bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma screening effects on inelastic Compton scattering of photons by hydrogenic ions in strongly coupled classical plasmas
Authors: Jung Young-Dae
Published: July 2002

Plasma screening effects on inelastic Compton scattering of photons by hydrogenic ions in uniformly distributed strongly coupled classical plasmas are investigated. The interaction potential in strongly coupled plasmas is given by the ion–sphere model. The screened radial atomic wave functions and energy eigenvalues for the 1s and 2p states of the hydrogenic target ion in strongly coupled plasmas are obtained by the Ritz variational and perturbational calculations. The expression for the lowest-order transition matrix element is obtained by a two photon process associated with terms quadratic in the vector potential A. The inelastic Compton scattering cross section from the 1s ground state to the 2p excited state is obtained as a function of the incident photon energy including the plasma screening effects through the ion–sphere radius. In strongly coupled plasmas, the plasma screening effect on the inelastic Compton scattering cross section is found to be stronger than that on the photoionization cross section.
Numerical studies of relativistic ion cyclotron instabilities
Authors: Chen K. R.
Published: July 2002

The novel physics of relativistic ion cyclotron instabilities is numerically investigated. The growth rate spectrums and the possibility being absolute instability of two fast ion cases (that the fast ions are energetic proton and alpha particle, respectively) are numerically studied and compared with the analytical theory. The fundamental difference in the characteristics of the instabilities due to a slight change in fast ion mass per nucleon is emphasized; it is determined by the relative normalized mass deficit per nucleon of fast and slow ions, and by the difference of their Lorentz factors. For the energetic proton case, both a cubic instability and a high harmonic quadratic instability can be driven; while, for the energetic alpha particle case, only the quadratic instability can occur at the high alpha cyclotron harmonics in the lower hybrid frequency regime and above; the threshold is determined by the dielectric constant of the slow ion. The peak growth rate is highest at the harmonics just over the threshold. Many new physics discovered by the numerical results are explained. A numerical polynomial expansion method with curve fitting is developed to conclude that the instabilities studied are absolute, because the analytical results cannot be used to address this important issue.
Two-dimensional electron-magnetohydrodynamic nonlinear structures
Authors: Jovanović D. , Pegoraro F.
Published: July 2002

Two-dimensional coherent electromagnetic structures involving the dynamics of the electron population in a quasineutral plasma with immobile ions are described. These structures consist of small scale magnetic vortices that propagate at a constant velocity. The dispersion relations of dipolar and tripolar vortices and of dipolar vortices and of vortex chains are given. The magnetic field of these vortices is three-dimensional. In addition, stationary quadrupolar vortices are described that are formed in the neighborhood of an X-point of the background configuration.
Three-dimensional reduced drift kinetic equation for neoclassical transport of helical plasmas in the ultralow collisionality regime
Authors: Wang Shaojie , Sanuki Heiji , Sugama Hideo
Published: July 2002

Based on the original five-dimensional drift kinetic equation, a three-dimensional reduced drift kinetic equation has been obtained to describe the neoclassical transport of helical plasmas in the ultralow collisionality regime, where the collision frequency is much lower than the poloidal bounce frequency of the superbanana particles. The reduced drift kinetic equation describes the evolution of the distribution function in terms of three constants of motion as independent variables. This reduced drift kinetic equation is suitable for studying the collisional transport of fast ions in helical magnetic confinement systems, since it includes the effects of finite superbanana width and finite aspect ratio, and it includes the collisional slowing-down and energy-scattering terms in addition to the pitch-angle-scattering term.
Velocity shear instabilities in the multicomponent plasma sheet region
Authors: Li Lu , Zhen-Xing Liu , Zhong-Yua Li
Published: July 2002

In the presence of ionospheric O+ ions, the velocity shear instability of ultralow frequency surface wave in the plasma sheet is investigated. By considering protons and O+ ions flow in the same macrospeed and keeping the velocity curl term in the magnetohydrodynamic equations, the general dispersion relation of surface wave perturbation was obtained. It is found that the velocity shear instability tends to happen in a range of wavelengths around a certain point, which was called the resonant point. At the resonant point, even in absence of velocity shear, the damping eigen-oscillations, which are caused by the dissipation, as well as the inhomogeneities of magnetic field and plasma density, coupled with the disturbed Alfvén wave and fed the energy into the disturbed wave through Alfvén resonance. The velocity shear makes the range of unstable wavelengths enlarge around the resonant point and even enhanced the coupling at the short wavelength side of the resonant point. The presence of ionospheric O+ ions could obviously enhance the coupling. These results may be helpful in understanding magnetic pulsation (Pc5) and vertical plasma cooling in the plasma sheet, as well as substorm processes.
Electron-drift driven ion-acoustic mode in a dusty plasma with collisional effects
Authors: Singh R. , Bora M. P.
Published: July 2002

Instabilities of ion-acoustic waves in a dusty plasma with electron-drift, collisional, and dust charge fluctuations effects, have been investigated. The regimes are clearly marked out where the theory is applicable. The critical electron-drift velocity required to drive the instability is predicted. It is also shown that electron thermal conductivity and charged grains concentration enhance the growth of the ion-acoustic mode, whereas ion viscosity, ion-thermal conductivity, and dust charge fluctuations have a stabilizing effect.
Electron parametric instabilities of relativistically intense laser light in under and overdense plasma
Authors: Barr H. C. , Mason P. , Parr D. M.
Published: July 2002

