Individual Collisions Research Articles

Models of Thermal Relaxation Mechanics for Particle Simulation Methods

An improved phenomenological microscopic model is introduced in the present study and compared to existing models for simulating molecular vibrational relaxation in rarefied flows. These models are employed in statistical particle simulation methods such as the direct simulation Monte Carlo (DSMC) technique. In the traditional Borgnakke-Larsen model, collision energies are partitioned among contributing energy modes as dictated by fractions sampled from equilibrium distributions. Application of this method to fully excited continuous energy modes alone, such as in translation-rotation (T-R) exchange, promotes the equilibrium state. However, application to translation-vibration (T-V) exchange is afforded by unrealistically approximating the quantized distribution as continuous and partially excited, and employing individual collision "temperatures" in an attempt to capture the temperature-dependence of the necessary distributions. As proven in theoretical and numerical analyses, such an implementation of the Borgnakke-Larsen method may fail to promote the equilibrium relaxed state exactly and poses computational difficulties. The improved technique of the present work iterates between translation-rotation and rotation-vibration exchanges which does promote equilibrium exactly if the model for the latter process is compatible with quantized oscillators. This may be achieved by quantizing the total internal energy of a molecule and dividing the quanta randomly among the rotational and vibrational energy modes. This iteration-equipartition model retains computational simplicity and promotes thermal equilibrium even when applied to multi-species gas mixtures of non-degenerate anharmonic quantized oscillators.

Particle production in interactions of 200 GeV/nucleon oxygen and sulfur nuclei in nuclear emulsion.

Oxygen and sulfur nuclei with energies of 200 GeV/nucleon have been allowed to interact in nuclear emulsions exposed at CERN. These emulsions have been scanned with a minimum bias so that essentially all the interactions occurring were detected. Nearly 1000 interactions of each projectile have been analyzed. We present results on the multiplicity distributions, the pseudorapidity distributions, and the fragmentation of the projectile and target nuclei. It is shown that the mean number of intranuclear collisions in each interaction, calculated from a superposition model, provides a useful parameter for organizing the data. We conclude that there are no significant deviations even at these energies from models, such as the venus model, describing the interactions as being the superposition of individual nucleon-nucleon collisions.

Erosion of Gaseous Suspension Flow Duct due to Particle Collision. (I). Experimental Determination of Erosion Rate by Individual Collision.

Abstract An experimental study is presented on the erosion process caused on the target specimens by solid particle impingement, in which precise quantifications are given as to the dependencies on collision angle and collision velocity. In order to eliminate fluid-dynamic drag force that retards the particles' motion before they impinge upon the targets, the experiments are performed in the vacuum vessel. In the process of erosion, the existence of so-called incubation period is confirmed. Measured exponents of the velocity are considerably smaller than those for the handy application and are rather close to semi-theoretical values of Bitter and Finnie, which suggests that they can be used in the numerical prediction. Regarding the dependency on collision angle, it was found clear discrepancy between the experimental results and predicted one by means of models of Bitter and Finnie as the collision angle increased. For the spherical particles, the cutting erosion actually ceases before the collision angl...

Screened Rutherford multiple scattering estimates for heavy ion backscattering applications

An analytical expression and an integral representation are presented for the contribution, Y ( n) = ϵ ( n) Δω n , of n-event multiple scattering chains to the observed backscattering spectrum in heavy-ion backscattering (HIBS) measurements. The approximations introduced in deriving the results are chosen such that an upper limit is placed on Y ( n) by the expressions. The Rutherford elastic scattering cross section is used to describe individual collisions between incident projectiles and target atoms. Screening of the Rutherford scattering cross section is included in an approximate fashion which maintains the upper limit estimate. Inelastic energy loss between collision events is assumed proportional to the projectile velocity. Specific application of these expressions is made to HIBS detection of trace amounts of heavy atom impurities on a Si surface by 200–400 keV C + beams. The predicted multiple scattering background for this application is compared with the predicted single scattering signal for 10 10 at./cm 2 of Fe, Cu, Zr, Sn, or Au, as a surface impurity. The comparison shows that the multiple scattering background poses no barrier to extending the sensitivity of HIBS detection of impurities in this mass range to levels as low as 10 8 at./cm 2 for the upper part of the energy range considered. Comparison of calculations with and without screening included show that the screening of the Rutherford cross section by atomic electrons is a significant factor in preventing multiple scattering effects from interfacing with HIBS spectrometry at impurity levels in the 10 10 at./cm 2 range.

