bluuebunny/crossref_metadata_2025 huggingface dataset:
Physical mobilization of secretory vesicles facilitates neuropeptide release by nerve growth factor‐differentiated PC12 Cells
Authors: Ng Yuen‐Keng , Lu Xinghua , Levitan Edwin S.
Published: July 2002

It has been speculated that neurosecretion can be enhanced by increasing the motion, and hence, the availability of cytoplasmic secretory vesicles. However, facilitator‐induced physical mobilization of secretory vesicles has not been observed directly in living cells, and recent experimental results call this hypothesis into question. Here, high resolution green fluorescent protein (GFP)‐based measurements in nerve growth factor‐differentiated PC12 cells are used to test whether altering dense core vesicle (DCV) motion affects neuropeptide release. Experiments with mycalolide B and jasplakinolide demonstrate that neuropeptidergic DCV motion at the ends of processes is proportional to F‐actin. Furthermore, Ba2+ increases DCV mobility without detectably modifying F‐actin. Finally, we show that altering DCV motion by changing F‐actin or stimulating with Ba2+ proportionally changes sustained neuropeptide release. Therefore, increasing DCV mobility facilitates prolonged neuropeptide release.
Ionotropic P2X purinoreceptors mediate synaptic transmission in rat pyramidal neurones of layer II/III of somato‐sensory cortex
Authors: Pankratov Y. , Lalo U. , Krishtal O. , Verkhratsky A.
Published: July 2002

Fast P2X receptor‐mediated excitatory postsynaptic current (EPSC) was identified in pyramidal neurones of layer II/III of somato‐sensory cortex in acutely isolated slices obtained from the brain of 17‐ to 22‐day‐old rats. The EPSCs were elicited by electrical stimulation of vertical axons originating from layer IV‐VI neurones at 0.1 Hz in the presence of bicuculline. When the glutamatergic EPSC was blocked by saturating concentrations of glutamate receptor inhibitors 2,3‐dioxo‐6‐nitro‐1,2,3,4‐tetrahydrobenzo‐[f]‐quinoxaline‐7‐sulphonamide (NBQX) and D‐(‐)‐2‐amino‐5‐phosphonopentanoic acid (D‐AP5), a small EPSC component was recorded from 90 % of neurones tested. This residual EPSC was not affected by selective blockers of nicotinic (hexamethonium) or serotonin (N‐(1‐azabicyclo‐[2.2.2]oct‐3‐yl)‐6‐chloro‐4‐methyl‐3‐oxo‐3,4‐dihydro‐2H‐1,4‐benzoxazine‐8‐carboxamide hydrochloride, Y‐25130) receptors, but it was reversibly inhibited by the antagonists of P2X receptors NF023 (8,8′‐[carbonylbis(imino‐3,1‐phenylenecarbonylimino)]bis‐1,3,5‐naphthalene‐trisulphonic acid), NF279 (8,8′‐[carbonylbis (imino‐4,1‐phenylenecarbonylimino‐4,1‐phenylenecarbonylimino)]bis‐1,3,5‐naphthalene‐trisulphonic acid) and PPADS (pyridoxal phosphate‐6‐azophenyl‐2′,4′‐disulphonic acid). Application of ATP (10 μm) or α,β‐methylene ATP (10 μm) to pyramidal neurones, acutely isolated from cortical slices, evoked inward currents (30 to 200 pA) in 65 % of cells tested. The relative calcium/caesium permeability (PCa/PCs) of P2X receptors was 12.3 as estimated from the reversal potential of ATP‐induced current measured at different extracellular calcium concentrations. We concluded that P2X purinoreceptors are activated during synaptic transmission in neocortex.
Process Parameters and Residual Stresses in Cylindrical Grinding
Authors: Capello Edoardo , Semeraro Quirico
Published: July 2002

The paper presents an experimental study on the relationship between process parameters and residual stresses in cylindrical grinding. In particular, the influence of the depth of cut (a) and the peripheral velocity of the workpiece vw was investigated. Residual stresses were found to depend on both parameters, but the effect of the peripherial velocity depends on the grinding conditions. In “easy to grind” conditions an increase in workpiece velocity leads to an increase in residual stresses, but in “difficult to grind” conditions an increase in workpiece velocity results in a decrease in residual stresses. Consequently, two joint parameters, Zw=as˙vw, and Kw=a/vw, were used to predict residual stresses. Regression analysis was then performed to determine the analytic relationship between the joint parameters and the residual stresses in “easy” and “difficult” grinding conditions. Finally, the influence of wheel wear, feed rate, sparkout and wheel dressing was investigated. The study showed that the workpiece feed rate does not seem to influence the residual stresses, while sparkout, wheel wear, and wheel dressing significantly affect the level of residual stresses generated during machining.
Investigation of Unsteady Aerodynamic Blade Excitation Mechanisms in a Transonic Turbine Stage—Part II: Analytical Description and Quantification
Authors: Laumert Bjo¨rn , Ma˚rtensson Hans , Fransson Torsten H.
Published: July 2002

