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The findings are presented, which reveal reaction enhancement in GaAs-A1GaAs IR detectors employing a doped substrate to increase reflection, thus intensifying the resonant cavity effect. This display of better response from application of a doped substrate presents the scope for improving reaction in heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors.

The bulk specific gravity ([G.sub.mb]) of compacted hot-mix asphalt (HMA) specimens is the basis for volumetric property calculations during mix design and quality control and assurance. For many years, [G.sub.mb] measurement has been accomplished by water-displacement procedures. Water displacement yields accurate [G.sub.mb] for most conventional dense-graded mixes, but [G.sub.mb] measurement errors using water displacement have been observed for coarser-graded mixes. Recently, vacuum-sealing technology, using the Corelok vacuum-sealing device has been shown to yield more accurate Grab values. This study's objectives were to illustrate Grab differences between water-displacement and Corelok procedures and resulting changes in volumetric properties and design asphalt contents. Results indicate significant [G.sub.mb] differences between Corelok and water-displacement procedures, with the Corelok procedure yielding slightly higher Grab values, resulting in an average increase in design asphalt content and voids in the mineral aggregate of 0.29 and 0.64%, respectively, relative to water-displacement procedures. DOI: 10.1061/(ASCE)0899-1561(2005)17:2(137) CE Database subject headings: Asphalt mixes; Quality control; Mixing; Displacement.

Achieving high internal quantum efficiency in GaAs/AlGaAs based light-emitting diodes (LEDs) for room-temperature operation at low current-density injection is crucial for applications such as optical up-converters based on the integration of LEDs and photodetectors. The internal up-conversion quantum efficiency of the device was measured to be 76%.

The superior performing asphalt pavement (Superpave) hot mix asphalt (HMA) mix design system was a principal development of the Strategic Highway Research Program (SHRP). In the Superpave mix design system, the Superpave gyratory compactor (SGC) was developed as a laboratory HMA compactor, targeted at better simulating the field compaction of HMA mixes. All SGCs are required to meet the American Association of State Highway and Transportation Officials (AASHTO) T312 specification parameter requirement of external gyration angle (1.25 [+ or -] 0.02[degrees]L Numerous agencies have reported significant differences in the HMA bulk specific gravity ([G.sub.mb]) of the same HMA mix compacted in different SGCs. These differences are hypothesized to be, in part, due to different internal gyration angles between SGCs. In response, a dynamic angle verification (DAV) procedure has been used to measure and verify the internal gyration angle during compaction. SGCs located in the central and district Mississippi Department of Transportation laboratories were evaluated with the DAV procedure to determine the internal gyration angle and the resulting Grab values. Substantial differences in the internal and external gyration angles, as well as the top and bottom DAV determined internal angles, were found for many of the SGCs evaluated. DOI: 10.1061/(ASCE)0899-1561(2004)16:5(444) CE Database subject headings: Asphalt mixes; Compaction; Verification; Construction equipment.

Results are presented of a study of the low-frequency noise behavior in two quantum well infrared photodetectors, with the first excited state near the top of the confining potential barriers.

The influence of well doping density on the performance of GaAs/AlGaAs p-type quantum well infrared photodetectors have been studied. Results show that detectors with 2D doping density from 1 to 2 X 10 to the 12th power per square cm can maximize background-limited infrared performance temperature and dark current-limited detectivity when operated at 100 K. It was also observed that peak absorption cannot be improved by merely increasing the 2D doping density in the well. However, increasing the doping density to 4 X 10 to the 12th power per square cm increases peak absorption by 1.5.

Results on a p-type intersubband infrared detector for the 3-5 micrometer wavelength region with optimal quantum well design were presented. Using the design, the fabricated detector showed good performance with a background limited operating temperature of 100 K. Findings also showed a clear polarization dependence for different intersubband transitions involving heavy hole to heavy hole and heavy hole to light hole processes.

The negative capacitance (NC) effect is seen in a range of electronic devices. The NC effect in homogeneous semiconductors has been considered theoretically. The NC effect is now discussed, with a focus on the theoretical interpretation of the phenomenon. This can be based on the analysis of the time-domain transient current, in response to a small voltage step. The time-domain transient current approach is demonstrated using simulation results of quantum well infrared photodetectors.

The emitter layer construction, cut-off wavelength, responsivity and dark current characteristics of p-GaAs homojunction interfacial workfunction internal photoemission far-infrared detectors have been studied. The study will lead to a better understanding of how doping affects the performance of these detectors. Results indicate that thermionic field emission is a major source of dark current. The experimental and theoretical values of leakage current and defect-induced current do not have significant deviations.

The possibility of covering the short wavelength infrared region using intersubband transitions in GaAs-based quantum wells, is explored. InGaAs wells were investigated with A1As thin confining barriers, and the dark current was found to decrease with increasing detection wavelength. Photocurrents due to intersubband transition were see down to a wavelength of around 1 mum. Physical effects related to the indirect band minima at the X point were also revealed.

