenuation as a function of frequency by processing backscattered signals obtained from a clinical imaging instrument. The signal processing steps are developed from a mathematical model of scattering in an attenuating medium with random inhomogeneities. Attenuation data are derived from the imaging system by recording amplitude-compressed ultrasonic echo waveforms along with transducer position information and time-varying gain values. The input-output characteristics of the receiver are employed to remove the effects of compression and gain. Attenuation values are calculated for selected regions within scans of two tissue phantoms and a normal breast. The values agree with other independent measurements and illustrate the requirements for incorporating quantitative attenuation measurements with clinical imaging.     

Comparisons of lesion detectability in ultrasound images acquired using time-shift compensation and spatial compounding

The effects of aberration, time-shift compensation, and spatial compounding on the discrimination of positive-contrast lesions in ultrasound b-scan images are investigated using a two-dimensional (2-D) array system and tissue-mimicking phantoms. Images were acquired within an 8.8 x 12-mm2 field of view centered on one of four statistically similar 4-mm diameter spherical lesions. Each lesion was imaged in four planes offset by successive 45 degree rotations about the central scan line. Images of the lesions were acquired using conventional geometric focusing through a water path, geometric focusing through a 35-mm thick distributed aberration phantom, and time-shift compensated transmit and receive focusing through the aberration phantom. The views of each lesion were averaged to form sets of water path, aberrated, and time-shift compensated 4:1 compound images and 16:1 compound images. The contrast ratio and detectability index of each image were computed to assess lesion differentiation. In the presence of aberration representative of breast or abdominal wall tissue, time-shift compensation provided statistically significant improvements of contrast ratio but did not consistently affect the detectability index, and spatial compounding significantly increased the detectability index but did not alter the contrast ratio. Time-shift compensation and spatial compounding thus provide complementary benefits to lesion detection.     

Spin Manipulation Using Magnetic II–VI Semiconductors

Recently, efficient spin injection, being the first step towards semiconductor spin electronics, by using BeMnZnSe as a spin filter was accomplished. Such a spin filter made it possible to align the spin orientation of conduction electrons and subsequently inject them into GaAs. However, controlling spin orientation of conduction electrons by an external voltage would be very desirable for semiconductor-based magnetoelectronics. This can be accomplished by using spin switch structures, based on resonant tunneling through magnetic quantum wells, with two separate spin-up and spin-down resonances. Here we summarize both our recent results on spin injection as well as on spin aligner and magnetic resonant tunneling structures. For accomplishing the latter, we have developed magnetic resonant tunneling diodes based on BeTe–ZnMnSe–BeTe structures. Resonant tunneling diode is meant to serve as a spin switch because of the existence of two separate spin-up and spin-down resonances. The tunneling carriers have subsequently been injected into a nonmagnetic GaAs p–i–n light emitting diode. Circular polarization of the emitted light is an indicator of the spin polarization of injected electrons. At constant magnetic field and current, degree of spin polarization could be changed from 81% to 38% by only varying the voltage across the magnetic resonant tunneling device.     

A singular-value method for reconstruction of nonradial and lossy objects

Efficient inverse scattering algorithms for nonradial lossy objects are presented using singular-value decomposition to form reduced-rank representations of the scattering operator. These algorithms extend eigenfunction methods that are not applicable to nonradial lossy scattering objects because the scattering operators for these objects do not have orthonormal eigenfunction decompositions. A method of local reconstruction by segregation of scattering contributions from different local regions is also presented. Scattering from each region is isolated by forming a reduced-rank representation of the scattering operator that has domain and range spaces comprised of far-field patterns with retransmitted fields that focus on the local region. Methods for the estimation of the boundary, average sound speed, and average attenuation slope of the scattering object are also given. These methods yielded approximations of scattering objects that were sufficiently accurate to allow residual variations to be reconstructed in a single iteration. Calculated scattering from a lossy elliptical object with a random background, internal features, and white noise is used to evaluate the proposed methods. Local reconstruction yielded images with spatial resolution that is finer than a half wavelength of the center frequency and reproduces sound speed and attenuation slope with relative root-mean-square errors of 1.09% and 11.45%, respectively.