Particle Cross Research Articles

The optical efficiency of flocs in shelf seas and estuaries

A correct understanding of the way in which light interacts with suspended particles is essential for quantitative interpretation of satellite visible band imagery of turbid shelf seas and estuaries. In this paper we describe new optical observations at 90 stations in the tidally energetic waters along the south and west coasts of Britain. The cross sectional area of the particles in suspension has been measured with a LISST laser diffraction instrument. Light scattering and absorption coefficients have been determined by applying Kirk’s method to radiometric measurements at 6 wavelengths. Results show that the scattering coefficient increases linearly with particle cross sectional area A per unit volume of water with a slope (scattering efficiency) of 1.96 (standard error 0.08) at 665 nm. Particle absorption coefficients a P also increase with particle cross sectional area but at the most turbid stations, particle absorption per unit area ( a P / A) is observed to increase with the mean size of the particles in suspension. The particles are mostly mineral flocs which become more opaque as they grow larger and the photon path length through them increases. The implication of these results for remote sensing is that reflectance in the red part of the spectrum, which mainly depends on light scattering, is proportional to the cross sectional area of particles in suspension. Reflectance measurements in the green and blue parts of the spectrum, where particle absorption becomes more important, depend on the diameter of the particles as well as their cross sectional area. We show that simultaneous measurements of reflectance in the red and green parts of the spectrum can be used to derive both the area and size of the particles in suspension.

Threshold expansion of massive coloured particle cross sections

Pair production of massive coloured particles in hadron collisions is accompanied by potentially large radiative corrections related to the suppression of soft gluon emission and enhanced Coulomb exchange near the production threshold. We recently developed a framework to sum both series of corrections for the partonic cross section using soft-collinear and non-relativistic effective theory. If it can be argued that the resummed cross section approximates the complete result over a significant kinematic range, an improvement of the hadronic cross section results, even when the production is not kinematically constrained to the threshold. This is discussed here for the case of top quark production.

Measuring DHEA-S in saliva: time of day differences and positive correlations between two different types of collection methods

BackgroundThe anabolic steroid, dehydroepiandosterone sulfate (DHEA-S), is secreted from the adrenal cortex. It plays a significant role in the body as a precursor to sex steroids as well as a lesser known role in the hypothalamic pituitary adrenal axis (HPA) response to stress. DHEA-S can be measured reliably in saliva, making saliva collection a valuable tool for health research because it minimizes the need for invasive sampling procedures (e.g., blood draws). Typical saliva collection methods include the use of plain cotton swab collection devices (e.g., Salivette®) or passive drool. There has been some speculation that the plain saliva cotton collection device may interfere with determination of DHEA-S by enzyme immunoassay (EIA) bringing this saliva collection method into question. Because of the increasing popularity of salivary biomarker research, we sought to determine whether the cotton swab interferes with DHEA-S determination through EIA techniques.FindingsFifty-six healthy young adult men and women aged 18-30 years came to the lab in the morning (0800 hrs; 14 men, 14 women) or late afternoon (1600 hrs; 14 men, 14 women) and provided saliva samples via cotton Salivette and passive drool. Passive drool collection was taken first to minimize particle cross contamination from the cotton swab. Samples were assayed for DHEA-S in duplicate using a commercially available kit (DSL, Inc., Webster, TX). DHEA-S levels collected via Salivette and passive drool were positively correlated (r = + 0.83, p < 0.05). Mean DHEA-S levels were not significantly different between collection methods. Salivary DHEA-S levels were significantly higher in males than in females, regardless of saliva collection method (p < 0.05), and morning DHEA-S values were higher than evening levels (p < 0.05).ConclusionsResults suggest that DHEA-S can be measured accurately using passive drool or cotton Salivette collection methods. Results also suggest that DHEA-S levels change across the day and that future studies need to take this time of day difference into account when measuring DHEA-S.

On the Quantum Mechanical Scattering Statistics of Many Particles

The probability of a quantum particle being detected in a given solid angle is determined by the S-matrix. The explanation of this fact in time-dependent scattering theory is often linked to the quantum flux, since the quantum flux integrated against a (detector-) surface and over a time interval can be viewed as the probability that the particle crosses this surface within the given time interval. Regarding many particle scattering, however, this argument is no longer valid, as each particle arrives at the detector at its own random time. While various treatments of this problem can be envisaged, here we present a straightforward Bohmian analysis of many particle potential scattering from which the S-matrix probability emerges in the limit of large distances.

