Neutral Projectiles Research Articles

Dynamic screening and two-center effects in neutral and partially dressed ion-atom collisions

In collisions involving projectiles with bound electrons, partial or total screening of their nuclear charge must be considered. Accordingly, a projectile dynamic effective charge, as a function of the collision momentum transfer, is included in the state-of-the-art continuum distorted wave with eikonal initial state model. This correction, applied in the exit channel, recovers a two-center description from collisions with neutral projectiles. Also, an improvement of both target and projectile ionization double differential cross sections is achieved, especially at the binary encounter and electron capture to the continuum peaks. The results calculated by our analytical model are shown in accordance with previous theoretical and experimental work.

Collisional excitation in Neon-like projectiles scattered from Al

We performed experiments of electron emission in the interaction of singly charged Na+ and Ne+ with Al surfaces. We compare Auger emission from sodium and neon projectiles excited in the 2p level by promotion of the 4fσ molecular orbital of the transient molecule, formed during a binary collision with an Al target atom. Doubly excited states are formed in single scattering events involving neutral Neon projectiles and neon-like Na+ ions. Measurements performed with Neon projectiles at impact energy up to 10 keV reveal the onset 2s excitation via promotion of the 3pσ molecular orbital. The fact, that several promotion processes become active with increasing ion energy, has implication on recent energy loss and energy deposition studies.

The viscoelastic-like response of a repulsive granular medium during projectile impact and penetration

We study experimentally the penetration of a projectile into a two-dimensional granular bed composed of magnetic repelling grains. The projectile can experience either repulsive, attractive or neutral interaction with the particles generating different dynamics: i) the repulsive intruder compresses the granular bed without contact and experiences rebounds before stopping; ii) the attractive intruder is first accelerated when it approaches the bed, then, some magnets attach to its surface increasing its effective diameter and the drag force acting on the intruder until it reaches the repose; and iii) the neutral projectile penetrates deeper into the granular bed than in the two previous cases because there is no magnetic interaction with the particles. In addition, we developed molecular dynamics simulations able to reproduce the experimental results and used to determine the effect of the magnetic strength and the role of friction between the particles and the container walls. The results show that the medium behaves similar to a viscoelastic micellar fluid on impact, and the projectile dynamics can be modelled by a spring-dashpot model. Our findings can be useful in the design of new magnetic granular dampers.

Calibration of 3 MV Tandetron™ accelerator over nominal energy range

The method based on the measurement of proton beam energies using the non-resonant direct-capture reaction 16O(p,γ)17F* with both charged and neutral projectiles was applied to perform the energy calibration of the 3 MV Tandetron™ accelerator throughout the nominal energy range. The uncertainty of the beam energy was found to be well within the accuracy specified by the manufacturer. No deviations from the linearity were revealed.

ANITA-NC: A code system for modelling material activation induced by neutral or charged particles

ANITA-NC (Analysis of Neutron Induced Transmutation and Activation – Neutral and Charged) is an inventory system (code and libraries) developed in ENEA-Bologna, capable of modelling material activation induced by neutral or charged projectiles (i.e. neutrons, protons, deuterons, alphas or gammas). It is an extension of the previous ANITA versions. ANITA-NC requires group-wise activation cross-section libraries in EAF format. It actually uses the most recent libraries in this format, that is the EAF-2010 libraries in the VITAMIN-J+ (211) energy group structure, up to 55 MeV. For neutrons, a new library in a 271 energy group structure up to 200 MeV, based on TENDL-2017, was produced. The decay data used in ANITA-NC are based on the JEFF-3.1.1 Radioactive Decay Data Library. This paper summarizes the main characteristics of the ANITA-NC activation system. Some application cases and an example of data and code validation on experimental results are shown.