A unified treatment of electron parametric instabilities driven by ultraintense laser light in plasma is described. The method is valid for laser light of any polarization and intensity, for any plasma density both rarefied and classically overdense, and for any scattering geometry. A fully relativistic self-consistent three-dimensional (3D) analysis, based on the cold electron fluid and Maxwell’s equations without further assumption, is carried out in the laser pulse proper frame allowing a concise description in terms of a system of six ordinary differential equations. The approach is applied to linearly polarized laser light in underdense plasma, and overdense plasma accessible by self-induced transparency. Electron charge displacement and relativistic mass fluctuations give rise to hybrids of stimulated Raman scattering, the two plasmon decay, the relativistic modulational and filamentation instabilities, and to stimulated harmonic generation. There is vigorous growth over a wide range of wave numbers and harmonics.
Semiclassical electron captures by protons from hydrogenic ions in nonideal classical plasmas
Authors: Jung Young-Dae
Published: July 2002

Electron capture processes by protons from hydrogenic ions in nonideal classical plasmas are investigated in accordance with a semiclassical version of the Bohr–Lindhard model using the impact parameter method. The screened electron capture radius is obtained by using the effective pseudopotential model, taking into account the plasma screening effect and the collective effect. The scaled semiclassical electron capture probability is obtained as a function of the impact parameter, nonideal plasma parameter, Debye length, and projectile velocity. The nonideal plasma effect on the scaled semiclassical capture probability for the intermediate energy projectile is found to be more significant than that for the high energy projectile. The maximum position of the electron capture probability is getting close to the target ion as the nonideal plasma parameter increases.
GaSb0.5As0.5/Al0.35Ga0.65Sb0.48As0.52 superlattice lattice matched to InP prepared by molecular beam epitaxy
Authors: Tsang W. T. , Chiu T. H. , Chu S. N. G. , Ditzenberger J. A.
Published: July 2002

We have shown, against conventional belief, for the first time that high quality GaSb0.5As0.5/Al0.35Ga0.65Sb0.48As0.52 superlattices lattice matched to InP substrate can be grown by molecular beam epitaxy in spite of the existence of extensive miscibility gap in this material system. The superlattices have excellent heterointerfaces as examined by in situ reflection high-energy electron diffraction during growth and transmission electron microscope afterwards. The surface morphology was smooth and mirrorlike as observed under Nomarski phase contrast microscope when lattice mismatch Δa/a was ≲1×10−3. These superlattices exhibited a 40 times more intense room-temperature photoluminescence than high quality GaSb05As05 single epilayers. Quantum size effect was also observed for the first time in these new superlattices.
Electric field selective optical data storage using persistent spectral hole burning
Authors: Bogner U. , Beck K. , Maier Max
Published: July 2002

The electric field domain is used as a storage dimension in optical data storage by persistent spectral hole burning. The memory locations in the electric field domain are addressed with the voltage applied to the sample consisting of the amorphous polymer polyvinyl-butyral doped with the dye 9-amino acridine. The information is written by burning spectral holes at different electric field strengths with a HeCd laser and read by detecting the presence or absence of holes with weak laser intensity.
Noncontact photothermal probe beam deflection measurement of thermal diffusivity in an unconfined hot gas
Authors: Loulergue J. C. , Tam A. C.
Published: July 2002

A pulsed CO2 laser beam is used to produce a transient thermal refractive index gradient in a nitrogen gas doped with trace amounts of absorbing Freon 12 at temperatures from 25 to 425 °C. The diffusion of this gradient is probed by a continuous helium-neon laser beam parallel to but displaced from the pulsed beam. The observed defletion signal agrees well with theory, and thermal diffusivity or gas temperature can be derived from the signal.
Laser-enhanced plasma etching of silicon
Authors: Holber W. , Reksten G. , Osgood R. M.
Published: July 2002

Laser-enhanced etching of Si in a CF4/O2 plasma is described. Both p-type and n-type Si were investigated for different dopant concentrations, at laser intensities from 0–2 W. Etch–rate enhancement, due to thermal and nonthermal effects, was observed.
Titanium silicide formation on BF+2-implanted silicon
Authors: Chow T. P. , Katz W. , Smith G.
Published: July 2002

Thin-film interaction between titanium and BF+2 -implanted silicon substrates at 650–900 °C was investigated. At 650 °C, the incomplete Ti/Si reaction led to formation of a surface layer (∼600 Å thick) of titanium-rich silicide (Si/Ti ∼1.6) on top of a near-stoichiometric silicide layer. Annealing at 700 °C or higher resulted in conversion of the titanium film into predominantly TiSi2 and a lower sheet resistance. After annealing, boron was found to redistribute into the silicide layer and fluorine was segregated onto the silicide/silicon interface, but neither species apparently affected the overall sheet resistance for BF+2 dosages up to 6×1015 cm−2. A sheet resistance of ∼0.7 Ω/⧠ was obtained after annealing between 700 and 900 °C for 30 min.
Photoresponse of the AsGa antisite defect in as-grown GaAs
Authors: Baeumler M. , Kaufmann U. , Windscheif J.
Published: July 2002

