bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiple magnetic transitions and large magnetoresistance of the Y0.5Ho0.5Mn6Sn6 compound
Authors: Zhang Shao-Ying , Zhao Tong-Yun , Shen Bao-Gen , Yao Jin-Lei , Zhang Li-Gang , Li Yun-Bao , Li Lu
Published: November 2002

Magnetic transitions and magnetoresistance of the HfFe6Ge6-type Y0.5Ho0.5Mn6Sn6 compound have been investigated in the temperature range of 5–380 K. It was found that the compound displays paramagnetism, ferrimagnetism, antiferromagnetism, and reentrant ferrimagnetism with decreasing temperature. The temperature range of the antiferromagnetic state can be narrowed by increasing the magnetic field, and the metamagnetic transition from the antiferro- to ferrimagnetic state can be induced by a fairly small threshold field (<6 kOe). The antiferro-ferrimagnetic transition is accompanied by a large magnetoresistance effect of about −16% at 100 K. The magnetic transitions with temperature and magnetic field are both of the second order.
Real-time heat capacity measurement during thin-film deposition by scanning nanocalorimetry
Authors: Zhang M. , Efremov M. Yu. , Olson E. A. , Zhang Z. S. , Allen L. H.
Published: November 2002

The scanning nanocalorimetry technique is utilized to characterize thin-film growth in real-time. The technique generates three-dimensional heat capacity data as a function of temperature and thickness that show the continuous change of indium film during deposition. The measurement interval is ∼4×10−3 nm in thickness. Indium thin films form nanoparticles on silicon nitride surfaces that show the phenomena of melting point depression and the formation of magic number size particles. The measured increment of the heat capacity ΔCp is ∼30 pJ/K and the temperature resolution is better than 0.5 K.
Ultralow threshold powers for optical pumping of homoepitaxial InGaN/GaN/AlGaN lasers
Authors: Ivanov V. Yu. , Godlewski M. , Teisseyre H. , Perlin P. , Czernecki R. , Prystawko P. , Leszczynski M. , Grzegory I. , Suski T. , Porowski S.
Published: November 2002

We report ultralow threshold powers for optically pumped laser emission from InGaN/GaN/AlGaN laser structures grown on bulk GaN substrates. The threshold powers at room temperature (for excitation with 355 nm third harmonic of Nd:YAG laser) are between 2.4 and 5.8 kW/cm2, depending on a cavity length. We believe that this is the consequence of a significant reduction of concentration of nonradiative centers in the active layer of homoepitaxial structures. We report also that spacing of the longitudinal laser modes exceeds by a factor of 6–7 the values predicted from the standard calculations.
Recombination and loss analysis in polythiophene based bulk heterojunction photodetectors
Authors: Schilinsky Pavel , Waldauf Christoph , Brabec Christoph J.
Published: November 2002

The monochromatic external quantum efficiency of a bulk heterojunction photodetector based on a blend of poly-3(hexylthiophene) with a methanofullerene is reported to be as high as 76% at the peak maximum at 25 °C. Analysis of the temperature dependence, the illumination intensity dependence together with absorption measurements in reflection geometry, allow calculation of the internal quantum efficiency of the device close to 100% at the peak maximum. Recombination of photoinduced carriers is negligible or even absent in these photodetectors when operated in the photovoltaic mode. Optical losses in these bulk heterojunction devices are analyzed.
Enhancement of third-harmonic generation in absorbing media
Authors: Veres G. , Matsumoto S. , Nabekawa Y. , Midorikawa K.
Published: November 2002

The efficiency of third-harmonic generation on glass–air interfaces near the focal point of a tightly focused laser beam is investigated for different material samples. It is found that highly absorbing materials give higher harmonic yield. Moreover, due to the systematic change of the third-harmonic yield with the absorption length, an estimation of the third-order susceptibility χ(3) of absorbing samples can be given from absorption coefficient measurements.
Stability and electronic structure of hydrogen–nitrogen complexes in GaAs
Authors: Orellana W. , Ferraz A. C.
Published: November 2002

We investigate the stability and electronic properties of defects formed by a substitutional nitrogen in GaAs (NAs) plus interstitial hydrogen atoms using first-principles total-energy calculations. We find the formation of strong NAs–H bond when a single H atom is incorporated in the lowest-energy bond centered (BC) position. This defect induces an electrically active level in the GaAs band gap. When two H atoms are incorporated, we find the stable NAs–H2* complex as the lowest-energy configuration, with one H atom at the BC position and the second H atom at an antibonding position. The electronic structure of this complex shows the passivation of the gap level restoring the GaAs band gap.
S doping in ZnO film by supplying ZnS species with pulsed-laser-deposition method
Authors: Yoo Y.-Z. , Jin Zheng-Wu , Chikyow T. , Fukumura T. , Kawasaki M. , Koinuma H.
Published: November 2002