Interaction of slow highly charged ions with a metal surface covered with a thin dielectric film. The role of the neutralization energy in the nanostructures formation

We consider the slow highly charged ions impinging upon a metal surface covered with a thin dielectric film, and formation of the surface nanostructures (craters) from the standpoint of the required energy. For the moderate ionic velocities, the size of the surface features depends on the deposited kinetic energy of the projectile and the ionic neutralization energy. The neutralization energy is calculated by employing the recently developed quasi-resonant two-state vector model for the intermediate Rydberg state population and the micro-staircase model for the cascade neutralization. The electron interactions with the ionic core, polarized dielectric and charge induced on the metal surface are modelled by the appropriate asymptotic expressions and the method for calculation of the effective ionic charges in the dielectric is proposed.The results are presented for the interaction of ions (velocity a.u.; ) with the metal surface (Co) covered with a thin dielectric film, for model values of dielectric constant inside the interaction region. In the absence of dielectric film, the neutralization energy is lower than the potential (ionization) energy due to the incomplete neutralization. The presence of dielectric film additionally decreases the neutralization energy. We calculate the projectile neutralization energy in the perturbed dielectric (perturbation is caused by the ionic motion and the surface structure formation). We correlate the neutralization energy added to the deposited kinetic energy with the experimentally obtained energy necessary for the formation of the nano-crater of a given depth.

NUMERICAL ANALYSIS OF EFFECTIVENESS FOR VEHICLE NET SYSTEMS PROTECTING AGAINST SHAPED CHARGE PROJECTILES

Analysis of effectiveness of two types of net systems protecting vehicles against projectiles with cumulative (shaped charge) warhead is presented in the article. Numerical models of net screens and PG7-WM projectile were created to simulate projectile penetration into the screen. Numerical analyses were used to determine a maximal distance in radial direction from projectile axis for which screen elements should come into contact with the projectile to neutralize it. Determined distances were used to calculate the probability of neutralization of pro-jectiles by net systems. Calculations were performed for elevation and azimuth angles of α = 0÷45°. The values calculated in such way were used to make graphs of PG-7WM projectile neutralization probability for both types of net systems in function of mesh size and azimuth and elevation angles at various mesh sizes. Received results may be used to optimize geometries of net systems to in-crease effectiveness of projectile neutralization.

Collinearly improved JIMWLK evolution in Langevin form

The high-energy evolution of Wilson line operators, which at leading order is described by the Balitsky-JIMWLK equations, receives large radiative corrections enhanced by single and double collinear logarithms at next-to-leading order and beyond. We propose a method for resumming such logarithmic corrections to all orders, at the level of the Langevin formulation of the JIMWLK equation. The ensuing, collinearly-improved Langevin equation features generalized Wilson line operators, which depend not only upon rapidity (the logarithm of the longitudinal momentum), but also upon the transverse size of the color neutral projectile to which the Wilson lines belong. This additional scale dependence is built up during the evolution, via the condition that the successive emissions of soft gluons be ordered in time. The presence of this transverse scale in the Langevin equation furthermore allows for the resummation of the one-loop running coupling corrections.

Electron loss in collisions of dressed projectiles with atoms and molecules

SynopsisEnergy distributions of electrons emitted with energies corresponding to the region of the electron-loss-cusp (ELP) were measured for collisions of several neutral projectiles with atoms and small molecules. Besides the well known projectile ELP cusp, a second structure is observed for Li and Na projectiles. Comparing the experimental data with four-body CTMC calculations we analise the origin of these structures in terms of nuclear-electron and electron-electron interactions.

Comparison of electronic energy loss in graphene and BN sheet by means of time-dependent density functional theory

Time-dependent density functional theory combined with Ehrenfest dynamics are employed to calculate electronic energy loss of energetic ions in two-dimensional graphene and white graphene (BN) targets. Special attention is paid to the effects of different electronic structures on their stopping power. Our results show that the energy transferred to the graphene target is much larger than to BN for both H+ and He2+ projectiles. Since the energy is mainly deposited into the electronic degree of freedom, it means that the electronic structure of the target plays an important role in determining the collision process. Our analysis indicates that more excited electrons are observed in graphene compared to BN. At low energies, a velocity proportional relation is found in the electronic energy loss of H+ and He2+ in both graphene and BN. In particular, a threshold velocity is observed for He2+. Finally, we have compared the energy transfer from neutral and charged projectiles when they collide with graphene and BN and the results show that charged projectiles damage the targets more severely.