Sequential Insertion of 3He and 4He in C

In a multicollision experiment in which the individual collision steps are separated from each other both spatially and temporally, two helium atoms can be inserted into C. The experiment allows the production of [4He2@C60]˙+ and [3He4He@C60]˙+ and therefore opens the way to the formation of mixed endohedral complexes of the general structure AB@Cx.

Statistical properties of sputtering from individual atomic collision cascades in solids

The correlation between sputtering and several features of individual cascades is investigated with the trim.sp program. A comparison with other simulation models indicates that the sputtering statistics is not deeply influenced by the target model. Also, the distributions of sputtering related quantities were found quite similar when resulting from cascades generated by particles which are implanted or backscattered, although the number and the distribution of displacements produced can be well distinguished. A partition of the individual cascades is made based on the sputtering frequency. The sputtering energy distributions are found similar in all classes. These features illustrate the similarity between the statistics of sputtering from individual and statistical cascades. The situation is different as the relation between the number of sputtered atoms and the surface deposited energy is concerned. The same number of sputtered atoms is associated with a broad distribution of deposited energies. Their profile is dependent on the number of sputtered atoms, and the sputtering frequency is not always linearly dependent on either the mean or the mode of the surface-deposited energy distribution. Finally, a surface-deposited energy threshold is emphasized for sputtering, which correlates to the anisotropy of the momentum flux distribution in the vicinity of the surface.

Numerical Simulation of Ball Motion in a Tumbling Ball Mill.

The two dimensional motion of the individual balls in a tumbling ball mill was analyzed numerically by a Distinct Element Method using the extended Kelvin model composed of nonlinear spring and nonlinear dashpot. A parameter related to the coefficient of viscosity is determined from the data of imperfect restitution between ball and mill shell. The simulation of the dynamic motion of the individual balls is based on the nonlinear perfect elasticity, nonlinear viscosity and Newton's law of motion. The dynamic motion of balls during milling calculated was compared with the actual three dimensional motion of balls observed in a mill. The effects of ball charge fraction and mill speed on the impact velocity of balls estimated were also discussed.It can be concluded that the model simulation developed in the present study predicts approximately the actual motion of the balls in a tumbling mill. The motion reproduced from the simulation is strongly influenced by the ball charge fraction and mill speed. The impact frequency of the individual balls and their impact velocities at individual collisions can be estimated by the simulation technique proposed.

Subthreshold pion production in nucleus-nucleus collisions within the quantum molecular dynamics approach

The subthreshold pion production from 12C+ 12C, 20Ne+ 20Ne, 40Ca+ 40Ca, 20Ne+ 64Cu and 93Nb+ 93Nb collisions is studied within in the quantum molecular dynamics (QMD) model. For the mean field part of the theory, we use both the soft and the hard equations of state (EOS) with and without the momentum dependence in the interaction, while for the in-medium NN cross section, the Brueckner G-matrix is utilized. The pion yield from a nucleus-nucleus collision is assumed to be the incoherent sum of the corresponding yields from the individual nucleon-nucleon collisions. For the latter process, parametrized cross sections are adopted. The dependence of the pion cross section on the different nuclear EOS, on the momentum dependence in the interaction and on the pion reabsorption factor is studied. The time sequence of the pion production is also analyzed. The theoretical results for 20 Ne+ 20 Ne ( E lab A = 183 MeV) and 20 Ne+ 64Cu ( E lab A = 250 MeV) collisions are compared with available experimental data.

Charge transfer during individual collisions in ice growing from vapor deposition

Measurements of charge transfer during individual collisions between ice spheres and a target growing from vapor deposition have been performed in order to establish the importance of fragmentation in the charging process. It is found that when the target is a few degrees colder than the environment, the charge pulses have mixed signs but the number of positive ones becomes dominant as the target temperature decreases. This behavior is reproduced in collisions with dielectric particles and a target in the same growth conditions. Reversal in the temperature gradient reversed the sign of the charge transfer. Direct evidence of fragments produced during collisions suggests that they are responsible for the measured charge transfer.