S-doped ZnO (ZnO:S) film was fabricated by supplying ZnS species from laser ablation of a ZnS target during ZnO growth. Variations of lattice constants and band gaps with respect to S content did not follow Vegard’s law. The ZnO:S film showed semiconducting behavior with lower activation energy and resistivity than those of ZnO owing to higher carrier concentration. Despite the absence of magnetic elements, the large magnetoresistance amount of 26% was observed at 3 K from ZnO:S film.
Laser-induced amorphization of silicon during pulsed-laser irradiation of TiN/Ti/polycrystalline silicon/SiO2/silicon
Authors: Chong Y. F. , Pey K. L. , Wee A. T. S. , Thompson M. O. , Tung C. H. , See A.
Published: November 2002

In this letter, we report on the complex solidification structures formed during laser irradiation of a titanium nitride/titanium/polycrystalline silicon/silicon dioxide/silicon film stack. Due to enhanced optical coupling, the titanium nitride/titanium capping layer increases the melt depth of polycrystalline silicon by more than a factor of 2. It is found that the titanium atoms diffuse through the entire polycrystalline silicon layer during irradiation. Contrary to the expected polycrystalline silicon growth, distinct regions of polycrystalline and amorphous silicon are formed instead. Possible mechanisms for the formation of these microstructures are proposed.
Picosecond pump–probe photoelectron spectroscopy as a probe of intramolecular dynamics in S1 para-fluorotoluene
Authors: Davies Julia A. , Reid Katharine L. , Towrie Michael , Matousek Pavel
Published: November 2002

Picosecond photoelectron spectroscopy is used to monitor the intramolecular dynamics in S1 parafluorotoluene prepared with ∼1200 cm−1 of internal energy from a jet cooled sample. The spectra evolve dramatically over a 100 ps timescale supporting results from chemical timing experiments. This suggests that initial temperature has little effect on the energy redistribution rate.
Reduction of buffer layer conduction near plasma-assisted molecular-beam epitaxy grown GaN/AlN interfaces by beryllium doping
Authors: Storm D. F. , Katzer D. S. , Binari S. C. , Glaser E. R. , Shanabrook B. V. , Roussos J. A.
Published: November 2002

Beryllium doping of epitaxial GaN layers is used to reduce leakage currents through interfacial or buffer conducting layers grown by plasma-assisted molecular-beam epitaxy on SiC. Capacitance–voltage measurements of Schottky barrier test structures and dc pinch-off characteristics of unintentionally doped GaN high-electron-mobility transistors indicate that these leakage currents are localized near the GaN/AlN interface of our AlGaN/GaN/AlN device structures. Insertion of a 2000 Å Be:GaN layer at the interface reduces these currents by three orders of magnitude.
Functoriality for the exterior square of 𝐺𝐿₄ and the symmetric fourth of 𝐺𝐿₂
Authors: Kim Henry
Published: November 2002

In this paper we prove the functoriality of the exterior square of cusp forms on

G

L

4

GL_{4}

as automorphic forms on

G

L

6

GL_{6}

and the symmetric fourth of cusp forms on

G

L

2

GL_{2}

as automorphic forms on

G

L

5

GL_{5}

. We prove these by applying a converse theorem of Cogdell and Piatetski-Shapiro to analytic properties of certain

L
L

-functions obtained by the Langlands-Shahidi method. We give several applications: First, we prove the weak Ramanujan property of cuspidal representations of

G

L

4

GL_{4}

and the absolute convergence of the exterior square

L
L

-functions of

G

L

4

GL_{4}

. Second, we prove that the fourth symmetric power

L
L

-functions of cuspidal representations of

G

L

2

GL_{2}

are entire, except for those of dihedral and tetrahedral type. Third, we prove the bound

3
26

\frac {3}{26}

for Hecke eigenvalues of Maass forms over any number field.
Packaging of children’s breakfast cereal: manufacturers versus children
Authors: Hill Helene , Tilley Jennifer
Published: November 2002

Based on primary research from both a child consumer and manufacturer perspective, this article explores the breakfast cereal market and the perceptions of packaging from the perspective of a child. Specific consideration is given to determining the overall role of packaging, what role packaging can play within integrated marketing communications and establishing the feasibility and effectiveness of packaging as a sole communications tool. Findings highlight some apparent inconsistencies between manufacturer and children’s views, and illustrate the possibility of adults underestimating how aware children are as consumers in today’s society.
Strategic management: an evaluation of the use of three learning methods
Authors: Jennings David
Published: November 2002

The article examines the use of three learning methods in the teaching of strategic management; the case method, simulation and action learning, in the form of a consultancy project. A survey of course members’ perceptions of learning outcomes indicates that simulation is the most effective method. Conclusions are provided for the design of strategic management programmes and the development of action learning projects.
Wave Propagation in a Piezoelectric Coupled Solid Medium
Authors: Wang Q.
Published: November 2002

Shear horizontal (SH) wave propagation in a semi-infinite solid medium surface bonded by a layer of piezoelectric material abutting the vacuum is investigated in this paper. The dispersive characteristics and the mode shapes of the deflection, the electric potential, and the electric displacements in the thickness direction of the piezoelectric layer are obtained theoretically. Numerical simulations show that the asymptotic phase velocities for different modes are the Bleustein surface wave velocity or the shear horizontal wave velocity of the pure piezoelectric medium. Besides, the mode shapes of the deflection, electric potential, and electric displacement show different distributions for different modes and different wave number. These results can be served as a benchmark for further analyses and are significant in the modeling of wave propagation in the piezoelectric coupled structures.
One, Two, and Three-Dimensional Universal Laws for Fragmentation due to Impact and Explosion
Authors: Carpinteri A. , Pugno N.
Published: November 2002

