bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnitude comparisons by children with specific language impairments: evidence of unimpaired symbolic processing
Authors: Donlan C. , Bishop D. V. M. , Hitch G. J.
Published: July 2002

A size judgement task was used to investigate number processing skills in children with specific language impairments (SLI). Previous work with unimpaired adults and children has shown that when comparing the size of written numbers and other ordinal stimuli, there is a symbolic distance effect (SDE) such that decision time decreases with the size distance between items. This study examined the ability of children to judge stimulus pairs which were varied to contrast the processing of symbolic material against direct perceptual judgement and to test processing of numeric versus non‐numeric material. Children with SLI were compared with a control group matched on verbal comprehension level. The children with SLI responded faster than the control subjects. The SLI and control groups showed similar SDE and a similar pattern of response across materials. No indication was found in the SLI data of any selective deficit in processing symbolic information. Findings are discussed in relation to theories of numeracy acquisition which acknowledge the importance of nonverbal representation of number meanings.
Regulation of Flagellar Dynein by Phosphorylation of a 138-kD Inner Arm Dynein Intermediate Chain
Authors: Habermacher Geoffrey , Sale Winfield S.
Published: July 2002

One of the challenges in understanding ciliary and flagellar motility is determining the mechanisms that locally regulate dynein-driven microtubule sliding. Our recent studies demonstrated that microtubule sliding, in Chlamydomonas flagella, is regulated by phosphorylation. However, the regulatory proteins remain unknown. Here we identify the 138-kD intermediate chain of inner arm dynein I1 as the critical phosphoprotein required for regulation of motility. This conclusion is founded on the results of three different experimental approaches. First, genetic analysis and functional assays revealed that regulation of microtubule sliding, by phosphorylation, requires inner arm dynein I1. Second, in vitro phosphorylation indicated the 138-kD intermediate chain of I1 is the only phosphorylated subunit. Third, in vitro reconstitution demonstrated that phosphorylation and dephosphorylation of the 138-kD intermediate chain inhibits and restores wild-type microtubule sliding, respectively. We conclude that change in phosphorylation of the 138-kD intermediate chain of I1 regulates dynein-driven microtubule sliding. Moreover, based on these and other data, we predict that regulation of I1 activity is involved in modulation of flagellar waveform.
Fission Yeast dim1+ Encodes a Functionally Conserved Polypeptide Essential for Mitosis
Authors: Berry Lynne D. , Gould Kathleen L.
Published: July 2002

In a screen for second site mutations capable of reducing the restrictive temperature of the fission yeast mutant cdc2-D217N, we have isolated a novel temperature-sensitive mutant, dim1-35. When shifted to restrictive temperature, dim1-35 mutant cells arrest before entry into mitosis or proceed through mitosis in the absence of nuclear division, demonstrating an uncoupling of proper DNA segregation from other cell cycle events. Deletion of dim1 from the Schizosaccharomyces pombe genome produces a lethal G2 arrest phenotype. Lethality is rescued by overexpression of the mouse dim1 homolog, mdim1. Likewise, deletion of the Saccharomyces cerevisiae dim1 homolog, CDH1, is lethal. Both mdim1 and dim1+ are capable of rescuing lethality in the cdh1::HIS3 mutant. Although dim1-35 displays no striking genetic interactions with various other G2/M or mitotic mutants, dim1-35 cells incubated at restrictive temperature arrest with low histone H1 kinase activity. Morevoer, dim1-35 displays sensitivity to the microtubule destabilizing drug, thiabendazole (TBZ). We conclude that Dim1p plays a fundamental, evolutionarily conserved role as a protein essential for entry into mitosis as well as for chromosome segregation during mitosis. Based on TBZ sensitivity and failed chromosome segregation in dim1-35, we further speculate that Dim1p may play a role in mitotic spindle formation and/or function.
Regulation of Cell Polarity by Microtubules in Fission Yeast
Authors: Sawin Kenneth E. , Nurse Paul
Published: July 2002

To investigate the role of microtubules in regulating cell polarity in Schizosaccharomyces pombe, we have developed a system in which normally cylindrical fission yeast synchronously form branched cells at high frequency upon treatment with the microtubule-depolymerizing drug thiabendazole (TBZ). Branching depends on both elevated temperature and cell cycle state and occurs at high frequency only when TBZ is added to cells that have not yet passed through New-End Take-Off (NETO), the normal transition from monopolar to bipolar growth. This suggests that microtubules may be of greatest physiological importance for the maintenance of cell shape at specific points in the cell cycle. The localization of three different proteins normally found at cell ends—cortical F-actin, tea1, and an ral3 (scd2)–green fluorescent protein (GFP) fusion—is disrupted by TBZ treatment. However, these proteins can eventually return to cell ends in the absence of microtubules, indicating that although their localization to ends normally depends on microtubules, they may recover by alternative mechanisms. In addition, TBZ induces a shift in ral3–GFP distribution from cell ends to the cell middle, suggesting that a protein complex containing ral3 may be part of the cue that specifies the position of branch formation.
Stonin 2
Authors: Martina José A. , Bonangelino Cecilia J. , Aguilar Rubén C. , Bonifacino Juan S.
Published: July 2002

