Individual Collisions Research Articles

Transverse energy distributions andJ/ψproduction in Pb+Pb collisions

We have analyzed the latest NA50 data on transverse energy distributions and $J/\psi$ suppression in Pb+Pb collisions. The transverse energy distribution was analysed in the geometric model of AA collisions. In the geometric model, fluctuations in the number of NN collisions at fixed impact parameter are taken into account. Analysis suggests that in Pb+Pb collisions, individual NN collisions produces less $<E_T>$, than in other AA collisions. The nucleons are more transparent in Pb+Pb collisions. The transverse energy dependence of the $J/\psi$ suppression was obtained following the model of Blaizot et al, where charmonium suppression is assumed to be 100% effective above a threshold density. With fluctuations in number of NN collisions taken into account, good fit to the data is obtained, with a single parameter, the threshold density.

Industrial tumbling mill power prediction using the discrete element method

The discrete element method models the motion of charge in tumbling mills using the physics of individual collision between balls, rock particles, and mill shell. Thus it differs from the approaches of the past wherein the power draft of the mill was based on the torque-arm formula. The discrete element method takes into account the number and dimensions of lifters and the number, density, and size of balls and rock particles in the charge. It is shown that this method successfully predicts power draft in a variety of semi-autogenous plant operations. Hence the simulation is an effective tool for designing lifters or evaluating the make-up ball size.

Low energy scattering of Cs + from Pt(1 1 1): A model study on angular and energy distributions

In a classical molecular dynamics simulation we have studied the in-plane angular intensity distributions and final energies for Cs + scattered from Pt(1 1 1) in the low incidence energy range (5–100 eV). The angular intensity distributions are predominantly symmetrical about the specular scattering angle and the absence of both, surface rainbow scattering and backscattering, indicate an important difference from lighter mass projectiles. The effective dislocation of the surface atoms upon impact accounts for this difference. The in-plane intensities are volatile due to out-of-plane scattering and we provide estimates of the amount of out-of-plane scattering. The initial perpendicular momentum determines the energy transfer to the surface, whereas the angular dependence of the final energies shows a transition towards individual binary collisions. This energy exchange can be understood in terms of a simple binary collision model.

Dynamical evolution and collisions of asteroids with the earth

We study the dynamical evolution of asteroids which are a potential danger for collisions with the Earth. The work is undertaken on basis of a simplified model of the planetary system including the perturbations from Venus to Saturn. For a whole grid of initial conditions (0.7⩽ a⩽1.45 and 0.1⩽ e⩽0.8) we integrated the equations of motion with the Lie-series integration for 0.5 Myrs for 720 fictitious objects. The analysis of the data provided the following results: (a) the dynamical evolution for such objects which are initially in four different groups namely Subatens, Atens, Apollos and Amors (b) the flow between these groups and (c) the individual collision probabilities.

Event indexing systems for efficient selection and analysis of HERA data

The design and implementation of two software systems introduced to improve the efficiency of offline analysis of event data taken with the ZEUS Detector at the HERA electron–proton collider at DESY are presented. Two different approaches were made, one using a set of event directories and the other using a tag database based on a commercial object-oriented database management system. These are described and compared. Both systems provide quick direct access to individual collision events in a sequential data store of several terabytes, and they both considerably improve the event analysis efficiency. In particular the tag database provides a very flexible selection mechanism and can dramatically reduce the computing time needed to extract small subsamples from the total event sample of the experiment.

On the elastic and inelastic collisions between precipitating energetic hydrogen atoms and Martian atmospheric neutrals

Properties of elastic and inelastic processes in the Martian upper atmosphere caused by precipitating hydrogen atoms have been studied. The energetic (≲ 1 keV) neutral atoms, ENAs, are produced by the charge exchange process between the solar wind protons and the atoms in the Martian exosphere. The process gives a mechanism to deposit energy to the Martian atmosphere in addition to the well‐known EUV radiation. The energy deposition is studied by both a microscopic and a macroscopic model. The microscopic approach is based on Monte Carlo (MC) simulations which make it possible to study individual collisions and trajectories. The macroscopic approach utilizes the continuous slowing down approximation (CSDA). The CSDA equations are derived starting from the first principles in order to analyze the role of the backscattered particles. The MC and CSDA models are used to study how the energy deposition depends on the energy of precipitating ENAs and on the solar zenith angle. The role of the adopted scattering cross‐section models is analyzed, as well as differences between elastic and inelastic collisions. MC simulations suggest that the backscattered ENAs may play an important role in the energy deposition process. Comparisons between MC and CSDA models also illustrate that the CSDA model can give a relatively good approximation for the energy deposition profiles with a proper choice of the free parameters.

