bluuebunny/crossref_metadata_2025 huggingface dataset:
Silicon carbide surface structure investigated by synchrotron radiation-based x-ray diffraction
Authors: Enriquez H. , D’angelo M. , Aristov V. Yu. , Derycke V. , Soukiassian P. , Renaud G. , Barbier A. , Chiang S. , Semond F.
Published: August 2003

We use synchrotron radiation based x-ray diffraction at grazing incidence to study the atomic structure of Si-rich β-SiC(100) 3×2 surface reconstruction. The latter includes three different Si atomic planes, in qualitative agreement with the theoretical two adlayers asymmetric dimer model. The measurements provide an accurate determination of the atomic bond, indicating asymmetric Si dimers in the first plane, and an alternating long and short Si dimers subsurface organization in the second atomic plane responsible for the lack of dimers buckling in the first plane, unlike corresponding silicon or germanium surfaces.
Plasma-assisted molecular beam epitaxy of GaN on porous SiC substrates with varying porosity
Authors: Sagar Ashutosh , Lee C. D. , Feenstra R. M. , Inoki C. K. , Kuan T. S.
Published: August 2003

We have grown GaN on porous SiC substrates and studied the effect of substrate porosity on the overgrown film quality in terms of defect structure and density and film strain. The growth was performed by plasma-assisted molecular beam epitaxy. The GaN films were characterized by x-ray diffraction, transmission electron microscopy (TEM), and wafer curvature measurements by surface profilometry. TEM images show that the GaN film grown on porous substrates contains open tubes and a relatively low dislocation density in regions between tubes. We discuss various growth mechanisms that can lead to these defect features in the GaN film. However, we do not find any overall improvement in the x-ray rocking curve full width at half maximum of the GaN films grown on porous substrates compared to those on nonporous substrates. It was found that the GaN films grown on porous SiC were significantly more strain relaxed compared to those grown on nonporous substrate. We attribute this strain relaxation in part to the observed half-loop dislocations which form at the walls of the open tubes in the GaN films.
High resolution synchrotron radiation-based x-ray photoemission spectroscopy study of the Si-rich β-SiC(100) 3×2 surface oxidation
Authors: Dunham D. , Mehlberg S. , Chamberlin S. , Soukiassian P. , Denlinger J. D. , Rotenberg E. , Tonner B. P. , Hurych Z. D.
Published: August 2003

We investigate the initial oxidation and interface formation of cubic silicon carbide for the silicon rich β-SiC(100) 3×2 surface reconstruction by high resolution synchrotron radiation-based soft x-ray photoemission spectroscopy. The surface is exposed to low doses of molecular oxygen ranging from 1 up to 10 000 L, at surface temperatures from 25 to 500 °C. Significant formation of SiO2 is found for the surface at room temperature, with the rate of oxidation increasing with temperature. Valence band data and Si 2p core level spectra show that even at low exposures, significant oxidation is taking place, with a surface reactivity to oxygen much larger than for silicon surfaces. The oxidation products, which are grown at very low temperatures (⩽500 °C) include SiO2 as a dominant feature but also substoichiometric oxides Si+1, Si+2, Si+3, and significant amounts of mixed oxide products involving C atoms (Si–O–C).
Study of InGaAsP/InP multiple quantum wells grown by solid source molecular beam epitaxy
Authors: Sun L. , Zhang D. H.
Published: August 2003

n -type strained InGaAsP/InP multiple quantum well (MQW) structures grown by solid source molecular beam epitaxy were characterized using high resolution x-ray diffraction and low temperature photoluminescence, and the effects of well width and doping density in the wells on the quality of the MQW structures were studied. Heavy Si doping in the wells degrades abruptness and causes more diffusion at the well-barrier interfaces. The PL results show the changes of peak energy and linewidth of the spectra with the doping concentration in the wells, which can be explained by band bending at the well-barrier interfaces and disordering caused by heavy Si doping. In addition, the samples with wider wells show a higher intensity and a slightly narrower zeroth-order peak, due likely to the longer growth time.
Theory of optical properties of 6.1 Å III–V superlattices: The role of the interfaces
Authors: Magri Rita , Zunger Alex
Published: August 2003

Interfacial interdiffusion in quantum wells and superlattices could alter the interfacial strain, band alignment, and even the atomic symmetry at the interface, thus potentially changing the electronic and optical properties. We consider the InAs/GaSb system describing the interdiffused interfaces via a simple kinetic model of molecular beam epitaxy growth. The predicted atomic positions after interdiffusion are then used in a pseudopotential theory to describe the electronic and optical consequences of interdiffusion. We determine (i) the effects of different interfacial bonding compositions on the electronic and optical properties; (ii) the segregation profiles at the normal and inverted interfaces; and (iii) the effect of structural disorder on band gaps. The application of our method to the InAs/GaSb superlattices allows us to explain numerous observed results and trends.
Mechanisms of current collapse and gate leakage currents in AlGaN/GaN heterostructure field effect transistors
Authors: Hasegawa Hideki , Inagaki Takanori , Ootomo Shinya , Hashizume Tamotsu
Published: August 2003

