Energetic Collisions Research Articles

Stardust silicate nucleation kick-started by SiO+TiO 2

Dust particles are quintessential for the chemical evolution of the Universe. Dust nucleates in stellar outflows of dying stars and subsequently travels through the interstellar medium, continuously evolving via energetic processing, collisions and condensation. Finally, dust particles are incorporated in the next-generation star or its surrounding planetary system. In oxygen-rich stellar outflows, silicates are observed in the condensation zone (1200-1000 K), but, in spite of several decades of experimental and theoretical study, the stardust nucleation process remains poorly understood. We have previously shown that under these conditions ternary Mg-Si-O clusters may start forming at high enough rates from SiO, Mg and H₂O through heteromolecular association processes. In this reaction scheme, none of the possible initial association reactions was thermodynamically favourable owing to the large entropy loss at these temperatures. Here, we follow a previous idea that the incorporation of TiO₂ could help to initiate stardust nucleation. In contrast to these studies, we find that there is no need for TiO₂ cluster seeds-instead, one molecule of TiO₂ is sufficient to kick-start the subsequent nucleation of a silicate dust particle.

Rb–Sr chronology of volatile depletion in differentiated protoplanets: BABI, ADOR and ALL revisited

A strong depletion in moderately volatile elements is a characteristic feature of many planetary bodies in the inner solar system and either reflects the rapid accretion of planetesimals from an incompletely condensed solar nebula, or is the result of energetic collisions during planetary accretion. To better constrain the origin and timescales of this volatile depletion, we have precisely measured Sr isotope compositions in angrites, eucrites and Ca–Al-rich inclusions (CAI). Angrites have an initial (87Sr/86Sr)ADOR=0.698978±0.000004, which is indistinguishable from (87Sr/86Sr)BABI=0.698970±0.000028 obtained for eucrites. In agreement with earlier studies we find that angrites and eucrites have higher initial 87Sr/86Sr ratios than CAI, at face value corresponding to model timescales for volatile loss of several millions of years. However, all the investigated CAI are characterized by elevated 84Sr/86Sr ratios compared to angrites and eucrites, which we interpret to reflect an excess of r-process Sr in the CAI. If this is correct, then the nucleosynthetic Sr isotope anomalies in the CAI require an upward correction of their measured 87Sr/86Sr. After this correction CAI have an initial (87Sr/86Sr)CAI=0.698975±0.000008, which is indistinguishable from ADOR and BABI. This implies volatile loss from the angrite and eucrite parent bodies within less than ~1Ma after formation of CAI. The volatile-depleted nature of these differentiated protoplanets thus most likely reflects their rapid accretion from volatile-poor dust in an incompletely condensed solar nebula.

A new observable to measure the top-quark mass at hadron colliders

A new method to measure the top-quark mass in high energetic hadron collisions is presented. We use theoretical predictions calculated at next-to-leading order accuracy in quantum chromodynamics to study the (normalized) differential distribution of the $t\bar{t} + 1\mbox{-jet}$ cross section with respect to its invariant mass $\sqrt{s_{t\bar{t} j}}$ . The sensitivity of the method to the top-quark mass together with the impact of various theoretical and experimental uncertainties has been investigated and quantified. The new method allows for a complementary measurement of the top-quark mass parameter and has a high potential to become competitive in precision with respect to established approaches. Furthermore we emphasize that in the proposed method the mass parameter is uniquely defined through one-loop renormalization.

Towards a nonequilibrium Green's function description of nuclear reactions

Semiclassical time-dependent approaches are nowadays able to describe energetic central collisions of heavy isotopes with minimal assumptions. Simulations in the mean-field picture have provided insight on both nuclear structure and low-energy many-body reaction mechanisms. In this context, nonequilibrium Greens functions techniques have the potential to improve the description of the time evolution of nuclear systems by introducing effects beyond the mean-field. We describe a first attempt to use the Kadanoff-Baym dynamics for one-dimensional nuclear systems, with a particular emphasis on the process of correlation buildup.