Quantum expulsion of impurities from a Bose–Einstein condensate

When a sizable, hard-sphere-like impurity is placed in a confined Bose–Einstein condensate, it will force a hole in the condensate wave function. Since it costs more energy to drill a hole at the wave function's maximum (near the trap center) than at its minimum (near the trap edge), the impurity will be expelled from the condensate. In sharp contrast to the classical case, the Bose condensed atoms can collectively expel impurities much more massive than themselves with essentially no individual collision with the impurity. For C 60 molecules freely falling through a condensate of atomic hydrogens, we estimate that they will be deflected with a radial acceleration comparable to g. At the extremely low density of the hydrogen condensate, only one out of two hundred C 60 will likely to encounter a single hydrogen atom during its fall. The scattered fullerenes would then form a detectable, banded density pattern outside of the trap.

Quantum expulsion of impurities from a Bose condensate

When a hard-sphere-like impurity is placed in a confined Bose condensate, because it forces a hole in the inhomogeneous condensate wave function, it will be expelled. In contrast to the classical case, Bose condensed atoms can collectively expel massive impurities at T=0 with essentially no individual collision with the impurity. We estimate that C 60 molecules falling through a condensate of atomic hydrogen will be deflected with a radial acceleration comparable to g. At the extremely low density of current hydrogen condensates, only one out of two hundred C 60 will likely encounter a single hydrogen atom during its fall.

Monte Carlo simulations of x-ray induced recombination in amorphous selenium

The photoconductor amorphous selenium (a-Se) has been used inmedical imaging since the 1950s, and is currently under studyfor use in digital radiography. The quantity of interest forradiographic applications is the energy required to create a detectable electron-hole pair, W±, which is governed by therecombination of electron-hole pairs. Both geminate and columnarrecombination theories have been invoked to describerecombination in a-Se, without success. In this work, we develop aMonte Carlo code which follows individual collisions along thex-ray/electron track structure in order to determine the initialpositions of the electron-hole pairs within a-Se. Wesubsequently simulate the transport of the pairs within anelectric field to calculate the amount of recombination. We usethese simulations to calculate W± as a function ofapplied electric field and incident x-ray energy. We findexcellent agreement between our simulations and experimentalresults. This indicates that the recombination occurs not onlybetween geminate pairs but also between other pairs created alongthe track. This inter-track recombination leads to an energydependent recombination probability which could not be explainedwith previous recombination theories.

Analogous Experiments on the Stickiness of Micron‐sized Preplanetary Dust

In the early solar nebula, the formation of planetesimals and cometesimals is believed to be due to inelastic collisions of initially micron-sized grains. The collisions are caused by relative velocities due to size-dependent interactions with the surrounding dilute gas. The grain growth process is determined by the velocity-dependent sticking efficiency upon collisions. Therefore, we performed experiments with eight samples of micron-sized particles consisting of monodisperse silica spheres, of irregularly shaped diamond, enstatite, and silicon carbide grains, and of silicon carbide whiskers. We determined the sticking probability and the energy loss upon bouncing collisions by studying individual grain-target collisions in vacuum. We found a sticking probability higher than predicted by previous theoretical work. Grain size, roughness, and primarily grain shape, i.e., the difference of spherical versus irregular grain shape, is important for the collisional behavior, whereas the material properties are rather unimportant. Our results indicate that the preplanetary dust aggregation is more effective than previously thought.

