bluuebunny/crossref_metadata_2025 huggingface dataset:
Voltage-controllable multiwavelength InAs quantum-dot infrared photodetectors for mid- and far-infrared detection
Authors: Ye Zhengmao , Campbell Joe C. , Chen Zhonghui , Kim Eui-Tae , Madhukar Anupam
Published: September 2002

We report a bias-controllable multiwavelength quantum dot infrared photodetector (QDIP). The active region of the QDIP consisted of five layers of InAs quantum dots with InGaAs cap layers. Photoresponse peaks at 5.5, 5.9, 8.9, and 10.3–10.9 μm were observed. The relative response of these peaks could be controlled through the applied bias. For 5.9 μm detection, a peak detectivity, D*, of 5.8×109 cm Hz1/2/W at 77 K and 0.3 V was achieved.
Low voltage electrowetting-on-dielectric
Authors: Moon Hyejin , Cho Sung Kwon , Garrell Robin L. , Kim Chang-Jin “CJ”
Published: September 2002

This article discusses and experimentally verifies how to lower the operating voltage that drives liquid droplets by the principle of electrowetting on dielectric (EWOD). A significant contact angle change (120°→80°) is desired to reliably pump the droplet in microchannels for applications such as lab-on-a-chip or micrototal analysis systems. Typically, much higher voltages (>100 V) are used to change the wettability of an electrolyte droplet on a dielectric layer compared with a conductive layer. The required voltage can be reduced by increasing the dielectric constant and decreasing the thickness of the dielectric layer, thus increasing the capacitance of the insulating layer. This dependence of applied voltage on dielectric thickness is confirmed through EWOD experiments for three different dielectric materials of varying thickness: Amorphous fluoropolymer (Teflon® AF, Dupont), silicon dioxide (SiO2) and parylene. The dependence on the dielectric constant is confirmed with two different dielectric materials of similar thickness: SiO2 and barium strontium titanate. In all cases, the surface is coated with a very thin (200 Å) layer of amorphous fluoropolymer to provide initial hydrophobicity. Limiting factors such as the dielectric breakdown and electrolysis are also discussed. By using very thin (700 Å) and high dielectric constant (∼180) materials, a significant contact angle change (120°→80°) has been achieved with voltages as low as 15 V. Based on these results, a microfluidic device has been fabricated and tested, demonstrating successful transporting (pumping) of a 460 nL water droplet with only 15 V.
Chemical and structural aspects of annealed ZnSe/GaAs(001) heterostructures
Authors: Mosca D. H. , Schreiner W. H. , Kakuno E. M. , Mazzaro I. , Silveira E. , Etgens V. H. , Eddrief M. , Zanelatto G. , Galzerani J. C.
Published: September 2002

The thermal evolution of a ZnSe epilayer grown by molecular beam epitaxy on GaAs(001) has been studied by high resolution x-ray diffraction as well as photoelectron and Raman spectroscopies. Sequential annealing of a relaxed epilayer reveals a fast migration of Ga towards the ZnSe cap layer with a significant accumulation of As atoms near the ZnSe-reacted interface. A Ga2Se3 compound appears as a predominant byproduct whereas Zn atoms are probably diffusing from the reacted interface into the GaAs substrate.
Extremal path approach to rate constant calculations by the linearized semiclassical initial value representation
Authors: Smedarchina Zorka , Fernández-Ramos Antonio
Published: September 2002

To extend the applicability of the linearized initial value representation (LIVR) method to lower temperatures and realistic potentials, a generalization to barriers other than the inverted parabola is proposed. The LIVR method calculates rate constants of chemical reactions involving quantum effects by weighting classical trajectories by the Wigner distribution function (WDF) of the Boltzmann-averaged flux operator. These calculations can be performed efficiently if the WDF is available in analytical form, which is the case for harmonic barriers only. The proposed generalization to anharmonic barriers is based on the recognition that above a critical temperature T*=ℏω/πkB, where ω is the curvature at the top of the barrier and kB is the Boltzmann constant, the WDF is dominated by an extremal trajectory. The evaluation of WDFs and thus of thermal rate constants is thereby reduced to the search for the extremal path defined by a steepest-descent condition for the WDF. This extremal trajectory is the high-temperature analogue of the instanton (bounce path), which exists for temperatures lower than T*/2. Explicit formulas are derived for the generation of realistic WDFs and barrier crossing rate constants for symmetric barriers of arbitrary shape. Approximations are introduced that will reduce the extra computational effort required for these anharmonic barriers. They are based on the fact that above the critical temperature the WDF of any anharmonic potential can be represented with good approximation in an analytical form analogous to that of the parabolic barrier by the introduction of effective parameters. Results obtained for two standard model potentials, the quartic potential and the symmetric Eckart barrier, are compared with the well-known parabolic barrier results. The formal and actual temperature limits for calculating tunneling rate constants and the extension of the method to asymmetric barriers are briefly discussed.
Wavelet formulation of path integral Monte Carlo
Authors: Cho Art E. , Doll J. D. , Freeman David L.
Published: September 2002