This paper presents a study of the blade pressure perturbation levels and the resulting blade forces in a high-pressure transonic turbine stage based on 3-D time dependent viscous computations. Globally, the blade pressure unsteadiness is quantified with the RMS of the pressure perturbations integrated in both time and along the blade surface. Operation point as well as spanwise variations are addressed. Locally, the relative strength of the pressure perturbation events on the vane and rotor blade surface is investigated. To obtain information about the relative strength of events related to the blade passing frequency and higher harmonics, the pressure field is Fourier decomposed in time at different radial positions along the blade arc-length. The amplitude peaks are then related to the pressure events in space-time maps. With the help of the observations and results from the blade pressure study, the radial variations of the unsteady blade force and torque acting on a constant span blade profile section are investigated. The connection between the first and second vane passing frequency pressure amplitudes on the rotor blade surface and the resulting force and the torque amplitudes for three selected blade modes was investigated in detail. In this investigation the pressure was integrated over defined rotor blade regions to quantify local force contributions. Spanwise as well as operation point variations are addressed.
Transition in Convective Flows Heated Internally
Authors: Nagata Masato , Generalis Sotos
Published: July 2002

The stability of internally heated convective flows in a vertical channel under the influence of a pressure gradient and in the limit of small Prandtl number is examined numerically. In each of the cases studied the basic flow, which can have two inflection points, loses stability at the critical point identified by the corresponding linear analysis to two-dimensional states in a Hopf bifurcation. These marginal points determine the linear stability curve that identifies the minimum Grashof number (based on the strength of the homogeneous heat source), at which the two-dimensional periodic flow can bifurcate. The range of stability of the finite amplitude secondary flow is determined by its (linear) stability against three-dimensional infinitesimal disturbances. By first examining the behavior of the eigenvalues as functions of the Floquet parameters in the streamwise and spanwise directions we show that the secondary flow loses stability also in a Hopf bifurcation as the Grashof number increases, indicating that the tertiary flow is quasi-periodic. Secondly the Eckhaus marginal stability curve, that bounds the domain of stable transverse vortices towards smaller and larger wavenumbers, but does not cause a transition as the Grashof number increases, is also given for the cases studied in this work.
Forced Convective Heat Transfer in Metallic Fibrous Materials
Authors: Angirasa Devarakonda
Published: July 2002

A numerical study is reported for high Reynolds number forced convection in a channel filled with rigid metallic fibrous materials of high porosity. The effects of convective and form inertia, viscous shear, and thermal dispersion are all considered together. Inertia and thermal dispersion are modeled. The numerical results suggest that heat transfer rate increases with increasing Reynolds number within a range, but not significantly beyond that range. The heat transfer rate also increases with stagnant thermal conductivity, and decreases with Darcy number. The fiber thickness was found to have significant influence on thermal dispersion. The range of applicability of the local volume averaging in terms of the significant parameters is discussed.
Numerical and Experimental Investigation of the Acoustic and Flow Performance of Intake Systems
Authors: Rusch P. A. , Dhingra A. K.
Published: July 2002

This paper investigates the acoustic and flow performance of an intake system using numerical and experimental techniques. The acoustic and flow performances are characterized by computing the Insertion Loss (IL) and the loss coefficient (LC) respectively. An indirect BEM formulation is used to predict the IL. The LC is computed by solving a one-dimensional fluid dynamics problem. For four simple cylindrical duct systems, numerical results for IL and LC are compared with experimental measurements. Finally, the acoustic and flow performance of an actual motorcycle intake is predicted and the results are compared to bench test results.
A Combined Eulerian and Lagrangian Method for Prediction of Evaporating Sprays
Authors: Burger M. , Klose G. , Rottenkolber G. , Schmehl R. , Giebert D. , Scha¨fer O. , Koch R. , Wittig S.
Published: July 2002

Polydisperse sprays in complex three-dimensional flow systems are important in many technical applications. Numerical descriptions of sprays are used to achieve a fast and accurate prediction of complex two-phase flows. The Eulerian and Lagrangian methods are two essentially different approaches for the modeling of disperse two-phase flows. Both methods have been implemented into the same computational fluid dynamics package which is based on a three-dimensional body-fitted finite volume method. Considering sprays represented by a small number of droplet starting conditions, the Eulerian method is clearly superior in terms of computational efficiency. However, with respect to complex polydisperse sprays, the Lagrangian technique gives a higher accuracy. In addition, Lagrangian modeling of secondary effects such as spray-wall interaction enhances the physical description of the two-phase flow. Therefore, in the present approach the Eulerian and the Lagrangian methods have been combined in a hybrid method. The Eulerian method is used to determine a preliminary solution of the two-phase flow field. Subsequently, the Lagrangian method is employed to improve the accuracy of the first solution using detailed sets of initial conditions. Consequently, this combined approach improves the overall convergence behavior of the simulation. In the final section, the advantages of each method are discussed when predicting an evaporating spray in an intake manifold of an internal combustion engine.
Combustion and Emission Characteristics of Direct-Injection Compression Ignition Engines by Means of Two-Stage Split and Early Fuel Injection
Authors: Yamane K. , Shimamoto Y.
Published: July 2002

