Collisional quenching of metastable x-ray emitting states in a fast beam of He-like fluorine

High resolution x-ray spectral measurements are used to determine the relative intensity of He-like fluorine x-ray transitions, 2 $sup 3$P$sub 1$ $Yields$ 1 $sup 1$S$sub 0$ and $sup 1$P$sub 1$ $Yields$ $sup 1$S$sub 0$, produced in fluorine collisions with Ne, Ar, and Kr gas targets at various pressures and in thin carbon foils and a thick carbon slab. The relative intensities are observed to be strong functions of both target density and incident charge Z. These effects are attributed to strong collisional quenching of both initial states by subsequent large impact parameter collisions. The data permit extraction for the first time of the total quenching cross sections (sigma/sub Q/ ) for fast fluorine ions in both states. A strong enhancement of the relative intensity of the 2 $sup 3$P$sub 1$ is observed for F$sup 7+$ projectiles. This strong enhancement is attributed to selective excitation of metastable states in the beam, i.e., 1s2s $sup 3$S$sub 1$, into the 1s2p $sup 3$P$sub 1$ state. Finally, the data for foil and solid targets are used to obtain new information on the excitation states of ions moving in solids. High resolution measurements of the radiative electron capture (REC) peak are reported and analyzed for the first time. 3 figs, 1 table (auth

A Hydrostratigraphic Framework for the Paleozoic Bedrock of Southern Ontario

Groundwater systems in the intermediate to deep subsurface of southern Ontario are poorly understood, despite their value for a number of societal uses. A regional hydrostratigraphic framework is a necessary precursor for improving our understanding of groundwater systems and enabling development of a 3-D hydrostratigraphic model to visualize these groundwater systems. This study is a compilation and integration of published and unpublished geological, hydrogeological, hydrochemical and isotopic data collected over the past 10 years to develop that framework.Bedrock is covered by a thin veneer of surficial sediments that comprise an aquifer/aquitard system of considerable local variability and complexity. Aquifers in the bedrock are thin and regionally extensive, separated by thick aquitards, within a well-defined lithostratigraphic framework and a well-developed hydrochemical depth zonation comprising a shallow fresh water regime, an intermediate brackish to saline sulphur water regime, and a deep brine regime of ancient, evaporated seawater. Occurrence and movement of groundwater in shallow bedrock is principally controlled by modern (Quaternary) karstic dissolution of subcropping carbonate and evaporite rocks, and in the intermediate to deep subsurface by paleokarst horizons developed during the Paleozoic. Flow directions in the surficial sediments of the shallow groundwater regime are down-gradient from topographic highs and down the regional dip of bedrock formations in the intermediate regime. Shallow karst is the entry point for groundwater penetration into the intermediate regime, with paleo-recharge by glacial meltwater and limited recent recharge by meteoric water at subcrop edges, and down-dip hydraulic gradients in confined aquifers. Hydraulic gradient is up-dip in the deep brine regime, at least for the Guelph Aquifer and the Cambrian Aquifer, with no isotopic or hydrochemical evidence of infiltration of meteoric water and no discharge to the surface.Fourteen bedrock hydrostratigraphic units are proposed, and one unit comprising all the surficial sediments. Assignment of lithostratigraphic units as hydrostratigraphic units is based principally on hydrogeological characteristics of Paleozoic bedrock formations in the intermediate to deep groundwater regimes, below the influence of modern meteoric water. Carbonate and evaporite rocks which form aquitards in the subsurface may form aquifers at or near the surface, due to karstic dissolution by acidic meteoric water, necessitating compromises in assignment of hydrostratigraphic units.Les systèmes d'eaux souterraines du sous-sol intermédiaire à profond du sud de l'Ontario sont mal compris, malgré leur valeur pour de nombreux usages par la société. Un cadre hydrostratigraphique régional est un préalable nécessaire à l’amélioration de notre compréhension des systèmes d'eaux souterraines et au développement d'un modèle hydrostratigraphique 3D pour visualiser ces systèmes d'eaux souterraines. Cette étude est une compilation et une intégration de données géologiques, hydrogéologiques, hydrochimiques et isotopiques publiées et non publiées recueillies au cours des 10 dernières années afin de développer ce cadre.Le substrat rocheux est recouvert d'un mince placage de sédiments de surface qui comprend un système d’aquifères et d’aquitards d'une variabilité et d'une complexité locales considérables. Les aquifères du substrat rocheux sont minces et étendus au niveau régional, séparés par des aquitards épais, dans un cadre lithostratigraphique bien défini et une zonation hydrochimique verticale bien développée comprenant un régime peu profond d'eau douce, un régime intermédiaire d'eau sulfureuse saumâtre à saline et un régime profond de saumure résultant de l’évaporation d'eau de mer ancienne. La présence et le mouvement des eaux souterraines dans le substrat rocheux peu profond sont principalement contrôlés par la dissolution karstique moderne (quaternaire) des roches carbonatées et évaporitiques sub-affleurantes, et dans le sous-sol intermédiaire à profond par les horizons paléokarstiques développés au Paléozoïque. Les directions d'écoulement des eaux dans les sédiments de surface du régime peu profond sont en aval des sommets topographiques et en aval du pendage régional des formations de substrat rocheux dans le régime intermédiaire. Le karst peu profond est le point d'entrée pour l’infiltration des eaux souterraines dans le régime intermédiaire, avec une paléo-recharge d'eau de fonte glaciaire et une recharge récente limitée d'eau météorique aux bords de sous-affleurement, et un gradient hydraulique en aval-pendage dans les aquifères confinés. Le gradient hydraulique est en amont-pendage dans le régime profond de saumure, au moins pour l'aquifère de Guelph et l'aquifère du Cambrien, sans indication isotopique ou hydrochimique d'infiltration d'eau météorique et sans déversement à la surface.Quatorze unités hydrostratigraphiques du substrat rocheux sont proposées, et une unité comprenant tous les sédiments de surface. L'attribution des unités lithostratigraphiques en tant qu’unités hydrostratigraphiques repose principalement sur les caractéristiques hydrogéologiques des formations du substrat rocheux du Paléozoïque dans les régimes intermédiaires à profonds des eaux souterraines, sous l'influence des eaux météoriques modernes. Les roches carbonatées et évaporitiques qui forment les aquitards dans le sous-sol peuvent former des aquifères à la surface ou près de la surface, en raison de la dissolution karstique par l'eau météorique acide, ce qui nécessite des compromis dans l'attribution des unités hydrostratigraphiques

