Range Of Impact Parameters Research Articles

Direct dynamics in a proton transfer reaction of isomer product competition. Insight into the suppressed formation of the isoformyl cation.

Proton transfer between HOCO+ and CO produces the formyl cation HCO+ and isoformyl cation HOC+ isomers initiating multiple astrochemical reaction networks. Here, the direct chemical dynamics simulations are performed to uncover the underlying atomistic dynamics of the above reaction. The simulations reproduce the measured product energy and scattering angle distributions and reveal that the reaction proceeds predominantly through a direct stripping mechanism which results in the prominent forward scattering observed in experiments. The reaction dynamics show propensity for the HCO+ product even at a collision energy larger than the threshold for HOC+ formation. This is a consequence of the larger opacity and impact parameter range for HCO+. In accordance with the revealed direct mechanistic feature, the reaction can be controlled by orienting the reactants into a reactive H-C orientation that also favors HCO+ formation. Considering the lack of equilibrated reactant complexes and the on the fly migration of the proton, the CO2-catalyzed isomerization is assumed to have insignificant impact on the isomer ratios. This work provides insights of dynamical effects besides energetics into the interesting finding of strongly suppressed formation of the metastable isoformyl cation for related proton transfer reactions in the measurements.

A fast centrality-meter for heavy-ion collisions at the CBM experiment

A new method of event characterization based on Deep Learning is presented. The PointNet models can be used for fast, online event-by-event impact parameter determination at the CBM experiment. For this study, UrQMD and the CBM detector simulation are used to generate Au+Au collision events at 10 AGeV which are then used to train and evaluate PointNet based architectures. The models can be trained on features like the hit position of particles in the CBM detector planes, tracks reconstructed from the hits or combinations thereof. The Deep Learning models reconstruct impact parameters from 2-14 fm with a mean error varying from -0.33 to 0.22 fm. For impact parameters in the range of 5-14 fm, a model which uses the combination of hit and track information of particles has a relative precision of 4-9% and a mean error of -0.33 to 0.13 fm. In the same range of impact parameters, a model with only track information has a relative precision of 4-10% and a mean error of -0.18 to 0.22 fm. This new method of event-classification is shown to be more accurate and less model dependent than conventional methods and can utilize the performance boost of modern GPU processor units.

A novel method for impact force estimation in composite plates under simulated environmental and operational conditions

During its lifetime, an aircraft structure is subjected to various impacts from various sources such as tool drops, hail, ground service equipment, etc. In modern composite structures, these impacts have a significant chance of generating barely visible damage (BVID) which may lead to catastrophic failure of a structure if left undetected to grow. However, BVID is difficult to detect during routine visual inspection without specialised non-destructive inspection and thus there is large interest in developing monitoring systems for estimating the location and severity of impact events. Currently, most systems and methods have been developed for controlled lab conditions and do not consider the wide range of impact parameters in real life operation (environmental conditions, vibration, impactor stiffness, angle, etc) which may severely compromise the accuracy of these methods. In this study we have explored two methods for maximum impact force estimation, deconvolution and a novel gradient method, for the purpose of reliable severity assessment in composite aircraft structures under simulated environmental and operational conditions. It is shown that both methods allow accurate and robust estimation of the maximum impact force from various cases of impacts (variation of impact energy, mass, stiffness, angle, temperature, source) using minimum initial data from a single impact case. From further testing it is demonstrated that the gradient method is robust towards the effects of impact localisation errors and noise. The gradient method also has much less computational and storage requirements and is thus more feasible to integrate with current data acquisition systems being developed for structural health monitoring. Thus, we conclude that the proposed gradient method is suitable for impact force monitoring and severity assessment in composite aircraft structures in the simulated environmental and operational conditions.

The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2

ABSTRACT We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ∼ 2. Using the spectra of ≃3000 galaxies with redshifts 〈z〉 = 2.3 ± 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3–500 arcsec and spectroscopic redshifts, with 〈zfg〉 = 2.23 and 〈zbg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess $\rm{H\,{\small I}}$$\rm Ly\,\alpha$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ Dtran/pkpc ≲ 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of $\rm{H\,{\small I}}$ gas as a function of projected distance Dtran. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of $\rm{H\,{\small I}}$. The best-fitting model suggests that galaxy-scale outflows with initial velocity vout ≃ 600 km s$^{-1}\,$ dominate the kinematics of circumgalactic $\rm{H\,{\small I}}$ out to Dtran ≃ 50 kpc, while $\rm{H\,{\small I}}$ at Dtran ≳ 100 kpc is dominated by infall with characteristic vin ≲ circular velocity. Over the impact parameter range 80 ≲ Dtran/pkpc ≲ 200, the $\rm{H\,{\small I}}$ line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame $\rm Ly\,\alpha$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond Dtran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.