The objective of this study was to experimentally clarify the effect of two-stage split and early injection on the combustion and emission characteristics of a direct-injection (DI) diesel engine. Engine tests were carried out using a single-cylinder high-speed DI diesel engine and an injection system, combining an ordinary jerk pump and an electronically controlled high-pressure injection system, KD-3. In these experiments to compare the combustion and exhaust emission characteristics with two-stage split and early injection, a single-stage and early injection was tested. The FT-IR exhaust-gas analyzer simultaneously measured the exhaust emissions of 26 components. The results showed that HCHO, CH3CHO, and CH3COOH were emitted during the very early stage of both single injection and two-stage injection. These concentrations were higher than those from diesel combustion with ordinary fuel injection timings. These exhaust emissions are characteristic components of combustion by premixed compression ignition with extremely early injection. In particular, the HCHO concentration in exhaust was reduced with an increase in the maximum rate of heat release after cool flame due to pre-reaction of pre-mixture. At extremely early injection, the NOx concentration was extremely low; however, the indicated specific fuel consumption (ISFC) was higher than that of ordinary diesel combustion. In the case of two-stage injection, the degree of constant volume is increased, so that ISFC is improved. These results also demonstrated the possibility of reducing HCHO, NOx, and smoke emissions by means of two-stage split and early injection.
Heat and Fluid Flow Within an Anisotropic Porous Medium
Authors: Nakayama A. , Kuwahara F. , Umemoto T. , Hayashi T.
Published: July 2002

A numerical experiment at a pore scale using a full set of Navier-Stokes and energy equations has been conducted to simulate laminar fluid flow and heat transfer through an anisotropic porous medium. A collection of square rods placed in an infinite two-dimensional space has been proposed as a numerical model of microscopic porous structure. The degree of anisotropy was varied by changing the transverse center-to-center distance with the longitudinal center-to-center distance being fixed. Extensive calculations were carried out for various sets of the macroscopic flow angle, Reynolds number and degree of anisotropy. The numerical results thus obtained were integrated over a space to determine the permeability tensor, Forchheimer tensor and directional interfacial heat transfer coefficient. It has been found that the principal axes of the permeability tensor (which controls the viscous drag in the low Reynolds number range) differ significantly from those of the Forchheimer tensor (which controls the form drag in the high Reynolds number range), The study also reveals that the variation of the directional interfacial heat transfer coefficient with respect to the macroscopic flow angle is analogous to that of the directional permeability. Simple subscale model equations for the permeability tensor, Forchheimer tensor and directional Nusselt number have been proposed for possible applications of VAT to investigate flow and heat transfer within complex heat and fluid flow equipment consisting of small scale elements.
The Zone Method: A New Explicit Matrix Relation to Calculate the Total Exchange Areas in Anisotropically Scattering Medium Bounded by Anisotropically Reflecting Walls
Authors: Goyhe´ne`che J. M. , Sacadura J. F.
Published: July 2002

A new explicit matrix relation for the calculation of the total exchange areas (TEA) in emitting, absorbing and anisotropically scattering semi-transparent medium bounded by emitting, absorbing and anisotropically reflecting walls has been established. It has been used to directly determine the TEA as a function of radiative properties and geometry of the medium and its boundaries. Computation calls for direct exchange areas (DEA) and indirect exchange areas (IEA). A new definition of these exchange areas reduces their integration order and provides practical energy balance relations for their computation in the case of complex geometry elements. The new formulation is applied in the case of an emitting, absorbing and linearly anisotropic scattering semi-transparent slab bounded by black surfaces. This method is also applicable to nongray medium using the weighted sum of gray gases model.
Axial Compressor Aerodesign Evolution at General Electric
Authors: Smith, Leroy H.
Published: July 2002

This paper traces the origins of the GE Design System and how it has evolved from early methods to underlie and supplement present CFD methods, which are not themselves discussed herein. The two main elements of the detailed aero design process are vector diagram establishment and airfoil design. Their evolution is examined, and examples of how they were used to design some early GE compressors of interest are given. By the late 1950s, some transonic airfoil shapes were being custom tailored using internal blade station data from more complete radial equilibrium solutions. In the 1960s, rules for shaping transonic passages were established, and by the 1970s, custom tailoring was done for subsonic blading as well. The preliminary design layout process for a new compressor is described. It involves selecting an annulus shape and blading overall proportions that will allow a successful detailed design to follow. This requires establishment of stage loading limits that permit stall-free operation, and an efficiency potential prediction method for state-of-the-art blading. As design methods evolved, the newer approaches were calibrated with data-match experience, a process that is expected to always be needed.
Influence of nitrogen and temperature on the plasma deposition of fluorinated amorphous carbon films
Authors: Valentini L. , Kenny J. M. , Montereali R. M. , Lozzi L. , Santucci S.
Published: July 2002

In this work, the effects of nitrogen addition on the properties of a-C:H:F films produced by rf plasma enhanced chemical vapor deposition at different temperatures are reported. The structural and optical properties were investigated by x-ray photoelectron spectroscopy, Raman spectroscopy, ultraviolet-visible transmittance and ellipsometry measurements. The dependence of both fluorine and nitrogen incorporation in the carbon matrix on the deposition temperature was analyzed. The main effect of progressive nitrogen incorporation is a decrease of transmittance and optical band gap of the samples grown at room temperature and at 400 °C. Raman spectra evidence that for films deposited at 400 °C a sudden loss of sp3 carbon bonding occurs. In particular, at fixed plasma composition, the decrease of the optical band gap is interpreted as a clustering of the existing sp2 carbon sites. Ellipsometry characterization indicates that nitrogen incorporation for the samples deposited at room temperature induces an increase of the refractive index while films deposited at 400 °C are characterized by a reduction of the refractive index; this has been correlated to changes in the graphitic cluster size of the network. In particular, the increase in the deposition temperature produces an increase in the size of the graphitic clusters.
Control of secondary phase segregations during CuGaSe2 thin-film growth
Authors: Balboul M. R. , Rau U. , Bilger G. , Schmidt M. , Schock H. W. , Werner J. H.
Published: July 2002