Competition between capillarity, layering and biaxiality in a confined liquid crystal

The effect of confinement on the phase behaviour and structure of fluids made of biaxial hard particles (cuboids) is examined theoretically by means of Onsager second-order virial theory in the limit where the long particle axes are frozen in a mutually parallel configuration. Confinement is induced by two parallel planar hard walls (slit-pore geometry), with particle long axes perpendicular to the walls (perfect homeotropic anchoring). In bulk, a continuous nematic-to-smectic transition takes place, while shape anisotropy in the (rectangular) particle cross-section induces biaxial ordering. As a consequence, four bulk phases, uniaxial and biaxial nematic and smectic phases, can be stabilised as the cross-sectional aspect ratio is varied. On confining the fluid, the nematic-to-smectic transition is suppressed, and either uniaxial or biaxial phases, separated by a continuous transition, can be present. Smectic ordering develops continuously from the walls for increasing particle concentration (in agreement with the supression of nematic-smectic second-order transition at confinement), but first-order layering transitions, involving structures with n and n + 1 layers, arise in the confined fluid at high concentration. Competition between layering and uniaxial-biaxial ordering leads to three different types of layering transitions, at which the two coexisting structures can be both uniaxial, one uniaxial and another biaxial, or both biaxial. Also, the interplay between molecular biaxiality and wall interactions is very subtle: while the hard wall disfavours the formation of the biaxial phase, biaxiality is against the layering transitions, as we have shown by comparing the confined phase behaviour of cylinders and cuboids. The predictive power of Onsager theory is checked and confirmed by performing some calculations based on fundamental-measure theory.

Study of Particle Interactions and Cross Sections for Modeling Energy Deposition and Cell Response(&lt;Special Issue&gt;INNOVATIVE BIOMEDICAL TECHNOLOGIES and INFORMATICS, BMFSA2008)

Study of Particle Interactions and Cross Sections for Modeling Energy Deposition and Cell Response(&lt;Special Issue&gt;INNOVATIVE BIOMEDICAL TECHNOLOGIES and INFORMATICS, BMFSA2008)

Cosmogenic nuclides in stony meteorites revisited

Abstract— We present new model calculations for depth and size dependent cosmogenic production rates in ordinary and carbonaceous chondrites by galactic cosmic rays. This model, essentially that of Leya et al. (2000a), folds together particle spectra and cross sections for the relevant nuclear reactions, but has been significantly improved due to major improvements in the neutron cross section database and better Monte Carlo modeling of the primary and the secondary particle spectra. The data presented here replace (and extend) the results of our earlier model predictions. Here we give for ordinary and carbonaceous chondrites elemental production rates for the cosmogenic radionuclides 10Be, 14C, 26Al, 36Cl, 41Ca, 53Mn, 60Fe, and 129I as well as for the noble gas isotopes 3He, 4He, 20Ne, 21Ne, 22Ne, 36Ar, and 38Ar. Using the new data and expressing size and depth scales to the unit [g/cm2], we are able to demonstrate that the matrix effect for both chondrite types is negligible for all target product combinations, except for those which are dominated by thermal or very low energy neutron reactions. Based on the new model predictions, we present a variety of elemental and isotopic production rate ratios allowing for a reliable determination of preatmospheric sizes, shielding depths, cosmic‐ray exposure ages, and diffusive losses.

Biaxial nematic and smectic phases of parallel particles with different cross sections

We have calculated the phase diagrams of one-component fluids made of five types of biaxial particles differing in their cross sections. The orientation of the principal particle axis is fixed in space, while the second axis is allowed to freely rotate. We have constructed a free-energy density functional based on fundamental-measure theory to study the relative stability of nematic and smectic phases with uniaxial, biaxial, and tetratic symmetries. Minimization of the density functional allows us to study the phase behavior of the biaxial particles as a function of the cross-section geometry. For low values of the aspect ratio of the particle cross section, we obtain smectic phases with tetratic symmetry, although metastable with respect to the crystal, as our Monte Carlo simulation study indicates. For large particle aspect ratios and in analogy with previous work [A. G. Vanakaras, M. A. Bates, and D. J. Photinos, Phys. Chem. Chem. Phys. 5, 3700 (2003)], we have found a four-phase point where four spinodals, corresponding to phase transitions between phases with different symmetries, meet together. The location of this point is quite sensitive to particle cross section, which suggests that optimizing the particle geometry could be a useful criterion in the design of colloidal particles that can exhibit an increased stability of the biaxial nematic phase with respect to other competing phases with spatial order.