Based on the fractal particle size distribution, a fragmentation theory for quasi-brittle materials is herein developed. The results are three simple and powerful universal laws for the multiscale energy dissipation under impact and explosion fragmentation for one, two, and three-dimensional bodies, respectively. The three-dimensional law unifies the most important and well-known fragmentation theories. As an example, it has been applied to the prediction of the devastated area due to asteroid impacts on earth as a function of the energy released in the collision.
On Mechanical Waves Along Aluminum Conductor Steel Reinforced (ACSR) Power Lines
Authors: Martin P. A. , Berger J. R.
Published: November 2002

The propagation of elastic waves along composite wire rope is considered. The rope is modeled as co-axial layers of cylindrically anisotropic material. Simple kinematical assumptions lead to a “rod theory” for the wire rope, consisting of three coupled one-dimensional wave equations. Solutions of these equations are found. Results for a particular aluminum conductor steel reinforced (ACSR) conductor are described in detail. The slowest mode is found to be mainly torsional and mainly nondispersive in character. The other two modes are dispersive and have small torsional components.
Transient Ultrasonic Waves in Multilayered Superconducting Plates
Authors: Niklasson A. J. , Datta S. K.
Published: November 2002

Transient response of multilayered superconducting tapes has been studied in this paper. These tapes are usually composed of layers of a superconducting material (like YBa2Cu3O7−δ, or YBCO, for simplicity) alternating between layers of a metallic material (like nickel or silver). The tapes are thin, in the range of 100–200 μm. The superconducting layer is orthotropic with a thickness of 5–10 μm. In applications, tapes are long and have a finite width. In this paper, attention has been focused on the transient response of homogeneous and three-layered tapes assuming that the width is infinite and that the thickness of the superconducting layer is much smaller than the metal layer. The problem considered here is of general interest for understanding the effect of anisotropy of thin coating or interface layers in composite plate structures on ultrasonic guided waves. Three plate geometries are considered as prototype examples: a homogeneous nickel (Ni) layer, a three-layered YBCO/Ni/YBCO, and a three-layered Ni/YBCO/Ni. Transient response due to a line force applied normal to the surface of the tape has been studied by means of Fourier transforms and direct numerical integration. Numerical results are presented using an exact model and a first-order approximation to the thin YBCO layer. The first-order approximation simplifies the problem to that of a homogeneous isotropic plate subject to effective boundary conditions representing the thin anisotropic layers. Both are seen to agree well (except when the center frequency of the force is high) and capture the coupling of the longitudinal, S, (or flexural, A) motion and the shear-horizontal (SH) motion. Detailed analysis of the influence of the thin layers, especially their anisotropy, on this coupling and the transient response shows significant differences among the three cases. The model results provide insight into the coupling phenomenon and indicate the feasibility of careful experiments to exploit the significant changes in the transient response caused by coupling for the determination of the in-plane elastic constants of thin coating or interface layers.
Axial Loading of Bonded Rubber Blocks
Authors: Horton J. M. , Tupholme G. E. , Gover M. J. C.
Published: November 2002

Axially loaded rubber blocks of long, thin rectangular and circular cross section whose ends are bonded to rigid plates are studied. Closed-form expressions, which satisfy exactly the governing equations and conditions based upon the classical theory of elasticity, are derived for the total axial deflection and stress distribution using a superposition approach. The corresponding relations are presented for readily calculating the apparent Young’s modulus, Ea, the modified modulus, Ea′, and the deformed lateral profiles of the blocks. From these, improved approximate elementary expressions for evaluating Ea and Ea′ are deduced. These estimates, and the precisely found values, agree for large values of the shape factor, S, with those previously suggested, but also fit the experimental data more closely for small values of S. Confirmation is provided that the assumption of a parabolic lateral profile is invalid for small values of S.
Investigation and simulation of XeF2 isotropic etching of silicon
Authors: Bahreyni Behraad , Shafai C.
Published: November 2002

Trenching and loading phenomena observed on XeF2-etched (100) silicon wafers are explained in this article. Trenching refers to deeper etching at the side of an etch feature with respect to the middle of the feature. Loading is the reduction in etch depth that adjacent etched regions impose on their respective etch profiles. These two phenomena are especially recognized at locations where the substrate is etched through large mask openings. Both phenomena were reported by other groups, but no explanation was given for them. A novel model explaining these phenomena is developed in this article, and the etching process is simulated in software. The results are compared to several etched samples with varying mask aperture size and etch depth. Good agreement is found between simulated profiles and actual measured etch profiles at given mean-free paths. Furthermore, our simulator predicts that the reaction probability between etchant and substrate molecules influences surface roughness of the etched regions.
Effects of plasma exposure on structural and optical properties of TiO2 films deposited by off-axis target sputtering
Authors: Takahashi T. , Nakabayashi H. , Mizuno W.
Published: November 2002