Endocytosis of cell surface proteins is mediated by a complex molecular machinery that assembles on the inner surface of the plasma membrane. Here, we report the identification of two ubiquitously expressed human proteins, stonin 1 and stonin 2, related to components of the endocytic machinery. The human stonins are homologous to the Drosophila melanogaster stoned B protein and exhibit a modular structure consisting of an NH2-terminal proline-rich domain, a central region of homology specific to the stonins, and a COOH-terminal region homologous to the μ subunits of adaptor protein (AP) complexes. Stonin 2, but not stonin 1, interacts with the endocytic machinery proteins Eps15, Eps15R, and intersectin 1. These interactions occur via two NPF motifs in the proline-rich domain of stonin 2 and Eps15 homology domains of Eps15, Eps15R, and intersectin 1. Stonin 2 also interacts indirectly with the adaptor protein complex, AP-2. In addition, stonin 2 binds to the C2B domains of synaptotagmins I and II. Overexpression of GFP–stonin 2 interferes with recruitment of AP-2 to the plasma membrane and impairs internalization of the transferrin, epidermal growth factor, and low density lipoprotein receptors. These observations suggest that stonin 2 is a novel component of the general endocytic machinery.
Regulation of cytokine-independent survival kinase (CISK) by the Phox homology domain and phosphoinositides
Authors: Xu Jun , Liu Dan , Gill Gordon , Songyang Zhou
Published: July 2002

PKB/Akt and serum and glucocorticoid–regulated kinase (SGK) family kinases are important downstream targets of phosphatidylinositol 3 (PI-3) kinase and have been shown to mediate a variety of cellular processes, including cell growth and survival. Although regulation of Akt can be achieved through several mechanisms, including its phosphoinositide-binding Pleckstrin homology (PH) domain, how SGK kinases are targeted and regulated remains to be elucidated. Unlike Akt, cytokine-independent survival kinase (CISK)/SGK3 contains a Phox homology (PX) domain. PX domains have been implicated in several cellular events involving membrane trafficking. However, their precise function remains unknown. We demonstrate here that the PX domain of CISK interacts with phosphatidylinositol (PtdIns)(3,5)P2, PtdIns(3,4,5)P3, and to a lesser extent PtdIns(4,5)P2. The CISK PX domain is required for targeting CISK to the endosomal compartment. Mutation in the PX domain that abolished its phospholipid binding ability not only disrupted CISK localization, but also resulted in a decrease in CISK activity in vivo. These results suggest that the PX domain regulates CISK localization and function through its direct interaction with phosphoinositides. Therefore, CISK and Akt have evolved to utilize different lipid binding domains to accomplish a similar mechanism of activation in response to PI-3 kinase signaling.
N-Propionylated Group B Meningococcal Polysaccharide Mimics a Unique Bactericidal Capsular Epitope in Group B Neisseria meningitidis
Authors: Pon Robert A. , Lussier Michele , Yang Qing-Ling , Jennings Harold J.
Published: July 2002

The N-propionylated group B meningococcal polysaccharide (NPrGBMP) mimics a unique protective epitope on the surface of group B meningococci (GBM) and Escherichia coli K1. Using a series of monoclonal antibodies (mAbs) induced by the NPrGBMP–monomeric tetanus toxoid (TT) conjugate vaccine it was demonstrated that mAbs having specificities for both extended and conventional short segments of the NPrGBMP were formed, but only the former were bactericidal, and/or gave passive protection against live challenge by GBM. The failure of mAbs specific for short epitopes to protect was further established when (NeuPr)4–TT was used as the vaccine. Of all the mAbs produced that were specific for short internal segments of the NPrGBMP, none were protective, despite the fact that most of them cross-react with the GBM capsular polysaccharide. In contrast, most of the protective mAbs produced by NPrGBMP– TT did not recognize the group B meningococcal polysaccharide (GBMP) unless it was present in its aggregated high molecular weight form. The bactericidal epitope mimicked by the NPrGBMP was shown to be ubiquitous in the capsule of both GBM and E. coli K1 using immunogold labeling techniques and, because of its unique properties, its identification could be significant in the development of a comprehensive conjugate vaccine against group B meningococcal meningitis. This is because most known human α(2–8)-polysialic acid self-antigens can be accommodated in 30–50 α(2–8)-linked sialic acid residues, which is roughly equivalent to an 11-kD length of the GBMP. It has been hypothesized that the formation of the protective epitope on the surface of GBM is due to the interaction of helical segments of the GBMP with another molecule and that the protective epitope is mimicked by the NPrGBMP. Support for the above hypothesis is provided by the fact that the protective NPrGBMP epitope has a similar unusual length dependency to that of the GBMP epitope.
An Apoptosis-inducing Isoform of Neu Differentiation Factor (NDF) Identified Using a Novel Screen for Dominant, Apoptosis-inducing Genes
Authors: Grimm Stefan , Leder Philip
Published: July 2002