Low Temperature Cu Etching Using CH4-Based Plasmas

Methane- (CH4) based subtractive plasma etching of Cu was investigated at low temperature. The Cu etch rate using a CH4 plasma (17 nm/min) was higher than that of an H2 plasma (13 nm/min) under the same plasma conditions, despite the fact that hydrocarbon deposition was evident with CH4. The higher Cu etch rate is ascribed to increased momentum transfer due to the heavier mass of CH+ and C+ relative to H+ and to the more thermodynamically stable etch products of CH3Cu or CH3CuH− relative to CuHx. Methyl-containing Cu etch product species were not detected by optical emission spectroscopy, most likely because Cu-CH3 was readily dissociated by energetic electron impact collisions after desorption from the Cu surface. Patterns generated in Cu displayed a sidewall angle of ∼80°, similar to that of H2 plasma patterning, but unlike the result in an H2 plasma, a photoresist mask was not significantly degraded during the etch process.

Active debris removal of multiple priority targets

Today’s space debris environment shows major concentrations of objects within distinct orbital regions for nearly all size regimes. The most critical region is found at orbital altitudes near 800km with high declinations. Within this region many satellites are operated in so called sun-synchronous orbits (SSO). Among those, there are Earth observation, communication and weather satellites. Due to the orbital geometry in SSO, head-on encounters with relative velocities of about 15km/s are most probable and would thus result in highly energetic collisions, which are often referred to as catastrophic collisions, leading to the complete fragmentation of the participating objects. So called feedback collisions can then be triggered by the newly generated fragments, thus leading to a further population increase in the affected orbital region. This effect is known as the Kessler syndrome.Current studies show that catastrophic collisions are not a major problem today, but will become the main process for debris generation within the SSO region in the near future, even without any further launches. In order to avoid this effect, objects with a major impact on collisional cascading have to be actively removed from the critical region after their end of life. Not having the capability to perform an end-of-life maneuver in order to transfer to a graveyard orbit or to de-orbit, many satellites and rocket bodies would have to be de-orbited within a dedicated mission. In such a mission, a service satellite would perform a de-orbit maneuver, after having docked to a specific target.In this paper, chemical and electric propulsion systems were analysed with the main focus on removing multiple targets within one single mission. The targets were chosen from a previously defined priority list in order to enhance the mission efficiency. Total mission time, ΔV and system mass were identified as key parameters to allow for an evaluation of the different concepts. It was shown that it is possible to remove up to five high priority targets per year using a chemical propulsion system, however, missions may result in too high ΔV and/or mission duration depending on the orbital distribution of the targets. When using an electric propulsion system, the required fuel mass is significantly reduced when compared to the chemical propulsion system, but it was shown that mission duration strongly depends on the mass of the selected targets. More powerful engines as well as out-of-plane thrust are thus required to achieve the defined mission goals.

Does Electron Capture Dissociation Cleave Protein Disulfide Bonds?

Peptide and protein characterization by mass spectrometry (MS) relies on their dissociation in the gas phase into specific fragments whose mass values can be aligned as ‘mass ladders’ to provide sequence information and to localize possible posttranslational modifications. The most common dissociation method involves slow heating of even-electron (M+n H)n+ ions from electrospray ionization by energetic collisions with inert gas, and cleavage of amide backbone bonds. More recently, dissociation methods based on electron capture or transfer were found to provide far more extensive sequence coverage through unselective cleavage of backbone N–Cα bonds. As another important feature of electron capture dissociation (ECD) and electron transfer dissociation (ETD), their unique unimolecular radical ion chemistry generally preserves labile posttranslational modifications such as glycosylation and phosphorylation. Moreover, it was postulated that disulfide bond cleavage is preferred over backbone cleavage, and that capture of a single electron can break both a backbone and a disulfide bond, or even two disulfide bonds between two peptide chains. However, the proposal of preferential disulfide bond cleavage in ECD or ETD has recently been debated. The experimental data presented here reveal that the mechanism of protein disulfide bond cleavage is much more intricate than previously anticipated.

Young-type interference in projectile-electron loss in energetic ion-molecule collisions

We propose an alternative and very promising scenario for studying the Young-type-interference phenomena in energetic ion-molecule collisions. In such a scenario, we focus on the energy spectra of electrons that are ionized from the bound states of fast moving projectiles. We argue that under certain conditions these (projectile) electrons interact mainly with the nuclei and inner-shell electrons of atoms forming the molecule. Due to their compact localization in space and distinct separation from each other, these molecular centers play a role similar to that of optical slits in light scattering leading to pronounced interference patterns in electron energy distributions.