In order to clarify the mechanisms of drain current collapse and gate leakage currents in the AlGaN/GaN heterostructure field effect transistor (HFET), detailed electrical properties of the ungated portion and Schottky-gated portion of the device were investigated separately, using a gateless HFET structure and an AlGaN Schottky diode structure. The gateless device was subjected to plasma treatments and surface passivation processes including our novel Al2O3-based surface passivation. dc I–V curves of gateless HFETs were highly nonlinear due to virtual gating by surface states. After drain stress, air-exposed, H2 plasma-treated and SiO2-deposited gateless HFETs showed an initial large-amplitude exponential current transient followed by a subsequent smaller, slow, and highly nonexponential response. The former was explained by emission from deep donors at Ec−0.37 eV, and the latter by emission from surface states. Capture transients with stress-dependent capture barriers were also observed. An x-ray photoelectron spectroscopy (XPS) study indicated that 0.37 eV-deep donors are N-vacancy related. On the other hand, no current transients took place in N2 plasma treated and Al2O3-passivated samples. Temperature dependences of I–V curves of Schottky diodes were extremely small and reverse currents were anomalously large. They were explained by the “thin surface barrier” (TSB) model where thermionic field emission and field emission through the TSB region formed by deep donors produce leakage current paths. By combining the results on gateless HFETs and Schottky diodes, a new unified model of near-surface electronic states for the free surface and Schottky interface of AlGaN is proposed. It consists of a U-shaped surface state continuum and N-vacancy related near-surface discrete deep donors. The model can explain the observed large gate leakage and drain current collapse in AlGaN/GaN HFETs in a unified way. It is also shown that our novel Al2O3 passivation, when also used as a gate insulator, can completely suppress current collapse and gate leakage.
Comparison of density functional theory methods as applied to compound semiconductor-oxide interfaces: Slab versus cluster models
Authors: Sexton J. Z. , Kummel A. C.
Published: August 2003

The comparison of density functional theory cluster and slab approaches is presented for modeling the formation of electrically pinned and unpinned metal oxide-III/V semiconductor interfaces. Thermodynamic stability, interfacial electrical properties, interfacial charge trap formation and bonding structures are examined critically in the case of gallium oxide formation on the GaAs(001)-β2(2×4) surface via direct oxidation of the surface with thermal O2(g) and by vapor deposition of Ga2O(g). It is seen in both cluster and slab models that the direct oxidation with thermal O2 will lead to an electrically pinned surface, while vapor deposition of Ga2O will electrically passivate the surface, effectively unpinning the interface. Fermi-level pinning and unpinning is observed in the local density of states (DOS) in the band-gap region, in the charge distribution per surface atom, and in the geometric structures. It is seen that the DOS can be accurately predicted using either cluster or slab DOS. When cluster DOS is calculated, band-gap states appear delocalized due to poor global convergence caused by the finite cluster size effect. The thermal smearing factor for the density of states needs to be decreased from the typical value of 0.2–0.1 eV to compensate for poor convergence to reproduce accurate DOS. While cluster and plane-wave slab models predict the experimentally observed phenomenon, the slab models more accurately predict the reaction thermodynamics. We have compared both linear combination of atomic orbital (LCAO) clusters to plane-wave slab models and plane-wave clusters to plane-wave slab models to investigate the most critical parameters in attaining accurate results. It is seen that both the LCAO and plane-wave cluster models are poorly converged with respect to total energy due to the finite cluster size effect, causing over 1 eV error in the total energy.
Electrical characterization of SiO2/n-GaN metal–insulator–semiconductor diodes
Authors: Nakano Yoshitaka , Jimbo Takashi
Published: August 2003

We have investigated the interface properties of SiO2/n-GaN metal–insulator–semiconductor (MIS) diodes by using capacitance–voltage (C–V) and capacitance transient techniques. The MIS diodes were fabricated by SiO2 sputtering onto an n-GaN epitaxial layer grown by atmospheric pressure metalorganic chemical-vapor deposition on a sapphire substrate. C–V characteristics show a total interface trap density of ∼2.2×1012 eV−1 cm−2 and display capacitance saturation in deep depletion (>15 V). The capacitance in deep depletion is found to significantly increase by incident white light. A capacitance transient is also seen after applying reverse voltages, reflecting thermal emission of carriers from the SiO2/GaN interface. Deep-level transient spectroscopic measurements reveal a dominant interface trap with an activation energy of ∼0.77 eV from the conduction band, corresponding to the capacitance transient. Therefore, this interface trap is considered to induce surface Fermi-level pinning, which results in the capacitance saturation in the measured C–V characteristics.
Fabrication of triode field emitter with planar carbon-nanoparticle cathode
Authors: Park Kyung Ho , Seo Woo Jong , Lee Soonil , Koh Ken Ha
Published: August 2003

Triode field emitters with planar-carbon-nanopaticle (CNP) cathodes were successfully fabricated using the conventional photolithography and the hot-filament chemical vapor deposition. Micro-Raman spectroscopy revealed that CNP cathodes, which had been deposited through tiny gate holes, consisted of good-quality graphitic carbons. Electron emission from a CNP triode emitter with a 12-μm-diam gate hole started at the gate voltage of 45 V, and the anode current reached the level of ∼120 nA at the gate voltage of 60 V, respectively. For the quantitative analysis of the Fowler–Nordheim (FN) type emission from a CNP triode emitter, we carried out two-dimensional numerical calculation of electrostatic potential using the finite element method. As it turned out, a radial variation of electric field was very important to account for the emission from a planar emitting layer. By assuming the graphitic work function of 5 eV for CNPs, we were able to extract a consistent set of FN parameters, together with the radial position of emitting sites.
Effects of stress on electron emission from nanostructured carbon materials
Authors: Poa C. H. P. , Lacerda R. G. , Cox D. C. , Marques F. C. , Silva S. R. P.
Published: August 2003