Using Computer Assisted Learning to Teach Molecular Reaction Dynamics

We have developed a Computer Assisted Learning program to aid in the teaching of molecular reaction dynamics to undergraduate students. Dynamic and steric factors affecting reaction outcomes are investigated by observing individual molecular collisions. Students control the reactions by experimenting with the initial trajectories of the molecules. Interactive questions emphasizing the role of the potential energy surface guide the students through this experiment and on to a detailed analysis of the results. This enables students to relate the new material they have learned to more familiar topics in chemical kinetics.

Improved methods of measurement and analysis of conversion electron and β-particle spectra

A general statistical test of the stability of measurement conditions was demonstrated on the β-spectra of 241Pu cumulated during four years. The α- and γ-ray spectroscopy indicated stability of the 241Pu source. Monte Carlo modelling of individual collision events clarified the role of electron scattering and energy losses within a radioactive source down to energies of several hundreds of eV. The impact ionization by β-particles of carbon and oxygen atoms in a surface contamination layer on the 241Pu and 63Ni sources was observed.

Results from the experiment E895 at the BNL AGS

We present some of the latest results from the E895 experiment conducted at the BNL AGS accelerator. Au+Au collisions were recorded by the EOS Time Projection Chamber (TPC) at beam energies of 2, 4, 6, and 8 A GeV. The TPC detector permitted the reconstruction of individual collision events with almost 4π acceptance and good particle identification. This capability allowed E895 to study global observables and two particle correlations with respect to symmetries of the event. Flow excitation functions are examined and discussed in the context of the Nuclear Equation of State.

The discrete element method for fine grinding scale-up in Hicom mills

The Hicom mill is a high-intensity grinding mill in which the grinding media tumbles in a strong centrifugal acceleration field. Typical applications include wet or dry fine grinding, liberation and concentration of diamonds, and breaking up mineral agglomerates and clay balls. When fine grinding, the intense grinding environment results in a rapid production of very fine material from most mineral ores, down to below 10 μm. However, scale-up from laboratory size data to industrial sized mills becomes increasingly difficult as the product size becomes finer. The discrete element method (DEM) is a computation technique which models the movement of collections of separate particles. DEM can be used to track individual collisions as particles tumble over each other, and to calculate the energy associated with each collision. It is shown how DEM techniques can be applied to the grinding media in a Hicom mill to produce frequency distribution plots of collision energies under different mill operating conditions, including ball size and density, mill speed and mill diameter. These results are shown to be well-correlated with conventional experimental and theoretical results for Hicom mills. The efficiency of fine grinding in the Hicom mill can be correlated with collision energies calculated by DEM. This has resulted in improved scale-up methods for fine grinding in the Hicom mill.

Condensed-history Monte-Carlo simulation for charged particles: what can it do for us?

Condensed-history (CH) Monte-Carlo (MC) groups together the vast number of individual charged-particle collisions using multiple scattering theory for elastic angular changes and stopping power for energy losses. CH codes such as EGS4 have been enormously successful in simulating the transport of electrons, for example, in radiotherapy. MC-derived values of the water-to-air stopping-power ratio, s(w/air), are used in all modern codes of practice for absolute dose determination in radiotherapy clinics. MC can also directly yield the dose ratio Dmed/Ddet for a dosimeter in a medium, and Correlated Sampling has been exploited to increase the efficiency, e. g., the central electrode in an ion chamber (aluminium vs. graphite). The extremely low density of the gas in an ion chamber poses problems for CH codes. However, multiple scattering can now be combined with single scattering and is expected to finally resolve important chamber perturbation effects. An exciting application of CH MC in radiotherapy is the computation of dose distributions in patients. Currently one can achieve an uncertainty around 1% (1 SD) in mm-sized voxels in several minutes for an electron beam and in around an hour for a photon treatment plan on hardware costing less than $20,000, and thus avoid all the various approximations conventionally used to account for inhomogeneities. In the microdosimetry/track structure field, CH codes have shown that the fluence (dPhi/dE) per unit dose at low electron energies is virtually independent of incident particle energy or depth, which simply explains the negligible RBE variation.