TiO 2 films with thickness of about 800–2300 nm have been reactively deposited on glass-slide substrates at an atmosphere of Ar and O2 mixture, using the off-axis target sputtering method. The maximum deposition rate of the film was 12.8 nm/min. The optical properties and the crystal structure of the films were measured using spectrophotometry, Raman spectroscopy, and x-ray diffractometry, respectively. Most of the films deposited in this study were transparent and had anatase crystal structure, while some of them were semitransparent. The A(101), A(112), A(211) and A(220) peaks were observed in the x-ray diffraction patterns, where A shows the anatase phase of TiO2. With increasing plasma exposure, the x-ray peak intensities IP of A(101) and A(112) gradually decreased, while the IP of A(211) and A(220) gradually increased. The IP depended strongly on the substrate position in this study. Moreover, the A(220) peak was significantly higher than other peaks when the TiO2 crystallites became larger. The anatase phase of the TiO2 films was also revealed in the Raman spectra with a Raman shift of 145 cm−1. With decreasing plasma exposure, the changes in the optical transmittance spectra of TiO2 films were very small at the wavelength of 350–900 nm. However, the transmittance spectra significantly depended on the wavelength with increasing plasma exposure. Consequently, it was found that the crystal structure and the optical properties of as-deposited TiO2 films were strongly affected by plasma exposure during film deposition.
Study on the characteristics of alumdinum thin films prepared by atomic layer deposition
Authors: Lee Yong Ju , Kang Sang-Won
Published: November 2002

The characteristics of aluminum (Al) films prepared by the atomic layer deposition (ALD) technique using trimethylaluminum (TMA) and a hydrogen plasma were examined for use as a wetting layer for the two-step cold/warm Al deposition. The most important role of a hydrogen plasma was to act as a reducing agent for TMA. The growth rate was saturated at 0.15 nm/cycle, demonstrating that the thickness can be precisely controlled by the number of cycles. Initial layer-by-layer growth was observed and this continued for up to 50 cycles (∼7.5 nm). The root-mean-square thickness variation of an Al film deposited in five cycles (∼0.75 nm) was found to be 0.188 nm. As the PH2/PTMA injection ratio was increased to 10 at a buffer line, the carbon concentration decreased to 1–3 at. %. The resistivity of a 100-nm-thick Al film was 10 μΩ  cm and this film was polycrystalline in nature. ALD Al films also had good step coverage on high aspect ratio trenches.
Refractive index control of silicon nitride films prepared by radio-frequency reactive sputtering
Authors: Nayar P. S.
Published: November 2002

Silicon nitride films were prepared on polished silicon wafers by radio-frequency reactive sputtering using a silicon target in nitrogen–argon plasma. The deposition rate was controlled from 2 to 7 nm/minute. The refractive index of the film decreased continuously and was varied from 2.8 to 1.96. The process described here is simple and does not require knowledge of the absolute magnitudes of partial pressures of the gases used. This method is well suited for fabricating multilayer structures of silicon nitride films with different refractive indices.
Subsurface interstitials as promoters of three-dimensional growth of Ti on Si(111): An x-ray standing wave, x-ray photoelectron spectroscopy, and atomic force microscopy investigation
Authors: Kuri G. , Schmidt Th. , Hagen V. , Materlik G. , Wiesendanger R. , Falta J.
Published: November 2002

The adsorption and growth of Ti on Si(111)-7×7 surface at room temperature was studied with x-ray standing waves and high energy x-ray photoelectron spectroscopy. The surface morphology of the specimens was analyzed using atomic force microscopy. We observed that at the submonolayer (0.08–0.24 ML) coverage a significant fraction of the metal atoms are coherently located in the near-surface interstitial positions of the Si lattice. For a higher Ti coverage (1.8 ML), the coherently ordered fraction almost vanishes and the surface is covered with three-dimensional islands. The Ti-coverage results are interpreted in terms of an interfacial transition layer containing Ti interstitials below the Si surface at low coverage, and a disordered mixed Ti–Si phase at higher Ti concentration.
Integrated AlN/diamond heat spreaders for silicon device processing
Authors: Yoganand S. N. , Jagannadham K. , Karoui A. , Wang H.
Published: November 2002

Growth and characterization of AlN and diamond films on the backside of a Si (100) wafer and the integration of AlN/diamond heat spreaders into silicon device technology is investigated. AlN film was deposited by pulsed dc reactive magnetron sputtering at 600 °C and diamond film was deposited by microwave plasma chemical vapor deposition at 900 °C. The films were characterized by x-ray diffraction and transmission electron microscopy for crystalline quality, by scanning electron microscopy for morphology, and by infrared thermography for heat spreading characteristics. The heat spreading characteristics of the silicon wafer with the composite AlN/diamond films were found to be superior to that of wafers with no heat spreaders or to the wafers with either single layer diamond or single layer AlN heat spreaders. Deep level transient spectroscopy and secondary ion mass spectroscopy were performed on the samples with and without the heat spreader to determine the concentration of the impurities that may have been introduced during deposition of AlN or diamond. The results showed that the purity of the wafers is not altered due to the deposition of AlN and diamond and subsequent device processing steps such as high temperature oxidation. The device characteristics were studied by fabrication of Schottky diodes on the wafers with the composite AlN/diamond heat spreader and compared with that of devices on wafers with no heat spreader. The device characteristics were found to be similar and unaffected by integration with an AlN/diamond heat spreader. Integration of AlN/diamond heat spreaders with silicon device processing has been shown to be successful.
In situ pressure measurements in small gettered volumes
Authors: Nemanič Vincenc , Žumer Marko , Zajec Bojan
Published: November 2002