The photoresponse of the As+Ga antisite electron-paramagnetic-resonance (EPR) has been studied in as-grown semi-insulating GaAs as a function of illumination time and photon energy hν. The As+Ga EPR signal intensity changes are nonmonotonic in time in the range 1.1≲hν≲1.4 eV. The spectral dependences of As+Ga enhancement and quenching show distinct similarities with the EL2 optical cross sections σ0n (hν) and σ0p (hν), respectively. These results demonstrate that the EL2 defect and the AsGa antisite have practically identical optical and photoelectronic properties.
Intensity oscillations of reflection high-energy electron diffraction during silicon molecular beam epitaxial growth
Authors: Sakamoto T. , Kawai N. J. , Nakagawa T. , Ohta K. , Kojima T.
Published: July 2002

Reflection high-energy electron diffraction (RHEED) intensity oscillations during Si molecular beam epitaxy (MBE) are observed for the first time. It is revealed that stable RHEED intensity oscillation exists during the MBE growth of a single-element semiconductor. The stable oscillations were observed only after preheating treatments. The substrate temperatures for the oscillation were from as low as room temperature to 1000 °C. One period of the oscillation corresponds to atomic-layer or biatomic-layer growth on the concerned surface. Oscillation of more than 2200 periods (∼6000 Å) was observed. One oscillation period can be as slow as 38 min.
Band-gap tailoring in amorphous germanium-nitrogen compounds
Authors: Chambouleyron I.
Published: July 2002

In this letter some properties of off-stoichiometric amorphous germanium-nitrogen compounds are presented. It is shown that the inclusion of nitrogen atoms in the Ge network produces important changes in the optical and electrical properties of the material. The samples were prepared by rf sputtering a Ge target in an argon plus nitrogen atmosphere. Optical transmission and dark conductivity versus temperature measurements are presented and discussed. The preparation conditions permitted the variation of the optical band gap in a continuous way between 0.9 and 2.7 eV. As expected for unhydrogenated samples, the dark conductivity measurements indicate a high density of states in the pseudogap, confirmed by an ill-defined activation energy. Preparation conditions that might improve the transport properties are indicated.
Structure and composition of NixGaAs
Authors: Sands T. , Keramidas V. G. , Washburn J. , Gronsky R.
Published: July 2002

Advanced compound semiconductor devices require increasingly stable, shallow, and uniform metallized layers for ohmic and Schottky contacts. However, the design of new multielemental contact metallization systems is limited by the paucity of information regarding the structure, composition, and stability of phases resulting from the interaction of single metal layers with compound semiconductors. In this letter, the results of a transmission electron microscopy investigation of the Ni/GaAs reaction are presented. The first reaction product is shown to have the composition Ni3GaAs. Based on this composition and lattice parameter measurements, it is proposed that the structure of Ni3GaAs is closely related to that of γ′ Ni3Ga2, a derivative of the hexagonal B8 structure type.
Significant enhancement of XeCl(B, C) and XeF(B, C) formation rate constants in reactions of Xe(5p56p) atoms with halogen donors
Authors: Ku J. K. , Setser D. W.
Published: July 2002

The quenching rate constants for Xe(6p) atoms by several halogens and halogen-containing molecules have been measured and they are substantially larger than for reactions of Xe(6s) metastable atoms. The effect is largest for molecules with small Xe(6s 3P2) quenching rate constants and the Xe(6p) rate constants for NF3 are 5–20 times larger. Moreover, the branching fractions for XeCl(B, C) and XeF(B, C) formation are also larger for Xe(6p) atoms than for Xe(6s 3P2) atoms; the branching fractions for HCl are enhanced by more than an order of magnitude. These findings may have important implications for the operation and modeling of rare gas halide lasers and other gas discharge devices.
Nondestructive depth profiling of carrier lifetimes in full silicon wafers
Authors: Guidotti D. , Batchelder J. S. , Van Vechten J. A. , Finkel A.
Published: July 2002

We demonstrate for the first time that the phase shift associated with amplitude modulated, near band-edge photoluminescence from Si at room temperature can be used to map defects and carrier lifetimes over full wafers with depth sensitivity.
Measurement of electrical characteristics and electron density in a fast discharge pumped XeCl excimer laser
Authors: Ford J. E. , Meyer J. , Houtman H.
Published: July 2002

A novel, fast, transverse electric discharge circuit is used to pump a 5-atm XeCl* excimer laser. The measured discharge characteristics agree with values predicted from critically damped circuit equations. Interferometric measurements of the electron density, n, as function of time indicate a maximum ne =1.6×1015 cm−3 and suggest a constant electron drift velocity vd =1.3×106 cm/s.
Electron traps in dislocation-free In-alloyed liquid encapsulated Czochralski GaAs and their annealing properties
Authors: Kitagawara Y. , Noto N. , Takahashi T. , Takenaka T.
Published: July 2002

Electron traps in dislocation-free In-alloyed liquid encapsulated Czochralski (LEC) GaAs have been studied by deep level transient capacitance spectroscopy. Five traps are observed with activation energies ranging from 0.26 to 0.79 eV below the conduction band. The energies are closely equal to the ones observed in undoped LEC GaAs. However, a notable difference between the In-alloyed crystal and the undoped crystal exists in effects of annealing on the deep levels. For the In-alloyed crystal, all levels except EL2(0.79 eV) are unstable under the annealing at 850 °C, while for the undoped crystal, levels EL5(0.41 eV) and EL2 remain stable.
Influence of the near-band-edge surface states on the luminescence efficiency of InP
Authors: Moison J. M. , Van Rompay M. , Bensoussan M.
Published: July 2002