The influence of the Se evaporation rate during deposition of CuGaSe2 thin films by coevaporation of the elements on Na-containing glass substrates is investigated. A high Se rate is found to avoid the segregation of a Na-containing secondary phase which, in the case of too low Se rates, is deleterious for the performance of the CuGaSe2 films. In addition, the article proposes a model for the phase segregation under low-Se conditions invoking sodium monoselenide as a precursor for the secondary phase. The segregation of the Na-containing compound in CuGaSe2 is further assisted by Ga diffusion from the CuGaSe2 film into the glass substrate. The present results also show that ionic exchange between Na from the glass and Ga from the absorber is active during the growth of Cu(In,Ga)Se2 alloys on Na-containing glass.
Properties and applications of cobalt-based material produced by electron-beam-induced deposition
Authors: Lau Y. M. , Chee P. C. , Thong J. T. L. , Ng V.
Published: July 2002

The deposition of cobalt-containing material from dicobalt octacarbonyl using a focused electron beam is reported. The material contains between 30% and 50% (atomic) cobalt, with the balance being carbon and oxygen, and comprises nanocrystalline metallic grains embedded in a carbonaceous matrix. Arches bridging two electrodes were fabricated to allow current–voltage (I–V) measurements to be conducted. The material resistivity decreases strongly with the deposition beam current, with values as low as 45 μΩ cm being attainable. Material growth by indirect irradiation is found to give rise to material of high resistivity and can result in highly resistive arches. At low temperatures, three distinctive conduction regimes are observed, with the I–V characteristics flattening out at the origin. At 20 K step-like features suggestive of Coulomb- blockade effects were observed. Magnetic force microscope (MFM) images of the material indicates that it is ferromagnetic in nature. The application of the technique and material to fabricate MFM tips is demonstrated. The use of selective material deposits as catalyst sites for the growth of carbon nanotubes is also demonstrated.
Plasma etching of SiC surface using NF3
Authors: Tasaka A. , Takahashi K. , Tanaka K. , Shimizu K. , Mori K. , Tada S. , Shimizu W. , Abe T. , Inaba M. , Ogumi Z. , Tojo T.
Published: July 2002

NF 3 was applied in the reactive ion etching of SiC. The effects of rf power and NF3 pressure on the etching rate and the surface morphology were investigated by means of scanning electron microscopy and atomic force microscopy. A procedure for getting the smooth and residue-free etched surface of SiC with a high etching rate of 87 nm/min was obtained under the conditions such as rf power of 100 W and NF3 pressure ranging from 0.5 to 1 Pa. A rough surface with spikes was obtained under the NF3 pressures higher than 3 Pa. It was found that the repetitive alternating treatment for the spike-formed and rough surface with the down flow etching using NF3 and Ar plasma sputtering enables us to obtain the smooth surface within the scale of ∼300 nm.
Adsorption process of chlorine on Si(111)7×7 studied by surface differential reflectivity and second harmonic generation
Authors: Tanaka M. , Shirao T. , Sasaki T. , Shudo K. , Washio H. , Kaneko N.
Published: July 2002

The adsorption process of chlorine on Si(111)7×7 has been studied by means of real-time surface differential reflectance (SDR) spectroscopy and second harmonic generation. Time evolution of adsorption on adatom dangling bonds and breaking of adatom back bonds are obtained from SDR spectra and second harmonic intensity. The initial sticking probability on the dangling bonds and the initial breaking probability of the back bonds are determined by fitting the time evolution to the solutions of the rate equations for the adsorption of atoms without migration. Temperature dependence of these probabilities reveals that both the adsorption on adatom dangling bonds and breaking of adatom back bonds proceed without potential barriers, and that a metastable state exists in the breaking process of the back bonds.
Amorphous silicon driver circuits for organic light-emitting diode displays
Authors: Servati P. , Prakash S. , Nathan A. , Py Christoph
Published: July 2002

This article presents design considerations pertinent to amorphous silicon (a-Si:H) pixel drive circuits for mobile display applications. We describe both pixel architectures and circuit topologies that are amenable for vertically stacked organic light-emitting diode (OLED) pixels in a-Si:H technology. Here, a dual-gate transistor structure is used to minimize the parasitic coupling between the OLED and the transistor layers. We consider both the two-transistor (2-T) voltage-programmed drive circuit and the five-transistor (5-T) current-programmed drive circuit. The latter provides compensation for shifts in device characteristics by virtue of metastable shifts in the threshold voltage of the thin-film transistor (TFT). Implementation of the 5-T drive circuit using dual-gate TFTs that enables high aperture ratio (∼100%), low leakage current, and surface emissive OLED pixels that are independent of scaling is also presented, along with simulation results of transfer characteristics.
Bakeable, all-metal demountable vacuum seal to a flat glass surface
Authors: Ng N. , Collins R. E. , Lenzen M.
Published: July 2002