Measurement of x-rays emitted from projectiles moving in solid targets

The results of three separate experiments all dealing with the production of x-rays in projectiles moving in solids are discussed. The first experiment deals with the measurement of line widths of x-rays emitted from projectiles moving in solid targets. The effect of collisional broadening of x- rays is found to dominate the line widths giving greater than an order of magnitude increase in the measured line widths. The second experiment studies ''solid target effects'' in producing non-binomial distributions of characteristic K x-ray spectra in heavy ion-atom collisions. The third experiment studies aluminum K x-ray production in Ar$sup +$ $Yields$ Al collisions in very thin aluminum foils as a function of foil thickness. Parameterization of the observed non-linear dependence enables the lifetime of the argon 2p vacancy and total ionization cross sections for the argon L-shell in Ar $Yields$ Al collisions to be measured. (auth

Formation of Organic Color Centers in Air-Suspended Carbon Nanotubes Using Vapor-Phase Reaction

Organic color centers in single-walled carbon nanotubes have demonstrated exceptional ability to generate single photons at room temperature in the telecom range. Combining the color centers with pristine air-suspended tubes would be desirable for improved performance, but all current synthetic methods occur in solution which makes them incompatible. Here we demonstrate formation of color centers in air-suspended nanotubes using vapor-phase reaction. Functionalization is directly verified on the same nanotubes by photoluminescence spectroscopy, with unambiguous statistics from more than a few thousand individual nanotubes. The color centers show a strong diameter-dependent emission intensity, which can be explained with a theoretical model for chemical reactivity taking into account strain along the tube curvature. We are also able to estimate the defect density by comparing the experiments with simulations based on a one-dimensional diffusion equation, whereas the analysis of diameter dependent peak energies gives insight to the nature of the dopant states. Time-resolved measurements show a longer lifetime for color center emission compared to E$_{11}$ exciton states. Our results highlight the influence of the tube structure on vapor-phase reactivity and emission properties, providing guidelines for development of high-performance near-infrared quantum light sources.Comment: 8 pages, 6 figure

Reverse Monte Carlo modeling of amorphous silicon

An implementation of the Reverse Monte Carlo algorithm is presented for the study of amorphous tetrahedral semiconductors. By taking into account a number of constraints that describe the tetrahedral bonding geometry along with the radial distribution function, we construct a model of amorphous silicon using the reverse monte carlo technique. Starting from a completely random configuration, we generate a model of amorphous silicon containing 500 atoms closely reproducing the experimental static structure factor and bond angle distribution and in improved agreement with electronic properties. Comparison is made to existing Reverse Monte Carlo models, and the importance of suitable constraints beside experimental data is stressed.Comment: 6 pages, 4 PostScript figure

The Structure, Dynamics and Electronic Structure of Liquid Ag-Se Alloys Investigated by Ab Initio Simulation