Research was conducted to examine the ac conductance and temperature dependence capacitance of multiquantum well structures at varying bias and frequency changes. These structures feature 4, 8, 16 and 32 wells and are made up of gallium arsenide or aluminum gallium arsenide. Results indicate the existence of two relaxation times at 80-100 K. However, relaxational properties are masked at temperatures above 100K.

Selective area chemical beam epitaxy was employed in developing faceted GaInAs/InP nanostructures between 20 and 2 micrometers. Different growth rates on various crystallographic planes caused the facets on the mesas edge and along the sidewalls. Field emission scanning electron microscopy and low temperature photoluminescence studies reveal that the lateral growth rate of the InP sidewalls in the orthogonal direction is large and and that mesas faceting is complicated.

Gallium arsenide quantum well infrared photodetectors (QWIPs) are ideally suited for infrared imagery due to their detection capability over a wide infrared wavelength region and the feasibility of fabrication of monolithic two-dimensional arrays of such detectors. This paper reports on the effects of high energy proton and alpha particle radiations on the performance of these devices. The particle energies ranged from 0.8 MeV to 10.0 MeV and the fluences used in this work ranged from [10.sup.11] to [10.sup.16] [cm.sup.-2]. The dark current and spectral response of these radiated devices were measured at different fluence levels. Using the spectral response as a measure of device performance, it is concluded that the device performance decreases with fluence and the degradation due to alpha particles is greater than for protons of the same energy. Further, the damage to device performance decreases with the increase in the energy of both types of these radiations. From prior work, it was expected that these devices would be sensitive to permanent radiation damage. The extent of damage, however, could not be pre-judged due to the device complexity. These are the first reported measurements of radiation hardness of QWIPs.

The concept of phased arrays can be used in noninvasive ultrasound surgery based on magnetic resonance imaging (MRI). The technique allows the focus to be moved electronically and the focal size to be increased. The arrays can provide enough power to necrose living tissues in 10 sec. The technique is based on two arrays which comprise a spherically curved concentric ring array and a spherically curved 16 square element array. The former helps to move the focus along the central axis, and the latter helps to make the focus larger.

A microwave-rectification method enables the study of the high-frequency capability of quantum-well infrared photodetectors (QWPI's). The intrinsic lifetime for QWIP's, with a barrier thickness of 234-466 angstroms and about 4-32 number of wells, in the high-biasing field regime is 5-6 ps. The photodetectors have a fairly flat frequency response up to around 30 GHz. The lifetime values compare well with heterodyne experiment.

Radiotracer techniques and nuclear magnetic resonance (NMR) spectroscopy are two complementary methods that are widely used to investigate cardiac metabolism. We have now developed a novel gamma photon detector system that will operate within a wide-bore, 9.4 T magnet. With this detector in position, it is possible to acquire radiotracer uptake data while simultaneously collecting NMR spectra. The advantages of this new system are firstly, that it enables correlations between radiotracer and NMR data to be made on individual rat hearts, and secondly that it allows the number of experiments required to obtain results of statistical significance to be greatly decreased. The extension of our system, to one in which positron emission tomography (PET) and magnetic resonance imaging (MRI) data are acquired simultaneously, clearly has enormous clinical potential. The detector consists of a NaI(Tl) scintillation crystal coupled to a magnetic field-insensitive photomultiplier tube by a 72.5 cm long, acrylic light pipe. This detector configuration satisfies the two, conflicting requirements of the crystal being near the sample, and thus in a high magnetic field, and the PMT being in a low magnetic field and thus far from the sample. The performance of the detector is not significantly altered by either the magnetic field or the collection of NMR data and, conversely, the performance of the NMR system is not affected by the presence of the detector. As expected, the light pipe causes a decrease in the amount of scintillation light reaching the photomultiplier tube; however, this does not constitute a significant problem for the sensitivity of the complete detector system. In this paper we present the technical specifications of our new system together with what we believe are the first examples of simultaneously acquired NMR spectra and 18F-fluorodeoxyglucose (18FDG) uptake data, obtained from isolated, perfused rat hearts.

An experimental analysis of photoconductive and noise gain in AlGaAs/GaAs quantum well infrared photodetectors for bound-to-quasibound, bound-to-continuum and bound-to-bound transitions revealed variations in the gain. Responsivity and dark current noise estimates represent variations. The gain values are consistent for bound-to-continuum and bound-to-quasibound transitions at certain rf bias voltages.

Multiple-quantum-well structures having a coupled double well as the period, exhibit perfectly regular periodic increases in capacitance, device current and intersubband photocurrent. The measurements obtained with scan direction of the applied voltage as a standard indicate the presence of magnetic hysteresis.

The use of focused-ion-beam implantation in the fabrication of nanostructures in strained InGaAs/GaAs quantum wells was evaluated using low-temperature photoluminescence spectroscopy. The results showed that implantation-induced compositional disordering in the quantum wells reached depths of 450 nm below the surface. The large extent of compositional disordering was attributed to strain-enhanced in-plane migration of defects during the annealing process.