A wavelet formulation of path integral Monte Carlo (PIMC) is constructed. Comparison with Fourier path integral Monte Carlo is presented using simple one-dimensional examples. Wavelet path integral Monte Carlo exhibits a few advantages over previous methods for PIMC. The efficiency of the current method is at least comparable to other techniques.
Magnetic properties of Mn-substituted Ni–Zn ferrites
Authors: Singh Amarendra K. , Goel T. C. , Mendiratta R. G. , Thakur O. P. , Prakash Chandra
Published: September 2002

Mn x Ni 0.5−x Zn 0.5 Fe 2 O 4 ferrites with x=0.05, 0.1, 0.2, 0.3, and 0.4, have been synthesized by the citrate precursor method and investigated for their magnetic properties. The initial relative permeability is observed to increase with increase in manganese concentration up to x=0.2 followed by a decrease, the maximum value being 632. An initial increase followed by a subsequent decrease of saturation magnetization with increase in x is observed. Curie temperature is observed to decrease continuously from 488 to 418 °C when x increases from 0.05 to 0.4. Frequency variation of complex initial permeability indicates that resonance peak due to domain wall oscillations is at a frequency above 13 MHz. Saturation magnetization is observed to increase by about 22% when the sintering temperature is increased from 1200 to 1400 °C. Initial permeability and B–H loops are observed to increase and constrict, respectively, with increase in sintering temperature.
Improving solar cell efficiencies by up-conversion of sub-band-gap light
Authors: Trupke T. , Green M. A. , Würfel P.
Published: September 2002

A system for solar energy conversion using the up-conversion of sub-band-gap photons to increase the maximum efficiency of a single-junction conventional, bifacial solar cell is discussed. An up-converter is located behind a solar cell and absorbs transmitted sub-band-gap photons via sequential ground state absorption/excited state absorption processes in a three-level system. This generates an excited state in the up-converter from which photons are emitted which are subsequently absorbed in the solar cell and generate electron-hole pairs. The solar energy conversion efficiency of this system in the radiative limit is calculated for different cell geometries and different illumination conditions using a detailed balance model. It is shown that in contrast to an impurity photovoltaic solar cell the conditions of photon selectivity and of complete absorption of high-energy photons can be met simultaneously in this system by restricting the widths of the bands in the up-converter. The upper limit of the energy conversion efficiency of the system is found to be 63.2% for concentrated sunlight and 47.6% for nonconcentrated sunlight.
Optical-transition properties of the Nd3+ ion in Gd0.8La0.2VO4 crystal
Authors: Jiang H. D. , Wang J. Y. , Zhang H. J. , Hu X. B. , Liu H.
Published: September 2002

Nd:Gd 0.8 La 0.2 VO 4 has been investigated as a Nd-doped laser crystal belonging to the zircon structural family. The optical absorption and emission properties have been characterized for the Nd3+ ion in Nd:Gd0.8La0.2VO4 crystal. It results in a good fit when the Judd–Ofelt theory of parity-forbidden electric-dipole transitions of rare earth ions on noncentrosymmetric sites is applied to the crystal. Based on the Judd–Ofelt theory, the following spectral parameters have been obtained: the phenomenological parameters Ωt (t=2, 4, and 6), the radiative lifetime, the fluorescence branching ratios and the quantum efficiency. The stimulated emission cross section σe for the 1.06 μm transition of Nd3+ in the crystal has been determined from fluorescence measurements. In comparison with those of other laser crystals, these results indicated that Nd:Gd0.8La0.2VO4 is a promising efficient laser material.
Simulation of crystallization of entangled polymer chains
Authors: Xu Liang , Fan Zhongyong , Zhang Hongdong , Bu Haishan
Published: September 2002

The crystallization of entangled polymer chains is studied by three-dimensional Monte Carlo simulations using the bond fluctuation model. To simulate entanglement, a mobile chain is assumed to move in a frozen environment through fixed obstacles. Simulation indicates that the temperature scale can be divided into three regions. In region I, high supercooling, the crystallization rate is high while disentanglement rate is low. In region III, low supercooling, the crystallization rate is much lower compared with disentanglement. Both rates can match each other in region II. Simulation also shows different morphologies in different temperature regions. In the simulation, all chains are mobile, but entanglement may still exist due to a high concentration of monomers. A new layer of crystal is observed to form on the surface of the seed at higher temperature, while chains crystallize in situ without large-scale movement at lower temperature. In moderate temperature range, disentanglement could match crystallization, and the new layer of crystal could still form on the seed but with smaller thickness. By close observation, the new layer of crystal consists of more than one chain when crystallization forms at higher supercooling because the crystallization rate is high in this case; a new chain deposits on the seed and crystallizes before the completion of crystallization of the former attached chain.
Ultrafast interferometric microscopy for laser-driven shock wave characterization
Authors: Gahagan K. T. , Moore D. S. , Funk D. J. , Reho J. H. , Rabie R. L.
Published: September 2002

We have applied ultrafast time-resolved two-dimensional interferometric microscopy to the measurement of shock wave breakout from thin metal films. This technique allows the construction of a two-dimensional breakout profile for laser generated impulsive shocks with temporal resolution of <300 fs and out-of-plane spatial resolution of 0.5 nm using 130 fs, 800 nm probe pulses. Constraints placed on the spatial extent of the probed region and on the spatial resolution of the technique by the short duration of the probe pulses are discussed. In combination with other techniques, such as spectral interferometry, this technique provides a powerful means of investigating shock dynamics in a variety of materials.
Influence of electrode configurations on the quality factor and piezoelectric coupling constant of solidly mounted bulk acoustic wave resonators
Authors: Lee Si-Hyung , Yoon Ki Hyun , Lee Jeon-Kook
Published: September 2002