Deep inelastic vortex scattering: A third outcome for head-on collisions

Results are presented from numerical simulations of the flat-space nonlinear Maxwell-Klein-Gordon equations demonstrating deep inelastic scattering of $m=1$ vortices for a range of Ginzburg-Landau (or Abelian-Higgs) parameters ($\kappa$), impact parameters ($b$), and initial velocities ($v_0$). The threshold ($v_0^*$) of right-angle scattering is explored for head-on ($b=0$) collisions by varying $v_0$. Solutions obey time-scaling laws, $T\propto \alpha\ln(v_0-v_0^*) $, with $\kappa$-dependent scaling exponents, $\alpha$, and have $v_0^*$ that appear not to have the previously reported upper bound. The arbitrarily long-lived static intermediate attractor at criticality ($v_0=v_0^*$) is observed to be the $\kappa$-specific $m=2$ vortex solution. Scattering angles are observed for off-axis ($b\neq 0$) collisions for a wide range of $b$, $v_0$, and $\kappa$. It is shown that for arbitrarily small impact parameters ($b\rightarrow 0$), the unstable %but arbitrarily long-lived $\kappa$-dependent $m=2$ "critical" vortex is an intermediate attractor and decays with a $\kappa$-\emph{independent} scattering angle of $135^{\circ}$, as opposed to either of the well-known values of $180^{\circ}$ or $90^{\circ}$ for $b=0$.

Impact parameter sensitive study of inner-shell atomic processes in Xe54+, Xe52+ → Xe collisions

SynopsisIn this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms. The projectile and target x-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35 - 70 fm.

Fast transient spray cooling of a hot thick target

Spray cooling of a hot target is characterized by strong heat flux and fast change of the temperature of the wall interface. The heat flux during spray cooling is determined by the instantaneous substrate temperature, which is illustrated by boiling curves. The variation of the heat flux is especially notable during different thermodynamic regimes: film, transitional and nucleate boiling. In this study transient boiling curves are obtained by measurement of the local and instantaneous heat flux produced by sprays of variable mass flux, drop diameter and impact velocity. These spray parameters are accurately characterized using a phase Doppler instrument and a patternator. The hydrodynamic phenomena of spray impact during various thermodynamic regimes are observed using a high-speed video system. A theoretical model has been developed for heat conduction in the thin expanding thermal boundary layer in the substrate. The theory is able to predict the evolution of the target temperature in time in the film boiling regime. Moreover, a remote asymptotic solution for the heat flux during the fully developed nucleate boiling regime is developed. The theoretical predictions agree very well with the experimental data for a wide range of impact parameters.

Anisotropy of electronic stopping power in graphite

The rate of energy transfer from ion projectiles onto the electrons of a solid target is hard to determine experimentally in the velocity regime between the adiabatic limit and the Bragg peak. First-principles simulations have lately offered relevant new insights and quantitative information for prototypical homogeneous materials. Here we study the influence of structural anisotropy on electronic stopping power with time-dependent density functional theory simulations of a hydrogen projectile in graphite. The projectile traveled at a range of angles and impact parameters for velocities between 0.1 and 1.4 a.u., and the electronic stopping power was calculated for each simulation. After validation with average experimental data, the anisotropic crystal structure was found to have a strong influence on the stopping power, with a difference between simulations parallel and perpendicular to the graphite plane of up to 25%, more anisotropic than expected based on previous work. The velocity dependence at low velocity displays clear linear behavior in general, except for projectiles traveling perpendicular to graphitic layers, for which a threshold-like behavior is obtained. For projectiles traveling along graphitic planes metallic behaviour is observed with a change of slope when the projectile velocity reaches the Fermi velocity of the electrons.

