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We have investigated Interparticle Coulombic Electron Capture (ICEC) using an ab initio approach for two systems, H+ + H2O and H + H2O+. In this work, we have determined the contribution of virtual photon exchange and electron transfer to the total ICEC cross section as a function of the distance between the charged and neutral particles. Furthermore, we have shown that the relative orientation of the electron acceptor and neighbor systems affects the magnitude of the ICEC cross sections by at least two orders at relatively small distances. This geometry dependence, present even for distances as large as 10 a0, is due to the electron transfer contribution. The relative magnitude of each contribution to ICEC seems to depend on the system studied. By replacing the projectile electron with a positron, we have confirmed that electron transfer also takes place in positron collisions and that the charge of the projectile has a noticeable effect on the process, particularly at low scattering energies. [ABSTRACT FROM AUTHOR]

The concentration of vacancies and impurities in semiconductors plays a crucial role in determining their electrical, optical, and thermal properties. This study aims to clarify the nature of the interaction between positrons and ionized p-type impurities, emphasizing the similarities they share with the interaction between holes and this type of impurity. An overall strategy for investigating defects in semiconductor crystals that exhibit a combination of vacancies and p-type impurities is presented. By using positron annihilation spectroscopy, in particular, Doppler broadening of the annihilation radiation, we quantify the concentration of vacancies in epitaxial Si crystals grown by low-energy plasma-enhanced chemical vapor deposition. The vacancy number densities that we find are (1.2 ± 1.0) × 1017 cm−3 and (3.2 ± 1.5) × 1020 cm−3 for growth rates of 0.27 and 4.9 nm/s, respectively. Subsequent extended annealing of the Si samples effectively reduces the vacancy density below the sensitivity threshold of the positron technique. Secondary ion mass spectrometry indicates that the boron doping remains unaffected during the annealing treatment intended for vacancy removal. This study provides valuable insights into the intricate interplay between vacancies and ionized impurities with positrons in semiconductor crystals. The obtained results contribute to advance the control and understanding of material properties in heterostructures by emphasizing the significance of managing vacancy and dopant concentrations. [ABSTRACT FROM AUTHOR]

In this article, we develop a vibronic theory of clocking in molecular quantum cellular automata (QCA). The clocking mechanism is considered for a trigonal trimeric mixed-valence (MV) system with one mobile electron, which is shown to act as the dimeric unit encoding binary information (Boolean states 0 or 1) coupled to a third redox center (Null state). The model includes the electron transfer between the three centers; vibronic coupling of the mobile charge with the "breathing" modes, forming a double degenerate Jahn–Teller vibration of the molecular triangle; and two electric fields, one collinear to the dimeric unit, which controls the binary states, and the other perpendicular to this unit, performing clocking. In the framework of the adiabatic approximation, the potential surface of the trimeric system has been studied and the condition determining switching and clocking has been analyzed in terms of the two controlling fields and the vibronic and transfer parameters. A thorough understanding of the site populations is achieved through the quantum-mechanical solution of the vibronic problem, maintaining the adiabatic condition for the controlling fields. It is shown that a MV trimer can act as a molecular clocked QCA cell, with favorable conditions being a positive electron transfer parameter and sufficiently strong vibronic coupling. [ABSTRACT FROM AUTHOR]

A theoretical investigation of positron scattering from phosphorus-containing compounds (viz., PH 3 , P 2 H 4 , PCl 3 , PF 3 , PBr 3 , POF 3 , POCl 3 , and H 2 PO 4) is reported in this article. The quantum mechanical potential scattering approach is utilized to calculate integral elastic, excitation, momentum transfer, direct ionization, positronium formation, total ionization, inelastic, differential, and total cross sections on a fine energy grid from 1 to 5000 eV. The ionization contribution in the inelastic scattering is estimated using the complex scattering potential-ionization contribution technique. Prior research on positron collisions with these targets is scarce; as a result, the purpose of this study is to make up, at least in part, for this deficiency in cross-section data. In addition to being pertinent to positron transport analyses, such as Monte Carlo methods, the current results should be useful to benchmark the accuracy and validity of positron molecule collision computations and, more significantly, to compare these calculations with related electron scattering outcomes. Furthermore, the calculated cross sections of PH 3 are compared with NH 3 and other phosphorus-containing compounds. The analysis makes it abundantly evident that the atoms on the periphery of a molecule have a substantially larger impact on the scattering process than the central atom. To analyze the scattering dynamics of positrons and their anti-particle electrons, a comparative study of cross sections of H 2 PO 4 and H 2 SO 4 is also presented. For most of these targets, positron calculations are carried out for the first time. [ABSTRACT FROM AUTHOR]