This article presents an electrode effect on the quality factor and effective coupling coefficient empirically and theoretically. The possible quality factor detractors in film bulk acoustic wave resonator devices are the acoustic wave attenuation in the metal and piezoelectric films, the wave scattering loss due to the surface roughness, and the electrode electrical loss. Due to the small acoustic wave attenuation of the Mo electrode, the composite Mo–AlN–Mo resonator with the thick electrode provides higher Q values than the resonator with the good quality AlN film and the thin Mo electrode. In such Mo electroded resonators, the effective coupling coefficient is kept relatively high. It is found that the Q value of solidly mounted resonators (SMRs) is not determined by only the piezoelectric film quality, but by a weighted average of the electrode and piezoelectric film quality. Of Q value detractors, the scattering loss originating from surface roughness is thought to be very important factor in SMR quality value.
Simultaneous observation of luminescence and dissociation processes of Mg–H complex for Mg-doped GaN
Authors: Koide Yasuo , Walker D. E. , White B. D. , Brillson L. J. , Murakami Masanori , Kamiyama S. , Amano H. , Akasaki I.
Published: September 2002

Both luminescence properties and dissociation kinetics of Mg–H complex for as-grown Mg-doped GaN are simultaneously investigated by low-energy electron-excited nanoluminescence (LEEN) spectroscopy. Ultraviolet luminescence at 3.2–3.3 eV and blue luminescence at 2.8–2.9 eV are observed as predominant LEEN emissions. In-depth profiles of LEEN emission show that the blue luminescence is the predominant emission for highly Mg-doped GaN. Electron-beam exposure less than 50 mC/cm2 produces an increase of the ultraviolet luminescence intensity and reduction of the blue luminescence intensity. These characteristics suggest that the blue luminescence is due to a transition from hydrogen-related deep donor to Mg acceptor and that the ultraviolet luminescence is due to transitions from conduction band and/or shallow hydrogen donor to Mg acceptor. We propose a kinetic model for dissociation reactions of Mg–H complex during electron exposure, and the reaction rate is evaluated to be (3.5±0.3)×10−3 s−1 for electron beams with 3.1×10−5 A/cm2 at 5.0 keV.
Influence of opacity on gain coefficients in static, and fast moving neon-like krypton plasmas
Authors: Masnavi Majid , Kikuchi Takashi , Nakajima Mitsuo , Horioka Kazuhiko
Published: September 2002

X-ray laser gains and the level populations in collisionally pumped neon (Ne)-like krypton (Kr XXVII) ions have been investigated considering 27 levels of the 2p6, 2p5 3s, 2p5 3p, and 2p5 3d configurations in fast moving cylindrical plasma. Effects of opacity of the 3d 1P1→2p6 1S0, 3d 3D1→2p6 1S0, 3d 3P1→2p6 1S0, 3s 3P1→2p6 1S0, and 3s 1P1→2p6 1S0 transitions on the gain coefficients are considered using escape probability factors in both the static, and dynamic plasmas under the Sobolev approximation, that includes effect of the large velocity gradient. Results indicate that the gain values are significantly affected by plasma opacity, and the velocity gradient allows us to obtain a broader gain region in dense plasmas.
Thermally stabilized photoinduced Bragg gratings
Authors: Nishii Junji , Kintaka Kenji , Nishiyama Hiroaki , Sano Tomokazu , Ohmura Etsuji , Miyamoto Isamu
Published: September 2002

Bragg gratings were printed in Ge–SiO2 and Ge–B–SiO2 thin glass films by KrF excimer laser irradiation through a phase mask. The diffraction efficiency of the grating printed in the Ge–B–SiO2 film was approximately one order of magnitude greater than that of the Ge–SiO2 film. Although the gratings were almost erased upon annealing at temperatures lower than 500 °C, a grating with much higher diffraction efficiency than before annealing was formed exclusively in the Ge–B–SiO2 film after annealing at 600 °C. The diffraction efficiency of the grating was unchanged upon repeated heating between room temperature and 600 °C.
Nonalloyed ohmic mechanism of TiN interfacial layer in Ti/Al contacts to (NH4)2Sx-treated n-type GaN layers
Authors: Lee Ching-Ting , Lin Yow-Jon , Lin Chun-Hung
Published: September 2002

We investigate the nonalloyed ohmic contact of Ti/Al contacts to (NH4)2Sx-treated n-type GaN layers using x-ray photoelectron spectroscopy analysis. The native oxide on the n-type GaN surface can be completely removed by (NH4)2Sx solution. The resultant Ga/N ratio was decreased. The deposited Ti is in intimate contact with the (NH4)2Sx-treated n-type GaN layer. As a result, the TiN interlayer, which has a low work function and low electrical resistivity, is formed easily. The nonalloyed ohmic mechanism results from the formation of a TiN interlayer and the creation of nitrogen vacancies due to the nitrogen outdiffusion from the n-type GaN layer. The nonalloyed ohmic behavior of the as-deposited Ti/Al contacts with (NH4)2Sx-treated GaN layer was associated with Al/Ti/TiN/n-type GaN.
Ion-to-CH3 flux ratio in diamond chemical-vapor deposition
Authors: Teii Kungen , Hori Masaru , Goto Toshio
Published: September 2002