A technique is described for making a demountable vacuum seal of reasonably good quality between an all-metal evacuation cup and a flat glass surface. The seal is bakeable to temperatures close to the softening point of glass. The stainless steel evacuation cup has two concentric and coplanar sealing surfaces. The regions defined by these surfaces are differentially pumped. Pressures typically below 1 Torr are achieved in the annular space between the two sealing surfaces using a conventional rotary pump. The center region inside the inner sealing surface is typically evacuated to below 10−4 Torr using conventional diffusion or turbomolecular pumps.
Multiple local minima in IMRT optimization based on dose–volume criteria
Authors: Wu Qiuwen , Mohan Radhe
Published: July 2002

Multiple local minima traps are known to exist in dose–volume and dose‐response objective functions. Nevertheless, their presence and consequences are not considered impediments in finding satisfactory solutions in routine optimization of IMRT plans using gradient methods. However, there is often a concern that a significantly superior solution may exist unbeknownst to the planner and that the optimization process may not be able to reach it. We have investigated the soundness of the assumption that the presence of multiple minima traps can be ignored. To find local minima, we start the optimization process a large number of times with random initial intensities. We investigated whether the occurrence of local minima depends upon the choice of the objective function parameters and the number of variables and whether their existence is an impediment in finding a satisfactory solution. To learn about the behavior of multiple minima, we first used a symmetric cubic phantom containing a cubic target and an organ‐at‐risk surrounding it to optimize the beam weights of two pairs of parallel‐opposed beams using a gradient technique. The phantom studies also served to test our software. Objective function parameters were chosen to ensure that multiple minima would exist. Data for 500 plans, optimized with random initial beam weights, were analyzed. The search process did succeed in finding the local minima and showed that the number of minima depends on the parameters of the objective functions. It was also found that the consequences of local minima depended on the number of beams. We further searched for the multiple minima in intensity‐modulated treatment plans for a head‐and‐neck case and a lung case. In addition to the treatment plan scores and the dose–volume histograms, we examined the dose distributions and intensity patterns. We did not find any evidence that multiple local minima affect the outcome of optimization using gradient techniques in any clinically significant way. Our study supports the notion that multiple minima should not be an impediment to finding a good solution when gradient‐based optimization techniques are employed. Changing the parameters for the objective function had no observable effect on our findings.
On cold spots in tumor subvolumes
Authors: Tomé Wolfgang A. , Fowler Jack F.
Published: July 2002

Losses in tumor control are estimated for cold spots of various “sizes” and degrees of “cold dose.” This question is important in the context of intensity modulated radiotherapy where differential dose–volume histograms (DVHs) for targets that abut a critical structure often exhibit a cold dose tail. This can be detrimental to tumor control probability (TCP) for fractions of cold volumes even as small as 1%, if the cold dose is lower than the prescribed dose by substantially more than 10%. The Niemierko–Goitein linear‐quadratic algorithm with slope 1–3 was used to study the effect of cold spots of various degrees (dose deficit below the prescription dose) and size (fractional volume of the cold dose). A two‐bin model DVH has been constructed in which the cold dose bin is allowed to vary from a dose deficit of 1%–50% below prescription dose and to have volumes varying from 1% to 90%. In order to study and quantify the effect of a small volume of cold dose on TCP and effective uniform dose (EUD), a four‐bin DVH model has been constructed in which the lowest dose bin, which has a fractional volume of 1%, is allowed to vary from 10% to 45% dose deficit below prescription dose. The highest dose bin represents a simultaneous boost. For fixed size of the cold spot the calculated values of TCP decreased rapidly with increasing degrees of cold dose for any size of the cold spot, even as small as 1% fractional volume. For the four‐subvolume model, in which the highest dose bin has a fractional volume of 80% and is set at a boost dose of 10% above prescription dose, it is found that the loss in TCP and EUD is moderate as long as the cold 1% subvolume has a deficit less than approximately 20%. However, as the dose deficit in the 1% subvolume bin increases further it drives TCP and EUD rapidly down and can lead to a serious loss in TCP and EUD. Since a dose deficit to a 1% volume of the target that is larger than 20% of the prescription dose may lead to serious loss of TCP, even if 80% of the target receives a 10% boost, particular attention has to be paid to small‐volume cold regions in the target. The effect of cold regions on TCP can be minimized if the EUD associated with the target DVH is constrained to be equal to or larger than the prescription dose.
Precise real‐time correction of Anger camera deadtime losses
Authors: Woldeselassie Tilahun
Published: July 2002

An earlier paper dealt with modeling of the camera in terms of the resolving times, and of the paralyzable detector and nonparalyzable computer system, respectively, for the case of a full energy window. A second paper presented a decaying source method for the accurate real‐time measurement of these resolving times. The present paper first shows that the detector system can be treated as a single device with a resolving time dependent on source distribution. It then discusses camera operation with an energy window, window fraction being where and are the detector and pulse‐height‐analyzer (PHA) outputs, respectively. The detector resolving time is shown to vary with window fraction according to while is unaffected, so that operation may be paralyzable or nonparalyzable depending on window setting and the ratio Regions of interest are described in terms of the ROI fraction, and resolving time, where and are the recorded count rates for the field‐of‐view and the region‐of‐interest, respectively. As and are expected to vary with input rate, it is shown that these can be measured in real‐time using the decaying source method. It is then shown that camera operation both with and can be described by the simple paralyzable equation where and and being the input rates within the energy window and the region of interest, respectively. An analytical solution to the paralyzable equation is then presented, which enables the input rates and to be obtained correct to better than 0.52% all the way up to the peak response point of the camera.
A comparison of physically and radiobiologically based optimization for IMRT
Authors: Jones Lois , Hoban Peter
Published: July 2002