Dynamical Properties of Merging Galaxy Clusters from Simulated Analogs

Abstract Merging galaxy clusters may provide a unique window into the behavior of dark matter and the evolution of member galaxies. To interpret these natural collider experiments, we must account for how much time has passed since pericenter passage (TSP), the maximum relative speed of the merging subclusters, merger phase (outbound after first pericenter or returning for second pericenter), and other dynamical parameters that are not directly observable. These quantities are often inferred from staged simulations or analytical timing arguments that include neither substructure, nor large-scale structure, nor a cosmologically motivated range of impact parameters. We include all these effects by extracting dynamical parameters from analog systems in a cosmological n-body simulation, and we present constraints for 11 observed systems. The TSP and viewing angles we derive are consistent with those of staged hydrodynamical simulations, but we find lower maximum speeds. Compared to the analytical MCMAC method, we find lower TSP and viewing angles that put the separation vector closer to the plane of the sky; we attribute this to the MCMAC assumption of zero pericenter distance. We discuss potential extensions to the basic analog method, as well as complementarities between methods.

Comparison between calculations with the AMD code and experimental data for peripheral collisions of Nb93+93Nb,116Sn at 38 MeV/nucleon

Experimental data concerning binary events in peripheral collisions for the systems 93Nb+93Nb and 93Nb+116Sn at 38 MeV/nucleon, collected with the Fiasco setup, are compared with calculations performed with the dynamic code AMD, coupled with the statistical code Gemini used as an afterburner. The comparison focuses on the properties of the quasiprojectile (QP) and on the total multiplicities of the emitted light charged particles. A good reproduction of the average mass ratio, charge Z and c.m. angle of the QP is obtained in the examined impact parameter range (b~7-12 fm). Concerning the light charged particles, a general agreement is found for the total emitted charge, while some discrepancy remains for the multiplicities of the various species, especially for the protons which are always overestimated by the calculations.

Classical theory of laser-assisted spontaneous bremsstrahlung

We study the process of laser-assisted spontaneous electron bremsstrahlung by running classical trajectories in a combined Coulomb and laser (ac) fields. Due to chaotic scattering in the combined Coulomb and ac fields, the radiation probability as a function of the impact parameter and the constant phase of the laser field exhibits fractal structures. However, these structures are smeared out when the cross section is integrated over the impact parameter and averaged over the phase. We analyze the role of different types of orbits, including the trapped orbits, and the dependence of the radiation probability on the impact parameter and the initial phase of the ac field. We show that, at low incident electron kinetic energy, the Coulomb focusing leads to a substantial extension of the range of impact parameters contributing to the bremsstrahlung cross section and results in a substantial increase (by one to two orders of magnitude) of the cross section as compared with the pure Coulomb case. As examples, we discuss the case of relatively high ponderomotive energy ${E}_{p}$ when we obtain an efficient production of photons with frequencies up to $2{E}_{p}$, and the case of low ${E}_{p}$ when only infrared photons are produced. Overall accuracy of the classical approach is estimated to be very good, although it does not describe resonant processes studied previously by quantum-mechanical methods.

Modification of nanodiamonds by xenon implantation: A molecular dynamics study

Xenon implantation into nanodiamonds is studied using molecular dynamics. The nanodiamonds range in size from 2 to 10 nm and the implantation energy extends up to 40 keV. For small nanodiamonds an energy-window effect occurs in which xenon energies of around 6 keV destroy the nanodiamond, while in larger nanodiamonds the radiation cascade is increasingly similar to those in bulk material. Destruction of the small nanodiamonds occurs due to thermal annealing associated with the small size of the particles and the absence of a heat-loss path. Simulations are also performed for a range of impact parameters, and for a series of double-nanodiamond systems in which a heat-loss path is present. The latter show that the thermal shock caused by the impact occurs on the timescale of a few picoseconds. These findings are relevant to ion-beam modification of nanoparticles by noble gases as well as astrophysics studies where implantation is proposed as the mechanism for xenon incorporation in pre-solar nanodiamonds.