Investigation of relative sputtering yields during ionoluminescence of Si

Atomic line emissions resulting from sputtered atoms and ions are investigated on Si(100) bombarded by 60 keV Ne +, 300 keV Xe + and 300 keV SF 5 + ions. It has been observed that the presence of oxygen enhances light emission due to radiative de-excitation. Relative sputtering yields of Si for 60 keV Ne +, 300 keV Xe + and 300 keV SF 5 + are estimated from the photon yields. The estimates of sputtering yields of adsorbed oxygen on Si for the above projectiles are also made from the transients of Si I 251.6 nm line. A non-linearity in the sputtering yield for SF 5 + ions was observed in comparison with the yields for Ne + and Xe + ions. The results are discussed with the model of overlapping of individual collision cascades.

The effect of the cloud-droplet spectrum on electrical-charge transfer during individual ice-ice collisions

The effect of the cloud-droplet spectrum on electrical-charge transfer during individual ice-ice collisions

Collisional Stellar Dynamics around Massive Black Holes in Active Galactic Nuclei

The dynamical evolution of nuclear star clusters containing a massive black hole is examined in the region in which physical collisions dominate other processes and the black hole dominates the potential. The numerical method features a discrete cluster of stars, fully relativistic calculation of the orbital trajectories (including an algorithm that searches for individual collisions between pairs of orbits), and the use of a series of smooth particle hydrodynamics simulations of high-velocity stellar impacts (computed independently of this work) to determine collision outcomes. In contrast to a Fokker-Planck analysis, this approach allows small-number statistics, relativistic effects, and collision dynamics to be accounted for directly and accurately. The versatility of the simulation techniques makes them usable in a range of problems; useful by-products of the routines include a simple, exact procedure for computing the integrals of motion of Kerr geodesics given their Keplerian orbital elements and a generalized form of Kepler's equation that is (asymptotically) valid in the Kerr geometry. We find that many grazing collisions produce very little mass loss—even when a head-on collision would lead to complete disruption—thereby creating an extended distribution of low-mass remnants. Collisional refilling of the loss cone is seen and generally dominates relaxation-induced disruptions in these systems. It is found that collisions preferentially produce a constant density core in the collisionally dominated region of the cusp, as opposed to the ρ∝r−½ profile found in Fokker-Planck studies; the discrepancy can be traced to the simplifying assumptions typically employed by the latter approach.

Experiments on the effects of dust flux exposure on ROSETTA spacecraft materials

During its comet rendezvous, the ROSETTA/ROLAND spacecraft will be exposed to a particle flux which is caused by the sublimation of volatile cometary material. The dust flux will be characterized by particle velocities of a few 100 ms −1 and particle sizes between 0.5 and 100 μm. The dust flux exposure requires the investigation of possible effects on spacecraft and scientific payload performance. It is of interest whether (1) the particles stick to the spacecraft and thereby form dust layers, (2) the particles cause erosive damage upon impact on sensitive parts, and (3) the impacts electrically charge the spacecraft. Furthermore, it will be possible to better decode dust flux characteristics from spacecraft data (e.g. internal temperatures, spacecraft charging) if the effects on the space probe are more intensively investigated. We present work of an ESA-funded pre-study which showed that an experimental method which consists of generating a dust jet in vacuum and of optically observing individual grain-target collisions is applicable for this purpose, and we point out design features of the future “Cometary Dust Flux Simulation Facility”.

New experiments on collisions of solid grains related to the preplanetary dust aggregation

The formation of planetesimals and cometesimals is due to inelastic collisions between dust grains and due to van der Waals and other attractive surface forces. These low-velocity collisions are believed to result from size-dependent friction with the dilute gas of the solar nebula. Any numerical simulation of the grain growth process must be based on assumptions about the collisional behaviour of these grains, which is a very complex theoretical problem. Therefore, we performed analogous laboratory experiments on individual grain-target collisions of micron-sized particles. We briefly summarize some important results, point out discrepancies to former work, and mention possible applications on solar nebula conditions with respect to the preplanetary dust aggregation, to a possible motion of charged particles due to magnetic forces, and to the formation of meteoritic chondrules due to ‘lightning’.