Researchers studied a series of GaAs/AlGaAs p-type quantum well infrared photodetectors (p-QWIPs) with differing barrier heights to compare their performance with n-QWIPs. The findings that detector cutoff wavelength, and dark current, decrease with increasing barrier height is of value in the optimization of devices. The researchers plan to undertake further studies of p-QWIPs.

Rapid thermal annealing process of an active quantum well infrared photodetector structure shifts the peak detection wavelength to red and partially mixes the barrier end well layers for postgrowth tuning of the detector. The peak spectral responsivity decreases at a 10 microamp device current after the maximum anneal time is reached. An increase in detection spectral bandwidth reduces broad-band responsivity by a factor of 3.

The dark current in quantum well infrared photodetectors (QWIP) is investigated. A numerical model capable of analyzing the physical parameters affectinng the behavior of the dark current is formulated based on Meshkov's theory. It used to obtain the final dark current expression that contains the relative contributions of various scattering mechanisms. Simulation results obtained from the application of the proposed model show strong agreement with the measured data.

The effect of the number of wells on the performance characteristics of multiple quantum well intersubband infrared detectors was investigated. The results showed that the number of wells determine the degree of absorption. However, current responsivity was not affected by the number of wells in the device. In addition, photoconductive gain was invrsely proportional to the number of wells.

A long-wavelength infrared photocurrent study was used to analyze a series of a Si-SiGe double-heterostructures. The results showed that the active region was a thin heavily p-type doped SiGe layer. In various SiGe compositions, photocurrent spectra featuring different cutoff wavelengths were observed. This was attributed to internal photoemission in theheterojunction.

The design, fabrication and characterization of a quantum well infrared photodetector capable of detecting near infrared (NIR), midwavelength infrared (MWIR) and long wavelength infrared (LWIR) simultaneously are described. The results have shown the possibility of the detection of separated wavelength bands, in a single pixel device, by using interband and intersubband transitions in quantum wells.

The room-temperature absorption and reflection spectra in the range of 5-100(micron)m for multilayer heterojunction interfacial work function internal photoemission (HEIWIP) GaAs/AlGaAs far-infrared (FIR) detectors are present. Results indicate that the behavior could be caused due to the free carrier absorption and interaction with optical phonons.

We investigated the etiology of anterior segment eye abnormalities caused by heterozygous null mutations in PAX6 by studying the small eye Pax6+/- mouse model. Histological analysis of Pax6+/- embryos showed that the lens was developmentally delayed, with signs of degeneration. Pax6+/+ [equivalence] Pax6+/- chimeras, which are mixtures of wild-type and heterozygous cells, revealed two cell autonomous defects of the heterozygous lens: Pax6+/- cells are less readily incorporated into the lens placode than wild type, and those which are incorporated into the lens are not maintained efficiently in the lens epithelium. The lens of chimeric embryos is therefore predominantly wild type by E16.5, while, in contrast, heterozygous cells contribute normally to all other chimeric eye tissues, into adulthood. Surprisingly, eye size and defects of the iris and cornea are corrected in fetal and adult chimeras with up to 90% mutant cells; correction of the whole eye phenotype correlates with correction of the lens genotype. The lens has been shown to produce growth factors and drive development of the ciliary body, iris, and cornea. We postulate that reduction of Pax6 dosage compromises the ability of the lens to instigate inductive interactions that organize the anterior segment. Our data suggest that small eyes, aniridia, and corneal opacities are all consequences of the lens defects. This has clinical relevance for human aniridia PAX6+/- phenotypes and suggests the possibility of therapy.

Permanent radiation damage was investigated in gallium arsenide (GaAs) quantum well light emitting diodes (QW LEDs) due to protons and alpha particles in the 1-15 MeV energy range. At room temperature, these devices under forward bias emit infrared radiation at 980 nm. Current-voltage (I-V) and light emission characteristics of these devices were studied as a function of fluence and energy of protons and alpha radiations. For a given particle, the radiation damage in these devices increased with increase in fluence at a given energy, and decreased with increase in energy of the radiation at a fixed fluence, for both protons and alpha particles. Further, the alpha particles were observed to be far more damaging than the protons. These results agree qualitatively with the principal concepts of dependence of permanent displacement damage on nonionizing energy loss (NIEL) in the device. They are however found to deviate from the widely accepted concepts of damage dependence on fluence and particle energy. Based on the decrease in damage with increase in energy observed in QW LEDs in the present work and the results of earlier workers on energy dependence of damage in GaAs LEDs, it is concluded that these QW LEDs are more radiation hard than the modern GaAs LEDs reported recently. Further, there is less device to device variability of radiation damage between the QW LEDs than in the double heterojunction LEDs. 3 MeV proton radiation effects on device characteristics of quantum well infrared photodetectors (QWIPs) operating in the short wavelength infrared region are also reported.