Characterization of Modified RNA by Top‐Down Mass Spectrometry

Characterization of Modified RNA by Top‐Down Mass Spectrometry

Shock vaporization of silica and the thermodynamics of planetary impact events

The most energetic planetary collisions attain shock pressures that result in abundant melting and vaporization. Accurate predictions of the extent of melting and vaporization require knowledge of vast regions of the phase diagrams of the constituent materials. To reach the liquid‐vapor phase boundary of silica, we conducted uniaxial shock‐and‐release experiments, where quartz was shocked to a state sufficient to initiate vaporization upon isentropic decompression (hundreds of GPa). The apparent temperature of the decompressing fluid was measured with a streaked optical pyrometer, and the bulk density was inferred by stagnation onto a standard window. To interpret the observed post‐shock temperatures, we developed a model for the apparent temperature of a material isentropically decompressing through the liquid‐vapor coexistence region. Using published thermodynamic data, we revised the liquid‐vapor boundary for silica and calculated the entropy on the quartz Hugoniot. The silica post‐shock temperature measurements, up to entropies beyond the critical point, are in excellent qualitative agreement with the predictions from the decompressing two‐phase mixture model. Shock‐and‐release experiments provide an accurate measurement of the temperature on the phase boundary for entropies below the critical point, with increasing uncertainties near and above the critical point entropy. Our new criteria for shock‐induced vaporization of quartz are much lower than previous estimates, primarily because of the revised entropy on the Hugoniot. As the thermodynamics of other silicates are expected to be similar to quartz, vaporization is a significant process during high‐velocity planetary collisions.

Explosion Welding High Temperature and Wear Resistant Metal Liners into 25 mm Gun Barrels

This paper describes the explosion welding process and how it has been adapted for welding high temperature and wear resistant metal liners into 25 mm gun barrels. Explosion welding is a process that is used to join two dissimilar metals together. Explosives are used to drive two plates together at a very high velocity. The energetic collision cleans both surfaces and produces a metallurgical joint between the plates. This process has been used commercially since the mid-1960s in many industrial and defense applications. The vast majority of explosion welding has been performed in two specific geometries, welding two flat plates together and welding a tube around the outside diameter of a solid rod. Explosion welding a liner into a gun barrel is unique since the explosive is detonated inside of the liner, expanding the liner out into the gun barrel. High Energy Metals has explosion welded Stellite, Ta-5 W-2Mo, and Ta-10 W on the inside of 12″ and 36″ long 25 mm gun barrel sections. Additionally, Ta-10W has been explosion welded on the inside diameter of 800″ long steel forgings. The lined forgings then have been rifled and machined into complete 25 mm Bushmaster guns.

THECHANDRAX-RAY SURVEY OF PLANETARY NEBULAE (CHANPLANS): PROBING BINARITY, MAGNETIC FIELDS, AND WIND COLLISIONS

We present an overview of the initial results from the Chandra Planetary Nebula Survey (ChanPlaNS), the first systematic (volume-limited) Chandra X-ray Observatory survey of planetary nebulae (PNe) in the solar neighborhood. The first phase of ChanPlaNS targeted 21 mostly high-excitation PNe within ~1.5 kpc of Earth, yielding 4 detections of diffuse X-ray emission and 9 detections of X-ray-luminous point sources at the central stars (CSPNe) of these objects. Combining these results with those obtained from Chandra archival data for all (14) other PNe within ~1.5 kpc that have been observed to date, we find an overall X-ray detection rate of ~70%. Roughly 50% of the PNe observed by Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing shocks formed by energetic wind collisions is detected in ~30%; five objects display both diffuse and point-like emission components. The presence of X-ray sources appears correlated with PN density structure, in that molecule-poor, elliptical nebulae are more likely to display X-ray emission (either point-like or diffuse) than molecule-rich, bipolar or Ring-like nebulae. All but one of the X-ray point sources detected at CSPNe display X-ray spectra that are harder than expected from hot (~100 kK) central star photospheres, possibly indicating a high frequency of binary companions to CSPNe. Other potential explanations include self-shocking winds or PN mass fallback. Most PNe detected as diffuse X-ray sources are elliptical nebulae that display a nested shell/halo structure and bright ansae; the diffuse X-ray emission regions are confined within inner, sharp-rimmed shells. All sample PNe that display diffuse X-ray emission have inner shell dynamical ages <~5x10^3 yr, placing firm constraints on the timescale for strong shocks due to wind interactions in PNe.