Many optimization techniques for intensity modulated radiotherapy have now been developed. The majority of these techniques including all the commercial systems that are available are based on physical dose methods of assessment. Some techniques have also been based on radiobiological models. None of the radiobiological optimization techniques however have assessed the clinically realistic situation of considering both tumor and normal cells within the target volume. This study considers a ratio‐based fluence optimizing technique to compare a dose‐based optimization method described previously and two biologically based models. The biologically based methods use the values of equivalent uniform dose calculated for the tumor cells and integral biological effective dose for normal cells. The first biologically based method includes only tumor cells in the target volume while the second considers both tumor and normal cells in the target volume. All three methods achieve good conformation to the target volume. The biologically based optimization without the normal tissue in the target volume shows a high dose region in the center of the target volume while this is reduced when the normal tissues are also considered in the target volume. This effect occurs because the normal tissues in the target volume require the optimization to reduce the dose and therefore limit the maximum dose to that volume.
X‐ray micro‐CT with a displaced detector array
Authors: Wang Ge
Published: July 2002

Because the sizes of samples differ in x‐ray micro‐CT applications, it is desirable to have a mechanism to change the field of view of a micro‐CT scanner. A well‐known way to double the diameter of the field of view is to displace a detector array by 50%. In this paper, we propose to displace a detector array by an amount of greater than 0% but less than 50% for a continuously adjustable field of view, and formulate a weighting scheme for artifact‐free reconstruction. Then, we perform numerical simulation with the Shepp–Logan phantom to demonstrate the feasibility in fan‐beam and cone‐beam geometry.
A simple model of quantum trajectories
Authors: Brun Todd A.
Published: July 2002

Quantum trajectory theory, developed largely in the quantum optics community to describe open quantum systems subjected to continuous monitoring, has applications in many areas of quantum physics. I present a simple model, using two-level quantum systems (q-bits), to illustrate the essential physics of quantum trajectories and how different monitoring schemes correspond to different “unravelings” of a mixed state master equation. I also comment briefly on the relationship of the theory to the consistent histories formalism and to spontaneous collapse models.
The Cartesian diver and the fold catastrophe
Authors: Güémez J. , Fiolhais C. , Fiolhais M.
Published: July 2002

The motion of the Cartesian diver is studied, both theoretically and experimentally, and interpreted as an example of a fold catastrophe, where the control parameter is the external pressure.
The variational principle and simple properties of the ground-state wave function
Authors: Mur-Petit J. , Polls A. , Mazzanti F.
Published: July 2002

The variational principle is used to show that the ground-state wave function of a one-body Schrödinger equation with a real potential is real, does not change sign, and is nondegenerate. As a consequence, if the Hamiltonian is invariant under rotations and parity transformations, the ground state must have positive parity and zero angular momentum.
Students do not overcome conceptual difficulties after solving 1000 traditional problems
Authors: Kim Eunsook , Pak Sung-Jae
Published: July 2002

The relation between traditional physics textbook problem solving and conceptual understanding was investigated. The number of problems a student solved, as estimated by students themselves, ranged from 300 to 2900 with an average of about 1500. The students did not have much difficulty in using physics formulas and mathematics. However, we found that they still had many of the well-known conceptual difficulties with basic mechanics, and there was little correlation between the number of problems solved and conceptual understanding. This result suggests that traditional problem solving has a limited effect on conceptual understanding.
Model analysis of fine structures of student models: An example with Newton’s third law
Authors: Bao Lei , Hogg Kirsten , Zollman Dean
Published: July 2002

In problem-solving situations, the contextual features of the problems affect student reasoning. Using Newton’s third law as an example, we study the role of context in students’ uses of alternative conceptual models. We have identified four contextual features that are frequently used by students in their reasoning. Using these results, a multiple-choice survey was developed to probe the effects of the specific contextual features on student reasoning. Measurements with this instrument show that different contextual features can affect students’ conceptual learning in different ways. We compare student data from different populations and instructions and discuss the implications.
Clicks from Cuvier’s beaked whales, Ziphius cavirostris (L)
Authors: Frantzis Alexandros , Goold John C. , Skarsoulis Emmanuel K. , Taroudakis Michael I. , Kandia Varvara
Published: July 2002

Recordings of pulsed sounds (clicks) from Cuvier’s beaked whales are presented. Such recordings have not been reported in the literature before. Spectrogram analysis of data collected off SW Crete (Greece) from 1998 to 2000 revealed numerous sequences of clicks. Click pulses had durations of about 1 ms and their energy content in the audible spectrum presented a narrow peak between 13 and 17 kHz. Sequences of 35–105 clicks, with duration 15–44 s, were separated by short intersequence pauses of 3–10 s. Interclick intervals appeared fairly constant, primarily oscillating between 0.40 and 0.50 s. Characteristics of Cuvier’s beaked whale clicks were consistent with echolocating cetaceans, suggesting that this species do echolocate.
Freshening of the Ross Sea During the Late 20th Century
Authors: Jacobs S. S. , Giulivi C. F. , Mele P. A.
Published: July 2002