Solving the Coagulation Equation by the Moments Method

We demonstrate an approach to solving the coagulation equation that involves using a finite number of moments of the particle size distribution. This approach is particularly useful when only general properties of the distribution, and their time evolution, are needed. The numerical solution to the integrodifferential Smoluchowski coagulation equation at every time step, for every particle size, and at every spatial location is computationally expensive and serves as the primary bottleneck in running evolutionary models over long periods of time. The advantage of using the moments method comes in the computational time savings gained from only tracking the time rate of change of the moments, as opposed to tracking the entire mass histogram which can contain hundreds or thousands of bins depending on the desired accuracy. The collision kernels of the coagulation equation contain all the necessary information about particle relative velocities, cross sections, and sticking coefficients. We show how arbitrary collision kernels may be treated. We discuss particle relative velocities in both turbulent and nonturbulent regimes. We present examples of this approach that utilize different collision kernels and find good agreement between the moment solutions and the moments as calculated from direct integration of the coagulation equation. As practical applications, we demonstrate how the moments method can be used to track the evolving opacity and also indicate how one may incorporate porous particles.

An accurate time advancement algorithm for particle tracking

We describe a particle position time advancement algorithm that is designed for use with several subgrid velocity reconstruction schemes used in LES/FDF methods, and potentially in other applications. These reconstruction schemes yield a subgrid velocity field with desirable divergence properties, but also with discontinuities across cell faces. Therefore, a conventional time advancement algorithm, such as second-order Runge–Kutta (RK2), does not perform as well as it does with a smooth velocity field. The algorithm that we describe, called Multi-Step RK2 (MRK2), builds upon RK2 by breaking up the time step into two or more substeps whenever a particle crosses one or more velocity discontinuities. When used in conjunction with the parabolic edge reconstruction method, MRK2 performs considerably better than RK2: both the final position of an advected particle, and the final area of a 2D infinitesimal area element are second-order accurate in time (as opposed to first-order accurate for RK2). Furthermore, MRK2 has the theoretical advantage that it better preserves the continuity of the mapping between initial and final particle positions.

Computer simulation of particle aggregates during sedimentation

This work concerns with a numerical investigation of particle velocity fluctuations in a blob undergoing gravity induced sedimentation at low Reynolds number. The simulations were performed by direct computation of the hydrodynamic interactions between a large collection of spherical particles. We focus our attention on both monodisperse and slightly polydisperse spherical aggregates comprised of inertialess particles whose radii have a Gaussian distribution about the mean. At vanishing particle Stokes number the dispersed particles undergo stochastic displacements arising from the random ambient field of the fluid velocity and not from direct solid-body collisions. As the sedimentation proceeds, a monodisperse blob persists as a cohesive entity with sporadic outward particle crossings of the blob boundary. A scaling argument for the rate at which the particles leak away from the aggregate was developed. When a sufficiently high degree of polydispersity is introduced the aggregate will behave differently from a monodisperse blob depending on the solid volume fraction. The observed fluctuating motion of the particles leads to the definition of a particle pressure associated with the particulate phase of the blob if it is regarded as an effective continuum. For polydisperse blobs, the tendency to a particle spreading induced by velocity fluctuations makes clear the effect of the particle pressure on resisting the formation of solid volume fraction inhomogeneities in suspension flows.

Fusion cross sections for 7Li + 12C system at near barrier energies

Elastic scattering and alpha particle angular distributions for the reaction 7Li + 12C have been measured at E lab = 7.5 , 9 , 12 , and 15 MeV. Statistical and direct reaction model calculations have been performed to deduce the fusion and direct reaction cross sections. The fusion cross sections measured in the present work are found to be consistent with those obtained by measurement of characteristic γ-rays, which are very different from those measured by direct detection of evaporation residues. The contribution from direct reactions to the alpha particle cross section has been quantified by exact finite range transfer calculations and found to be negligible ( < 6 % ) at all the bombarding energies studied. The measured fusion cross sections at all the energies studied are nearly equal to the reaction cross sections which imply that the direct reaction contributions are less and fusion is the dominant process.

