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At least 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Helical Tomotherapy Delivery of an IMRT Boost in Lieu of Interstitial Brachytherapy in the Setting of Gynecologic Malignancy: Feasibility and Dosimetric Comparison

Interstitial brachytherapy is an important means by which to improve local control in gynecologic malignancy when intracavitary brachytherapy is untenable. Patients unable to receive brachytherapy have traditionally received conventional external beam radiation alone with modest results. We investigated the ability of Tomotherapy (Tomotherapy Inc., Madison, WI) to replace interstitial brachytherapy. Six patients were selected. The planning CT of each patient was contoured with the planning target volume (PTV), bladder, rectum, femoral heads, and bowel. Identical contour sets were exported to Tomotherapy and Nucletron PLATO (Nucletron B.V., Veenendaal, The Netherlands). With Tomotherapy, the PTV was prescribed 31 Gy in 5 fractions to 90% of the volume. With PLATO, 600 cGy x 5 fractions was prescribed to the surface of the PTV. Dose delivered was normalized to 2 Gy fractions (EQD2) and added to a hypothetical homogenous 45-Gy pelvic dose. Tomotherapy achieved a D90 of 87 Gy EQD2 versus 86 Gy with brachytherapy. PTV dose was more homogeneous with tomotherapy. The dose to the most at-risk 2 mL of bladder and rectum with Tomotherapy was of 78 and 71 Gy EQD2 versus 81 and 75 Gy with brachytherapy. Tomotherapy delivered more dose to the femoral heads (mean 1.23 Gy per fraction) and bowel. Tomotherapy was capable of replicating the peripheral dose achieved with brachytherapy, without the PTV hotspots inherent to interstitial brachytherapy. Similar maximum doses to bowel and bladder were achieved with both methods. Excessive small bowel and femoral head toxicity may result if previous pelvic irradiation is not planned accordingly. Significant challenges related to interfraction and intrafraction motion must be overcome if treatment of this nature is to be contemplated.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2010

Confocal Microscopy on Light-emitting Nanostructures and X-ray Imaging Detectors Based on Color Centers in Lithium Fluoride

Confocal Laser Scanning Microscope (CLSM) is a versatile and powerful optical instrument which is gaining a strong increase of interest for biological investigations and also for the characterization of materials, microstructures and devices. We exploit its capability for the characterization of light-emitting micro and nano-structures based on color centers in lithium fluoride. CLSM was successfully used as an advanced optical reading system to detect X-ray micro-radiographies of biological specimens stored in LiF imaging detectors.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2010

Aging of magnetic properties in MgO films

In this work we report on the magnetic behavior of MgO thin films prepared by sputtering. A severe aging process of the ferromagnetic properties is detected in magnetic samples exposed to ambient atmosphere. However, ferromagnetism can be successively switched on again by annealing samples in vacuum. We suggest this behavior reflects the key role played by defects in stabilizing ferromagnetism in MgO films and is likely to be closely related to the hydrogen-driven instability of V-type centers in this material.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Characterization of Vacancy Defects in Carbon Ion Irradiated Graphite Using Positrons

Highly Oriented Pyrolytic Graphite samples are irradiated with 200 keV Carbon ions to fluences of 10{sup 14} and 10{sup 15} C{sup +} ions/cm{sup 2}. Depth resolved Doppler lineshape S-parameter exhibited large increase in peak damage regions of the sample, indicating the existence of irradiation induced vacancy defects. The depth profile of the defect region has been deduced from the analysis of the experimental data. It is found that divacancies are the dominant defects in the irradiated samples.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Impact of hollow-atom formation on coherent x-ray scattering at high intensity

X-ray free-electron lasers (FELs) are promising tools for structural determination of macromolecules via coherent x-ray scattering. During ultrashort and ultraintense x-ray pulses with an atomic-scale wavelength, samples are subject to radiation damage and possibly become highly ionized, which may influence the quality of x-ray scattering patterns. We develop a toolkit to treat detailed ionization, relaxation, and scattering dynamics for an atom within a consistent theoretical framework. The coherent x-ray scattering problem including radiation damage is investigated as a function of x-ray FEL parameters such as pulse length, fluence, and photon energy. We find that the x-ray scattering intensity saturates at a fluence of {approx}10{sup 7} photon/A{sup 2} per pulse but can be maximized by using a pulse duration much shorter than the time scales involved in the relaxation of the inner-shell vacancy states created. Under these conditions, both inner-shell electrons in a carbon atom are removed, and the resulting hollow atom gives rise to a scattering pattern with little loss of quality for a spatial resolution >1 A. Our numerical results predict that in order to scatter from a carbon atom 0.1 photon per x-ray pulse, within a spatial resolution of 1.7 A, a fluence of 1x10{sup 7} photons/A{sup 2} per pulse is required at a pulse length of 1 fs and a photon energy of 12 keV. By using a pulse length of a few hundred attoseconds, one can suppress even secondary ionization processes in extended systems. The present results suggest that high-brightness attosecond x-ray FELs would be ideal for single-shot imaging of individual macromolecules.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Reply to 'Comment on 'Increase in specific heat and possible hindered rotation of interstitial C{sub 2} molecules in neutron-irradiated graphite''