Tidal disruptions by rotating black holes: effects of spin and impact parameter

Abstract We present the results of relativistic smoothed particle hydrodynamics simulations of tidal disruptions of stars by rotating supermassive black holes, for a wide range of impact parameters and black hole spins. For deep encounters, we find that: relativistic precession creates debris geometries impossible to obtain with the Newtonian equations; part of the fluid can be launched on plunging orbits, reducing the fallback rate and the mass of the resulting accretion disc; multiple squeezings and bounces at periapsis may generate distinctive X-ray signatures resulting from the associated shock breakout; disruptions can occur inside the marginally bound radius, if the angular momentum spread launches part of the debris on non-plunging orbits. Perhaps surprisingly, we also find relativistic effects important in partial disruptions, where the balance between self-gravity and tidal forces is so precarious that otherwise minor relativistic effects can have decisive consequences on the stellar fate. In between, where the star is fully disrupted but relativistic effects are mild, the difference resides in a gentler rise of the fallback rate, a later and smaller peak, and longer return times. However, relativistic precession always causes thicker debris streams, both in the bound part (speeding up circularization) and in the unbound part (accelerating and enhancing the production of separate transients). We discuss various properties of the disruption (compression at periapsis, shape and spread of the energy distribution) and potential observables (peak fallback rate, times of rise and decay, duration of super-Eddington fallback) as a function of the impact parameter and the black hole spin.

Chemical Dynamics Simulations on Association and Ensuing Dissociation of a Benzene-Hexafluorobenzene Molecular System.

Chemical dynamics simulations are performed to study the association of benzene (Bz) and hexafluorobenzene (HFB) followed by the ensuing dissociation of the Bz-HFB complex. The calculations are done for 1000, 1500, and 2000 K with an impact parameter ( b) range of 0-10 Å at each temperature. Almost no complexes are observed to form at b = 8 and 10 Å. Following three different methods of calculation of the temperature-dependent association rate constant kasso( T), the values obtained are 1.67 × 10-10, 1.86 × 10-10, and 2.05 × 10-10 cm3/molecule·s with a standard deviation of approximately 0.1 × 10-10 cm3/molecule·s for T = 1500 K. Among those values of kasso( T), the middle one is obtained by considering a relative translational energy of 3 RT/2 at T = 1500 K, and the same is followed to calculate kasso( T) at 1000 and 2000 K. The Arrhenius parameters, using the kasso( T) values at three temperatures, are 0.203 × 10-10 cm3/molecule·s for the pre-exponential factor and -5.79 kcal/mol for the activation energy. The absolute value of the latter is similar to the Bz + HFB association energy of 5.93 kcal/mol. The ensuing dissociation dynamics of the complex is significantly different from the unimolecular dissociation dynamics, and an exponential function fits the N( t - t0)/ N( t0) curves comparatively well. The ensuing dissociation is also observed to be independent of time for a statistically large sample size.

A Mean-field Approach to Simulating the Merging of Collisionless Stellar Systems Using a Particle-based Method

Abstract We present a mean-field approach to simulating merging processes of two spherical collisionless stellar systems. This approach is realized with a self-consistent field (SCF) method in which the full spatial dependence of the density and potential of a system is expanded in a set of basis functions for solving Poisson’s equation. To apply this SCF method to a merging situation where two systems are moving in space, we assign the expansion center to the center of mass of each system, the position of which is followed by a massless particle placed at that position initially. Merging simulations over a wide range of impact parameters are performed using both an SCF code developed here and a tree code. The results of each simulation produced by the two codes show excellent agreement in the evolving morphology of the merging systems and in the density and velocity dispersion profiles of the merged systems. However, comparing the results generated by the tree code with those obtained with the softening-free SCF code, we found that in large impact parameter cases, a softening length of the Plummer type introduced in the tree code has an effect of advancing the orbital phase of the two systems in the merging process at late times. We demonstrate that the faster orbital phase originates from the larger convergence length to the pure Newtonian force. Other application problems suitable to the current SCF code are also discussed.

Lights and (some) shadows in the comparison among experimental data of heavy ion collisionat Fermi energies and the dynamical model AMD

The simulation of heavy ion collisions in the Fermi energy region is a challenge for the theoretical models; in particular it is difficult to obtain a coherent description in all the impact parameter range and to reproduce all the experimental observables. In this contribution we will show the very good job done by the dynamical model AMD [1] followed by the statistical code GEMINI [2, 3] as an afterburner. The model is able to reproduce the main characteristics of peripheral and semiperipheral collisions, although some discrepancies still persist.