The electron field emission properties of highly graphite like (sp2 rich) amorphous carbon films have been investigated. These films were prepared by dual ion beam-assisted deposition technique, where the assisting energies were varied from 0 to 800 eV. Threshold fields as low as 8 V/μm is observed at an assisting energy of 400 eV, which is comparable to the best threshold fields observed in high sp3 carbon films. Surface nanostructures are found on these films during growth, but are thought not to be the primary reason for the observed low threshold fields. The combination of a highly graphite-like structure with a high intrinsic compressive stress and a high local (electronic) density, obtained from x-ray photoelectron spectroscopy, is identified as the source for the field enhancement. The controllable stress is thought to modify the band structures of the graphite-like sp2 rich component in the films, which results in high dielectric inhomogeneity. This analysis is in agreement with the concept of an internal or nongeometric field enhancement from sp2 nanostructures within the carbon thin films. The effect of stress induced band structure modification can also be extended to explain the field emission behavior of carbon nanotubes under stress.
Excellent field emission from carbon nanotubes grown by microwave-heated chemical vapor deposition
Authors: Huang J. H. , Chuang C. C. , Tsai C. H. , Chen W. J.
Published: August 2003

The growth of carbon nanotubes using a microwave-heated chemical vapor deposition system is reported. The material properties including morphology and emission behavior of carbon nanotubes were studied as a function of the Ni layer thickness, applied microwave power, and substrate types. It was found that the diameter and length of carbon nanotubes increase with the Ni layer thickness. The emission current density versus field characteristics of carbon nanotubes grown at 1000 W for 10 min show clearly two groups of characteristics marked off with the Ni thickness of 50 nm. The microwave power, which determines the resulting substrate temperature, also affects the adhesion, morphology, and emission property of carbon nanotubes. There is essentially no difference in the tube’s appearance for carbon nanotubes grown on different types of Si substrates, while carbon nanotubes grown on glass substrates are smaller and more uniform in diameter. A carbon nanotube emitter, grown at 900 W for 18 min on a p-type Si(100) coated with an 80-nm-thick Ni layer, has shown an excellent emission characteristic with extremely low turn-on and threshold fields, respectively, at 0.056 and 1.50 V/μm.
Emission characteristics of ultrathin layer solid-state emitters, temperature, and thickness dependence
Authors: Nakane H. , Dupin J. P. , Binh Vu Thien
Published: August 2003

Ultrathin semiconductor solid-state emitters (SSEs) have specific electron emission properties. In particular, the values of the turn-on field are 100 times less than in conventional field emission and there is a very rapid increase of the current at high field leading to high emission current densities. In this study, the emission behavior from SSE as a function of the cathode temperature (from room temperature to 425 K) and thickness of the ultrathin semiconductor layer (from 2 to 5 nm) are reported.
Simulations of transport and field-emission properties of carbon nanotubes
Authors: Mayer A. , Miskovsky N. M. , Cutler P. H.
Published: August 2003

We present three-dimensional simulations of transport and field-emission properties of multiwall carbon nanotubes. The structure considered for the transport properties is the (5,5)@(10,10)@(15,15)@(20,20) multiwall nanotube. When electrons are injected into the inner (5,5) or outer (20,20) layer of this structure, it is observed that around 70% of the current keeps propagating in the shell it is injected into and that the fraction of the current that reaches the opposite shell is of a few percents at most, even after propagation over micron-long distances. For the simulations of field emission, the (5,5)@(10,10)@(15,15) structure is considered. For an extraction field of 2.5 V/nm, the emission obtained with a convex termination is around eight times larger than that obtained with a flat one. The emission from these convex and flat-terminated structures is, respectively, 35 and 21 times smaller than the total current obtained by considering the single-wall components separately.
Two processing techniques to sandwich a 100 nm GaAs layer between ferromagnetic metallic electrodes
Authors: Aziz A. , Smith C. G. , Winiecki G. P. , Beere H. E. , Ritchie D. A.
Published: August 2003

We report two processing techniques to sandwich a thin (tens of nanometers) GaAs layer between ferromagnetic metallic electrodes. Such devices are valuable in the study of the spin properties of electrons in semiconductors. In these processes an AlAs layer is selectively etched from underneath a GaAs/NiFe/Au heterostructure using hydrofluoric acid. Subsequently, the rest of the structure which comprises GaAs/NiFe/Au is inverted in such a way that the GaAs layer lies on the top of the NiFe layer. The mean roughness of the inverted GaAs surface is 1.2 nm and it is crack free. In the first process, structures with an area of tens of microns are inverted on the same chip. In the second process, small disks with an area of a few microns are inverted on a different host chip.
Role of nanostructure on electron field emission from amorphous carbon thin films
Authors: Carey J. D. , Forrest R. D. , Poa C. H. , Silva S. R. P.
Published: August 2003