We report the first simultaneous measurements of the photoluminescence yield and of the basic electronic properties (density of states, position of the Fermi level) of InP surfaces subjected to various treatments under ultrahigh vacuum conditions. The surface densities of states located near the band edges are found to govern the surface recombination process. Annealing under As pressure which moves these densities out of the band gap is shown to yield a surface with low surface recombination, in agreeement with this framework.
Electroreflectance study of HgCdTe in the metal-insulator-semiconductor configuration at 77 K
Authors: Ksendzov A. , Pollak Fred H. , Wilson J. A. , Cotton V. A.
Published: July 2002

Electroreflectance from Hg1−xCdxTe in the metal-insulator-semiconductor configuration at 77 K as a function of gate voltage has been investigated. We find the spectra consist of contributions from two considerably different compositions, one of which corresponds to semimetallic material (x≊0.1). The x≊0.1 component is a consequence of the passivation of the HgCdTe surface. There is some evidence that these two regions are distributed across the surface rather than in depth. In addition, the two components have different responses to the gate voltage. The higher composition region can be driven through a flat band while the lower composition portion cannot.
Nonradiative transition dynamics in alexandrite
Authors: Gayen S. K. , Wang W. B. , Petričević V. , Alfano R. R.
Published: July 2002

The first direct picosecond time-resolved measurement of the nonradiative transition dynamics between the excited 4T2 pump band and the metastable 2E storage level of the trivalent chromium ion in alexandrite is reported. The nonradiative relaxation times of 17 ps for intra-4T2 vibrational transitions, and 27 ps for 4T2→2E electronic transition are obtained. The thermal repopulation rate of the 4T2 state from the metastable 2E level is of the order 3.5×109 s−1.
High-speed circuit measurements using photoemission sampling
Authors: Bokor J. , Johnson A. M. , Storz R. H. , Simpson W. M.
Published: July 2002

High-speed sampling of the voltage waveform on a microstrip transmission line is performed by exploiting the multiphoton photoelectric effect induced by a visible cw mode-locked laser source. Energy analysis of the electrons emitted from the surface of the strip line is used to infer the emission point potential at the arrival time of the laser pulse. The technique may be applied to measure voltage waveforms on metallization lines of any integrated circuit or electronic device and is capable of picosecond time resolution and millivolt sensitivity.
Effect of size and material of liquid spherical particles on laser-induced breakdown
Authors: Chýlek Petr , Jarzembski M. A. , Chou N. Y. , Pinnick R. G.
Published: July 2002

We have used the Nd:YAG laser output at λ=0.532 μm to investigate the effect of the size and the material of liquid spherical aerosols on the breakdown threshold. The breakdown threshold intensity decreases with increasing droplet size; however, no simple general dependence on the refractive index has been found.
Correlation of stress with light-induced defects in hydrogenated amorphous silicon films
Authors: Kurtz Sarah R. , Tsuo Y. Simon , Tsu Raphael
Published: July 2002

No correlation was found between the stress in hydrogenated amorphous silicon films and the light-induced effect, as measured by the photoconductivity. An equation is derived for calculation of the external stress applied to a film. The light-induced degradation in a ‘‘zero-stress’’ film (one removed from the substrate) was shown to be equivalent to that of an as-deposited film.
Equivalence between resonant tunneling and sequential tunneling in double-barrier diodes
Authors: Weil T. , Vinter B.
Published: July 2002

Resonant tunneling is known to lead to negative differential resistance in double-barrier diodes. Sequential tunneling has been proposed by S. Luryi [Appl. Phys. Lett. 47, 490 (1985)] as an alternative mechanism for the negative differential resistance observed. We show that the two interpretations lead to the same predictions for the dc current.
Picosecond temporal resolution photoemissive sampling
Authors: Weiner A. M. , Lin P. S. D. , Marcus R. B.
Published: July 2002

Sampling measurements of electrical transients generated photoconductively on a 5-μm gold coplanar transmission line on GaAs were performed via multiphoton photoemission. A temporal resolution of 5 ps was achieved, with a voltage sensitivity of 10 mV/(Hz)1/2. These results confirm that the temporal resolution attainable by this technique is enhanced when the dimensions of the structure under examination are reduced.
Influence of microstructure on the Urbach edge of amorphous SiC:H and amorphous SiGe:H alloys
Authors: Mahan A. H. , Menna P. , Tsu R.
Published: July 2002

Infrared measurements have been used as a means of quantifying the amount of hydrogenated amorphous silicon and amorphous silicon alloy microstructure. Using a parameter obtained from these infrared measurements, the Urbach edge of amorphous silicon (a-Si:H), amorphous silicon carbon (a-SiC:H), and amorphous silicon germanium (a-SiGe:H) obtained from photothermal deflection spectroscopy measurements fall on the same curve. This suggests that the decreasing steepness of the Urbach edge, observed to occur with increasing alloying, is due primarily to microstructural effects and not to increased structural or compositional disorder. Based upon this correlation, we suggest an explanation for the observed decrease in alloy material photoconductivity with increasing alloy content.
Photothermal absorption microprobe: Infrared spectroscopy of a single 1 μm organic particle
Authors: Taubenblatt Marc A.
Published: July 2002