Apoptosis is a genetically programmed series of events that results in cell death. As a consequence, it is difficult to identify dominant genes that play a role in this process using genetic selections in conventional cell culture systems. Accordingly, we have established an efficient expression screen to isolate dominant, apoptosis-inducing genes. The assay is based on the apoptotic morphology induced in the human kidney cell line 293 after transient transfection of small plasmid pools from normalized cDNA expression libraries. Using this assay, we isolated a novel isoform of the proto-oncogene Neu differentiation factor (NDF), a ligand for erbB receptor tyrosine kinases. Several lines of experimental evidence indicate that this gene kills in a cell-autonomous fashion and independently of known erbB receptors. This apoptotic property of an NDF isoform is readily contrasted with NDF's transforming potential and might balance the tendency to tumorigenesis in cells that overexpress NDF.
Light-scattering studies on suspensions of orientated attapulgite
Authors: H Plummer  , B R Jennings
Published: July 2002

A study is reported of the light scattered by aqueous suspensions of the clay mineral attapulgite when the dispersions were subjected to alternating electric fields. Prior to the application of these fields, conventional measurements indicated that the dispersed particles were of weight average molecular weight 5·9×109 and of 1·1 μm in length. Electron micrographs confirmed the approximate cylindrical form of the particles. The theory for the change in the intensity and shape of the angular scattering diagram when alternating electric fields are applied to the suspensions and the requirements of a suitable apparatus to detect these changes are briefly outlined. The observed intensity changes for fields of magnitude up to 170 v cm−1 and frequencies below 150 Hz indicated that the attapulgite particles were characterized by an effective permanent dipole moment of magnitude 4·2×10−6 e.s.u. across the particles and an electrical anisotropy of 2·4×10−13 e.s.u. along the rods. The magnitude of the latter indicates that a surface rather than a bulk conductivity is involved and that attapulgite is probably synonymous with palygorskite. The frequency dependence of the intensity changes enabled a value of the rotary diffusion constant of 60 s−1 to be determined. The study is closed with a brief comparison between the electric field light scattering as used in these experiments and the similar but better-known Kerr electro-optical effect.
Ion optics of mass spectrometers IIB. Oblique incidence on magnetic fields
Authors: B S Prahallada Rao
Published: July 2002

In Part IIA ion-optical properties, including magnification, dispersion, aberrations and resolution, for homogeneous magnetic sector fields were considered for the case of normal incidence. In this part these properties have been re-examined for an ion beam incident obliquely on the field boundary. All calculations are presented in graphical form.
On the analysis of ionization growth times in neon
Authors: R D Davies  , C J Evans
Published: July 2002

In calculating the secondary photoelectron yield in a gas discharge it is not always possible to consider relatively long-lived resonance or metastable states separately because of transitions between them (natural or collision-induced). A theory is first developed for any collection of excited states, giving formulae for the secondary electron yield and for the temporal growth of ionization. It is shown how the formulae were used to analyse experimental data for neon and so to calculate the relative contributions of the various secondary ionization processes.
Ionization in the cathode-fall region of moving arcs
Authors: D J Hamilton  , A E Guile
Published: July 2002

Measurements have been made of the sum of the cathode- and anode-fall voltages of arcs moving in a transverse magnetic field between copper and brass electrodes in air, argon and helium. The experiments have covered a range of arc current of 40-700 A and a velocity range up to 470 m s−1. The gas pressure was mainly about 1 atm but a check at 11 atm showed no major difference. Comparison between these voltages, the excitation and ionization potentials of the gases and recent data on cathode-fall voltages of static arcs show that even with arcs moving at speeds up to nearly 500 m s−1 on cold-cathode emitting materials, ionization of electrode vapour rather than gas appears to have occurred in the cathode-fall region. The ionized vapour is generally that of the cathode, but it is possible for anode material carried by vapour jets to be ionized in the cathode-fall region when the arc is moving sufficiently slowly.
Column recovery of wall-stabilized d.c. arcs in air
Authors: K R Hearne  , M Rasul
Published: July 2002

Detailed results are given of the steady-state characteristics, over the current range 4-40 A, for d.c. arcs in air burning through a 5 mm bore in a stack of water-cooled disks. The corresponding decay of the conductance has been determined over a period of 300 μs following rapid interruption (similar 1 μs). These measurements have also been obtained under externally imposed gas-flow conditions (0-2·5 1. min−1).

It has been found that both the steady state and recovery are dependent upon humidity and, furthermore, that the recovery is sensitive to the radial gas flow which is generated after interruption.
The activation energies of chromium, iron and nickel in gallium arsenide
Authors: G A Allen
Published: July 2002

Measurements have been made of the thermal and optical activation energies of chromium, iron and nickel in gallium arsenide.

The temperature dependence of the broad chromium level may be written in the form E=0·81±0·02−8×10−4
T ev for T>100°K E=0·75±0·01 ev for T->0°K where E is measured from the conduction band.