Particle densities in SS and PbPb collisions at RHIC and LHC energies in the dual parton model

The density of charged particles in nucleus-nucleus collisions is computed in the framework of the dual parton model including the effect of multiple inelastic scattering in the individual nucleon-nucleon collisions. Numerical calculations for SS and PbPb collisions at mid rapidities are presented.

Electron-atom collisions in a laser field

Cross sections tor the 1S–2S and 1S–2P 0 transitions in laser assisted e-H(1S) collisions are calculated in both the multichannel eikonal and the Born-wave treatments as a function of impact energy and laser field intensity and phase. The laser considered is a monotonic, plane polarized CO 2 laser (photon energy = 0.117 eV), with the polarization direction parallel to the initial projectile velocity. Floquet dressing of the hydrogen atom in the soft-photon weak-field limit reveals a concise description of the laser assisted electron-atom collision. This model also links the microscopic detail of the individual collisions with the macroscopic considerations of experimental analysis.

On the Relaxation of Energetic Electrons in Plasmas

AbstractThe impact of individual collision processes on the relaxation of the velocity distribution function of a group of electrons, initially localized in a narrow region at relatively high energies, has been studied. By having recourse to solutions of the non‐stationary Boltzmann equation and to corresponding Monte‐Carlo simulations, the temporal behaviour of electrons in CO2 plasmas, both in the absence and the presence of an external dc field, has been investigated. A microphysical interpretation of observed relaxation phenomena, based on the data relevant to the individual collision processes, is also given.

Irradiation Damage in Zr3 Fe

ABSTRACTIon bombardments, transmission-electron-microscopy (TEM) and analytical-electron-microscopy (AEM) techniques were used to obtain information on the effect of irradiation on the phase stability of Zr3 Fe, which has an orthorhombic crystal structure. Irradiations were performed at 35–773 K using 0.5–1.5 MeV 40Ar ions and at 40 K using 15–120 keV 209Bi ions.At low ion fluences (1011–1013 ions cm−2), most of the visible damaged regions (∼1.0–5.0 nm diameter) exhibited strong structure-factor contrast features and it is believed that they are probably amorphous. With increasing ion energy, multiple damaged regions were observed to form within individual collision cascades, and the fraction of the theoretical cascade volume occupied by these heavily-damaged regions decreased rapidly. Irradiations at higher ion fluences showed that an irradiation-induced phase change from the crystalline-to-amorphous state occurred at 35–573 K, but not above 623 K. The effect of varying the ion flux was also investigated. The results showed that the fluence required to amorphize a given fraction of the irradiated volume was quite dependent upon the amount of annealing that occurred within the collision cascade at the temperature of the bombardment. The crystalline-to-amorphous transformation appeared to be the result of direct amorphization within the collision cascades and a critical defect concentration being reached in the cascade overlap regions, thus producing additional amorphous regions.

Transform-limited collisions of K Rydberg atoms.

By reducing the collision velocity of Rydberg atoms in resonant energy-transfer collisions, we have observed collisional resonances as narrow as 1 MHz, or collision times as long as 1.0 {mu}s. Restricting the collision to a time less than the natural collision time results in transform broadening of the resonances, allowing us to know when an individual collision begins and ends. The latter point is verified by observing the dependence of the collision line shape on the phase of an oscillating field of frequency equal to the width of the collisional resonance.

Digital acoustical analysis of normal and bimodal Björk-Shiley 60 ° convexo-concave heart valves

Fracture of the outlet strut of the Björk-Shiley 60 ° convexo-concave (BS60CC) valve has been attributed to a bimodal closing pattern in certain valves in which the closing disk rotates about the inlet strut, causing upward displacement of the outlet strut and its eventual fracture. This article reports the in vivo studies of the normal BS60CC valve and the in vitro studies of the normal and bimodal BS60CC valves, using a digitial acoustical signal processing technique, in which the individual collisions (impact history) of the occluder disk with the components of the valve body are revealed during each closing cycle. In vitro analysis of the closing acoustical signals of normal BS60CC valves showed impact history cluster width (IHCW) means of 2.07 ± 0.85 ms (standard error), not significantly different from those of 1.86 ± 0.58 ms (standard error) observed in 38 clinically normal patients with BS60CC valves (p >0.1). The bimodal valves showed IHCW of 6.14 ± 0.98 ms (standard error), in vitro, which was significantly greater than those observed in the normal in vitro valve group and in the normal patient population (p < 0.0001).