Ab initio molecular-dynamics simulations have been used to investigate the structure, dynamics and electronic properties of the liquid alloy Ag(1-x)Se(x) at 1350 K and at the three compositions x=0.33, 0.42 and 0.65. The calculations are based on density-functional theory in the local density approximation and on the pseudopotential plane-wave method. The reliability of the simulations is confirmed by detailed comparisons with very recent neutron diffraction results for the partial structure factors and radial distribution functions (RDF) of the stoichiometric liquid Ag2Se. The simulations show a dramatic change of the Se-Se RDF with increasing Se content. This change is due to the formation of Se clusters bound by covalent bonds, the Se-Se bond length being almost the same as in pure c-Se and l-Se. The clusters are predominantly chain-like, but for higher x a large fraction of 3-fold coordinated Se atoms is also found. It is shown that the equilibrium fractions of Se present as isolated atoms and in clusters can be understood on a simple charge-balance model based on an ionic interpretation. The Ag and Se diffusion coefficients both increase with Se content, in spite of the Se clustering. An analysis of the Se-Se bond dynamics reveals surprisingly short bond lifetimes of less than 1 ps. The changes in the density of states with composition arise directly from the formation of Se-Se covalent bonds. Results for the electronic conductivity obtained using the Kubo-Greenwood approximation are in adequate agreement with experiment for l-Ag2Se, but not for the high Se contents. Possible reasons for this are discussed.Comment: 14 pages, Revtex, 14 Postscript figures embedded in the tex

Experiencing neutropenia: Quality of life interviews with adult cancer patients

BACKGROUND: Neutropenia is a common toxicity in chemotherapy but detailed information about how neutropenia is associated with changes in patients' quality of life is not readily available. This prospective study interviewed patients with grade 4 neutropenia to provide qualitative information on patients' experience of developing and coping with grade 4 neutropenia during a cycle of chemotherapy. METHODS: A sample of 34 patients who developed grade 4 neutropenia during the first cycle of chemotherapy completed a total of 100 structured clinical interviews. Interviews were transcribed, and 2 raters inductively developed 5 broad categories comprising 80 specific complaint domains nominated by patients. Thirty-five patient-nominated problems were mentioned in 5% or more of the interviews. RESULTS: Fatigue was the most common physical symptom. Interference in daily routine, negative self-evaluation, negative emotion, and social isolation were other common complaints associated with neutropenia. CONCLUSION: Neutropenia is associated with a number of negative experiences among cancer patients undergoing chemotherapy, and these negative experiences have an adverse effect on the patient's quality of life. Oncology nurses can play a key role in helping patients manage adverse effects to maintain their quality of life

Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications

Elemental compositions of organic aerosol (OA) particles provide useful constraints on OA sources, chemical evolution, and effects. The Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is widely used to measure OA elemental composition. This study evaluates AMS measurements of atomic oxygen-to-carbon (O : C), hydrogen-to-carbon (H : C), and organic mass-to-organic carbon (OM : OC) ratios, and of carbon oxidation state ([bar over OS][subscript C]) for a vastly expanded laboratory data set of multifunctional oxidized OA standards. For the expanded standard data set, the method introduced by Aiken et al. (2008), which uses experimentally measured ion intensities at all ions to determine elemental ratios (referred to here as "Aiken-Explicit"), reproduces known O : C and H : C ratio values within 20% (average absolute value of relative errors) and 12%, respectively. The more commonly used method, which uses empirically estimated H[subscript 2]O[superscript +] and CO[superscript +] ion intensities to avoid gas phase air interferences at these ions (referred to here as "Aiken-Ambient"), reproduces O : C and H : C of multifunctional oxidized species within 28 and 14% of known values. The values from the latter method are systematically biased low, however, with larger biases observed for alcohols and simple diacids. A detailed examination of the H[subscript 2]O[superscript +], CO[superscript +], and CO[subscript 2][superscript +] fragments in the high-resolution mass spectra of the standard compounds indicates that the Aiken-Ambient method underestimates the CO[superscript +] and especially H[subscript 2]O[superscript +] produced from many oxidized species. Combined AMS–vacuum ultraviolet (VUV) ionization measurements indicate that these ions are produced by dehydration and decarboxylation on the AMS vaporizer (usually operated at 600 °C). Thermal decomposition is observed to be efficient at vaporizer temperatures down to 200 °C. These results are used together to develop an "Improved-Ambient" elemental analysis method for AMS spectra measured in air. The Improved-Ambient method uses specific ion fragments as markers to correct for molecular functionality-dependent systematic biases and reproduces known O : C (H : C) ratios of individual oxidized standards within 28% (13%) of the known molecular values. The error in Improved-Ambient O : C (H : C) values is smaller for theoretical standard mixtures of the oxidized organic standards, which are more representative of the complex mix of species present in ambient OA. For ambient OA, the Improved-Ambient method produces O : C (H : C) values that are 27% (11%) larger than previously published Aiken-Ambient values; a corresponding increase of 9% is observed for OM : OC values. These results imply that ambient OA has a higher relative oxygen content than previously estimated. The [bar over OS][subscript C] values calculated for ambient OA by the two methods agree well, however (average relative difference of 0.06 [bar over OS][subscript C] units). This indicates that [bar over OS][subscript C] is a more robust metric of oxidation than O : C, likely since [bar over OS][subscript C] is not affected by hydration or dehydration, either in the atmosphere or during analysis.National Science Foundation (U.S.) (CHE-1012809)National Science Foundation (U.S.) (ATM-1238109