In modern small optoelectronic devices like field emitter displays, miniature cathode ray tubes (CRTs), channel photomultipliers, etc., the vacuum requirements are much more stringent than in conventional electron beam devices. As there should be a pressure in the ultrahigh vacuum region and the volume is only a few cm3, a direct measurement is not feasible and is often estimated on the basis of the expected pumping speed of the getter. The present study was arranged to investigate the pressure in small CRTs (25 cm3) during a period of several months, namely after the conventional pumping and bakeout procedure, immediately after the activation of Ba getters and after the accumulation of some months. All the CRTs were equipped by a spinning rotor gauge ball. Two barium getter sizes were studied: St15/AM/O/9.5 and St15/AM/O/5, both made by SAES. After the evaporation by the prescribed procedure the pressure did not drop, but increased from p∼1×10−5 mbar up to p∼1×10−3 mbar, showing that the pumping speed was completely suppressed by forming of a nongetterable gas. When the same experiments were repeated inside identical glass bulbs connected with a valve to a quadrupole mass spectrometer, a formation of methane was observed. The initial rate just after the activation was Q∼10−8 mbar l s−1, but even after several hours it was still as high as Q∼10−9 mbar l s−1. By switching-on the cathode heater, methane was pumped by the getter after a precedent cracking procedure. The virtual pumping speed was directly related to the heater power, but independent of the getter area. Therefore, within the cathode warm-up period, methane was “pumped” and did not represent a harmful gaseous contaminant in a small electron beam device with a thermionic cathode.
Fluorocarbon-based plasma etching of SiO2: Comparison of C4F6/Ar and C4F8/Ar discharges
Authors: Li Xi , Hua Xuefeng , Ling Li , Oehrlein Gottlieb S. , Barela Marcos , Anderson Harold M.
Published: November 2002

A gas phase and surface chemistry study of inductively coupled plasmas fed with C4F6/Ar and C4F8/Ar intended for SiO2 etching processes was performed. Adding Ar to those fluorocarbon gases results in a strong increase of the ion current, by up to a factor of 5 at 90% Ar relative to the pure fluorocarbon gases. The fluorocarbon deposition rate is higher for C4F6/Ar than for C4F8/Ar, whereas the fluorocarbon etching rate is lower, and both quantities decrease as the amount of Ar is increased. For both C4F6/Ar and C4F8/Ar, the CF2 density is more than an order of magnitude greater than the CF density. The CF2 partial pressure decreases as more Ar is added to the C4F6/Ar plasmas. A comparison of these data with corresponding results obtained with C4F8/Ar shows that the CF2 partial pressure in C4F6 is higher for Ar-lean gas mixture than for C4F8/Ar. This remains true up to 40% Ar. Above 40% Ar the CF2 partial pressure in C4F8 is higher than for C4F6. The CF and COF2 partial pressures in C4F8 are higher than for C4F6. The SiO2 etch rate is higher for C4F8/Ar than for C4F6/Ar. This may be attributed in part to the higher F/C ratio of the steady-state fluorocarbon film formed on SiO2 surfaces for C4F8/Ar which was determined by x-ray photoemission spectroscopy (XPS). The etching selectivity of SiO2 over resist and silicon is increased by the addition of Ar to the fluorocarbon gases. Overall, the SiO2/resist and SiO2/Si etching selectivity are higher for C4F6/Ar (i.e., 4 and 9, respectively) at 90% Ar than for C4F8/Ar (i.e., 2 and 5, respectively) at 90% Ar and otherwise identical conditions. Both ellipsometry and XPS measurements show that the steady-state fluorocarbon layer thickness is greater for C4F6/Ar (∼4 nm) than for C4F8/Ar (∼2.8 nm). Argon addition leads to a strong decrease of the fluorine content of the steady-state fluorocarbon layers on both Si and SiO2 surfaces relative to films produced in pure fluorocarbon discharges, and this effect is related to the increase of the SiO2/Si and SiO2/resist etching selectivity.
On the degeneracy of the IMRT optimization problem
Authors: Alber M. , Meedt G. , Nüsslin F. , Reemtsen R.
Published: November 2002