It is argued that some of the confusion concerning activation energies in the literature results from equating values which are measures of different features of the impurity band and which give significantly different results if the impurity band is broad.
Analysis of multiple relaxation processes
Authors: R Holmes  , M A Stott
Published: July 2002

A method has been developed for analysing multiple relaxation processes. It is based on the least-squares criterion of fit and automatically calculates relaxation times and strengths so that the sum f of the squares of the deviations of the measured points from the theoretical curve is a minimum. The method may be used with or without restrictions on the relaxation parameters and is designed to give rapid convergence on the minimum. Errors in the relaxation parameters may be estimated by calculating the change in a parameter necessary to produce some chosen change in f.
Electron penetration and the atomic number correction in electron probe microanalysis
Authors: J Philibert  , R Tixier
Published: July 2002

The number of ionizations on a given j shell in any electron-bombarded target has been calculated analytically by means of the Bethe formulae. A general formula for the atomic number correction has been derived in the form of an equation using parameters that are functions of target composition and accelerating voltage. This equation allows the calculation of the penetration effect to be readily carried out. The electron range, in terms of beam energy, has also been calculated. Finally, it is shown that the correction is different in the case of thin-target analysis.
Spectroscopic study during the extinction of an electric arc
Authors: H Kafrouni  , S Vacquie
Published: July 2002

The decay of the electron density is measured during extinction of an arc discharge at atmospheric pressure. Measurements are made by spectroscopic methods. Complete local thermodynamic equilibrium is not attained for an electron density lower than 3*1016cm-3. The apparent recombination coefficient for electrons and ions is studied as a function of temperature between 10000 and 13000K.
Parametric interaction of radially gaussian modes in the absence of self-focusing
Authors: L A Patel  , M S Sodha
Published: July 2002

The authors present an investigation on the effect of transverse non-uniformity on the parametric interaction of radially gaussian modes of arbitrary type in the absence of self-focusing. The propagation-distance dependences of the real-valued axial amplitudes, phases, beamwidth parameters and phase gradients have been derived by using the variational technique. The analysis has been numerically illustrated by considering the stimulated Raman scattering in a plasma. It is seen that the pump node propagates like a trapped filament whereas the remaining modes undergo a periodic focusing or defocusing on account of the parametric-interaction-induced energy exchange.
Response of Schottky barrier solar cells at higher temperatures
Authors: S Srivastava  , N K Swami  , G P Srivastava
Published: July 2002

The responses of Cu/Cr-p-type silicon and Au-n-type silicon Schottky barrier solar cells at higher temperatures have been studied. The variations of different factors, such as intrinsic carrier density, energy gap, absorption coefficient, barrier height, effective number of photons etc., have been taken into account. The results show that in the temperature range 300-450K, except for the short-circuit current, all other factors (e.g. open-circuit voltage, fill-factor and conversion efficiency) decrease with increase in temperature.
Selfconsistent calculation of the hydrogen plasma parameters. I. Stationary plasmas
Authors: N N Ljepojevic  , J M Labat
Published: July 2002

Hydrogen plasma parameters have been calculated for a glow discharge driven by 0.1 A current in a 0.4 cm tube at 270 Pa and 530 Pa initial gas pressures. The set of differential equations that includes levels population, electron density and electron temperature rate equations has been solved simultaneously and the stationary plasma parameters have been evaluated.
The thermal conductivity of Cl2and O2in the dissociation region measured by optical interferometry. II. Evaluation of the thermal conductivity from interferometric data
Authors: A Kurtz  , J Mentel
Published: July 2002

For pt.I see ibid., vol.17, p.1335 (1984). The authors describe a method to evaluate temperatures and the thermal conductivity in the dissociation region of molecular gases from interferometric measurements in arc tubes. The lack of accuracy of the absolute value of optical path length data and the unknown polarisability of chlorine atoms is compensated by fitting the experimental results to the cross-section normalised shape of the thermal conductivity dependence on temperature, which is described by kinetic theory. The thermal conductivity of Cl2is given up to 3500K and of O2up to 6000K at atmospheric pressure.
Thermal recovery and thermoluminescence of room-temperature X-irradiated NaCl:Pb2+single crystals
Authors: A Navarro  , F J Lopez
Published: July 2002

The thermoluminescence (TL) spectrum of NaCl:Pb2+single crystals X-irradiated at room temperature consists of seven glow peaks that obey second-order kinetics. The bands in the wavelength spectrum of the emitted light correspond to different emissions of Pb2+ions. A parallel study of the thermal stability of defects produced by the irradiation shows that the Pb+ions and F-centres annihilate during TL; at the same time Pb2+ions are recovered. A model is proposed in which holes and interstitials are trapped together by different defects. When they are thermally activated the hole recombines with a Pb+yielding a Pb2+ion in an excited state which gives the observed emission; simultaneously the interstitial recombines with an F-centre and regenerates the perfect lattice.
On the Balmer recombination continuum in dense hydrogen plasma
Authors: R Radtke  , F Serick  , R Spanke  , R Zimmermann
Published: July 2002

An electric pulse discharge has been used to generate dense hydrogen plasmas, and measurements were made of the Balmer recombination continuum between 260 and 400 nm. The results show that the effect of finite electron density on bound-scattering transitions is negligible over the whole range of experimental conditions.
New evidence against the presence of peak IV in the TL and PTTL glow curves of heavily annealed TLD-100
Authors: A Delgado  , J M Gomez Ros  , J C Portillo
Published: July 2002

Presents some additional evidence against the recent identification of peak IV in the TL and PTTL glow curves of heavily annealed TLD-100. Deconvolution analysis of the glow curves, produced by annealing for progressively increasing durations at 150 degrees C, permits the identification of a new and relatively broad peak, centered at 162 degrees C. This should not be confused with peak IV (182 degrees C), despite their relative proximity in temperature.
A large-signal treatment of various acoustoelectric phenomena
Authors: R K L Gay  , H L Hartnagel
Published: July 2002

A treatment of various acoustoelectric problems, using the large-signal expressions for gain and current saturation which were derived by the authors in an earlier paper, is presented.