Ocean measurements in the Ross Sea over the past four decades, one of the longest records near Antarctica, reveal marked decreases in shelf water salinity and the surface salinity within the Ross Gyre. These changes have been accompanied by atmospheric warming on Ross Island, ocean warming at depths of ∼300 meters north of the continental shelf, a more negative Southern Oscillation Index, and thinning of southeast Pacific ice shelves. The freshening appears to have resulted from a combination of factors, including increased precipitation, reduced sea ice production, and increased melting of the West Antarctic Ice Sheet.
Spin-Polarized Resonant Tunneling in Magnetic Tunnel Junctions
Authors: Yuasa S. , Nagahama T. , Suzuki Y.
Published: July 2002

Insertion of a thin nonmagnetic copper Cu(001) layer between the tunnel barrier and the ferromagnetic electrode of a magnetic tunnel junction is shown to result in the oscillation of the tunnel magnetoresistance as a function of the Cu layer thickness. The effect is interpreted in terms of the formation of spin-polarized resonant tunneling. The amplitude of the oscillation is so large that even the sign of the tunnel magnetoresistance alternates. The oscillation period depends on the applied bias voltage, reflecting the energy band structure of Cu. The results are encouraging for the development of spin-dependent resonant tunneling devices.
Proterozoic Modular Biomineralized Metazoan from the Nama Group, Namibia
Authors: Wood Rachel A. , Grotzinger John P. , Dickson J. A. D.
Published: July 2002

We describe a Proterozoic, fully biomineralized metazoan from the Omkyk Member (∼549 million years before the present) of the northern Nama Group, Namibia.
Namapoikia rietoogensis
gen. et sp. nov. is up to 1 meter in diameter and bears a complex and robust biomineralized skeleton; it probably represents a cnidarian or poriferan.
Namapoikia
encrusts perpendicular to the walls of vertical synsedimentary fissures in microbial reefs. This finding implies that large, modular metazoans with biologically controlled mineralization appeared some 15 million years earlier than previously documented.
Requirement for Hippocampal CA3 NMDA Receptors in Associative Memory Recall
Authors: Nakazawa Kazu , Quirk Michael C. , Chitwood Raymond A. , Watanabe Masahiko , Yeckel Mark F. , Sun Linus D. , Kato Akira , Carr Candice A. , Johnston Daniel , Wilson Matthew A. , Tonegawa Susumu
Published: July 2002

Pattern completion, the ability to retrieve complete memories on the basis of incomplete sets of cues, is a crucial function of biological memory systems. The extensive recurrent connectivity of the CA3 area of hippocampus has led to suggestions that it might provide this function. We have tested this hypothesis by generating and analyzing a genetically engineered mouse strain in which the
N
-methyl-
d
-asparate (NMDA) receptor gene is ablated specifically in the CA3 pyramidal cells of adult mice. The mutant mice normally acquired and retrieved spatial reference memory in the Morris water maze, but they were impaired in retrieving this memory when presented with a fraction of the original cues. Similarly, hippocampal CA1 pyramidal cells in mutant mice displayed normal place-related activity in a full-cue environment but showed a reduction in activity upon partial cue removal. These results provide direct evidence for CA3 NMDA receptor involvement in associative memory recall.
Ongoing Modification of Mediterranean Pleistocene Sapropels Mediated by Prokaryotes
Authors: Coolen Marco J. L. , Cypionka Heribert , Sass Andrea M. , Sass Henrik , Overmann Jörg
Published: July 2002

Late Pleistocene organic-rich sediments (sapropels) from the eastern Mediterranean Sea harbor unknown, metabolically active chemoorganotrophic prokaryotes. As compared to the carbon-lean intermediate layers, sapropels exhibit elevated cell numbers, increased activities of hydrolytic exoenzymes, and increased anaerobic glucose degradation rates, suggesting that microbial carbon substrates originate from sapropel layers up to 217,000 years old. 16
S
ribosomal RNA gene analyses revealed that as-yet-uncultured green nonsulfur bacteria constitute up to 70% of the total microbial biomass. Crenarchaeota constitute a smaller fraction (on average, 16%). A slow but significant turnover of glucose could be detected. Apparently, sapropels are still altered by the metabolic activity of green nonsulfur bacteria and crenarchaeota.
Intercellular Communication in the Mammalian Ovary: Oocytes Carry the Conversation
Authors: Matzuk Martin M. , Burns Kathleen H. , Viveiros Maria M. , Eppig John J.
Published: July 2002

The production of functional female gametes is essential for the propagation of all vertebrate species. The growth of oocytes within ovarian follicles and their development to mature eggs have fascinated biologists for centuries, and scientists have long realized the importance of the ovarian follicle's somatic cells in nurturing oogenesis and delivering the oocyte to the oviduct by ovulation. Recent studies have revealed key roles of the oocyte in folliculogenesis and established that bidirectional communication between the oocyte and companion somatic cells is essential for development of an egg competent to undergo fertilization and embryogenesis. The challenge for the future is to identify the factors that participate in this communication and their mechanisms of action.
Rapid Regulation of Light Harvesting and Plant Fitness in the Field
Authors: Külheim Carsten , Ågren Jon , Jansson Stefan
Published: July 2002