Methyl radicals (CH3) and positive ionic species in a low-pressure inductively coupled plasma under diamond-depositing conditions have been detected by using a quadrupole mass spectrometer. Absolute calibration of the fluxes of CH3 and ionic species was made by the threshold ionization technique and Langmuir probe measurement, respectively. The CH3 density increased by two to three times with a small addition of carbon monoxide to a methane–hydrogen plasma and was on the order of 1011–1012 cm−3. As the pressure decreased from 60 to 10 mTorr, the ion-to-CH3 flux ratio increased from 0.2 to 4.3, accompanied by an increase in the fraction of light ions such as Hx+ (x=1–3). The average ion energy in the ion energy distribution at a grounded electrode was compared with the sheath potential and the discrepancy was found to be 0.5–2 eV depending on pressure and ion mass. The results were used to describe the specific surface process dominated by energetic (∼ several eV) ions rather than thermal neutrals.
Photoluminescence of Er in strained Si on SiGe layer
Authors: Ishiyama T. , Nawae S. , Komai T. , Yamashita Y. , Kamiura Y. , Hasegawa T. , Inoue K. , Okuno K.
Published: September 2002

We studied photoluminescence of Er in strained Si on a SiGe layer (Si:Er:O/SiGe) and unstrained Si on a Si layer (Si:Er:O/Si) grown by molecular beam epitaxy. Er-related photoluminescence was observed in both Si:Er:O/SiGe and Si:Er:O/Si samples. The peak intensity of Si:Er:O/SiGe at 1.54 μm was higher than that of Si:Er:O/Si. Moreover, the spectrum of strained Si (Si:Er:O/SiGe) was much broader than that of unstrained Si (Si:Er:O/Si). These differences between Si:Er:O/SiGe and Si:Er:O/Si suggest that the optical activation of Er can be enhanced by the presence of strain.
Third-order nonlinear optical properties of in-backbone substituted conjugated polymers
Authors: Gubler U. , Concilio S. , Bosshard Ch. , Biaggio I. , Günter P. , Martin R. E. , Edelmann M. J. , Wytko J. A. , Diederich F.
Published: September 2002

We present an alternative approach to tune the third-order nonlinear optical properties of organic molecules by inserting a functional group into the path of the π-electron conjugation instead of at its ends. We show that this scheme has a beneficial influence on the second-order hyperpolarizability for short molecules, but that in two instances where such molecules were polymerized into longer molecules the overall hyperpolarizability is reduced.
All-angle negative refraction in a three-dimensionally periodic photonic crystal
Authors: Luo Chiyan , Johnson Steven G. , Joannopoulos J. D.
Published: September 2002

We introduce a photonic crystal that has a large range of effective negative refractive index in three dimensions (3D) and demonstrate its negative-refraction property by numerical simulations. With slight modifications, our design is also amenable to layer-by-layer fabrication. This work should enable experimental observation of negative refraction and related phenomena in 3D at optical frequencies.
Room-temperature magnetism in Cr-doped AlN semiconductor films
Authors: Yang S. G. , Pakhomov A. B. , Hung S. T. , Wong C. Y.
Published: September 2002

Synthesis and characterization of magnetic semiconductors Al1−xCrxN, in which the atomic fraction of chromium x is up to 0.357, are reported. The films, grown by reactive co-sputtering on silicon, glass, and kapton substrates, have a crystal structure of aluminum nitride. Magnetic and transport properties were studied in the temperature range of 50 to 340 K. The materials are in the dielectric regime and have variable-range-hopping type of conductance. The films are ferromagnetic at temperatures over 340 K.
Thermal energy barrier distribution measurements in perpendicular media
Authors: Wu X. W. , Zhou H. , van de Veerdonk R. J. M. , Ju G. , Lu B. , Weller D.
Published: September 2002

dc demagnetized remanent curves at different applied field time durations have been applied to measure the thermal energy barrier distribution of two sets of perpendicular alloy media. Sharrock’s formula is applied at all levels of percentages of magnetization switching. The measured energy barrier distribution width decreases with increasing intergranular exchange coupling and with reducing intergranular magnetostatic interactions. The results agree well with micromagnetic simulations.
Improved theory for remote-charge-scattering-limited mobility in metal–oxide–semiconductor transistors
Authors: Saito Shin-ichi , Torii Kazuyoshi , Hiratani Masahiko , Onai Takahiro
Published: September 2002

Transport theory is extended to include the remote-charge-scattering-limited electron mobility of metal–oxide–semiconductor field-effect transistors. We evaluated remote-charge-scattering from the depletion charge in the polycrystalline silicon gate. We obtained an analytical expression for the scattering potential, by taking image charge, screening, and quantization effects into account. The potential increases with decreasing gate-oxide thickness, which results in a mobility degradation at lower vertical electric fields. The calculated mobility agrees well with recent measurements.
Factors hampering the formulation of a national information policy in Kenya
Authors: Ayoo Philip , Otike Japhet
Published: September 2002