The phenomena dealt with are:

Travelling high-field domains in semiconducting CdS.

Damped and continuous oscillations in photoconductive CdS.

The current-voltage characteristics of thin platelets of photoconductive CdS.

The essential characteristics of the above phenomena are obtained in the numerical calculations carried out for specified cases.
Photoeffects in metal-insulator-gallium arsenide diodes
Authors: T L Tansley  , J E Ralph
Published: July 2002

The application of evaporated gold-amorphous silica-gallium arsenide electroluminescent diodes to photodetection has been investigated. Detectors sensitive in the wavelength range 0·38-0·9 μm with a peak internal quantum efficiency of about 80% have been produced. Wavelength and temperature dependence are accounted for on a minority carrier diffusion model, room temperature diffusion lengths of 1·5 μm are indicated. Some device switching phenomena are reported.
Mobility, diffusion and attachment of electrons in oxygen
Authors: M S Naidu  , A N Prasad
Published: July 2002

D/μ values (diffusion coefficient/mobility), drift velocities and attachment coefficients of electrons in oxygen have been measured over the range 5<E/p<50 V cm−1torr−1at 20 °C with gas pressures varying between 3 and 5 torr using a modified drift tube. ForE/p>25, the measurements have also yielded values of Townsend's primary ionization coefficient. If electron detachment is neglected, theD/μ values and drift velocities are in good agreement with the available data, while the attachment coefficients differ substantially from some of the reported values. The overall accuracy of theD/μ values and the drift velocities is estimated to be better than ±3%, while the accuracy of the attachment coefficients obtained varies from ±3% forE/p<20 to about ±10% at higherE/pvalues.
Penetration and energy-loss theory of electrons in solid targets
Authors: K Kanaya  , S Okayama
Published: July 2002

Starting from a simple atomic model giving the potential between electrons and atoms asV(r) =Ze2as−1/srswith the empirical values=fraction six-fifths, we combine the diffusion effect due to multiple collisions and the energy retardation in accordance with a modified Thomson-Whiddington law, with the scattering cross section in the Lenard absorption law. On this basis, consistent expressions are obtained for the fraction of transmitted electrons in amorphous solid targets, the backscattering fraction with depth, the fraction of electrons absorbed per unit mass-thickness and the depth-dose function, which are in good agreement with experiments over the energy range 10-1000 keV.

A diffusion model represented by a sphere whose centre is located at the maximum energy dissipation depth, related to the diffusion depth and the range, is found to agree well with experiments.
Activation energy for the slow states in the aluminium-germanium contact
Authors: A Thanailakis  , D S Chan  , D C Northrop
Published: July 2002

An earlier paper by Thanailakis and Northrop in 1971 reported measurements of the slow reversible barrier height changes which take place in metal-germanium Schottky diodes at constant applied voltage.

Further experiments have now been performed on the temperature dependence of these barrier height changes in aluminium-germanium contacts. They have led to the following significant advances in our understanding:

There is an activation energy for the barrier height change which varies from 0·05 to 0·75 eV, depending on preparation of the germanium slice, but which is independent of the applied voltage.

This result shows that the charge exchange is between the localized oxide states and the conduction band of the semiconductor, since charge exchange with the valence band is not an activated process.

The proposed model has been refined to take account of the shape of the oxide barrier and is now fully consistent with all the experimental results.
Ion mobilities and ion-molecule reaction rates in oxygen
Authors: L Harrison  , J L Moruzzi
Published: July 2002

The mobilities and ion-molecule reaction rates for negative oxygen ions have been measured in a drift-tube mass spectrometer as a function ofE/Nusing a pulsed electron source which allows ion transients to be recorded. By curve-fitting these transients using a small digital computer to generate calculated waveforms, the mobilities and reaction rates are obtained by inspection. The rate coefficients for the three-body reaction producing O3from O−were measured over theE/Nrange 30·0-63·9 Td and were found to agree well with earlier data. The two-body reaction producing O2−from O−was investigated over theE/Nrange 86·5-252 Td and again good agreement with earlier work was found. The mobilities of O−, O2−and O3−ions were measured over as large a range ofE/Nas possible and were found to extrapolate back to zero-field mobilities of 9·2×1019, 5·6×1019and 6·8×1019V−1s−1cm−1.
Measurement of the space charges produced in streamers
Authors: Wolf Reininghaus
Published: July 2002

An investigation was made of the slow phase of cathode-directed streamers in gas discharges, which developed according to the streamer mechanism. A 3 cm gap was used with a uniform electric field and low overvoltages (about 20% above the DC breakdown voltage). All experiments were carried out in an N2-CH4mixture (total about 100 Torr). By means of a probe electron avalanche, the ionic space charge which accumulates in the gap during the development of the streamer was measured. Streak pictures of the discharge were recorded simultaneously, so it was possible to show the correlation of the ionic space charge with the growth of the streamer.
The prospects for an improved absorption correction in electron probe microanalysis
Authors: H E Bishop
Published: July 2002

It is shown that the absorption correctionf(χ) is dependent on two parameters: the mean depth which essentially determines the correction forf(χ)>0·8 and which accounts for variations with incident beam energy, and a shape function that is a function of overvoltage ratio and the mean atomic number of the specimen. The commonly used modified Philibert correction represents the mean depth reasonably well but has a very inadequate shape function.