One approach to the computation of photon IMRT treatment plans is the formulation of an optimization problem with an objective function that derives from an objective density. An investigation of the second‐order properties of such an objective function in a neighborhood of the minimizer opens an intuitive access to many traits of this approach. A general finding is that only a small subset of the parameter space has nonzero curvature, while the objective function is entirely flat in a neighborhood of the minimizer in most directions. The dimension of the subspace of vanishing curvature serves as a measure for the degeneracy of the solution. This finding is important both for algorithm design and evaluation of the mathematical model of clinical intuition, expressed by the objective function. The structure of the subspace of great curvature is found to be imposed on the problem by conflicts between objectives of target and critical structures. These conflicts and their corresponding modes of resolution form a common trait between all reasonable treatment plans of a given case. The high degree of degeneracy makes the use of a conjugate gradient optimization algorithm particularly favorable since the number of iterations to convergence is equivalent to the number of different eigenvalues of the curvature tensor and is hence independent from the number of optimization parameters. A high level of degeneracy of the fluence profiles implies that it should be possible to stipulate further delivery‐related conditions without causing severe deterioration of the dose distribution.
Blurring artifacts in megavoltage radiography with a flat‐panel imaging system: Comparison of Monte Carlo simulations with measurements
Authors: von Wittenau A. E. Schach , Logan C. M. , Aufderheide M. B. , Slone D. M.
Published: November 2002

Originally designed for use at medical‐imaging x‐ray energies, imaging systems comprising scintillating screens and amorphous Si detectors are also used at the megavoltage photon energies typical of portal imaging and industrial radiography. While image blur at medical‐imaging x‐ray energies is strongly influenced both by K‐shell fluorescence and by the transport of optical photons within the scintillator layer, at higher photon energies the image blur is dominated by radiation scattered from the detector housing and internal support structures. We use Monte Carlo methods to study the blurring in a notional detector: a series of semi‐infinite layers with material compositions, thicknesses, and densities similar to those of a commercially available flat‐panel amorphous Si detector system comprising a protective housing, a gadolinium oxysulfide scintillator screen, and associated electronics. We find that the image blurring, as described by a point‐spread function (PSF), has three length scales. The first component, with a submillimeter length scale, arises from electron scatter within the scintillator and detection electronics. The second component, with a millimeter‐to‐centimeter length scale, arises from electrons produced in the front cover of the detector. The third component, with a length scale of tens of centimeters, arises from photon scatter by the back cover of the detector. The relative contributions of each of these components to the overall PSF vary with incident photon energy. We present an algorithm that includes the energy‐dependent sensitivity and energy‐dependent PSF within a ray‐tracing formalism. We find quantitative agreement (∼2%) between predicted radiographs with radiographs of copper step wedges, taken with a 9 MV bremsstrahlung source and a commercially available flat‐panel system. The measured radiographs show the blurring artifacts expected from both the millimeter‐scale electron transport and from the tens‐of‐centimeters length scale arising from the scattered photon transport. Calculations indicate that neglect of the energy‐dependent blurring would lead to discrepancies in the apparent transmission of these wedges of the order of 9%.
Quantification of the degree of mineralization of bone in three dimensions using synchrotron radiation microtomography
Authors: Nuzzo Stefania , Peyrin Françoise , Cloetens Peter , Baruchel José , Boivin Georges
Published: November 2002

The availability of three‐dimensional measuring techniques coupled to specific image processing methods opens new opportunities for the analysis of bone structure. In particular, synchrotron radiation microtomography may provide three‐dimensional images with spatial resolution as high as one micrometer. Moreover, the use of a monoenergetic synchrotron beam, which avoids beam‐hardening effects, allows quantitative measurements of the degree of mineralization in bone samples. Indeed, the reconstructed gray levels of tomographic images correspond directly to a map of the linear attenuation coefficient within the sample. Since the absorption depends on the amount of mineral content, we proposed a calibration method to evaluate the three‐dimensional distribution of the degree of mineralization within the sample. First a theoretical linear relationship modeling the linear attenuation coefficient as a function of the hydroxyapatite concentrations was derived. Then, an experimental validation on phantoms confirmed both the accuracy of the image processing tools and the experimental setup used. Finally, the analysis of the degree of mineralization in four iliac crest bone biopsy samples was reported. Our method was compared to the reference microradiography technique, currently used for this quantification in two dimensions. The concentration values of the degree of mineralization were found with both techniques in the range 0.5–1.6 g of mineral per cubic centimeter of bone, both in cortical and in trabecular region. The mean difference between the two techniques was around 4.7%, and was slightly higher in trabecular region than in cortical bone.
Methods for improving limited field‐of‐view radiotherapy reconstructions using imperfect a priori images
Authors: Ruchala Kenneth J. , Olivera Gustavo H. , Kapatoes Jeffrey M. , Reckwerdt Paul J. , Mackie Thomas R.
Published: November 2002