This letter describes a technique, based on photothermal absorption, for performing infrared spectroscopy with high sensitivity to a single micrometer-sized particle on a surface. The heat absorbed as a result of irradiation with a tunable infrared laser is monitored by observing the induced thermal expansion with a phase-sensitive microscope (an interferometer with a beam focused to a few micrometer spot diameter). The technique has a high specificity to a particle over background from substrate materials because of the poor heat conductance and small thermal mass of the particle. Spectra have been obtained from particles as small as 1 μm in diameter (which corresponds to 0.5 pg of material) with the use of only a few milliwatts of power from a tunable CO2 laser.
Raman scattering in (AlxGa1−x)0.51In0.49P quaternary alloys
Authors: Kubo Minoru , Mannoh Masaya , Takahashi Yasuhito , Ogura Mototsugu
Published: July 2002

We report a Raman study of the phonon mode of (AlxGa1−x)0.51In0.49P quaternary alloys lattice matched to GaAs over the whole range of compositions. From the composition dependence of spectra it has been confirmed that the three-mode behavior of the alloy corresponding to the three binary compositions: AlP-, GaP-, and InP-like phonon modes may occur.
Near-ideal transport in an AlGaAs/GaAs heterostructure bipolar transistor by Na2S⋅9H2O regrowth
Authors: Nottenburg R. N. , Sandroff C. J. , Humphrey D. A. , Hollenbeck T. H. , Bhat R.
Published: July 2002

The deposition of sodium sulfide nonahydrate (Na2S⋅9H2O) onto mesa AlGaAs/GaAs heterostructure bipolar transistors confers near-ideal transport characteristics to the device structure. By reducing the GaAs surface recombination velocity, sulfide regrowth leads to current gain ( β) almost independent of collector current, and β>1 at collector current density below 5×10−7 A/cm−2. Furthermore, we obtain by passivation an emitter junction ideality factor of n=1.03.
Ligand-independent assembly of recombinant human CD1 by using oxidative refolding chromatography
Authors: Altamirano Myriam M. , Woolfson Adrian , Donda Alena , Shamshiev Abdijapar , Briseño-Roa Luis , Foster Nicholas W. , Veprintsev Dmitry B. , De Libero Gennaro , Fersht Alan R. , Milstein César
Published: July 2002

CD1 is an MHC class I-like antigen-presenting molecule consisting of a heavy chain and β2-microglobulin light chain. Thein vitrorefolding of synthetic MHC class I molecules has always required the presence of ligand. We report here the use of a folding method using an immobilized chaperone fragment, a protein disulphide isomerase, and a peptidyl-prolylcis-transisomerase (oxidative refolding chromatography) for the fast and efficient assembly of ligand-free and ligand-associated CD1a and CD1b, starting with material synthesized inEscherichia coli. The results suggest that “empty” MHC class I-like molecules can assemble and remain stable at physiological temperatures in the absence of ligand. The use of oxidative refolding chromatography thus is extended to encompass complex multisubunit proteins and specifically to members of the extensive, functionally diverse and important immunoglobulin supergene family of proteins, including those for which a ligand has yet to be identified.
Synaptic sprouting increases the uptake capacities of motoneurons in amyotrophic lateral sclerosis mice
Authors: Millecamps Stéphanie , Nicolle Delphine , Ceballos-Picot Irène , Mallet Jacques , Barkats Martine
Published: July 2002

Using adenoviruses encoding reporter genes as retrograde tracers, we assessed the capacity of motoneurons to take up and retrogradely transport adenoviral particles injected into the muscles of transgenic mice expressing the G93A human superoxide dismutase mutation, a model of amyotrophic lateral sclerosis. Surprisingly, transgene expression in the motoneurons was significantly higher in symptomatic mice than in control or presymptomatic mice. Using botulinum toxin to induce nerve sprouting at neuromuscular junctions, we showed that the unexpectedly high level of motoneurons retrograde transduction results, at least in part, from newly acquired uptake properties of the sprouts. These findings demonstrate the remarkable uptake properties of amyotrophic lateral sclerosis motoneurons in response to denervation and the rationale of using intramuscular injections of adenoviruses to overexpress therapeutic proteins in motor neuron diseases.
Archaeal-type lysyl-tRNA synthetase in the Lyme disease spirochete
Borrelia burgdorferi
Authors: Ibba Michael , Bono James L. , Rosa Patricia A. , Söll Dieter
Published: July 2002