The general effect of the shape function can be demonstrated conveniently by the behaviour of the ratiof(χ)/fsq(χ) wherefsq(χ) is the absorption correction predicted by a square model for the depth distribution of ionization. For many practical purposes the overvoltage variation of the shape function can be neglected, but there is always a substantial atomic number dependence in the range 0·05<f(χ)<0·5.

Although there are not sufficient consistent experimental data available on which to base an accurate absorption correction covering the full range off(χ), possible forms for such a correction procedure are discussed. A simple expression for use in the case of high absorption,f(χ)<0·2, is proposed.
An empirical treatment of fast-electron scattering in semi-infinite targets
Authors: Vilho Lantto
Published: July 2002

A simple empirical treatment is given of the absorption of fast electrons and their energy at normal incidence in semi-infinite targets. It is based on the consideration of the multiple reflection of electrons at imaginary planes parallel to the surface. Experimental data measured on films of various thicknesses for transmitted and backscattered electrons can be used for calculations for semi-infinite targets. Analytical expressions are also set up to describe the behaviour of electrons transmitted through or backscattered from a film. From these it is possible to analyse the multiple reflection of 10-100 keV electrons for nearly all elements. As an application, the energy dissipation is calculated for 20 keV electrons in copper and gold, and the results are compared with those of other investigators. The present values resemble those calculated by Spencer and by Bishop.
Cathode spot motion in high current vacuum arcs on copper electrodes
Authors: J C Sherman  , R Webster  , J E Jenkins  , R Holmes
Published: July 2002

The motion of vacuum arc cathode spots under the influence of the self-generated magnetic field has been investigated for copper electrodes at currents up to 6.9 kA. The spots were photographed with a high-speed framing camera. The magnetic flux density was measured. Surface contamination caused large changes in spot velocity. For velocities up to 15 ms-1and corresponding flux densities up to 2*10-2T the velocity is proportional to the flux density. Above 20 ms-1and 4*10-2T the velocity increases relatively slowly with increasing flux density. Velocities of more than 30 ms-1were not observed.
Strict definiteness of integrals via complete monotonicity of derivatives
Authors: Mattner L.
Published: July 2002

Let

k
k

be a nonnegative integer and let

ϕ
:
(
0
,
∞
)
→

R

\phi : (0,\infty ) \rightarrow \mathbb {R}

be a

C
∞

C^\infty

function with

(
−

)
k

⋅

ϕ

(
k
)

(-)^k\cdot \phi ^{(k)}

completely monotone and not constant. If

σ
≠
0

\sigma \neq 0

is a signed measure on any euclidean space

R

d

\mathbb {R}^d

, with vanishing moments up to order

k
−
1

k-1

, then the integral
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="integral Underscript double-struck upper R Superscript d Endscripts integral Underscript double-struck upper R Superscript d Endscripts phi left-parenthesis double-vertical-bar x minus y double-vertical-bar squared right-parenthesis d sigma left-parenthesis x right-parenthesis d sigma left-parenthesis y right-parenthesis">

∫

R

d

∫

R

d

ϕ
(
‖
x
−
y

‖
2

)

d
σ
(
x
)
d
σ
(
y
)

\int _{\mathbb {R}^d} \int _{\mathbb {R}^d} \phi ( \|x-y\|^2 ) \, d\sigma (x) d\sigma (y)

is strictly positive whenever it exists. For general

d
d

no larger class of continuous functions

ϕ
\phi

seems to admit the same conclusion. Examples and applications are indicated. A section on ”bilinear integrability” might be of independent interest.
Generic lattice ideals
Authors: Peeva Irena , Sturmfels Bernd
Published: July 2002

A concept of genericity is introduced for lattice ideals (and hence for ideals defining toric varieties) which ensures nicely structured homological behavior. For a generic lattice ideal we construct its minimal free resolution and we show that it is induced from the Scarf resolution of any reverse lexicographic initial ideal.
Transversely projective structures on a transversely holomorphic foliation
Authors: Biswas Indranil
Published: July 2002

The space of transversely projective structures on a transversely holomorphic foliation is described. Some applications are given.
Evaluation of gene prediction software using a genomic data
set: application to Arabidopsis thalianasequences
Authors: Pavy Nathalie , Rombauts Stephane , Déhais Patrice , Mathé Catherine , V. V.Ramana Davuluri , Leroy Philippe , Rouzé Pierre
Published: July 2002

Motivation: The annotation of the Arabidopsis thalianagenome remains a problem in terms of time and quality. To improve the annotation process, we want to choose the most appropriate tools to use inside a computer-assisted annotation platform. We therefore need evaluation of prediction programs with Arabidopsis sequences containing multiple genes.

Results: We have developed AraSet, a data set of contigs of validated genes, enabling the evaluation of multi-gene models for the Arabidopsis genome. Besides conventional metrics to evaluate gene prediction at the site and the exon levels, new measures were introduced for the prediction at the protein sequence level as well as for the evaluation of gene models. This evaluation method is of general interest and could apply to any new gene prediction software and to any eukaryotic genome. The GeneMark.hmm program appears to be the most accurate software at all three levels for the Arabidopsis genomic sequences. Gene modeling could be further improved by combination of prediction software.