The mechanism of electron field emission from different forms of amorphous carbon (a-C) thin films is discussed. We show that it is possible to explain electron emission from a variety of amorphous carbon films by understanding the nature of electron states near the Fermi level. The films can be described as consisting of conductive sp2 C clusters lying within a more insulating sp3 C matrix. We show that the trend in the threshold field for emission can be explained in terms of improvements in the connectivity between these sp2 clusters and that cluster concentration and size can be in turn controlled by the choice of deposition conditions. The presence of the dielectric inhomogeneity between the two regions of sp2 clusters and sp3 C matrix is also shown to be very important in understanding the apparently low barrier heights that appear in the analysis of the current-voltage emission characteristics using the Fowler–Nordheim theory. This article attempts to set the framework for a unified model for electron emission from carbon films.
Electron field emission properties of carbon nanotubes converted from nanodiamonds
Authors: Lin I-Nan , Teng Mao-Ying , Liu Kuo-Shung , Hsu Tung , Huang Jin-Hua , Tsai Chun-Horng , Cheng Hsiu-Fung
Published: August 2003

The characteristics of nanotubes (nanorods) converted from diamond films were investigated. The microstructures and Raman spectroscopy are markedly altered when the diamond films were coated with a thin layer of Fe/Co films (<10 nm) and then postannealed at high enough temperature (>950 °C), which is owing to the formation of carbon nanotubes (nanorods) resulted from the heat treatment. The same phase transform behavior was observed no matter whether the starting materials are diamond nuclei, nanodiamonds, or faceted diamonds. The main reaction is presumably the dissolution and reprecipitation of carbon species in the diamond films through the Fe–Co catalyst layer. The carbon nanotubes (nanorods) derived from faceted diamonds show electron field emission current density 1 order of magnitude larger than those converted from diamond nuclei, that is, (Je)nuclei=320 μA/cm2 and (Je)diamond=3800 μA/cm2, with the turn-on field about the same [(E0)=5.2–7.2 V/μm].
Cobalt growth on InGaP(001)(2×4): Interface formation
Authors: Lüdge K. , Vogt P. , Braun W. , Richter W. , Esser N.
Published: August 2003

The interface formation between Cobalt and In0.48Ga0.52P(001) has been studied using soft x-ray photoemission spectroscopy (SX́PS), low energy electron diffraction (LÉED), and scanning tunneling microscopy. For room temperature deposition the first monolayer of Co decorates homogeneously the InGaP(001)(2×4) dimer rows. The surface morphology, e.g., step distribution, is conserved leading to a smooth (0.8 nm roughness) surface, however no evidence for crystalline bcc Co was observed with LEED. SXPS spectra reveal that the In 4d and Ga 3d core level contain metallic components shifted to lower binding energies, that are related to InCo/GaCo compounds forming at the interface. In contrast to Co reactions with GaAs where As is diffusing to the top we found that the intensity of the P 2p core level decreases after deposition of 20 ML Co. The In and Ga intensity instead remain prominent, thus even after the deposition of 20 ML of Co the reactions have not stopped. Growth experiments at higher substrate temperature (150 °C) showed that the enhanced mobility of the species favors island growth and therefore leads to a higher surface roughness of 2–3 nm, whereas the film does not show epitaxial quality as confirmed with LEED.
Characterization of advanced complementary metal–oxide–semiconductor processes with reverse secondary ion mass spectrometry profiling
Authors: Jiang Z. X. , Lerma J. , Lee J. J. , Sieloff D. , Chen S. , Beck J. , Backer S. , Taylor W. , Tseng H. , Tobin P. , Svedberg L.
Published: August 2003

Secondary ion mass spectrometry (SIMS) profiling from the backside of a wafer containing the silicon-on-insulator structure has proven to be a unique way to achieve high accuracy in the characterization of a through-gate-oxide phosphorous profile in a complementary metal–oxide–semiconductor (CMOS) gate stack [P. Ronsheim et al., J. Vac. Sci. Technol. B 20, 448 (2002)]. In this work, we extended the sample preparation method for reverse SIMS by including the traditional transmission electron microscopy dimpling technique and generated large area openings with good reproducibility. Details about the flatness of sample surfaces, electrical conductivity near the surface of polycrystalline Si films, and difference between frontside and reverse SIMS data were elaborated by profiling an annealed P-type metal–oxide–semiconductor gate sample with polycrystalline Si doped with boron. The capability of reverse SIMS was further evaluated by characterizing the Hf distribution at the interface of the HfO2/Si substrate and Cu diffusion in a Cu/Ta/SiO2/Si stack, two typical analytical requests from CMOS front-end and back-end processes that have been challenges to SIMS. It was demonstrated that reverse SIMS was a promising solution to the characterization of abrupt interfaces that used to be hindered by the large decay lengths dominated by beam-bombardment-induced diffusion, segregation, preferential sputtering, etc.
In situ x-ray photoelectron spectroscopy measurement during Ta deposition on low-k dielectric SiLK™
Authors: Uchibori Chihiro. J. , Kimura Takahiro
Published: August 2003