Lysyl-tRNAs are essential for protein biosynthesis by ribosomal mRNA translation in all organisms. They are synthesized by lysyl-tRNA synthetases (EC
6.1.1.6
), a group of enzymes composed of two unrelated families. In bacteria and eukarya, all known lysyl-tRNA synthetases are subclass IIc-type aminoacyl-tRNA synthetases, whereas some archaea have been shown to contain an unrelated class I-type lysyl-tRNA synthetase. Examination of the preliminary genomic sequence of the bacterial pathogen
Borrelia burgdorferi
, the causative agent of Lyme disease, indicated the presence of an open reading frame with over 55% similarity at the amino acid level to archaeal class I-type lysyl-tRNA synthetases. In contrast, no coding region with significant similarity to any class II-type lysyl-tRNA synthetase could be detected. Heterologous expression of this open reading frame in
Escherichia coli
led to the production of a protein with canonical lysyl-tRNA synthetase activity
in vitro
. Analysis of
B. burgdorferi
mRNA showed that the lysyl-tRNA synthetase-encoding gene is highly expressed, confirming that
B. burgdorferi
contains a functional class I-type lysyl-tRNA synthetase. The detection of an archaeal-type lysyl-tRNA synthetase in
B. burgdorferi
and other pathogenic spirochetes, but not to date elsewhere in bacteria or eukarya, indicates that the gene that encodes this enzyme has a common origin with its orthologue from the archaeal kingdom. This difference between the lysyl-tRNA synthetases of spirochetes and their hosts may be readily exploitable for the development of anti-spirochete therapeutics.
From residue matching patterns to protein folding topographies: General model and bovine pancreatic trypsin inhibitor
Authors: Fernández Ariel , Kostov Konstantin , Berry R. Stephen
Published: July 2002

A coarse-grained model for protein-folding dynamics is introduced based on a discretized representation of torsional modes. The model, based on the Ramachandran map of the local torsional potential surface and the class (hydrophobic/polar/neutral) of each residue, recognizes patterns of both torsional conformations and hydrophobic-polar contacts, with tolerance for imperfect patterns. It incorporates empirical rates for formation of secondary and tertiary structure. The method yields a topological representation of the evolving local torsional configuration of the folding protein, modulo the basins of the Ramachandran map. The folding process is modeled as a sequence of transitions from one contact pattern to another, as the torsional patterns evolve. We test the model by applying it to the folding process of bovine pancreatic trypsin inhibitor, obtaining a kinetic description of the transitions between the contact patterns visited by the protein along the dominant folding pathway. The kinetics and detailed balance make it possible to invert the result to obtain a coarse topographic description of the potential energy surface along the dominant folding pathway, in effect to go backward or forward between a topological representation of the chain conformation and a topographical description of the potential energy surface governing the folding process. As a result, the strong structure-seeking character of bovine pancreatic trypsin inhibitor and the principal features of its folding pathway are reproduced in a reasonably quantitative way.
Crystal structure of the functional domain of the splicing factor Prp18
Authors: Jiang Jianzhong , Horowitz David S. , Xu Rui-Ming
Published: July 2002

The splicing factor Prp18 is required for the second step of pre-mRNA splicing. We have isolated and determined the crystal structure of a large fragment of the
Saccharomyces cerevisiae
Prp18 that lacks the N-terminal 79 amino acids. This fragment, called Prp18Δ79, is fully active in yeast splicing
in vitro
and includes the sequences of Prp18 that have been evolutionarily conserved. The core structure of Prp18Δ79 is compact and globular, consisting of five α-helices that adopt a novel fold that we have designated the five-helix X-bundle. The structure suggests that one face of Prp18 interacts with the splicing factor Slu7, whereas the more evolutionarily conserved amino acids in Prp18 form the opposite face. The most highly conserved region of Prp18, a nearly invariant stretch of 19 aa, forms part of a loop between two α-helices and may interact with the U5 small nuclear ribonucleoprotein particles. The structure is consistent with a model in which Prp18 forms a bridge between Slu7 and the U5 small nuclear ribonucleoprotein particles.
Experimental determination of the anisotropy function and anisotropy factor for model 6711 I‐125 seeds
Authors: Sloboda Ron S. , Menon Geetha V.
Published: July 2002

Model 6711 seeds are commonly used in permanent brachytherapy implants. While recommended dose distribution parameters for these sources have been published by AAPM TG‐43, several investigators have recently questioned the presence of seemingly unphysical fluctuations in the anisotropy function data. Seeking to understand these, we measured the dimensions of eight model 6711 seeds radiographically, and made a new experimental determination of the anisotropy function and factor using thermoluminescent dosimeters (TL‐100 chips and minicubes) in a solid water phantom. It was found that variations in seed capsule end weld thickness, and movement within the capsule of the Ag rod onto which the is adsorbed, were present for all sources and thus contributed to experimental uncertainty. Averaging results from two different sources, from data acquired at the same time from diametrically opposed quadrants of the phantom, and from repeated measurements yielded anisotropy function values similar to those of TG‐43 but characterized by greater precision (⩽3% for radial distance and ⩽1% for an absence of sharp fluctuations, and reduced magnitudes at The measured values can be well represented by an analytic function similar to that proposed by Furhang and Anderson to fit the TG‐43 data. Values of the anisotropy factor derived from this function by integration exhibit little variation with r, in agreement with earlier diode data but in contrast to TG‐43 data, and can also be represented by an analytic function. Finally, a difference in TLD chip and minicube reproducibility, observed here and by earlier investigators, is explained by reference to recent work (done concurrently with ours) as stemming from variations in dosimeter orientation and automated reader positioning/heating for minicubes.
Beta versus gamma dosimetry close to Ir‐192 brachytherapy sources
Authors: Baltas D. , Karaiskos P. , Papagiannis P. , Sakelliou L. , Loeffler E. , Zamboglou N.
Published: July 2002