Availability: The AraSet sequence set, the Perl programs and complementary results and notes are available at http://sphinx.rug.ac.be:8080/biocomp/napav/.

Contact: Pierre.Rouze@gengenp.rug.ac.be
Telomerase subunit overexpression suppresses telomere‐specific checkpoint activation in the yeast yku80 mutant
Authors: Teo Soo‐Hwang , Jackson Stephen P
Published: July 2002

Ku is a conserved heterodimeric DNA‐binding protein that plays critical roles in DNA repair and telomere homeostasis. In Saccharomyces cerevisiae, deletion of YKU70 or YKU80 results in an inability to grow at 37°C. This is suppressed by overexpression of several components of telomerase (EST1, EST2 and TLC1). We show that overexpression of EST2 or TLC1 in yku80 mutants does not restore efficient DNA repair, or restore normal telomere function, as measured by telomere length, single‐stranded G‐rich strand or transcriptional silencing. Instead, yku80 mutants activate a Rad53p‐dependent DNA‐damage checkpoint at 37°C and this is suppressed by overexpression of EST2 or TLC1. Indeed, deletion of genes required for Rad53p activation also suppresses the yku80 temperature sensitivity. These results suggest that activation of the DNA‐damage checkpoint in yku mutants at 37°C does not result from reduced telomere length per se, but reflects an alteration of the telomere structure that is recognized as damaged DNA.
Speciation through temporal segregation of Madeiran storm petrel (Oceanodroma castro) populations in the Azores?
Authors: Monteiro L. R. , Furness R. W.
Published: July 2002

Madeiran storm petrelsOceanodroma castrobreed on three small islets in the Azores: Vila, off Santa Maria, and Praia and Baixo, off Graciosa. Analysis of data on brood patch, incubation periods, chick body size and recaptures of adults provides evidence of the existence of two distinct populations (hot– and cool–season) breeding annually on Baixo and Praia, out of phase by four to five months and overlapping in colony attendance during August and early September; on Vila only the cool–season population is present. Analyses of adult morphology indicate highly significant phenotypic differentiation between the sympatric hot– and cool–season breeders, whereas an almost complete phenotypic uniformity characterizes allopatric breeders within the same season. The hot–season birds are 10% smaller in egg and body mass but have longer wings and tails than cool–season birds. The two groups were readily separated by discriminant analysis. The preference to breed in the cooler season is interpreted as a consequence of greater food availability in that period. Morphological differentiation between seasonal populations is interpreted as an adaptative response to different environmental conditions in the two seasons. The hypothesis is given that the hot–season population has evolved from the cool–season population owing to density–dependent constraints on crowded colonies, forcing birds to time–share nest sites. These populations may represent a case of sympatric speciation through temporal partitioning of reproduction and may be better treated as sibling species.
Lsh, a member of the SNF2 family, is required for genome-wide methylation
Authors: Dennis Kathleen , Fan Tao , Geiman Theresa , Yan Qingsheng , Muegge Kathrin
Published: July 2002

Methylation patterns of the mammalian genome are thought to be crucial for development. The precise mechanisms designating specific genomic loci for methylation are not known. Targeted deletion ofLsh results in perinatal lethality with a rather normal development. We report here, however, that Lsh−/−mice show substantial loss of methylation throughout the genome. The hypomethylated loci comprise repetitive elements and single copy genes. This suggests that global genomic methylation is not absolutely required for normal embryogenesis. Based on the similarity of Lsh to other SNF2 chromatin remodeling proteins, it suggests that alteration of chromatin affects global methylation patterns in mice.
Microwave‐induced thermoacoustic tomography using multi‐sector scanning
Authors: Xu Minghua , Ku Geng , Wang Lihong V.
Published: July 2002

A study of microwave‐induced thermoacoustic tomography of inhomogeneous tissues using multi‐sector scanning is presented. A short‐pulsed microwave beam is used to irradiate the tissue samples. The microwave absorption excites time‐resolved acoustic waves by thermoelastic expansion. The amplitudes of the acoustic waves are strongly related to locally absorbed microwave‐energy density. The acoustic waves may propagate in all spatial directions. A focused ultrasonic transducer is employed to acquire temporal acoustic signals from multiple directions. Each detected signal is converted into a one‐dimensional (1D) image along the acoustic axis of the transducer. The cross‐sectional images of the tissue samples are calculated by combining all of the 1D images acquired in the same planes.
Total scatter factors and tissue maximum ratios for small radiosurgery fields: Comparison of diode detectors, a parallel‐plate ion chamber, and radiographic film
Authors: Zhu X. Ron , Allen J. Joseph , Shi Jie , Simon William E.
Published: July 2002