There are many benefits to having an online CT imaging system for radiotherapy, as it helps identify changes in the patient's position and anatomy between the time of planning and treatment. However, many current online CT systems suffer from a limited field‐of‐view (LFOV) in that collected data do not encompass the patient's complete cross section. Reconstruction of these data sets can quantitatively distort the image values and introduce artifacts. This work explores the use of planning CT data as a priori information for improving these reconstructions. Methods are presented to incorporate this data by aligning the LFOV with the planning images and then merging the data sets in sinogram space. One alignment option is explicit fusion, producing fusion‐aligned reprojection (FAR) images. For cases where explicit fusion is not viable, FAR can be implemented using the implicit fusion of normal setup error, referred to as normal‐error‐aligned reprojection (NEAR). These methods are evaluated for multiday patient images showing both internal and skin‐surface anatomical variation. The iterative use of NEAR and FAR is also investigated, as are applications of NEAR and FAR to dose calculations and the compensation of LFOV online MVCT images with kVCT planning images. Results indicate that NEAR and FAR can utilize planning CT data as imperfect a priori information to reduce artifacts and quantitatively improve images. These benefits can also increase the accuracy of dose calculations and be used for augmenting CT images (e.g., MVCT) acquired at different energies than the planning CT.
Dose discrepancies between Monte Carlo calculations and measurements in the buildup region for a high‐energy photon beam
Authors: Ding George X.
Published: November 2002

This study investigates a possible cause of reported significant dose discrepancies between Monte Carlo calculations and measurements in the buildup region for high‐energy photon beams in large fields. A proposed hypothesis was that the discrepancy was caused by a source of electrons in the accelerator head that was not fully accounted for in the treatment head simulation. In this investigation, a lead foil is added just below the accelerator head in order to study this hypothesis. The lead foil effectively removes charged particles generated inside the accelerator head. The charged particles generated by the lead foil can be accounted for fully because the simple geometry can be simulated accurately. An 18 MV photon beam from a Varian Clinac‐2100EX is measured using a WELLHOFER WP700 beam scanner with an IC‐10 ionization chamber (cavityThe BEAM Monte Carlo code is used in the incident beam simulations. Both EGS4/DOSXYZ and EGSnrc/DOSRZnrc are used in the dose calculations in a water phantom. The Monte Carlo calculated depth‐dose curve is scaled so that it has the same values at 10 cm depth as the measured curve. It is found that the discrepancies between Monte Carlo calculations and measurements remain significant in the buildup region even after applying necessary corrections to the measured data. The discrepancies have only been modestly decreased with the lead foil in place compared to theopen field. At a depth of 1 cm, discrepancies of about 5% are still observed in the buildup region for the field with the lead foil. Therefore a new explanation for the unresolved discrepancy remains to be found.
The use of an extra‐focal electron source to model collimator‐scattered electrons using the pencil‐beam redefinition algorithm
Authors: Boyd Robert A. , Hogstrom Kenneth R. , White R. Allen , Antolak John A.
Published: November 2002

Currently, the pencil‐beam redefinition algorithm (PBRA) utilizes a single electron source to model clinical electron beams. In the single‐source model, the electrons appear to originate from a virtual source located near the scattering foils. Although this approach may be acceptable for most treatment machines, previous studies have shown dose differences as high as 8% relative to the given dose for small fields for some machines such as the Varian Clinac 1800. In such machines collimation‐scattered electrons originating from the photon jaws and the applicator give rise to extra‐focal electron sources. In this study, we examined the impact of modeling an additional electron source to better account for the collimator‐scattered electrons. The desired dose calculation accuracy in water throughout the dose distribution is 3% or better relative to the given dose. We present here a methodology for determining the electron‐source parameters for the dual‐source model using a minimal set of data, that is, two central‐axis depth‐dose curves and two off‐axis profiles. A Varian Clinac 1800 accelerator was modeled for beam energies of 20 and 9 MeV and applicator sizes of and The improvement in the accuracy of PBRA‐calculated dose, evaluated using measured two‐dimensional dose distributions in water, was characterized using the figure of merit, which represents the fractional area containing dose differences greater than 3%. For the field the evaluation was restricted to the penumbral region, and for the field the central region of the beam was included as it was impacted by the penumbra. The greatest improvement in dose accuracy was for the applicator. At 9 MeV, decreased from 15% to 0% at 100 cm SSD and from 34% to 4% at 110 cm SSD. At 20 MeV, decreased from 17% to 2% at 100 cm SSD and from 41% to 10% at 110 cm SSD. In the penumbra of the applicator, the improvement was less, but still significant. At 9 MeV, changed from 11% to 1% at 100 cm SSD and from 10% to 12% at 110 cm SSD. At 20 MeV, decreased from 12% to 8% at 100 cm SSD and from 14% to 5% at 110 cm SSD. Results demonstrate that use of a dual‐source beam model can provide significantly improved accuracy in the PBRA‐calculated dose distribution that was not achievable with a single‐source beam model when modeling the Varian Clinac 1800 electron beams. Time of PBRA dose calculation was approximately doubled; however, dual‐source beam modeling of newer accelerators (e.g., the Varian Clinac 2100) may not be necessary because of less impact of collimator‐scattered electrons on dosimetry.
DISTRIBUTIONAL SOURCES IN GENERAL RELATIVITY: TWO POINT-LIKE EXAMPLES REVISITED
Authors: PANTOJA N. R. , RAGO H.
Published: November 2002