The relative importance of the dose rate component owing to the beta spectrum emitted by brachytherapy sources at the short radial distances of interest in intravascular and endobronchial applications is investigated. Separate dosimetric calculations, using Monte Carlo simulations, were performed for the gamma and beta dose rate components of an ideal point source as well as real source designs used in clinical practice including wire and seed sources and both Nucletron and Varian, old and new, high dose rate (HDR) source designs. A significant dose rate enhancement due to the beta spectrum emitted by greater than 50% for radial distances was observed for an ideal point source. For real source designs, however, the magnitude of this enhancement was found to depend strongly on the sources' geometric as well as compositional details of the active core and encapsulation. A detectable effect was found for the majority of the investigated sources at radial distances less than 1 mm, but overall findings suggest that the contribution of beta particles is not significant in clinical intravascular applications that are currently carried out. However, since treatment of vessels with smaller diameters, in the future, may lead to the development of sources and catheters of reduced diameters, the potential effect of the beta spectrum in terms of dose enhancement to tissues in close proximity to sources should not be ignored.
Performance characteristics of a Kodak computed radiography system
Authors: Bradford Carla D. , Peppler Walter W. , Dobbins James T.
Published: July 2002

The performance characteristics of a photostimulable phosphor based computed radiographic (CR) system were studied. The modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE) of the Kodak Digital Science computed radiography (CR) system (Eastman Kodak Co.‐model 400) were measured and compared to previously published results of a Fuji based CR system (Philips Medical Systems‐PCR model 7000). To maximize comparability, the same measurement techniques and analysis methods were used. The DQE at four exposure levels (30, 3, 0.3, 0.03 mR) and two plate types (standard and high resolution) were calculated from the NPS and MTF measurements. The NPS was determined from two‐dimensional Fourier analysis of uniformly exposed plates. The presampling MTF was determined from the Fourier transform (FT) of the system's finely sampled line spread function (LSF) as produced by a narrow slit. A comparison of the slit type (“beveled edge” versus “straight edge”) and its effect on the resulting MTF measurements was also performed. The results show that both systems are comparable in resolution performance. The noise power studies indicated a higher level of noise for the Kodak images (∼20% at the low exposure levels and 40%–70% at higher exposure levels). Within the clinically relevant exposure range (0.3–3 mR), the resulting DQE for the Kodak plates ranged between 20%–50% lower than for the corresponding Fuji plates. Measurements of the presampling MTF with the two slit types have shown that a correction factor can be applied to compensate for transmission through the relief edges.
Bioacoustic spatial perception by humans: A controlled laboratory measurement of spatial resolution without distal cues
Authors: Kay Leslie
Published: July 2002

The angular spatial resolution of a wide-angle air sonar using a continuous transmission frequency-modulated radiation, with the output coupled binaurally to the auditory system of a user, was measured under restrained controlled conditions. This was done to determine the effect of adding a narrow central field of view of 9 deg to a wide-angle sonar. The target objects were three equidistant vertical rods initially spaced apart by 10 deg. This was varied down to a spacing of 4 deg. Ten nonvisual subjects achieved an angular resolution of 6 deg. Four of these ten subjects continued learning to achieve an unexpected spatial resolution of 4 deg within the 9 deg central field. A mean error of approximately 1 deg in direction accuracy was achieved. It is inferred that the unique variations in the octave band ultrasonic echoes within the narrow field, and the invariance of the on-axis echo as one’s head is turned, enables this angular resolution and accuracy to be achieved within the wide binaural field of view of 50 deg. This ability to resolve specula objects within a narrow angular resolution element of 9 deg is linked to the bat’s ability to seemingly resolve object glints within a distal resolution element of less than 2 wavelengths.
Spontaneous classification of complex tones at high and ultrasonic frequencies in the bat, Megaderma lyra
Authors: Preisler Annemarie , Schmidt Sabine
Published: July 2002

Megaderma lyra, a bat species using harmonically structured calls for echolocation, exploits the spectral content of its echoes for texture discrimination. It is the aim of the present study to test according to which sensory qualities harmonic complex tones are spontaneously classified by this bat. The applied experimental paradigm is especially adapted to the preference of M. lyra to use absolute pitch cues. Three animals were trained in a 2-AFC procedure to classify three-component stimuli as low or high, with all their harmonics below or above a pure tone reference of 33 kHz, respectively. Later, the original tones were interspersed with “incomplete” test stimuli, with their fundamentals (and lower harmonics) missing. These were ambiguous in that their possible virtual, i.e., collective pitches were below the reference whereas their pure tone pitches were above it. Bat 1 classified 22 of 23 test stimuli with missing fundamentals between 5.3 and 28.3 kHz according to their collective pitches, whereas bat 2 judged all presented ambiguous tones on the basis of their pure tone pitches. Bat 3 failed the pitch control criterion which is why in this case results cannot be interpreted unequivocally. The implications of these findings are discussed with respect to the bats’ behavioral context, as well as to psychoacoustical models of the formation of the pitch of complex tones.
Effects of higher-order modes and harmonics in single-bubble sonoluminescence
Authors: Seeley Fred B.
Published: July 2002