The bonding energy of Ta and C was measured by in situ x-ray photoelectron spectroscopy during sequential tantalum (Ta) deposition on low-k SiLK™. A tantalum carbide layer was found to be formed at the interface between Ta and SiLK™ and the thickness was estimated to be less than 1.2 nm. Since the energy peak of π-π* shakeup caused by an aromatic ring in SiLK™ disappeared after Ta deposition, tantalum carbide was thought to be formed by a minute reaction between Ta and de-bonded aromatic ring in SiLK™.
Submicron poly-Si gate Si field-emitter array for generation of a collimated electron beam
Authors: Shimawaki Hidetaka , Shibuya Syunpei , Mimura Hidenori , Yokoo Kuniyoshi
Published: August 2003

Reducing the emitter size in a field-emitter array is the most effective method to reduce the extraction-gate voltage for electron emission. This method results in reduction of power consumption and spot size on the screen for a field-emission display (FED) application. The transverse energy of the extracted electron at the gate hole is kept almost at the same value while the electron travels between the gate and the screen in the FED. Then, the lower extraction-gate voltage causes the lower transverse energy of the electron and the smaller spot size on the screen. By a fully self-aligned silicon process, a Si field-emitter array with a submicron poly-Si gate was fabricated for generation of a fine electron beam. This beam’s emission characteristics were examined. Without using the submicron lithography, the submicron size gate aperture was formed by reactive-ion etching, thermal oxidation, chemical-vapor deposition, and the etchback technique. The experimental results show that the electrons are emitted at the relatively low extraction-gate voltage of around 30 V and due to the reduction of the gate voltage, an emission image is observed with a relatively small spot size on the phosphor screen.
Fabrication of a self-aligned microtip field emission array
Authors: Forrest R. D. , Cox D. C. , Tang Y. F. , Shannon J. M. , Silva S. R. P.
Published: August 2003

In this work we describe the development of a process to fabricate a self-gated cold cathode microtip emitter array from laser crystallized silicon using mature large area techniques and materials familiar to the microelectronics industry. A scanning electron microscopy study demonstrates the evolution of the devices with reactive ion etch time and indicates how they may be tailored to optimize field emission performance. The surface density of the gated emitter structures was estimated to be 108–109 per cm2, similar to that reported for emission arrays fabricated using cutting edge methods. A photoresist masking technique to potentially enhance device performance was investigated. Two terminal field emission operation of the device is demonstrated with a threshold field of 18 V/μm for a current of 10 nA.
Sequential tunneling model of field emission through dielectric deposits on nanotips
Authors: Filip V. , Nicolaescu D. , Tanemura M. , Okuyama F.
Published: August 2003

A model of sequential (incoherent) tunneling for the electron field emission was built up in order to explain some peculiarities of the electronic emission from relatively thick dielectric layers covering nanometer-range tips, particularly carbon nanotubes. The emission current as a function of applied voltage, dielectric layer thickness, polarizability, and temperature was computed. Various experimentally detected trends were thereby modeled, leading to the conclusion that incoherent tunneling might be a competitive mechanism for electron field emission from dielectric layers on the tips of nanometer-sized cathodes.
Feasibility study on epithermal neutron field for cyclotron‐based boron neutron capture therapy
Authors: Yonai Shunsuke , Aoki Takao , Nakamura Takashi , Yashima Hiroshi , Baba Mamoru , Yokobori Hitoshi , Tahara Yoshihisa
Published: August 2003

To realize the accelerator‐based boron neutron capture therapy (BNCT) at the Cyclotron and Radioisotope Center of Tohoku University, the feasibility of a cyclotron‐based BNCT was evaluated. This study focuses on optimizing the epithermal neutron field with an energy spectrum and intensity suitable for BNCT for various combinations of neutron‐producing reactions and moderator materials. Neutrons emitted atfrom a thick (stopping‐length) Ta target, bombarded by 50 MeV protons of 300 μA beam current, were selected as a neutron source, based on the measurement of angular distributions and neutron energy spectra. As assembly composed of iron,and lead was chosen as moderators, based on the simulation trials using theMCNPXcode. The depth dose distributions in a cylindrical phantom, calculated with theMCNPXcode, showed that, within 1 h of therapeutic time, the best moderator assembly, which is 30‐cm‐thick iron, 39‐cm‐thickand 1‐cm‐thick lead, provides an epithermal neutron flux ofThis results in a tumor dose of 20.9 Gy‐eq at a depth of 8 cm in the phantom, which is 6.4 Gy‐eq higher than that of the Brookhaven Medical Research Reactor at the equivalent condition of maximum normal tissue tolerance. The beam power of the cyclotron is 15 kW, which is much lower than other accelerator‐based BNCT proposals.
Limitations of a convolution method for modeling geometric uncertainties in radiation therapy. II. The effect of a finite number of fractions
Authors: Craig Tim , Battista Jerry , Van Dyk Jake
Published: August 2003