In a recent paper [T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 (2010)], we have interpreted the irradiation-induced increase in the low-temperature specific heat of neutron-irradiated graphite as due to the hindered rotation of interstitial C{sub 2} molecules with a rotational frequency of 1.39x10{sup 12} s{sup -1} in the periodic potential with a height of 0.040 eV, and concluded that the C{sub 2} molecules do not form covalent bonds with atoms in the adjacent graphite layers. In their Comment [preceding paper, C. D. Latham et al., Phys. Rev. B 82, 056101 (2010)] on this paper, based on the recent first-principles theoretical calculations and related experiments, Latham et al. assert that self-interstitial atoms cannot exist as clusters of nearly free C{sub 2} molecules. In this Reply, based on the experiments, we address the issues raised by Latham et al. in their Comment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

First-Principles Simulations of Chemical Reactions in an HCl Molecule Embedded inside a C or BN Nanotube Induced by Ultrafast Laser Pulses

We show by first-principles simulations that ultrafast laser pulses induce different chemical reactions in a molecule trapped inside a nanotube. A strong laser pulse polarized perpendicular to the tube axis induces a giant bond stretch of an encapsulated HCl molecule in semiconducting carbon nanotube or in a BN nanotube. Depending on the initial orientation of the HCl molecule, the subsequent laser-induced dynamics is different: either complete disintegration or rebonding of the HCl molecule. Radial motion of the nanotube is always observed and a vacancy appears on the tube wall when the HCl is perpendicular to the tube axis. Those results are important to analyze confined nanochemistry and to manipulate molecules and nanostructures encapsulated in organic and inorganic nanotubes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Simulation of radiation-defect formation processes in heterostructures with self-assembled Ge(Si)/Si(001) nanoislands under neutron irradiation

TRISQD software is developed for the computer simulation of processes in which radiation defects are formed under the corpuscular irradiation of semiconductor heterostructures with lenticular nanoinclusions of various shapes. The computer program is used to study defect-formation processes in p-i-n diodes with the i region having a built-in 20-period lattice of self-assembled Ge(Si) nanoislands formed under irradiation with high-energy neutrons. It is found that the fraction of Ge(Si) nanoislands in which point radiation defects are formed under the impact of atomic-displacement cascades is ≤3% of their total number in the lattice. More than 94% of the defects are localized in the bulk of the p, n, and i regions of the diode and in silicon layers that separate sheets of Ge(Si) nanoislands.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Kinetic lattice Monte Carlo model for oxygen vacancy diffusion in praseodymium doped ceria: Applications to materials design

Kinetic lattice Monte Carlo (KLMC) model is developed for investigating oxygen vacancy diffusion in praseodymium-doped ceria. The current approach uses a database of activation energies for oxygen vacancy migration, calculated using first-principles, for various migration pathways in praseodymium-doped ceria. Since the first-principles calculations revealed significant vacancy-vacancy repulsion, we investigate the importance of that effect by conducting simulations with and without a repulsive interaction. Initially, as dopant concentrations increase, vacancy concentration and thus conductivity increases. However, at higher concentrations, vacancies interfere and repel one another, and dopants trap vacancies, creating a 'traffic jam' that decreases conductivity, which is consistent with the experimental findings. The modeled effective activation energy for vacancy migration slightly increased with increasing dopant concentration in qualitative agreement with the experiment. The current methodology comprising a blend of first-principle calculations and KLMC model provides a very powerful fundamental tool for predicting the optimal dopant concentration in ceria related materials. -- graphical abstract: Ionic conductivity in praseodymium doped ceria as a function of dopant concentration calculated using the kinetic lattice Monte Carlo vacancy-repelling model, which predicts the optimal composition for achieving maximum conductivity. Display Omitted Research highlights: {yields} KLMC method calculates the accurate time-dependent diffusion of oxygen vacancies. {yields} KLMC-VR model predicts a dopant concentration of {approx}15-20% to be optimal in PDC. {yields} At higher dopant concentration, vacancies interfere and repel one another, and dopants trap vacancies. {yields} Activation energy for vacancy migration increases as a function of dopant content