The importance of a national information policy to a developing country is explained and the factors hindering its development in Kenya are discussed. These factors relate to poor recognition of the importance of information, funding shortages, weak infrastructure, lack of qualified manpower, information illiteracy and lack of commitment within the information profession.
Yes, but how do we know if it’s working? Evidence regarding impact on clinical practice of access for health service staff to bibliographic databases and full text electronic journals
Authors: Marriott Richard
Published: September 2002

Discusses the development of regionally organised electronic library services within the NHS. Examines evidence for the effectiveness of electronic library services both in the UK and abroad. Draws attention to the inter‐dependence of electronic and conventional library services.
A history of the Birmingham University library
Authors: Glasgow Eric
Published: September 2002

Recounts the origins of Birmingham University in Mason College, and the early development of its library under William Henry Cope. The growth of collections, particularly by donations from Thomas Pretious Heslop and the services based on them into the early twentieth century are discussed. The importance of the appointment of Charles Grant Robertson as Vice‐Principal is examined and the growth of the collections described, as is the further service development under Fenwick Mills and Wilfred Bonser. The continuing development of the collections and opening of a new library building in 1960 are discussed in a review of the history of the library and its services into the modern period.
Sharing the wealth – resource sharing developments in the Pacific making for a resource richer region
Authors: Cohen Arlene
Published: September 2002

Individual libraries in the islands of the Pacific are notoriously viewed as resource poor, although if the resources of each library are viewed collectively, there is a wealth of materials ready to be shared. This paper will discuss the growing use of technology, such as document scanners, telecommunications capabilities and electronic document delivery using the Internet, to drastically reduce the time to deliver documents and the costs for this service. It will also discuss low‐technology projects fostering resource sharing throughout the region and directions for future growth.
Assumptions versus reality: user behaviour in sourcing scholarly information
Authors: McGrath Mike
Published: September 2002

Reviews recent interlending and document supply literature. Considers the implications of open accesss archiving and the ongoing developments in consortia and electronic journals. A number of other issues are reviewed briefly.
Opening interlending services to end users: the Catalogue Collectif de France
Authors: Creff Christelle
Published: September 2002

An important role is played by academic libraries in French interlibrary loan and the huge volume of heritage collections and rare holdings in public libraries have also to be considered. This paper presents the interlibrary loan strategy that the Bibliothèque Nationale de France has engaged in for ten years to improve end users’ access to collections. It also presents the Catalogue Collectif de France features and demonstrates how this catalogue makes available to Internet users around 14 million records of documents held by different kinds of libraries in France. In conclusion, the paper presents some of the expected consequences for end users and French libraries when free interlending services are opened via the Internet.
Interlending and document supply developments in South Korea
Authors: Shin Eun‐Ja , Oh Kyung‐Mook
Published: September 2002

This paper aims to introduce the present situation of the interlibrary loan and document supply services among libraries and information centers in South Korea. The Korean ILL network has relied for a long time on two major structures: cooperation among college and university libraries and cooperation among regional research libraries. The library cooperation networks in each sector are encouraging more resource sharing methods such as cooperative acquisition, union cataloguing and consortia for E‐journal services. If the union catalogue systems of all library sectors were combined as a single national union catalogue, they could share over 100,000,000 holdings in South Korea.
Transcending digital, performance, and innovation divides in the preparation of new teachers
Authors: Dexter Sara
Published: September 2002

In this article I argue that teacher educators must prepare future classroom teachers to bridge innovation, digital and performance divides so that they are supported to develop new pedagogy that incorporates technology to increase student performance. In most of the teacher licensure programs at the University of Minnesota, where I direct its Ed‐U‐Tech project, a Preparing Tomorrow’s Teachers to Use Technology (PT3) grant, we are working towards these ends. In particular, I describe the eTIP Cases, an instructional tool that others and I have created to develop preservice teachers’ technology integration abilities. These instructional cases are powered by IMMEX software, which provides embedded assessment capabilities. It is felt that the learning environments created by these cases and the response they demand help preservice teachers to develop the instructional planning and decision‐making skills necessary to bridge innovation, digital and performance divides in K12 schools.
A multi‐attribute preventive replacement model
Authors: Azaiez M.N.
Published: September 2002

We formulate a multi‐attribute decision model for preventive replacement of a “magnetic sealing head” in a soft‐drink producing factory in the Kingdom of Saudi Arabia. In case of failure of this part, the opportunity cost (for production losses) is very important, as the entire production line will be idle. We determine in a first case the replacement policy that minimizes the total expected unit cost of replacement (preventive and corrective). Next, we determine the optimal policy that maximizes the expected multi‐attribute utility of the decision‐maker in the factory. Four attributes are considered in the replacement problem, namely cost, quality, labor productivity, and cash flow availability. The optimal policy in each case outperforms by far the one applied in the plant, which turns out to be costly and inefficient with respect to the utility of the decision‐maker.
On the effects of production and maintenance variations on machinery performance
Authors: Tumer Irem Y. , Huff Edward M.
Published: September 2002