Convolution methods can be used to model the effect of geometric uncertainties on the planned dose distribution in radiation therapy. This requires several assumptions, including that the patient is treated with an infinite number of fractions, each delivering an infinitesimally small dose. The error resulting from this assumption has not been thoroughly quantified. This is investigated by comparing dose distributions calculated using the Convolution method with the result of Stochastic simulations of the treatment. Additionally, the dose calculated using the conventional Static method, a Corrected Convolution method, and a Direct Simulation are compared to the Stochastic result. This analysis is performed for single beam, parallel opposed pair, and four‐field box techniques in a cubic water phantom. Treatment plans for a simple and a complex idealized anatomy were similarly analyzed. The average maximum error using the Static method for a 30 fraction simulation for the three techniques in phantoms was 23%, 11% for Convolution, 10% for Corrected Convolution, and 10% for Direct Simulation. In the two anatomical examples, the mean error in tumor control probability for Static and Convolution methods was 7% and 2%, respectively, of the result with no uncertainty, and 35% and 9%, respectively, for normal tissue complication probabilities. Convolution provides superior estimates of the delivered dose when compared to the Static method. In the range of fractions used clinically, considerable dosimetric variations will exist solely because of the random nature of the geometric uncertainties. However, the effect of finite fractionation appears to have a greater impact on the dose distribution than plan evaluation parameters.
Point dose verification for intensity modulated radiosurgery using Clarkson's method
Authors: Zhu Jingeng , Yin Fang‐Fang , Kim Jae Ho
Published: August 2003

In clinical radiation physics chart checking, the dose calculation results generated by computer treatment planning software are usually verified by an independent computerized monitor unit calculation routine, or by “hand calculation” using percent depth dose (PDD), tissue phantom ratio (TPR), scatter factors, and the machine calibration factors. For intensity‐modulated radiosurgery (IMRS) or intensity‐modulated radiation therapy (IMRT), the “hand calculation” becomes not feasible due to the sophisticated multileaf collimator (MLC) segments created for intensity‐modulated dose delivery. Therefore, an independent computerized dose calculation routine is needed for fast and reliable dose verification. In this work, a point dose calculation routine for IMRS/IMRT plan verification is developed by directly applying Clarkson's method. The method includes preparing data table by measuring TPRs for circular fields with diameters ranging 6 to 98 mm, extrapolating TPR for the zero field size (TPR0) from measured data and generating scatter phantom ratio (SPR) for each individual circular field. The segmented MLC sequences created by IMRS/IMRT inverse planning are converted into irregular fields for Clarkson's calculation. This method has been tested using 29 IMRS/IMRT cases. The results indicate that it is reliable, fast, and accurate. The average time to calculate one field is about 2 s with a 300 Mhz CPU.
A contribution to improved radiotherapy for muscle‐invading urinary bladder cancer
Authors: Muren Ludvig Paul
Published: August 2003

Due to the possible side‐effects of radical cystectomy for muscle‐invading urinary bladder cancer (UBC), radiotherapy remains an attractive organ‐sparing treatment option, either alone or combined with increasingly effective chemotherapy. Radiation dose escalation may further improve the results obtained with radiotherapy. This Ph.D. program aimed at developing improved conformal radiotherapy techniques required for dose escalation in bladder irradiation. Initially, computer‐controlled movement of the collimators during beam delivery was applied to shape partially wedged beams (PWBs), designed to conform the dose distribution to bladder targets. The dosimetric verification and treatment planning implementation of PWBs were addressed. Particular attention was given to dynamic beam isodose verification with the BMS‐96 diode array. The theoretical clinical impact of PWBs in bladder irradiation was evaluated in a planning study. Using PWBs the dose homogeneity inside bladder targets improved and normal tissue (small intestine and rectum) doses were reduced. Judged from normal tissue complication probability (NTCP) modeling, PWBs would allow radiation dose escalation with 2–6 Gy in up to 60% of the patients without increasing the estimated combined NTCP relative to the standard setup. This work also demonstrated the uncertainty in intestine and rectum tolerance data and the differences between the various NTCP models. Finally, the internal bladder motion and patient setup variation were quantified from weekly repeat CT scans and electronic portal images, and new bladder treatment margins were derived. Currently, a bladder dose escalation trial using the PWB principle is performed, testing if the whole bladder target dose can be increased from 64 to 68 Gy while maintaining a low level of treatment‐induced complications.
Ultrasound tomography imaging of radiation dose distributions in polymer gel dosimeters: Preliminary study
Authors: Mather Melissa L. , Baldock Clive
Published: August 2003

A novel imaging system for investigation of absorbed dose distributions in radiotherapy polymer gel dosimeters using ultrasound is introduced. A prototype transmission ultrasound computed tomography (UCT) imaging system is developed and evaluated. The imaging capabilities of the system are assessed through investigation of an irradiated polyacrylamide gel test phantom. Images based on transmitted signal amplitude and time of flight (TOF) of the ultrasonic signal through the phantom are reconstructed using a filtered backprojection technique. In general, the reconstruction of the square field in the TOF image was superior to the transmission image, however, transmission images displayed superior contrast to TOF images. The image quality achieved with this prototype system is promising and could be significantly enhanced through improvements, in particular through the development of more sophisticated experimental equipment. It is concluded that UCT is a viable technique for imaging absorbed dose distributions in polymer gel dosimeters and investigations are continuing to further improve the system.
Nontumor integral dose variation in conventional radiotherapy treatment planning
Authors: D'Souza Warren D. , Rosen Isaac I.
Published: August 2003