Exploring the Barkas effect with keV-electron scattering

The energy loss of fast ions at close collision is mainly due to electron-ion collisions. The electrons are approximately stationary and they collide with a fast-moving ion. Here we study the same collision experimentally, in a reference system where the ions (or atoms) are stationary and interacting with keV electrons. Scattering cross sections under these conditions deviate from Rutherford, and we link these deviations, at higher energies, to the ${Z}^{3}$ contributions to the electronic stopping and the related Barkas effect and, at lower energies, also to quantum interference. The present measurements are well described by partial-wave calculations of the elastic cross section of electrons scattering from atoms. Encouraged by this agreement we use these calculations to estimate the Barkas factor for all elements and many energies. A universal curve for the Barkas factor due to close collisions is obtained for neutral projectiles and similar curves with smaller magnitude are found for ions.

Organic Synthesis in the Interstellar Medium by Low-Energy Carbon Irradiation

We present a first principles molecular dynamics (FPMD) study of the interaction of low-energy neutral carbon projectiles with amorphous solid water clusters at 30 K. Reactions involving the carbon atom at an initial energy of 11 and 1.7 eV with 30-molecule clusters have been investigated. Simulations indicate that the formation of hydroxymethylene, an intermediate in formaldehyde production, dominates at the higher energy. The reaction proceeds by fragmenting a water molecule, binding the carbon to the OH radical, and saturating the C valence with a hydrogen atom that can arise from the originally dissociated water molecule, or through a chain of proton transfer events. We identified several possible pathways for the formation of HCOH. When the initial collision occurs at the periphery of the cluster, we observe the formation of CO and the evaporation of water molecules. At the lower energy water fragmentation is not favorable, thus leading to the formation of weakly bound carbon-water complexes.

Functional Polymer Laminates from Hyperthermal Hydrogen Induced Cross-Linking

The use of a hyperthermal hydrogen induced cross-linking process to prepare laminates comprising polypropylene, poly(isobutylene-co-isoprene), and poly(vinyl acetate) is described. In this new, milder alternative to conventional plasma techniques, neutral molecular hydrogen projectiles were used to create carbon radicals on impacted surfaces by collision-induced dissociation of C-H bonds, and this process was used to cross-link polymers on a polypropylene surface. It was demonstrated that multiple layers of cross-linked materials could be added, creating polymer laminates with each layer introducing new functionalities and properties. In particular, the present work shows that the process is largely nondestructive toward ester functionalities. First, the esters were grafted to become nonleachable. Then, the esters were subsequently hydrolyzed to convert the surface from hydrophobic to hydrophilic. Afterward, the esters could be recovered by simple esterification demonstrating that further chemical transformations were possible.

Charge State Dependent Energy Deposition by Ion Impact

We report on a measurement of craters in thin dielectric films formed by Xe(Q+) (26 ≤ Q ≤ 44) projectiles. Tunnel junction devices with ion-irradiated barriers were used to amplify the effect of charge-dependent cratering through the exponential dependence of tunneling conductance on barrier thickness. Electrical conductance of a crater σ(c)(Q) increased by 4 orders of magnitude (7.9 × 10(-4) μS to 6.1 μS) as Q increased, corresponding to crater depths ranging from 2 to 11 Å. By employing a heated spike model, we determine that the energy required to produce the craters spans from 8 to 25 keV over the investigated charge states. Considering energy from preequilibrium nuclear and electronic stopping as well as neutralization, we find that at least (27 ± 2)% of available projectile neutralization energy is deposited into the thin film during impact.

Negative ions produced in multicharged ions and C60 collisions

We have measured the differential cross-sections in F3+–C60 collisions at low velocity (v=0.18 a.u.). Using coincidence measurements between the ejected electron number and recoil ions, we have estimated the initial charge state of C60 parent ions associated with neutral outgoing projectiles to range from 4+ to 7+. We found that the formation of anions is correlated with the strong excitation of C60 (average excitation energy about 160 eV). The cross-section of the production of the anion has been found to be 25 a.u. This value is in agreement with the scenario of anion formation in two steps. In the first step, F3+ ions are neutralized by multicollision on the C60 surface. In the second step and at the exit part of the collision, the electron attachment process occurs. For the production of a stable anion, the capture of electrons on carbon must occur directly to the ground states of F−, giving many electron vacancies on C60. This process explains the observation of strong electronic and vibronic excitations of C60 leading to electron emission and multifragmentation processes.