The variations introduced during the production and maintenance of rotating machinery components are correlated with the vibration and noise emanating from the final system during its operational lifetime. Vibration and noise are especially unacceptable elements in high‐risk systems such as helicopters and aircraft engines, resulting in premature component degradation and a potentially unsafe flying environment. In such applications, individual components often are subject to 100 per cent inspection following production and during operation through rigorous maintenance, resulting in increased product development cycles and high production and operation costs. In this work, the aim is to provide engineers with a technique to evaluate vibration modes and levels for each component or subsystem prior to putting them into operation. This paper presents a preliminary investigation of the correlation of manufacturing and assembly variations with vibrations, using an experimental test rig. A factorial design is used to study the effects of various factors. Challenges in developing a process monitoring and inspection methodology to predict performance quality are identified, followed by a discussion of future work.
Laser treatment of LTCC for 3D structures and elements fabrication
Authors: Kita Jaroslaw , Dziedzic Andrzej , Golonka Leszek J. , Zawada Tomasz
Published: September 2002

This paper presents possibility of laser application for fabrication of 3D elements and structures. The Aurel NAVS‐30 Laser Trimming and Cutting System with special software was used. It was applied successfully for fabrication of vias (minimum diameter – 50 μm) in fired and unfired LTCC ceramics and channels with width between 100 μm and 5 mm. The achievements and problems are presented and discussed. The influence of lamination process on quality of vias and channels as well as the problems connected with interaction of laser beam with ceramic tapes are shown. Three‐dimensional resistors and microfluidic system were successfully designed and fabricated based on our investigations. Chosen electrical and thermal parameters of constructed devices are shown, too.
Dosimetric performance of an enhanced dose range radiographic film for intensity‐modulated radiation therapy quality assurance
Authors: Olch Arthur J.
Published: September 2002

Film‐based quality assurance (QA) is an important element of any intensity modulated radiation therapy (IMRT) program. XV2 film is often used for IMRT QA, however, it has saturation and energy response limitations which hinder accurate film dosimetry. A new commercially released ready‐pack film has been introduced that has an extended dose range (EDR2), reportedly allowing measured doses above 600 cGy without saturation. Also, this film may have less energy dependence due to its composition. The purpose of this paper is to study and compare the two types of film with respect to absolute dose accuracy for IMRT plans, percent depth dose accuracy for square fields between 2 and 20 cm, ability to measure composite plan isodoses and single beam fluence maps for IMRT cases, and sensitivity to processor variations over time. In 19 IMRT patient QA tests, the EDR2 film was able to achieve an absolute dose accuracy of better than 2% vs over 4% for XV2 film. The EDR2 film was able to reproduce ionization chamber and diode‐measured percent depth doses to 20 cm depth generally to within 1% over the range of field sizes tested compared to about 10% for the XV2 film. When compared to calculations, EDR2 film agreed better than XV2 film for both composite plan isodoses and single beam fluence intensity maps. The EDR2 film was somewhat more resistant to processor changes over time than the XV2 film, with a standard deviation of dose reproducibility of less than 2% compared to 6%, respectively.
Suppression of dark current radiation in step‐and‐shoot intensity modulated radiation therapy by the initial pulse‐forming network
Authors: Cheng Chee‐Wai , Das Indra J. , Ndlovu Alois M.
Published: September 2002

The effect of the initial pulse forming network (IPFN) on the suppression of dark current is investigated for a Siemens Primus accelerator. The dark current produces a spurious radiation, which is referred to as dark current radiation (DCR) in this study. In the step‐and‐shoot delivery of an intensity modulated radiation therapy (IMRT), the DCR could be of some concern for whole body dose along with leakage radiation through collimator jaws or multileaf collimator. By adjusting the IPFN‐to‐PFN ratio to >0.8, the DCR can be measured with an ion chamber during the “PAUSE” state of the accelerator in the IMRT mode. For 15 MV x rays, the magnitude of the DCR is approximately equal to 0.7% of the dose at for a field. The DCR has a similar central axis depth dose as a 15 MV beam as determined from a water phantom scan. When the IPFN‐to‐PFN ratio is lowered to <0.8, no DCR is detected. For low energy x rays (6 MV), no DCR is detected regardless of the IPFN‐to‐PFN ratio. Although the DCR is studied only for the Siemens Primus model accelerator, the same precaution applies to other models of modern accelerators from other vendors. Due to the large number of field segments used in a step‐and‐shoot IMRT, it is imperative therefore, that dark current evaluation be part of machine commissioning and annual calibration for high‐energy photon beams. Should DCR be detected, the medical physicist should work with a service engineer to rectify the problem. In view of DCR and whole body dose, low‐energy photon beams are advisable for IMRT.
Microcalcification detectability for four mammographic detectors: Flat‐panel, CCD, CR, and screen/film
Authors: Rong Xiujiang J. , Shaw Chris C. , Johnston Dennis A. , Lemacks Michael R. , Liu Xinming , Whitman Gary J. , Dryden Mark J. , Stephens Tanya W. , Thompson Stephen K. , Krugh Kerry T. , Lai Chao‐Jen
Published: September 2002