Treatment planning involves selecting delivery parameters that distribute the dose to nontumor tissue in such a way as to minimize the risk of complications. This work studied the relationship between nontumor integral dose (NTID), the fractional energy deposited in nontumor tissue, and a variety of delivery parameters for three clinical cases: nasopharynx, pancreas, and prostate. Integral dose for an organ of uniform density is simply the product of the organ density, volume, and mean dose. For each case, conventional plans were generated with 2, 4, 8, 12 and 36 equally spaced beams. All plans were normalized to the same tumor mean dose which is equivalent to the same tumor integral dose. For the pancreas and prostate cases, the patients were assumed to be uniform density. For the nasopharynx case, bones and air cavities were outlined and each assigned a uniform non‐unit density. With four or more beams and clinical margin values, the variation in NTID was as a function of number of beams. With eight or more beams, the variation was Reducing the beam margin decreased the NTID because less normal tissue was irradiated. However, the effect of the number of beams on NTID was independent of margin size. Higher energy beams reduced the NTID, as expected, and the effect was independent of the number of beams. With four or more beams, variation in beam direction changed NTID by less than 1.5%. Changing beam weights changed NTID by for plans with four to eight beams. For the body sites studied, the majority of energy was deposited in nontumor tissue, ranging from 72% in the nasopharynx case to 97% for the prostate case. The NTID decreased with increasing tumor size for similar anatomic sizes and increased with increasing size of anatomical region for similar tumor size. Finally, the effect of heterogeneity‐corrected doses on the NTID was found to be for the nasopharynx case. These data support the hypothesis that the NTID is approximately independent of beam orientation or relative weighting when many beams are used. Optimization, therefore, can only find the best distribution of dose; it cannot reduce the energy imparted. NTID may be useful in establishing an upper bound on the quality of plan that can be achieved by optimization.
Tissue‐equivalent materials for construction of tomographic dosimetry phantoms in pediatric radiology
Authors: Jones A. K. , Hintenlang D. E. , Bolch W. E.
Published: August 2003

Tissue equivalent materials have a variety of uses, including routine quality assurance and quality control in both diagnostic and therapeutic physics. They are frequently used in a research capacity to measure doses delivered to patients undergoing various therapeutic procedures. However, very few tissue equivalent materials have been developed for research use at the low photon energies encountered in diagnostic radiology. In this paper, we present a series of tissue‐equivalent (TE) materials designed to radiographically mimic human tissue at diagnostic photon energies. These tissue equivalent materials include STES–NB (newborn soft tissue substitute), BTES–NB (newborn bone tissue substitute), LTES (newborn as well as a child/adult lung tissue substitute), STES (child/adult soft tissue substitute), and BTES (child/adult bone tissue substitute). In all cases, targeted reference elemental compositions are taken from those specified in the ORNL stylized computational model series. For each material, reference values of mass density, mass attenuation coefficients (10–150 keV), and mass energy‐absorption coefficients (10–150 keV) were matched as closely as permitted by material selection and manufacturing constraints. Values of μ/ρ and for the newborn TE materials are noted to have maximum deviations from their ORNL reference values of from 0 to −3% and from +2% to −3%, respectively, over the diagnostic energy range 10–150 keV. For the child/adult TE materials, these same maximal deviations of μ/ρ and are from +1.5% to −3% and from +3% to −3%, respectively. Simple calculations of x‐ray fluence attenuation under narrow‐beam geometry using a 66 kVp spectrum typical of newborn CR radiographs indicate that the tissue‐equivalent materials presented here yield estimates of absorbed dose at depth to within 3.6% for STES–NB, 3.2% for BTES–NB, and 1.2% for LTES of the doses assigned to reference newborn soft, bone, and lung tissue, respectively.
Initial evaluation of commercial optical CT‐based 3D gel dosimeter
Authors: Islam K. T. S. , Dempsey James F. , Ranade Manisha K. , Maryanski Marek J. , Low Daniel A.
Published: August 2003

We evaluated the OCTOPUS‐ONE™ research laser CT scanner developed and manufactured by MGS Research, Inc. (Madison, CT). The scanner is designed for imaging 3D optical density distributions in BANG® gels. The scanner operates in a translate‐rotate configuration with a single scanning laser beam. The rotating cylindrical gel phantom is immersed in a refractive index matching solution and positioned at the center of a square tank made of plastic and glass. A stationary polarized He–Ne laser beam (633 nm) is reflected from a mirror moving parallel to the tank wall and scans the gel. Another mirror moves synchronously along the opposite side of the tank and collects the transmitted light and sends it to a single stationary silicon photodetector. A filtered backprojection algorithm is used to reconstruct projection data in a plane. The laser‐mirrors‐detector assembly is mounted on a horizontal platform that moves vertically for slice selection. We have tested the mechanical and optical setup, projection centering on the axis of rotation, linearity, and spatial resolution. We found the optical detector to respond linearly to transmitted light from control samples. The spatial resolution of the scanner was determined by employing a split field resolution technique. We obtained the horizontal and vertical full widths at half maxima of the laser beam intensity profiles as 0.6 and 0.8 mm, respectively. Dose calibration tests of the gel were performed using a nine‐field each) dose pattern irradiated at different dose levels. Finally, we compared gel‐derived 2D planar dose distribution against radiochromic film measured dose distribution for both the nine‐field and a uniform field of 6 MV x rays. Very similar dose distributions were observed in gel and radiochromic film except in regions of steep dose gradient and highest dose. A dose normalization of 15.6% was required between the two dosimeters due to differences in overall radiation response. After normalization, analysis using the γ evaluation showed that the radiochromic film and gel‐measured dose distributions differed by a maximum γ of 1.3 using 5% and 1.5 mm dose difference and distance‐to‐agreement criteria. The optical CT scanner has great potential as a 3D dosimeter, but a few refinements and further testing are necessary before its routine clinical use.
Memory and Learning: Cdk5 tells a tail to remember
Authors: Davenport R. John
Published: August 2003