Mechanistic investigation of the sonochemical synthesis of zinc ferrite

In this investigation, an attempt has been made to establish the physical mechanism of sonochemical synthesis of zinc ferrite with concurrent analysis of experimental results and simulations of cavitation bubble dynamics. Experiments have been conducted with mechanical stirring as well as under ultrasound irradiation with variation of pH and the static pressure of the reaction medium. Results of this study reveal that physical effects produced by transient cavitation bubbles play a crucial role in the chemical synthesis. Generation of high amplitude shock waves by transient cavitation bubbles manifest their effect through in situ micro-calcination of metal oxide particles (which are generated through thermal hydrolysis of metal acetates) due to energetic collisions between them. Micro-calcination of oxide particles can also occur in the thin liquid shell surrounding bubble interface, which gets heated up during transient collapse of bubbles. The sonochemical effect of production of OH radicals and H2O2, in itself, is not able to yield ferrite. Moreover, as the in situ micro-calcination involves very small number of particles or even individual particles (as in intra-particle collisions), the agglomeration between resulting ferrite particles is negligible (as compared to external calcination in convention route), leading to ferrite particles of smaller size (6nm).

Hard-photon flow and photon-photon correlation in intermediate-energy heavy-ion collisions

Hard photons emitted from energetic heavy-ion collisions are very interesting since they do not experience nuclear interaction, and therefore they are useful to explore properties of nuclear matter. We investigated hard-photon production and its properties in intermediate-energy heavy-ion collisions with the help of the Blotzmann-Uehling-Ulenbeck model. Two components of hard photons are discussed: direct and thermal. The positive directed flow parameter and negative elliptic flow parameter of direct photons are demonstrated and they are anticorrelated to the flows of free protons. The dependencies of hard-photon production and anisotropic parameters on impact parameter, beam energy, nuclear equation of state, and symmetry energy are also discussed. Furthermore, we investigated the two-photon momentum correlation function, from which the space-time structure information of the photon source could be extracted, as well as the two-photon azimuthal correlation, which could provide another good method to determine the elliptic flow parameter ${v}_{2}$ of direct hard photons.

BLACK HOLES AND ASYMPTOTICALLY SAFE GRAVITY

Quantum gravitational corrections to black holes are studied in four and higher dimensions using a renormalisation group improvement of the metric. The quantum effects are worked out in detail for asymptotically safe gravity, where the short-distance physics is characterized by a nontrivial fixed point of the gravitational coupling. We find that a weakening of gravity implies a decrease of the event horizon, and the existence of a Planck-size black hole remnant with vanishing temperature and vanishing heat capacity. The absence of curvature singularities is generic and discussed together with the conformal structure and the Penrose diagram of asymptotically safe black holes. The production cross-section of mini-black holes in energetic particle collisions, such as those at the Large Hadron Collider, is analysed within low-scale quantum gravity models. Quantum gravity corrections imply that cross-sections display a threshold, are suppressed in the Planckian, and reproduce the semiclassical result in the deep trans-Planckian region. Further implications are discussed.

The onset of inelastic collapse in a one-dimensional granular gas

We examine the onset of inelastic collapse for a cluster of 10 ≤ N ≤ 50 grains colliding with a fixed wall. When the restitution coefficient ẽ is less than a threshold, the cluster does not bounce off the wall, but comes to rest after an infinite number of collisions. Near the collapse threshold, a nearly periodic sequence of collisions is established that can be identified by tracking the most energetic collisions. The length of this sequence grows exponentially with N. The dispersal of a colliding cluster can be compared to the decay of a radioactive nucleus: the cluster has a fixed probability per unit time (measured in collisions) of dispersing. The “half-life” of the cluster grows exponentially as the restitution coefficient is decreased. Thus the onset of inelastic collapse is characterized by an exponentially growing time scale.

Molecular-Dynamics-Based Study of the Collisions of Hyperthermal Atomic Oxygen with Graphene Using the ReaxFF Reactive Force Field