A regularization procedure, that allows one to relate singularities of curvature to those of the Einstein tensor without some of the shortcomings of previous approaches, is proposed. This regularization is obtained by requiring that (i) the density [Formula: see text], associated to the Einstein tensor [Formula: see text] of the regularized metric, rather than the Einstein tensor itself, be a distribution and (ii) the regularized metric be a continuous metric with a discontinuous extrinsic curvature across a non-null hypersurface of codimension one. In this paper, the curvature and Einstein tensors of the geometries associated to point sources in the (2 + 1)-dimensional gravity and the Schwarzschild spacetime are considered. In both examples the regularized metrics are continuous regular metrics, as defined by Geroch and Traschen, with well defined distributional curvature tensors at all the intermediate steps of the calculation. The limit in which the support of these curvature tensors tends to the singular region of the original spacetime is studied and the results are contrasted with the ones obtained in previous works.
INFLATON FIELD AND PRIMORDIAL BLACKHOLE
Authors: PAUL B. C. , CHAKRABORTY S.
Published: November 2002

Primordial black hole formation has been studied using an inflaton field with a variable cosmological term as the potential.
A FORMAL APPROACH TO MACHIAN GENERAL RELATIVITY
Authors: WESSON PAUL S. , SEAHRA SANJEEV S. , LIU HONGYA
Published: November 2002

We take Mach's principle to mean that the local properties of a test particle should depend on the global properties of the geometry. Using a complex wave-like metric and an appropriate redefinition of the energy-momentum tensor, we show this to be possible in principle within the context of general relativity. We outline implications for higher-dimensional theories of gravity.
TECHNOLOGY CHOICES FOR SUSTAINABLE INDUSTRIAL PRODUCTION: TRANSITIONS IN METAL MAKING
Authors: MOORS ELLEN H.M. , VERGRAGT PHILIP J.
Published: November 2002

Many firms have been taking up the environmental challenge, primarily by implementing incremental technological changes in their production systems. We believe that incremental innovations are no longer enough. Instead, radical innovations in industrial production are necessary for achieving high-level sustainability goals. Accordingly, the aim of this paper is to assess how industrial production can be managed towards sustainability goals, by focusing on technological innovations of large-scale, process-oriented firms. We focus on technology choice processes at the micro level that lead to innovations in the metals production industry. Insights into these choice processes could inform policy for promoting more sustainable industrial production.

We first conceptualise "incremental" and "radical" innovations according to a technological criterion. Next, a systems-network conceptual framework is presented and applied to the analysis of technology choice processes within two steel producing firms. We argue that the awareness of a crisis, due to increased external pressures upon the production system, is a necessary (but not sufficient) condition for radical innovations to be developed and implemented. Furthermore, the availability of a mature firm-internal technology network is a necessary (but not always sufficient) condition for the development of radical innovations. Heterogeneous, informal interactions with firm-external actors are also important. A high degree of technical or organisational embeddedness of the industrial production system seems to complicate the implementation of radical innovations. These options are often implemented when firms are growing.

The empirical studies and conceptual framework generate conclusions about policy interventions aimed at transition to radical innovations. These interventions should be focused on the sensitivity of large firms to external pressure. In addition, more outsiders should be enrolled in the technology networks within firms. Further, the firm management should reinforce existing, firm-internal technology networks by including a high density of heterogeneous specialists. Finally, governments should tune their technology policy on business expanding programs.
The Evolution and Maintenance of Androdioecy
Authors: Pannell John R.
Published: November 2002

▪ Abstract  Examples of androdioecy, the coexistence of males and hermaphrodites, was unknown when the subject was last reviewed about two decades ago. Since then, several examples have been discovered in both plants and animals, and we are now in a position to reappraise theoretical work on the subject. Whereas early ideas were framed largely in terms of the invasion of males into hermaphroditic populations, all of the clearest examples of androdioecy now known appear to have evolved from dioecy. There are strong indications that this has occurred repeatedly as a result of the selection of self-fertile hermaphroditism for reproductive assurance during colonization. Male frequencies in these species are highly variable, self-fertilization in hermaphrodites is delayed, and mating opportunities appear to depend strongly on population density. Results from theoretical work on the evolution and maintenance of androdioecy in single populations and in metapopulations are summarized, and several case studies of androdioecious plants and animals are reviewed.
The Causes and Consequences of Ant Invasions
Authors: Holway David A. , Lach Lori , Suarez Andrew V. , Tsutsui Neil D. , Case Ted J.
Published: November 2002

▪ Abstract  Invasions by non-native ants are an ecologically destructive phenomenon affecting both continental and island ecosystems throughout the world. Invasive ants often become highly abundant in their introduced range and can outnumber native ants. These numerical disparities underlie the competitive asymmetry between invasive ants and native ants and result from a complex interplay of behavioral, ecological, and genetic factors. Reductions in the diversity and abundance of native ants resulting from ant invasions give rise to a variety of direct and indirect effects on non-ant taxa. Invasive ants compete with and prey upon a diversity of other organisms, including some vertebrates, and may enter into or disrupt mutualistic interactions with numerous plants and other insects. Experimental studies and research focused on the native range ecology of invasive ants will be especially valuable contributions to this field of study.