Continuous cytometric bead processing within a microfluidic device for bead based sensing platforms

A microfluidic device for continuous biosensing based on analyte binding with cytometric beads is introduced. The operating principle of the continuous biosensing is based on a novel concept named the "particle cross over" mechanism in microfluidic channels. By carefully designing the microfluidic network the beads are able to "cross-over" from a carrier fluid stream into a recipient fluid stream without mixing of the two streams and analyte dilution. After crossing over into the recipient stream, bead processing such as analyte-bead binding may occur. The microfluidic device is composed of a bead solution inlet, an analyte solution inlet, two washing solution inlets, and a fluorescence detection window. To achieve continuous particle cross over in microfluidic channels, each microfluidic channel is precisely designed to allow the particle cross over to occur by conducting a series of studies including an analogous electrical circuit study to find optimal fluidic resistances, an analytical determination of device dimensions, and a numerical simulation to verify microflow structures within the microfluidic channels. The functionality of the device was experimentally demonstrated using a commercially available fluorescent biotinylated fluorescein isothiocyanate (FITC) dye and streptavidin coated 8 microm-diameter beads. After, demonstrating particle cross over and biotin-streptavidin binding, the fluorescence intensity of the 8 microm-diameter beads was measured at the detection window and linearly depends on the concentration of the analyte (biotinylated FITC) at the inlet. The detection limit of the device was a concentration of 50 ng ml(-1) of biotinylated FITC.

Anisotropy beneath an active collision orogen of Taiwan: Results from across islands array observations

We examined shear wave splitting in SKS phases from a large event recorded by a temporary array across southern Taiwan. The span extends from the western plain to the east coast. We applied particle motion and cross correlation methods to estimate its polarization directions and delay times. Analysis shows clear evidence of splitting except stations on the east coast. The fast split‐shear wave along this transect are approximately parallel to the strike of the mountain belt (NE‐SW). The delay times show a short‐wavelength variation and are well correlated to the surface geology. The largest split time is about 1.6 sec located at Eastern Central Mountain Range and indicates its possible mantle origin. Implications suggest that anisotropy is related to the collision tectonics which built the Taiwan islands, and that this tectonic compression involved the lithosphere and is characterized by a strong coherent deformation of the upper mantle and the crust.

Excitation functions of (p,x) reactions on natural nickel between proton energies of 2.7 and 27.5 MeV

Excitation functions have been measured for a number of proton induced nuclear reactions on natural nickel in the energy range from 27.5 MeV down to their threshold energy, using the activation method on stacked foils. Excitation functions for the reactions leading to the formation of 60Cu, 61Cu, 56Ni, 57Ni, 55Co, 56Co, 57Co and 58Co are presented and compared with earlier reported experimental data. Comparison with the recommended data reported by the International Atomic Energy Agency [Gul et al., 2001. Charged particle cross section database for medical radioisotope production. IAEA-TECDOC-1211, IAEA Vienna, Austria] is also presented when possible.

Modeling of absolute spatial emission profiles of Townsend discharges at high E/N in methane by a Monte Carlo technique

In this paper we present preliminary comparisons between experimental data and Monte Carlo simulations for the spatial profiles of excitation coefficients for the molecular band CH (A2Δ − X2Π) produced by dissociative excitation by electron swarms in methane. Measurements were made in parallel plate drift tube for E/N values between 500 Td and 11 000 Td (E-electric field, N-gas density, 1 Td = 10−21 Vm2). The spatial profiles of emission reveal significant heavy particle excitation even at moderately high E/N. Calculated absolute profiles are in excellent agreement with the experimental results. The calculations were based on the heavy particle cross sections of Petrovic and Phelps.