Splashing characteristics of diesel exhaust fluid (AdBlue) droplets impacting on urea-water solution films

The rapid implementation of Selective-Catalytic-Reduction (SCR) technologies into light passenger and commercial vehicles, led to the omission of fundamental research creating several reliability issues that are largely related to the research field of droplet dynamics. In this study, the splashing behaviour of an AdBlue droplet impacting onto thin urea-water solution films is experimentally investigated over a range of impact parameters. In particular, the crown-type splashing threshold, the number of fingers and the characteristics of the ejected secondary droplets are evaluated for various drop impact velocities, wall-film thicknesses and urea concentrations in the liquid film. The results show that impact parameters that are able to enhance the energy dissipation in the wall-film, e.g. film thickness and viscosity, influence negatively the intensity of splashing. On the other hand, as the droplet kinetic energy increases or the wall-film thickness decreases, more energy is available to intensify the splashing outcome, and consequently, the upward ejected secondary droplet volume. The obtained trends are correlated in simple empirical expressions providing a remarkable industrial design tool, and they are also compared to the state-of-the-art literature of single- and binary-droplet/wall-film interactions aiming to draw generalised theories and paradigms that will support the connection between SCR applications and academic research.

Formation and incidence of shell galaxies in the Illustris simulation

Shells are low surface brightness tidal debris that appear as interleaved caustics with large opening angles, often situated on both sides of the galaxy center. In this paper, we study the incidence and formation processes of shell galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We identify shells at redshift z=0 using stellar surface density maps, and we use stellar history catalogs to trace the birth, trajectory and progenitors of each individual star particle contributing to the tidal feature. Out of a sample of the 220 most massive galaxies in Illustris ($\mathrm{M}_{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}_{\odot}$), $18\%\pm3\%$ of the galaxies exhibit shells. This fraction increases with increasing mass cut: higher mass galaxies are more likely to have stellar shells. Furthermore, the fraction of massive galaxies that exhibit shells decreases with increasing redshift. We find that shell galaxies observed at redshift $z=0$ form preferentially through relatively major mergers ($\gtrsim$1:10 in stellar mass ratio). Progenitors are accreted on low angular momentum orbits, in a preferred time-window between $\sim$4 and 8 Gyrs ago. Our study indicates that, due to dynamical friction, more massive satellites are allowed to probe a wider range of impact parameters at accretion time, while small companions need almost purely radial infall trajectories in order to produce shells. We also find a number of special cases, as a consequence of the additional complexity introduced by the cosmological setting. These include galaxies with multiple shell-forming progenitors, satellite-of-satellites also forming shells, or satellites that fail to produce shells due to multiple major mergers happening in quick succession.

Binary collision of a burning droplet and a non-burning droplet of xylene: Outcome regimes and flame dynamics

The binary collisions of a burning droplet and a non-burning droplet of xylene are experimentally investigated. The experimental parameters span an extensive range of Weber number and impact parameter, covering the collision outcome regimes of coalescence, reflexive separation, and stretching separation. A high-speed camera captures the temporal details of the collision process, involving flame spread, visible radiation, and flame distributions around droplets. For reflexive separation and stretching separation, the flame from the droplet spreads to the ligament, surrounding it during the interaction process, and then spreads around separated droplets and satellite droplets. Highly-interactive flames are formed in-between the droplets, with very sooty flames generated for most collisions. For the coalescence case, a swirling flame forms around the rotating coalesced droplet. For similar Weber numbers, visible flame radiation is compared for different collision regimes. The visible flame radiation changes more significantly for the reflexive and stretching separation cases than it does for the coalescence case. The change of the averaged visible flame radiation for reflexive separation and stretching separation is more than two times higher than that for coalescence. The map of three different collision regimes is plotted in the Weber number versus impact parameter domain and compared with available theoretical model predictions. Although the different outcomes of collision with the presence of flame can be well predicted by the model, using fluid properties determined by the averaged properties of the two droplets, the dynamics of the detailed processes involved in the collisions are very interesting and have strong implications on overall combustion behavior that go well beyond the mapped regimes.

Fragment distribution in 78,86Kr+181Ta reactions**Supported by Youth Research Foundation of Shanxi Datong University (2016Q10)

Within the framework of the isospin-dependent quantum molecular dynamics model, along with the GEMINI model, the 86Kr+181Ta reaction at 80, 120 and 160 MeV/nucleon and the 78Kr+181Ta reaction at 160 MeV/nucleon are studied, and the production cross sections of the generated fragments are calculated. More inter-mediate and large mass fragments can be produced in the reactions with a large range of impact parameter. The production cross sections of nuclei such as the isotopes of Si and P generally decrease with increasing incident energy. Isotopes near the neutron drip line are produced more in the neutron-rich system 86Kr+181Ta.