Theoretical discussions are given on issues in relativistic molecular orbital theory to which the quantum electrodynamics (QED) Hamiltonian is applied. First, several QED Hamiltonians previously proposed are sifted by the orbital rotation invariance, the charge conjugation and time reversal invariance, and the nonrelativistic limit. The discussion on orbital rotation invariance shows that orbitals giving a stationary point of total energy should be adopted for QED Hamiltonians that are not orbital rotation invariant. A new total energy expression is then proposed, in which a counter term corresponding to the energy of the polarized vacuum is subtracted from the total energy. This expression prevents the possibility of total energy divergence due to electron correlations, stemming from the fact that the QED Hamiltonian does not conserve the number of particles. Finally, based on the Hamiltonian and energy expression, the Dirac–Hartree–Fock (DHF) and electron correlation methods are reintroduced. The QED-based DHF equation is shown to give information on positrons from negative-energy orbitals while having the same form as the conventional DHF equation. Three electron correlation methods are derived: the QED-based configuration interactions and single- and multireference perturbation methods. Numerical calculations show that the total energy of the QED Hamiltonian indeed diverged and that the counter term is effective in avoiding the divergence. The relativistic molecular orbital theory presented in this article also provides a methodology for dealing with systems containing positrons based on the QED Hamiltonian. [ABSTRACT FROM AUTHOR]

We here report theoretical triply differential cross sections (TDCS) for 250 eV electron and positron impact ionization of the methane molecule calculated within the second-order distorted-wave Born approximation (DWBA2) for various momentum transfer conditions. The experimental data taken from Işık et al. [J. Phys. B: At., Mol. Opt. Phys. 49, 065203 (2016)] will be compared with the current theoretical predictions as well as molecular three body distorted wave (M3DW) approximation and generalized Sturmian function (GSF) theoretical models in a non-coplanar geometry. In the low analyzer scattering plane, the results obtained within the DWBA2 theory show better agreement with the experimental results compared to the GSF results. The M3DW results also exhibit agreement with the experimental results, in particular in the perpendicular plane geometry. Furthermore, significant differences between electron and positron TDCS were observed. [ABSTRACT FROM AUTHOR]

We studied the stability of a system consisting of a positron (e+) and two lithium anions, [Li−; e+; Li−], using first-principles quantum Monte Carlo calculations combined with the multi-component molecular orbital method. While diatomic lithium molecular dianions Li 2 2 − are unstable, we found that its positronic complex can form a bound state with respect to the lowest energy decay into the dissociation channel Li 2 − and a positronium (Ps). The [Li−; e+; Li−] system has the minimum energy at the internuclear distance of ∼3 Å, which is close to the equilibrium internuclear distance of Li 2 − . At the minimum energy structure both an excess electron and a positron are delocalized as orbiting around the Li 2 − molecular anion core. A dominant feature of such a positron bonding structure is described as the Ps fraction bound to Li 2 − , unlike the covalent positron bonding scheme for the electronically isovalent [H−; e+; H−] complex. [ABSTRACT FROM AUTHOR]

Radiation-induced property changes in materials originate from the energy transfer from an incoming particle to the existing lattice, displacing atoms. The displaced atoms can cause the formation of extended defects including dislocation loops, voids, or precipitates. The non-equilibrium defects created during damage events determine the extent of these larger defects and are a function of dose rate, material, and temperature. However, these defects are transient and can only be probed indirectly. This work presents direct experimental measurements and evidence of irradiated non-equilibrium vacancy formation, where in situ positron annihilation spectroscopy was used to prove the generation of non-equilibrium defects in silicon. [ABSTRACT FROM AUTHOR]

A theoretical investigation of the phase-mixing phenomenon of Langmuir oscillations (LOs) in a multi-component dusty plasma is presented. The plasma under study is a four-component plasma consisting of electrons, positrons, singly ionised positive ions and positive (or negative) polarity dust grains. The massive dust grains are assumed to form a fixed charge neutralising background, while the electrons, positrons and ions are considered to take part in the wave dynamics. A simple perturbative analysis of the governing fluid-Maxwell's equations leads us to an approximate relation for phase-mixing time of LOs, improving some earlier existing results. It is revealed that even though the dust grains remain motionless in the background, they still have significant impact on the phase-mixing process of LOs in such plasmas. It is found that the nature of the charge on the dust grains and the equilibrium dust density profoundly affect the phase-mixing time. The results of this investigation is expected to have relevance in space-plasma environments. [ABSTRACT FROM AUTHOR]