Stellar occultations at UV wavelengths by the SPICAM instrument: Retrieval and analysis of Martian haze profiles

Observations made by the SPICAM ultraviolet spectrometer on board the Mars Express orbiter are presented. We focus on several hundreds of atmospheric profiles which have been collected over 3/4 of a Martian year by making use of the stellar occultation technique. The typical structure of the Martian haze possesses at least one discrete layer (60% of all cases) standing over an extended portion wherein opacity continuously increases down to the surface. Differences of morphology are, however, noted between profiles observed near the equator and profiles collected elsewhere. The Martian haze exhibits a pronounced seasonal signal manifested by variations of the maximum elevation at which particles are observed. For reasons related to both convective activity and changes in the hygropause level, cold regions display a much lower hazetop than warm regions. Using UV spectrometry data, we put constraints on haze microphysical properties. Vertical variations of particle size are keyed to variations of opacity; e.g., an increase of particle size is systematically observed near extinction peaks. This is the likely consequence of cloud formation which results into a local increase of particle cross section. Despite marked differences of aerosol profiles between low and high latitudes, haze properties above 60 km remain invariant, possibly reflecting the long‐term presence of a background submicronic particle population. Several profiles have been analyzed in more detail to extract properties of detached cloud layers lofted above 40 km. Their optical depth ranges between 0.01 and 0.1 in the visible. Estimation of cloud particle size is technically restricted because of SPICAM wavelength sampling, but it generally yields a minimum radius value of about 0.3 μm, while several estimates are consistent with a robust 0.1–0.2 μm. This crystal size, significantly smaller than the 1 to 4 μm associated with recently classified type I and II clouds, suggests that a different class of clouds, henceforth type III clouds, can be extracted from our data. Observations made in the southern winter polar night indicate a very distinct aerosol behavior where particles are less abundant (τ &lt; 0.1), confined to lower heights (vertical profile consistent with a Conrath parameter exceeding 0.04) and made of particles having a radius on the order of 0.1 μm. This shows that the Martian polar night is a region with a very clean atmosphere and with a distinct type of aerosols.

Microstructural Evolution in AA7449 Plate Subject to Friction Stir Welding and Post Weld Heat Treatment

The effect of friction stir welding (FSW) and post weld heat treatment (PWHT) on the second phase particle distribution and cross weld hardness profile in AA7449 plate has been investigated. The alloy was received in an underaged condition, welded, then PWHT to give an overaged condition (in the parent material) . The effect of this complex treatment on the precipitate distribution in the weld and parent plate has been investigated over a range of length scales using small angle X-ray scattering (SAXS), TEM and FEGSEM. It is shown that the PWHT does not improve the hardness in the heat affected zone (HAZ), which is the location of the strength minimum after welding, but it does reduce the difference between the hardness in the HAZ and the nugget and parent hardness. The reduction in nugget strength after PWHT is particularly marked and is due to replacement of fine GP zones formed on post weld natural ageing by coarse overaged precipitates.

Soft gamma-ray background and light dark matter annihilation

The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the ($S$- or $P$-wave) annihilation of light dark matter particles into ${e}^{+}{e}^{\ensuremath{-}}$. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard x-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. Confronting these expected contributions to observation, we find that any extra contribution to this unresolved background around 511 keV should be lower than about $4\text{ }\text{ }\mathrm{keV}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$. We also estimate analytically the extragalactic background due to dark matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be 1 order of magnitude lower so that the hypothesis of a light dark matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross sections.

Relativistic electron scattering by electrostatic upper hybrid waves in the radiation belt

[ 1] Observations of particle precipitation into the Earth's atmosphere performed on low-altitude satellites have excited substantial interest in relativistic electron precipitation bursts from the outer zone of the radiation belt. As underlying mechanism of such bursts, we suggest particle scattering into the loss cone due to higher-order cyclotron resonance interaction between relativistic electrons and intense narrow-band upper hybrid waves, which are frequently observed outside the plasmapause. The case of a single wave and the case of a wide wave number spectrum are considered, and approximate expressions for the relativistic particle diffusion coefficients in phase space are calculated for conditions of an inhomogeneous plasma. It is found that relativistic electrons have a preference over lower-energy electrons from the viewpoint of the number of cyclotron resonances that a particle crosses during each bounce period. Owing to the predominantly longitudinal direction of the upper hybrid wave group velocity, the resonant wave-particle interaction can take place over many electron bounce periods, which facilitates the particle scattering into the loss cone. The theory developed in the present paper accounts for two main features of the relativistic electron precipitation bursts, namely, a strong energy dependence in the electron precipitation process and the small-scale burst structure, which is primarily attributed to the localization and strong inhomogeneity of the growth region of the upper hybrid modes responsible for the scattering. The existence of intrinsic temporal structure of the precipitation on timescales comparable to the bounce period is also consistent with the theory due to the high efficiency of the scattering process.