Scientists have added to the list of proteins that might regulate learning and memory. New work hints that Cdk5--a protein with numerous other functions in the brain--also helps a class of glutamate receptors known as
N
-methyl-D-aspartate (NMDA) receptors function properly as they facilitate learning. Li and colleagues report that Cdk5, a member of the cyclin-dependent kinase family of proteins (which have achieved fame for coordinating the cell cycle), is located near NMDA receptors at synapses. With fluorescent labels that distinguish Cdk5 and the NMDA receptor, the team visualized these proteins in cultured neurons and found that the patterns overlapped. This result suggests that Cdk5 and the NMDA receptor physically associate with each other. To test this idea, the researchers used antibodies to grab Cdk5 from brain extracts. This procedure captured the receptor in addition to Cdk5, indicating that the two bind to each other. The researchers next applied a chemical inhibitor of Cdk5 to slices of hippocampus, the memory center of the brain. This treatment decreases the flow of electrical current through the receptor and blocks long-term potentiation: the persistent strengthening of neuronal signals that many believe is the initial step in making new memories. Other results hint at a possible mechanism: In vitro, Cdk5 can chemically modify the long tail of an NMDA receptor subunit. Although this work is preliminary, it provides an impetus to figure out whether Cdk5 chemically alters NMDA receptors in vivo and whether the modifications alter receptor function.

--R. John Davenport

(At deadline time, full hyperlink and reference information was not yet available.)

B.-S. Li, H. C. Pant, M.-K. Sun, L. Zhang, S. Takahashi, A. B. Kulkarni, W. Ma, L. Vinade, R. O. Brady, Regulation of NMDA receptors by cyclin-dependent kinase-5.
PNAS Early Edition 2
(issue no. 40) (2001).
[Full Text]
Detangling Alzheimer’s Disease
Authors: Helmuth Laura
Published: August 2003

Alzheimer’s disease (AD) afflicts 4 million people in the United States and is expected to strike 14 million by the year 2050, as the population ages. Researchers are scrambling to find genetic risk factors, decipher disease mechanisms, and develop reliable diagnostic tests that detect the illness at its earliest, potentially most treatable stage. Using these findings, they hope to devise new therapeutic approaches. Current clinical trials are testing novel techniques that stall or reverse AD-like neuropathology in mice.
The Hunt for a Cure for Parkinson’s Disease
Authors: Andersen Julie K. , Kumar Jyothi , Srinivas Bharath , Kaur Deepinder , Hsu Michael , Rajagopalan Subramanian
Published: August 2003

Several exciting new scientific advances have been made in the past decade toward both understanding the causes of and finding a cure for Parkinson’s disease. Heartened by an acceleration in research findings in the past several years, the government has recently called for an infusion of funds from both the National Institutes of Health and private foundations into this burgeoning area of biomedical research. Most currently available conventional treatments for the disease only temporarily delay symptom presentation while doing nothing to halt disease progression. However, the rapidly accelerating pace of research in this field has left researchers hopeful that Parkinson’s will be the first major age-related neurodegenerative disease for which we have a viable cure. In this article, advances in various areas of Parkinson’s disease research are reviewed.
Mining for Genes: Comparative sequence analysis unearths hidden gem
Authors: Aschwanden Christie
Published: August 2003

With the Human Genome Project nearing completion, finding the genes involved in aging-related disorders and other unwelcome afflictions is closer than ever. But obtaining the sequence is only the first step; locating genes among the sea of 3 billion nucleotides still poses a formidable challenge.

Pennacchio and colleagues now illustrate the power of comparative sequence analysis to pinpoint genes, even in well-studied chromosomal regions. Their find: a gene that influences risk for a common malady of old age, namely coronary artery disease. The method hinges on the fact that natural selection maintains sequences that perform important functions. As a result, genes--as opposed to the stretches of DNA that lie between them--tend to be conserved among species. With this principle in mind, the researchers staked out a region of chromosome 11 that carries an apolipoprotein gene cluster; they sought sequences shared by humans and mice, hoping to find spans of DNA that regulate the expression of these previously identified genes. Instead, their comparison turned up an entirely new member of the group, a gene they call
APOAV
. Like other apolipoproteins, APOAV affects the concentrations of lipids in the bloodstream. Blood plasma triglyceride concentrations in mice engineered to overproduce
Apoav
are a third as great as those in wild-type animals; conversely, triglyceride concentrations in mice that lack the gene are four times as high. Results from additional experiments suggest that certain versions of
APOAV
correlate strongly with elevated triglyceride levels in humans.

The discovery could reach beyond fighting heart disease: Elevated triglyceride levels are also a risk factor for strokes, so therapies that target
APOAV
or the protein it encodes could potentially reduce both strokes and the vascular dementia that commonly accompanies them.

--Christie Aschwanden

L. A. Pennacchio, M. Olivier, J. A. Hubacek, J. C. Cohen, D. R. Cox, J.-C. Fruchart, R. M. Krauss, E. M. Rubin, An apolipoprotein influencing triglycerides in humans and mice revealed by comparative sequencing.
Science
294
, 169-173 (2001).
[Abstract]
[Full Text]