Amorphous silicon/cesium iodide (a‐Si:H/CsI:Tl) flat‐panel (FP)‐based full‐field digital mammography systems have recently become commercially available for clinical use. Some investigations on physical properties and imaging characteristics of these types of detectors have been conducted and reported. In this perception study, a phantom containing simulated microcalcifications (μCs) of various sizes was imaged with four detector systems: a FP system, a small field‐of‐view charge coupled device (CCD) system, a high resolution computed radiography (CR) system, and a conventional mammography screen/film (SF) system. The images were reviewed by mammographers as well as nonradiologist participants. Scores reflecting confidence ratings were given and recorded for each detection task. The results were used to determine the average confidence‐rating scores for the four imaging systems. Receiver operating characteristics (ROC) analysis was also performed to evaluate and compare the overall detection accuracy for the four detector systems. For calcifications of 125–140 μm in size, the FP system was found to have the best performance with the highest confidence‐rating scores and the greatest detection accuracy in the ROC analysis. The SF system was ranked second while the CCD system outperformed the CR system. The p values obtained by applying a Student t‐test to the results of the ROC analysis indicate that the differences between any two systems are statistically significant Differences in μC detectability for the large (150–160 μm) and small (112–125 μm) size μC groups showed a wider range of p values (not all p values are smaller than 0.005, ranging from 0.6 to <0.001) compared to the p values obtained for the medium (125–140 μm) size μC group. Using the p values to assess the statistical significance, the use of the average confidence‐rating scores was not as significant as the use of the ROC analysis p value for p value.
Development and evaluation of a case‐based reasoning classifier for prediction of breast biopsy outcome with BI‐RADS™ lexicon
Authors: Bilska‐Wolak Anna O. , Floyd Carey E.
Published: September 2002

Approximately 70–85 % of breast biopsies are performed on benign lesions. To reduce this high number of biopsies performed on benign lesions, a case‐based reasoning (CBR) classifier was developed to predict biopsy results from BI‐RADS™ findings. We used 1433 (931 benign) biopsy‐proven mammographic cases. CBR similarity was defined using either the Hamming or Euclidean distance measure over case features. Ten features represented each case: calcification distribution, calcification morphology, calcification number, mass margin, mass shape, mass density, mass size, associated findings, special cases, and age. Performance was evaluated using Round Robin sampling, Receiver Operating Characteristic (ROC) analysis, and bootstrap. To determine the most influential features for the CBR, an exhaustive feature search was performed over all possible feature combinations (1022) and similarity thresholds. Influential features were defined as the most frequently occurring features in the feature subsets with the highest partial ROC areas For CBR with Hamming distance, the most influential features were found to be mass margin, calcification morphology, age, calcification distribution, calcification number, and mass shape, resulting in an of 0.33. At 95% sensitivity, the Hamming CBR would spare from biopsy 34% of the benign lesions. At 98% sensitivity, the Hamming CBR would spare 27% benign lesions. For the CBR with Euclidean distance, the most influential feature subset consisted of mass margin, calcification morphology, age, mass density, and associated findings, resulting in of 0.37. At 95% sensitivity, the Euclidean CBR would spare from biopsy 41% benign lesions. At 98% sensitivity, the Euclidean CBR would spare 27% benign lesions. The profile of cases spared by both distance measures at 98% sensitivity indicates that the CBR is a potentially useful diagnostic tool for the classification of mammographic lesions, by recommending short‐term follow‐up for likely benign lesions that is in agreement with final biopsy results and mammographer's intuition.
Mammogram synthesis using a 3D simulation. I. Breast tissue model and image acquisition simulation
Authors: Bakic Predrag R. , Albert Michael , Brzakovic Dragana , Maidment Andrew D. A.
Published: September 2002

A method is proposed for generating synthetic mammograms based upon simulations of breast tissue and the mammographic imaging process. A computer breast model has been designed with a realistic distribution of large and medium scale tissue structures. Parameters controlling the size and placement of simulated structures (adipose compartments and ducts) provide a method for consistently modeling images of the same simulated breast with modified position or acquisition parameters. The mammographic imaging process is simulated using a compression model and a model of the x‐ray image acquisition process. The compression model estimates breast deformation using tissue elasticity parameters found in the literature and clinical force values. The synthetic mammograms were generated by a mammogram acquisition model using a monoenergetic parallel beam approximation applied to the synthetically compressed breast phantom.
Mammogram synthesis using a 3D simulation. II. Evaluation of synthetic mammogram texture
Authors: Bakic Predrag R. , Albert Michael , Brzakovic Dragana , Maidment Andrew D. A.
Published: September 2002

We have evaluated a method for synthesizing mammograms by comparing the texture of clinical and synthetic mammograms. The synthesis algorithm is based upon simulations of breast tissue and the mammographic imaging process. Mammogram texture was synthesized by projections of simulated adipose tissue compartments. It was hypothesized that the synthetic and clinical texture have similar properties, assuming that the mammogram texture reflects the 3D tissue distribution. The size of the projected compartments was computed by mathematical morphology. The texture energy and fractal dimension were also computed and analyzed in terms of the distribution of texture features within four different tissue regions in clinical and synthetic mammograms. Comparison of the cumulative distributions of the mean features computed from 95 mammograms showed that the synthetic images simulate the mean features of the texture of clinical mammograms. Correlation of clinical and synthetic texture feature histograms, averaged over all images, showed that the synthetic images can simulate the range of features seen over a large group of mammograms. The best agreement with clinical texture was achieved for simulated compartments with radii of 4–13.3 mm in predominantly adipose tissue regions, and radii of 2.7–5.33 and 1.3–2.7 mm in retroareolar and dense fibroglandular tissue regions, respectively.
Mammographic x‐ray unit kilovoltage test tool based on k‐edge absorption effect
Authors: Napolitano Mary E. , Trueblood Jon H. , Hertel Nolan E. , David George
Published: September 2002