We used
Arabidopsis thaliana
mutants to examine how a photosynthetic regulatory process, the qE-type or ΔpH-dependent nonphotochemical quenching, hereafter named feedback de-excitation, influences plant fitness in different light environments. We show that the feedback de-excitation is important for plant fitness in the field and in fluctuating light in a controlled environment but that it does not affect plant performance under constant light conditions. Our findings demonstrate that the feedback de-excitation confers a strong fitness advantage under field conditions and suggest that this advantage is due to the increase in plant tolerance to variation in light intensity rather than tolerance to high-intensity light itself.
Disassembly of Transcriptional Regulatory Complexes by Molecular Chaperones
Authors: Freeman Brian C. , Yamamoto Keith R.
Published: July 2002

Many biological processes are initiated by cooperative assembly of large multicomponent complexes; however, mechanisms for modulating or terminating the actions of these complexes are not well understood. For example, hormone-bound intracellular receptors (IRs) nucleate formation of transcriptional regulatory complexes whose actions cease promptly upon hormone withdrawal. Here, we show that the p23 molecular chaperone localizes in vivo to genomic response elements in a hormone-dependent manner, disrupting receptor-mediated transcriptional activation in vivo and in vitro; Hsp90 weakly displayed similar activities. Indeed, p23 and Hsp90 also disrupted the activities of some non–IR-containing transcriptional regulatory complexes. We suggest that molecular chaperones promote disassembly of transcriptional regulatory complexes, thus enabling regulatory machineries to detect and respond to signaling changes.
Distribution of Hydrogen in the Near Surface of Mars: Evidence for Subsurface Ice Deposits
Authors: Boynton W. V. , Feldman W. C. , Squyres S. W. , Prettyman T. H. , Brückner J. , Evans L. G. , Reedy R. C. , Starr R. , Arnold J. R. , Drake D. M. , Englert P. A. J. , Metzger A. E. , Mitrofanov Igor , Trombka J. I. , d'Uston C. , Wänke H. , Gasnault O. , Hamara D. K. , Janes D. M. , Marcialis R. L. , Maurice S. , Mikheeva I. , Taylor G. J. , Tokar R. , Shinohara C.
Published: July 2002

Using the Gamma-Ray Spectrometer on the Mars Odyssey, we have identified two regions near the poles that are enriched in hydrogen. The data indicate the presence of a subsurface layer enriched in hydrogen overlain by a hydrogen-poor layer. The thickness of the upper layer decreases with decreasing distance to the pole, ranging from a column density of about 150 grams per square centimeter at –42° latitude to about 40 grams per square centimeter at –77°. The hydrogen-rich regions correlate with regions of predicted ice stability. We suggest that the host of the hydrogen in the subsurface layer is ice, which constitutes 35 ± 15% of the layer by weight.
A Continuous Source of Bose-Einstein Condensed Atoms
Authors: Chikkatur A. P. , Shin Y. , Leanhardt A. E. , Kielpinski D. , Tsikata E. , Gustavson T. L. , Pritchard D. E. , Ketterle W.
Published: July 2002

A continuous source of Bose-Einstein condensed sodium atoms was created by periodically replenishing a condensate held in an optical dipole trap with new condensates delivered using optical tweezers. The source contained more than 1 × 10
6
atoms at all times, raising the possibility of realizing a continuous atom laser.
Potential Therapeutic Role of Cationic Peptides in Three Experimental Models of Septic Shock
Authors: Giacometti Andrea , Cirioni Oscar , Ghiselli Roberto , Mocchegiani Federico , Del Prete Maria Simona , Viticchi Claudio , Kamysz Wojciech , Łempicka Elżbieta , Saba Vittorio , Scalise Giorgio
Published: July 2002

The therapeutic efficacies of buforin II, indolicidin, and KFFKFFKFF were investigated in three rat models of septic shock: (i) rats injected intraperitoneally with 10 μg ofEscherichia coliO111:B4 lipopolysaccharide, (ii) rats given an intraperitoneal injection of 2 × 1010CFU ofEscherichia coliATCC 25922, and (iii) rats in which intra-abdominal sepsis was induced via cecal ligation and single puncture. All animals were randomized to receive parenterally isotonic sodium chloride solution, 1 mg of buforin II per kg of body weight, 1 mg of indolicidin per kg, 1 mg of KFFKFFKFF per kg, and 20 mg of imipenem per kg. The main outcome measures were bacterial growth in abdominal exudate and plasma, endotoxin and tumor necrosis factor alpha (TNF-α) concentrations in plasma, and lethality. Treatment with all peptides resulted in significant reductions in plasma endotoxin and TNF-α concentrations compared with those resulting from the imipenem and saline treatments. On the other hand, imipenem treatment significantly reduced the levels of bacterial growth compared with the reductions achieved with the peptide and saline treatments. All compounds reduced the rates of death compared to that for the controls. Although the peptides demonstrated lower levels of antimicrobial activity than imipenem, they exhibited the dual properties of antimicrobial and antiendotoxin agents.