Two ‐type diode detectors, a parallel‐plate ion chamber, and radiographic film were used to measure total scatter factors and tissue maximum ratios (TMRs) for a stereotactic radiosurgery system with circular fields ranging from 5 to 50 mm in diameter. One diode has a square detection diagonal of 2.3 mm and the other diode has a circular detection diameter of 1 mm. It is found that the two diodes measured essentially the same total scatter factors for all field sizes. Total scatter factors measured by film are within 3% of diode values. Our results also suggest that the parallel‐plate ion chamber could underestimate total scatter factors for fields as large as 15 mm in diameter, although it is recommended for field diameters The total scatter factors used in our clinic are combined from data measured with the ion chamber and the 2‐mm‐diam diode. The combined total scatter factors generally agree with published data. While film overestimates TMRs for the smallest fields at large depths because of energy dependence of the film, the measurements with the 1‐mm‐diam diode agree with published data measured with thermoluminescent dosimeters. It is demonstrated that the accurate measurements of total scatter factors and TMRs for small fields can be obtained by combining results of the commercially available detectors used in this study.
Optimization of radiosurgery treatment planning via mixed integer programming
Authors: Lee Eva K. , Fox Tim , Crocker Ian
Published: July 2002

An automated optimization algorithm based on mixed integer programming techniques is presented for generating high‐quality treatment plans for LINAC radiosurgery treatment. The physical planning in radiosurgery treatment involves selecting among a large collection of beams with different physical parameters an optimal beam configuration (geometries and intensities) to deliver the clinically prescribed radiation dose to the tumor volume while sparing the nearby critical structure and normal tissue. The proposed mixed integer programming models incorporate strict dose restrictions on tumor volume, and constraints on the desired number of beams, isocenters, couch angles, and gantry angles. The model seeks to deliver full prescription dose coverage and uniform radiation dose to the tumor volume while minimizing the excess radiation to the periphery normal tissue. In particular, it ensures that proximal normal tissues receive minimal dose via rapid dose fall‐off. Preliminary numerical tests on a single patient case indicate that this approach can produce exceptionally high‐quality plans in a fraction of the time required using the procedure currently employed by clinicians. The resulting plans provide highly uniform prescription dose to the tumor volume while drastically reducing the irradiation received by the proximal critical normal tissue.
Resource Letter: TACMB-1: The theory of anisotropies in the cosmic microwave background
Authors: White Martin , Cohn J. D.
Published: July 2002

This Resource Letter provides a guide to the literature on the theory of anisotropies in the cosmic microwave background. Journal articles, web pages, and books are cited for the following topics: discovery, cosmological origin, early work, recombination, general CMB anisotropy references, primary CMB anisotropies (numerical, analytical work), secondary effects, Sunyaev-Zel’dovich effect(s), lensing, reionization, polarization, gravity waves, defects, topology, origin of fluctuations, development of fluctuations, inflation and other ties to particle physics, parameter estimation, recent constraints, web resources, foregrounds, observations and observational issues, and Gaussianity.
Interaction-free measurement
Authors: DeWeerd Alan J.
Published: July 2002

Interaction-free measurement is introduced through a set of problems suitable for undergraduates. Both the original scheme suggested by Elitzur and Vaidman and an improved one proposed by Kwiat et al. are considered. Theoretical predictions are compared to experimental data.
Motion of a wave packet in the Klein paradox
Authors: Nitta H. , Kudo T. , Minowa H.
Published: July 2002

Computer simulations for the scattering of a relativistic wave packet by a step potential are performed. Under the condition of the Klein paradox, where the height of the potential barrier becomes supercritical, the scattering induces the creation of electron–positron pairs. The “pair creation” is demonstrated in the present simulation by making use of the hole theory.
Random walks and effective resistances on toroidal and cylindrical grids
Authors: Jeng Monwhea
Published: July 2002

A mapping between random walk problems and resistor network problems is described and used to calculate the effective resistance between any two nodes on an infinite two-dimensional square lattice of unit resistors. The superposition principle is then used to find effective resistances on toroidal and cylindrical square lattices.
Transmission of sound across a trusslike periodic panel; 2-D analysis
Authors: El-Raheb Michael , Wagner Paul
Published: July 2002

Transmission of sound across a 2-D trusslike periodic panel in contact with an acoustic fluid on both its faces is analyzed. Elastic frequency response of the panel made of repeated cells is expressed using transfer matrices of the unit cell, and acoustic pressure of the fluid is expressed using a source boundary element method. Transmission loss (TL) of a panel is computed for two geometries and compared with measurements. These comparisons reveal differences that result from attenuation zones due to periodicity along one axis of the panel and response that is predominantly along the other axis. Conditions under which these differences disappear are explained. A nondimensional parameter, including the ratio of sound speeds in solid and fluid and the ratio of panel length to radius of gyration of the cross-section controls the drop in TL at coincidence, called strength of coincidence.
Acoustic bullets: Transient Bessel beams generated by planar apertures
Authors: Stepanishen Peter R. , Sun Jie
Published: July 2002

Acoustic bullets are defined to be waves which maintain their peak amplitude and/or general shape as they propagate in free space. A general theoretical approach is presented to investigate the space-time properties of a new class of acoustic bullets which are defined here as acoustic transient Bessel beams. The approach encompasses both baseband and modulated carrier signal wave fields. Closed form space-time expressions are developed using Fourier and Hilbert transform methods for several special cases of transient Bessel beams. The space-time source distributions for generating such wave fields from planar apertures are developed from the free space wave fields via the use of the Rayleigh surface integral solution for time-dependent fields. Finally, numerical results are presented to illustrate the general space-time characteristics of the acoustic transient Bessel beams.