Negative ions and their behavior in collisions

AbstractWe present a review of electron detachment cross sections of atomic, molecular, and cluster anionic projectiles colliding with atoms and molecules, the main focus being on reporting systematic experimental studies. These studies use an experimental method developed at our laboratory, which uses the gas cell of standard tandem accelerators as a collision target. The experimental data show that the velocity dependence of electron detachment cross sections can be described modeling negative ions as a quasi‐free electron bound to a neutral core. Using this model, a velocity‐independent contribution to cross sections is associated with the neutral projectile core, while a velocity‐dependent curve is associated to the projectile quasi‐free electron. This latter curve is the convolution of experimental free‐electron cross sections, for the same target, with the momentum distribution of the quasi‐free projectile electron. This model is useful to obtain cross section estimates, as exemplified by its application in several collision systems relevant for inertial fusion and plasma diagnostics. It is also a powerful tool for verifying the accuracy of anion structure calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Influence of the projectile charge state on the ionization probability of sputtered particles

We present a computational study of the effect of the projectile charge state on secondary ion formation in sputtering. A molecular dynamics simulation of an atomic collision cascade is combined with a kinetic excitation model including electronic friction and electron promotion in close atomic collisions. The model is extended to account for potential excitation following the bombardment with a highly charged ion (HCI). The spatial spreading of the excitation generated in the cascade is treated in an diffusive approach. The excitation energy density profile obtained this way is parametrized via an effective electron temperature, which is then used to calculate the ionization probability of each sputtered atom in terms of a simple charge exchange model. The results obtained for the impact of a 5 keV Ag atom onto a solid silver surface show that the average ionization probability increases from 4.7 × 10 - 4 for a neutral projectile to 5.4 × 10 - 4 for a highly charged projectile ion with a total ionization energy of 576 eV.

Mass spectrometry (fragmentation ratios) of DNA base molecules following 80 keV proton impact with separation of direct ionization and electron capture processes

The first fragmentation ratios are presented for the ionization of gas-phase DNA bases by 80 keV (1.8 v 0 in Bohr velocity units) proton impact. Event-by-event determination of the projectile charge state post-collision enables branching ratios to be determined for electron capture (EC) by the projectile and for direct ionization (DI) of the target molecule (without projectile neutralization). Results are compared with similar experiments on uracil [11] and water [14]. In all cases, whereas both processes (EC and DI) produce the same fragment ion groups, greater fragmentation ratios are observed for EC than for DI. Moreover the fragmentation ratio is greater for thymine than for adenine, cytosine, and uracil.

20–150-keV proton-impact-induced ionization of uracil: Fragmentation ratios and branching ratios for electron capture and direct ionization

Fragmentation ratios and branching ratios are measured for ionization and dissociative ionization for 20–150 keV (0.9–2.4v0) proton collisions with gas-phase uracil molecules. Through event-by-event determination of the postcollision projectile charge, it is possible for such a key biomolecule to distinguish between electron capture (EC) by the incident proton and direct ionization (DI) without projectile neutralization. While the same fragment ion groups are observed in the mass spectra for both processes, EC induces dissociation with greater efficiency than DI in the impact energy range of 35–150 keV (1.2–2.4v0). In this range EC is also less abundant than DI with a branching ratio for EC/total ionization of <50%. Moreover, whereas fragmentation ratios do not change with energy in the case of EC, DI mass spectra show a tendency for increased fragmentation at lower impact energies.

Single and multiple projectile electron-loss and capture of low charge states of oxygen beams colliding with the methane molecule

The projectile electron loss and capture are important ionization components collision channels in dressed ion-atom and ion-molecule collisions. They play important roles in several processes connected to the penetration of neutral and charged projectiles in gases, in radiation damage and upper atmospheric physics. In this work, cross sections for total single and multiple electron loss and capture of O-, Oo, O+, O2+ projectiles impinging on the methane molecule were measured in the 15 keV ≤ E ≤ 1000 keV energy range. In addition, an extended version of the free-collision model was employed for the calculation of the electron loss cross sections. Multiple electron loss was analyzed within the independent particle model.