Internal nuclear momentum and subthreshold anti-proton production in p-nucleus and nucleus-nucleus collisions

We investigate subthreshold anti-proton production in p-nucleus and nucleus-nucleus collisions in terms of the internal nuclear momentum of the nucleons in the colliding nuclei. We employ a first collision model which contains the kinematics of individual NN collisions, with the inclusion of internal nuclear momentum distributions extracted from data on quasi-elastic electron scattering and backward proton production. Calculations for subthreshold particle production in p-nucleus collisions show excellent agreement with experimental data on subthreshold anti-proton and K + production in p + Cu collisions. An extension of the model to nucleus-nucleus collisions includes also the internal momentum for the incoming projectile nucleons, and contains the parameters obtained from a best fit to the p + Cu → p + X data. Despite the excellent agreement with the p + Cu → p + X data, this model underestimates recent experimental data on subthreshold p production in nucleus-nucleus collisions by large factors. Implications for collective phenomena at large nuclear densities and temperatures are discussed.

Classical trajectory calculations of non-linear sputtering: SiCl 4 sputtering of a Cu(001) surface using ab initio potentials

Classical trajectory calculations are carried out on SiCl 4 sputtering of a Cu(001) surface. The repulsive Cu Si and Cu Cl interaction potentials are calculated using ab initio electronic structure theory. At energies of 120 eV or greater, the calculated sputtering yields are found to be greater than the sum of the component atomic sputtering yields. This effect cannot be explained by a thermal spike such as has been invoked to explain nonlinear sputtering at higher energies. Examination of individual trajectories suggest that different sputtering mechanisms become operational as the kinetic energy of the incident molecule is increased. The observed enhancement of the molecular sputtering yield can be explained by the overlap of the individual atomic collision cascades.

A mycoplasmal protein influences tumour cell invasiveness and contact inhibition in vitro

Fab fragments of a monoclonal antibody directed against p37, a protein associated with the surface of FS9 mouse sarcoma cells, were previously found to inhibit the highly invasive behaviour of FS9 cells in vitro. We show that p37 originates from Mycoplasma hyorhinis. Infecting various cell lines with the mycoplasma consistently increased their invasiveness when confronted with chicken heart fibroblasts using Abercrombie's confronted explant technique. Conversely, removing the mycoplasmas or blocking p37 with specific Fab fragments reduced invasiveness. Analysis of individual cell collisions using time-lapse filming showed that the addition of Fab fragments to cells infected with M. hyorhinis greatly increased the level of contact inhibition. The antibody also reduced the invasiveness of transformed cells that did not express the p37 antigen. Hence, a cellular protein or proteins that are structurally related to p37 apparently influence invasive behavior.

Radial Energy Transfer Density Distribution around the Fast Ion Tracks in Silicon and Germanium

AbstractThe radial energy transfer density distributions around the tracks of fast protons (p), α‐particles, and C‐ions in Si and Ge targets are obtained by the Monte‐Carlo method using individual collision scheme. A semiempirical formula for D(r) is proposed based on these calculations. Some aspects of its application are discussed.

Collision Cascades in Zr3Al

AbstractZr3Al is an ordered f.c.c. alloy of the L12 type, which under various irradiation conditions can be disordered and then eventually amorphized. In the present transmission-electron-microscopy (TEM) investigation, the nature of the damaged regions formed in individual collision cascades was investigated using low fluence (&lt; 1 × 1011 ions cm−2) bombardments at 290 K with 15 − 120 keV 63Cu ions.Imaging under dynamical conditions with the fundamental 111, 200, 220 and 113 reflections revealed damaged regions (5.1 - 8.1 nm average diameter) that exhibited black-white strain contrast features. These damaged regions have essentially a spherically symmetrical strain field and probably are three-dimensional clusters of point defects. Imaging with 110 superlattice reflections revealed additional damaged regions (2.4 - 3.7 nm average diameter), which appeared as black spots, that represent disordered regions in the ordered lattice. For both the strain-contrast regions and the disordered regions, as the incident ion energy increased, the number of damaged regions produced per incident ion increased and the fraction of the theoretical collision cascade volume occupied by these regions decreased.