A simple tool to determine the peak kilovoltage (kVp) of a mammographic x‐ray unit has been designed. Tool design is based on comparing the effect of k‐edge discontinuity of the attenuation coefficient for a series of element filters. Compatibility with the mammography accreditation phantom (MAP) to obtain a single quality control film is a second design objective. When the attenuation of a series of sequential elements is studied simultaneously, differences in the absorption characteristics due to the k‐edge discontinuities are more evident. Specifically, when the incident photon energy is higher than the k‐edge energy of a number of the elements and lower than the remainder, an inflection may be seen in the resulting attenuation data. The maximum energy of the incident photon spectra may be determined based on this inflection point for a series of element filters. Monte Carlo photon transport analysis was used to estimate the photon transmission probabilities for each of the sequential k‐edge filter elements. The photon transmission corresponds directly to optical density recorded on mammographic x‐ray film. To observe the inflection, the element filters chosen must have k‐edge energies that span a range greater than the expected range of the end point energies to be determined. For the design, incident x‐ray spectra ranging from 25 to 40 kVp were assumed to be from a molybdenum target. Over this range, the k‐edge energy changes by approximately 1.5 keV between sequential elements. For this design 21 elements spanning an energy range from 20 to 50 keV were chosen. Optimum filter element thicknesses were calculated to maximize attenuation differences at the k‐edge while maintaining optical densities between 0.10 and 3.00. Calculated relative transmission data show that the kVp could be determined to within ±1 kV. To obtain experimental data, a phantom was constructed containing 21 different elements placed in an acrylic holder. MAP images were used to determine appropriate exposure techniques for a series of end point energies from 25 to 35 kVp. The average difference between the kVp determination and the calibrated dial setting was 0.8 and 1.0 kV for a Senographe 600 T and a Senographe DMR, respectively. Since the k‐edge absorption energies of the filter materials are well known, independent calibration or a series of calibration curves is not required.
Methodological issues in radiation dose–volume outcome analyses: Summary of a joint AAPM/NIH workshop
Authors: Deasy Joseph O. , Niemierko Andrzej , Herbert Donald , Yan Di , Jackson Andrew , Ten Haken Randall K. , Langer Mark , Sapareto Steve
Published: September 2002

This report represents a summary of presentations at a joint workshop of the National Institutes of Health and the American Association of Physicists in Medicine (AAPM). Current methodological issues in dose–volume modeling are addressed here from several different perspectives. Areas of emphasis include (a) basic modeling issues including the equivalent uniform dose framework and the bootstrap method, (b) issues in the valid use of statistics, including the need for meta‐analysis, (c) issues in dealing with organ deformation and its effects on treatment response, (d) evidence for volume effects for rectal complications, (e) the use of volume effect data in liver and lung as a basis for dose escalation studies, and (f) implications of uncertainties in volume effect knowledge on optimized treatment planning. Taken together, these approaches to studying volume effects describe many implications for the development and use of this information in radiation oncology practice. Areas of significant interest for further research include the meta‐analysis of clinical data; interinstitutional pooled data analyses of volume effects; analyses of the uncertainties in outcome prediction models, minimal parameter number outcome models for ranking treatment plans (e.g., equivalent uniform dose); incorporation of the effect of motion in the outcome prediction; dose‐escalation/isorisk protocols based on outcome models; the use of functional imaging to study radio‐response; and the need for further small animal tumor control probability/normal tissue complication probability studies.
Four-bubble clusters and Menelaus’ theorem
Authors: Fischer Fred
Published: September 2002

We discuss a relatively easy way to construct a stable cluster of four soap bubbles using the radii of four selected spherical films out of a total of ten. To this end, we extend Menelaus’ theorem, a geometrical relation between a triangle and a straight line in the plane, to three and higher dimensions.
Application of Padé interpolation to stationary state problems
Authors: Leung C. N. , Wong Yvonne Y. Y.
Published: September 2002

If the small and large coupling behavior of a physical system can be computed perturbatively and expressed, respectively, as power series in a coupling parameter g and 1/g, a Padé approximant embracing the two series can interpolate between these two limits and provide an accurate estimate of the system’s behavior in the generally intractable intermediate coupling regime. The methodology and validity of this approach is illustrated by considering several stationary state problems in quantum mechanics.
Buffered Tree Population Changes in a Quaternary Refugium: Evolutionary Implications
Authors: Tzedakis P. C. , Lawson I. T. , Frogley M. R. , Hewitt G. M. , Preece R. C.
Published: September 2002

A high-resolution pollen record from western Greece shows that the amplitude of millennial-scale oscillations in tree abundance during the last glacial period was subdued, with temperate tree populations surviving throughout the interval. This provides evidence for the existence of an area of relative ecological stability, reflecting the influence of continued moisture availability and varied topography. Long-term buffering of populations from climatic extremes, together with genetic isolation at such refugial sites, may have allowed lineage divergence to proceed through the Quaternary. Such ecologically stable areas may be critical not only for the long-term survival of species, but also for the emergence of new ones.