36 MATERIALS SCIENCE↗
DOE OSTI2010

Role of oxygen atoms in the growth of magnetron sputter-deposited ZnO films

The role of oxygen atoms in the growth of magnetron sputter-deposited ZnO films was studied by alternating the deposition of a several-nanometer-thick ZnO layer and the O{sub 2}/Ar mixture plasma exposure, i.e., layer-by-layer technique. The film crystallization promoted with suppressing the oxygen vacancy and interstitial defects by adjusting the exposure condition of O{sub 2}/Ar plasma. These findings suggest that the chemical potential of oxygen atom determine the film crystallization as well as the electronic state. The diffusion and effusion of oxygen atoms at the growing surface play a role of thermal annealing, promoted the film crystallization as well as the creation and the annihilation of oxygen and zinc related defects. The role of oxygen atoms reaching at the film-growing surface is discussed in term of chemical annealing. The possible oxygen diffusion mechanism is proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Diluted manganese on the bond-centered site in germanium

The functional properties of Mn-doped Ge depend to large extent on the lattice location of the Mn impurities. Here, we present a lattice location study of implanted diluted Mn by means of electron emission channeling. Surprisingly, in addition to the expected substitutional lattice position, a large fraction of the Mn impurities occupies the bond-centered site. Corroborated by ab initio calculations, the bond-centered Mn is related to Mn-vacancy complexes. These unexpected results call for a reassessment of the theoretical studies on the electrical and magnetic behavior of Mn-doped Ge, hereby including the possible role of Mn-vacancy complexes.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Nanofabricated solid immersion lenses registered to single emitters in diamond

We describe a technique for fabricating micro- and nanostructures incorporating fluorescent defects in diamond with a positional accuracy better than hundreds of nanometers. Using confocal fluorescence microscopy and focused ion beam etching, we initially locate a suitable defect with respect to registration marks on the diamond surface then etch a structure using these coordinates. We demonstrate the technique by etching an 8 {mu}m diameter hemisphere positioned with single negatively charged nitrogen-vacancy defect lies at its origin. Direct comparison of the fluorescence photon count rate before and after fabrication shows an eightfold increase due to the presence of the hemisphere.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Dynamics of Water Confined in Synthetic Saponite Clays

The dynamics of water in two synthetic saponite clays (treated by microwave) with different interlayer spacing has been studied using the quasielastic neutron scattering technique. A model in which the molecules undergo random jump diffusion describes the dynamics of water in both clays. The dynamics of water confined in both saponite clays considered here is found to be hindered in comparison to the bulk phase. It is found that diffusivity increases for samples having higher interlayer spacing.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Influence of Ge content on the optical properties of X and W centers in dilute Si-Ge alloys

Photoluminescence (PL) measurements, performed in Si and Si{sub 1-x}Ge{sub x} alloys (x= 0.0069 and 0.0125) irradiated with protons and annealed between 100 and 650 deg. C, are combined with first-principles calculations to assess the assignment of the W and X lines to the trigonal and tetragonal forms of the tri- and tetra-interstitial defects, denoted I{sub 3} and I{sub 4}, respectively. It is found that for the W line the annealing temperature that maximizes the PL intensity is independent of x, whereas for the X line a {approx}50 deg. C shift to higher temperatures was observed in the alloys when compared to pure Si. Analysis of the PL quenching mechanisms along with the calculations show that the radiative excited state for both defects comprises a pseudodonor state, where a trapped exciton combines a tightly bound hole and a diffuse electron. The broadening rate as a function of x for the X line is greater than that of the W line by about a factor of two. This is consistent with a stronger electronic localization in the X center than in the W center, as well as with the relative positions of the donor levels as obtained from the calculations for the I{sub 3} and I{sub 4} defects, respectively. The calculated change rates of donor levels with x are in qualitative agreement with the hole binding energy shifts obtained from the experiments. Our results support the assignment of I{sub 3} and I{sub 4} to the W and X centers, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Nitrogen vacancy and ferromagnetism in Cr-doped GaN: First-principles calculations

Nitrogen defects and their effect on the ferromagnetism (FM) in Cr-doped GaN have been systematically investigated by first-principles. Four considered configurations including one N vacancy (V{sub N}), single substitutional Cr, double substitutional Cr, and complex of Cr-V{sub N} are all ferromagnetic. The lowest energy arrangements for double Cr-doped (or Cr-V{sub N}) GaN occur at the nearest Cr-Cr (or Cr-V{sub N}) distance. One Cr contributes 84.3% of the total magnetic moment (2.533 {mu}{sub B}), while one Cr-Cr pair leads to 5.998 {mu}{sub B} moment (more than twice moment of one Cr) by the strong d-d spin coupling, which is mediated by two Cr 3d states antiferromagnetically coupling with the 'commonly shared' N 2p states, and driven by a double exchange mechanism. The V{sub N} can enhance the FM by adding about 1 {mu}{sub B} to the Cr moment but reduce the FM spin exchange interaction between the nearest Cr-Cr pairs, so experimentally, high-performance samples may be synthesized by controlling N pressure. - Graphical abstract: Nitrogen defects and their effect on the ferromagnetism in Cr-doped GaN were investigated by first-principles. Given are the PDOS of Ga{sub 36}N{sub 36} with one Cr-Cr pair and one complex of Cr-V{sub N}. Display Omitted