In this work, we have investigated the hyperthermal collisions of atomic oxygens with graphene through molecular dynamics simulations using the ReaxFF reactive force field. First, following Paci et al. (J. Phys. Chem. A 2009, 113, 4677 - 4685), 5-eV energetic collisions of atomic oxygen with a 24-atom pristine graphene sheet and a sheet with a single vacancy defect, both functionalized with oxygen atoms in the form of epoxides, were studied. We found that the removal of an O(2) molecule from the surface of the graphene sheet occurs predominantly through an Eley-Rideal-type reaction mechanism. Our results, in terms of the number of occurrences of various reactive events, compared well with those reported by Paci et al. Subsequently, energetic collisions of atomic oxygen with a 25-times-expanded pristine sheet were investigated. The steady-state oxygen coverage was found to be more than one atom per three surface carbon atoms. Under an oxygen impact, the graphene sheet was always found to buckle along its diagonal. In addition, the larger sheet exhibited trampoline-like behavior, as a result of which we observed a much larger number of inelastic scattering events than those reported by Paci et al. for the smaller system. Removal of O(2) from the larger sheet occurred strictly through an Eley-Rideal-type reaction. Investigation of the events leading to the breakup of a pristine unfunctionalized graphene sheet and the effects of the presence of a second layer beneath the graphene sheet in an AB arrangement was done through successive impacts with energetic oxygen atoms on the structures. Breakup of a graphene sheet was found to occur in two stages: epoxide formation, followed by the creation and growth of defects. Events leading to the breakup of a two-layer graphene stack included epoxide formation, transformation from an AB to an AA arrangement as a result of interlayer bonding, defect formation and expansion in the top layer, and finally erosion of the bottom layer. We observed that the breakup of the two-layer stack occurred through a sequential, layer-by-layer, erosion process.

Constraints on the density dependence of the symmetry energy from heavy-ion collisions

Constraints on the equation of state (EoS) for symmetric matter (equal neutron and proton numbers) have been extracted from energetic collisions of heavy ions over a range of energies. Collisions of neutron-deficient and neutron-rich heavy ions now provide initial constraints on the EoS of neutron-rich matter at subsaturation densities from isospin diffusions and neutron proton ratios. This article reviews the experimental constraints on the density dependence of symmetry energy at subsaturation density.

Effect of deposition power on structural and electrical properties of Al-doped ZnO films using pulsed direct-current magnetron sputtering with single cylindrical target

The change of stoichiometric ratio caused by the increase in sputtering power in Al-doped ZnO (AZO) films was investigated, where the films were grown by pulsed dc magnetron sputtering with a cylindrical target. The properties of the films were strongly affected by the sputtering power. Lower sputtering power was found suitable for the growth of c-axis oriented AZO films. At 2 kW, assisted proper energetic particles, the high quality films having a proper stoichiometric ratio of ZnO were obtained. Whereas, at 4 kW, highly accelerated oxygen ions and recoiling Ar atoms strongly collide with target and these energetic collisions then degrade the quality of films. The average carrier concentrations of all the samples were about 1.5×10 21 cm −3. For a sputtering power of 2 kW, the lowest value of resistivity was estimated to be 7.69×10 −4 Ω cm along with the highest Hall mobility of 6.2 cm 2 V −1 s −1. The deposition rate of AZO films at this power was 130.8 nm/min. The lower sputtering power (2 kW) was found suitable for the fabrication of low-cost transparent conductive oxide layer for futuristic electronic devices with a transmittance of 85%.

Neutralization phenomena observed with secondary ions originating from inner‐shell excitation

AbstractThe observation of comparatively high yields of low‐energy secondary ions, ejected from clean ion‐bombarded solids, has been a long‐standing miracle. The measured velocity dependence of the ionization probability is at variance with the predicted exponential velocity dependence. It has been proposed that the recorded high yields are due to electronic excitations initiated by the collision cascade. Here, the issue is addressed by exploring energy spectra of ions generated by inner shell electron promotion during energetic collisions. Using new experimental results and literature data, ionization probabilities were determined using two different approaches. The evaluated data suggest that doubly charged ions of Mg, Al and Si (and possibly, Ca) can escape neutralization, presumably as a consequence of Auger deexcitation taking place at sufficiently large distances from the surface. The ion fractions for Sc, Ti, V and Cr are in accordance with an exponential velocity dependence at energies &gt; 300 eV, but approach a constant level at low energies. Given the fact that the electronic‐excitation model does not apply in the case of ionization by electron promotion, it is suggested that the unusually high ionization probabilities observed at low energies are due to suppressed neutralization. The idea is that in a small fraction of impact events the collision cascade may generate a very high perturbation of the surface (‘dynamic randomization’) so that a well‐defined band structure may not exist, locally and for a short period of time. In these rare cases, the conduction electrons required for neutralization are not available. Copyright © 2010 John Wiley &amp; Sons, Ltd.