This paper presents the results of a series of experiments that was performed to study the higher-order modes and harmonics that exist within water-filled resonators during single-bubble sonoluminescence [SBSL]. These experiments demonstrated that: (1) the spatial position and the phase of the light flashes of an SBSL bubble relative to the fundamental sound field could be significantly changed by adding higher harmonics, (2) SBSL could be produced in spherical flasks at eigenfrequencies of the higher-order modes and higher harmonics of the spherical Bessel functions, j1, and j2, thus producing noncentered flashing bubbles, and (3) it is possible to produce the SBSL effect at eigenfrequencies at which the eigenmodes are degenerate. These experiments show that a rich, complex mode structure evidently exists within the water-filled resonator. The externally generated higher modes and higher harmonics influence the position, spatial stability, and phase of the light-flash characteristic of single-bubble sonoluminescence. These experiments suggest that any higher-order modes and harmonics, self-generated by the collapsing bubble, may spatially shift their position within the resonator and thus may play a significant role in destabilizing the bubble’s motion.
Vocal tract normalization for midsagittal articulatory recovery with analysis-by-synthesis
Authors: McGowan Richard S. , Cushing Steven
Published: July 2002

A method is presented that accounts for differences in the acoustics of vowel production caused by human talkers’ vocal-tract anatomies and postural settings. Such a method is needed by an analysis-by-synthesis procedure designed to recover midsagittal articulatory movement from speech acoustics because the procedure employs an articulatory model as an internal model. The normalization procedure involves the adjustment of parameters of the articulatory model that are not of interest for the midsagittal movement recovery procedure. These parameters are adjusted so that acoustic signals produced by the human and the articulatory model match as closely as possible over an initial set of pairs of corresponding human and model midsagittal shapes. Further, these initial midsagittal shape correspondences need to be generalized so that all midsagittal shapes of the human can be obtained from midsagittal shapes of the model. Once these procedures are complete, the midsagittal articulatory movement recovery algorithm can be used to derive model articulatory trajectories that, subsequently, can be transformed into human articulatory trajectories. In this paper the proposed normalization procedure is outlined and the results of experiments with data from two talkers contained in the X-ray Microbeam Speech Production Database are presented. It was found to be possible to characterize these vocal tracts during vowel production with the proposed procedure and to generalize the initial midsagittal correspondences over a set of vowels to other vowels. The procedure was also found to aid in midsagittal articulatory movement recovery from speech acoustics in a vowel-to-vowel production for the two subjects.
An acoustic model of a laminar sand bed
Authors: Chotiros Nicholas P. , Yelton Dennis J. , Stern Morris
Published: July 2002

It was postulated that a laminar sand bed may be modeled as an ensemble of randomly layered Biot media. The thickness of each layer was approximately half a grain diameter. The porosity variations in the vertical direction were matched to the mean and standard deviation of that of a structure of packed spherical grains. The effect of large-scale lateral variations in porosity was simulated by performing a coherent ensemble average of the acoustic output from several realizations of the randomly layered medium. The medium parameters were chosen to represent water-saturated sand. Specifically, the sand bed was modeled as bounded by a homogeneous water halfspace above, and a homogeneous poroelastic halfspace of equal average porosity below. Reflected and transmitted signals were computed. Coherent and random components of the reflected signal were calculated. The coherent parts were directly related to the reflected and the transmitted waves. Results showed significant differences between the modeled sand bed and an equivalent uniform Biot medium. In the modeled sand bed, the fast wave attenuation was found to be anisotropic, and a propagating slow wave was excited at most incident angles, except at normal incidence, which may explain the apparent failure to detect the slow wave of certain experiments.
Nonlinear oscillation of a spherical gas bubble in acoustic fields
Authors: Kameda Masaharu , Matsumoto Yoichiro
Published: July 2002

Radial motion of a spherical air bubble in acoustic fields is observed experimentally. The radius-time curves and frequency responses are obtained from the experiment for comparison with a numerical calculation. The calculation is based on a mathematical model in which the thermo-fluid mechanics of the gas in the bubble is precisely described. An oscillatory pressure field is generated in a cylindrical cell, which consists of two piezoceramic transducers and a glass cylinder. A new bubble generator is developed. It is able to generate a bubble filled with an arbitrary kind of gas. The bubble motion is observed by high-speed photography. The time history of the bubble radius is measured from the pictures. The pressure field has a frequency of 19.2 kHz and its amplitude is up to 40 kPa. The bubble has an initial radius within the range from 0.1 mm to 0.25 mm. A highly viscous silicone oil, whose kinematic viscosity is 100 mm2/s, is used for the liquid to keep the spherical shape of the bubble. A quantitatively good agreement between the experimental and numerical results is obtained. The difference between experiment and theory based on the polytropic approximation for the gas is briefly discussed.
Response to “Flexural edge waves and Comments on ‘A new bending wave solution for the classical plate equation’ ” [J. Acoust. Soc. Am. 104, 2220–2222 (1998)]
Authors: Kauffmann Christiaan
Published: July 2002

We reply to the preceding letter.
The Waves Generated by Brittle Fracture in Glass
Authors: Kolsky H.
Published: July 2002

This paper is concerned with the stress pulses emitted when glass specimens fracture under various types of loading. In particular, the pulses emitted under the following conditions are dealt with in detail: (a) fractures produced by Hertzian impacts; (b) fractures produced in rods under conditions of pure tension; (c) fractures produced in rods by ‘Hopkinson’ spalling; and (d) fractures produced in rods under conditions of pure bending. The information which these emitted pulses give with respect to the fracture process, and the interaction between the emitted pulses and the applied stress field are considered; the information which these results give with regard to the process of brittle fracture is also discussed.