36 MATERIALS SCIENCE↗
DOE OSTI2010

Origin of the improved photo-catalytic activity of F-doped ZnWO{sub 4}: A quantum mechanical study

Two different mechanisms for improving photo-catalytic activity in different types of F-doped ZnWO{sub 4} are tentatively proposed, based on density function theory calculations. When the lattice O atom is substituted by one F atom, our calculations show that a reduced W{sup 5+} center adjacent to the doped F atom will act as a trap for the photo-induced electron, and will thus result in a reduction of electron-hole recombination and improvement of the photo-catalytic activity. For the interstitial F-doped model, partial F 2p states mixing with O 2p states localize above the top of the valence band and act as the frontier orbital level. Electronic transitions from these localized states induce a red shift of about 54 nm of the optical absorption edge. This work shows that F-doped ZnWO{sub 4} will be a promising photo-catalyst with favorable photo-catalytic activity in the UV region. -- Graphical Abstract: DFT calculations are used to investigate the origin of the improved photo-activity of monoclinic ZnWO{sub 4} induced by the substituted and interstitial F-doping. Two possible mechanisms are tentatively put forward according to the F-doping types. Display Omitted

36 MATERIALS SCIENCE↗
DOE OSTI2010

Reinforcing multiwall carbon nanotubes by electron beam irradiation

We study the effect of electron beam irradiation on the bending modulus of multiwall carbon nanotubes grown by chemical vapor deposition. Atomic force microscopy observations of the nanotube deflection in the suspended-beam geometry suggest an internal, reversible stick-slip motion prior to irradiation, indicating presence of extended defects. Upon electron beam irradiation, nanotubes with an initial bending modulus exceeding 10 GPa initially get stiffer, before softening at high doses. Highly defective nanotubes with smaller initial bending moduli do not exhibit the initial reinforcement. These data are explained by ab initio molecular dynamics calculations suggesting a spontaneous cross-linking of neighboring nanotube walls at extended vacancy defects created by the electron beam, in agreement with electron microscopy observations. At low defect concentration, depending on the edge morphology, the covalent bonds between neighboring nanotube walls cause reinforcement by resisting relative motion of neighboring walls. At high concentration of defects that are present initially or induced by high electron beam dose, the structural integrity of the entire system suffers from increasing electron beam damage.

36 MATERIALS SCIENCE↗
DOE OSTI2011

The synthesis and complex anion-vacancy ordered structure of La{sub 0.33}Sr{sub 0.67}MnO{sub 2.42}

The low-temperature topotactic reduction of La{sub 0.33}Sr{sub 0.67}MnO{sub 3} with NaH results in the formation of La{sub 0.33}Sr{sub 0.67}MnO{sub 2.42}. A combination of neutron powder and electron diffraction data show that La{sub 0.33}Sr{sub 0.67}MnO{sub 2.42} adopts a novel anion-vacancy ordered structure with a 6-layer OOTOOT' stacking sequence of the 'octahedral' and tetrahedral layers (Pcmb, a=5.5804(1) A, b=23.4104(7) A, c=11.2441(3) A). A significant concentration of anion vacancies at the anion site, which links neighbouring 'octahedral' layers means that only 25% of the 'octahedral' manganese coordination sites actually have 6-fold MnO{sub 6} coordination, the remainder being MnO{sub 5} square-based pyramidal sites. The chains of cooperatively twisted apex-linked MnO{sub 4} tetrahedra adopt an ordered -L-R-L-R- arrangement within each tetrahedral layer. This is the first published example of a fully refined structure of this type which exhibits such intralayer ordering of the twisted tetrahedral chains. The rationale behind the contrasting structures of La{sub 0.33}Sr{sub 0.67}MnO{sub 2.42} and other previously reported reduced La{sub 1-x}Sr{sub x}MnO{sub 3-y} phases is discussed. - Graphical Abstract: The topotactic reduction of the perovskite phase La{sub 0.33}Sr{sub 0.67}MnO{sub 3} with NaH yields La{sub 0.33}Sr{sub 0.67}MnO{sub 2.42(3)}, which adopts a novel anion vacancy ordered structure with a 6-layer OOTOOT' stacking sequence of the 'octahedral' and tetrahedral layers. The anion site that links the neighbouring octahedral layers is partially occupied so only 25% of the 'octahedral' manganese sites actually have 6-fold MnO{sub 6} coordination. Highlights: > Topotactic reduction with NaH to form a mixed valent Mn{sup 2+}/Mn{sup 3+} phase. > Novel layered anion-vacancy ordered structure. > Comparison to related brownmillerite structure types.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