We propose a new "disappearing positron track" channel at Belle II to search for dark matter, in which a positron that is produced at the primary interaction vertex scatters with the electromagnetic calorimeter to produce dark matter particles. Such scatterings can occur via either annihilation with atomic electrons, or the bremsstrahlung process with target nuclei. The main backgrounds are due to photons and neutrons that are produced in the same scatterings and then escape detection. We require a large missing energy and further veto certain activities in the KLM detector to suppress such backgrounds. To illustrate the sensitivity of the new channel, we consider a new physics model where dark matter interacts with the standard model via a dark photon, which decays predominantly to dark matter; we find that our proposed channel can probe some currently unexplored parameter space, surpassing both the mono-photon channel at Belle II and the NA64 constraints. [ABSTRACT FROM AUTHOR]

We have proposed a compact scheme for generating high-density and high-energy positrons by irradiating different shaped targets with an ultra-intense laser pulse, using 2D particle-in-cell simulations and numerical analysis. Our simulations show that the Breit–Wheeler process dominates positron production during laser-target interaction when a laser with an intensity of 4 × 10 23 W cm − 2 is used. We obtain a positron beam with an energy of 2.5 GeV and a total number exceeding 109, which is an improvement by two orders of magnitude compared to previous work with the same laser intensity. By using different shaped targets, we are able to generate a high-energy positron beam with low divergence. The yield of positron pairs depends on both the target and the laser parameters, and we have investigated how the shape of the target can enhance pair production and reduce divergence. Both of our analytical and simulation results demonstrate that this high-flux and low-divergence positron beam has direct applications in the field of medical and astro-physics that can be explored in the upcoming high intensity laser facilities. [ABSTRACT FROM AUTHOR]

Collisionless shocks are generated via the magnetic field mediated by Weibel instability in astrophysical systems. In this work, by performing particle-in-cell (PIC) simulations, Weibel instability-mediated magnetic field amplification is investigated for initially unmagnetized, spatially uniform, counter-streaming electron–positron (e−/e+) plasma flows and compared with the magnetic amplification for nonuniform counter-streaming e−/e+ plasma flows by considering their drift velocity of 0.5 c . Our simulation results show that initially, the magnetic field grows exponentially in the linear regime and then decays further after saturation for homogeneous e−/e+ plasma flows. However, in the case of inhomogeneous counter-streaming e−/e+ plasma flow, the magnetic field re-amplifies in the post-saturation region after the first saturation. It is found that the amplification magnitude of magnetic field energy in the post-saturation region is related to the density fluctuations for upstream plasma. Our calculations show that temperature anisotropy is the reason behind the second saturation of the magnetic field energy in the case of inhomogeneous plasma distribution. Such inhomogeneous media in astrophysical systems like Gamma-ray bursts are common. Therefore, this study will be useful for understanding collisionless shocks' formation and their effects. [ABSTRACT FROM AUTHOR]

We explore the dynamic characteristics of positron acoustic multiple-solitons in an unmagnetized plasma containing mobile cold positrons, Kappa-distributed superthermal hot electrons and positrons, and stationary positive ions. This study investigates the overtaking collisional effects, various parametric impacts, and phase shifts in the electron–positron-ion (e-p-i) plasma. Through the reductive perturbation technique, we derived the Korteweg-de Vries (KdV) equation and the modified Korteweg-de Vries (mKdV) equation. The multiple-soliton solutions (MSS) are then obtained using the simplified Hirota method and Cole-Holf transformation. The investigation shows that the amplitudes and widths of multi-solitons decrease with the increasing hot positron concentration but increase with the increasing hot electron index parameter. The results are expected to help us understand the dynamics of waves propagation in the pulsar magnetosphere, the active galactic nuclei, the quasar's relativistic jet, the inner region of the accretion disk surrounding a black hole, etc. [ABSTRACT FROM AUTHOR]

Background: The elemental concentration (especially oxygen and carbon) and mass density must be accurately assigned to perform Monte Carlo (MC) simulations for predicting proton‐induced nuclear reactions in the human body. We recently proposed an approach to quantify elemental concentrations and mass densities of human soft tissues from water content (WC) data obtained by quantitative magnetic resonance (MR) imaging (which we called "MRWC"). Purpose: This study presents the first implementation of MRWC‐derived elemental concentrations and mass densities as complementary inputs into MC simulations on a virtual head phantom, and demonstrates the simulation of positron emitter production yields in proton therapy. Methods: An MC code, PHITS, was used to simulate proton therapy with a monoenergetic 140 MeV beam for a digital head phantom provided by BrainWeb. Three different head images were synthesized as inputs: a conventional CT image, an ideal CT image as a reference, and a WC image coupled with the bone‐only CT image for a hybrid approach (MRWC/CT). Thereafter, the performance of the MRWC/CT method was evaluated by comparing its accuracy in predicting the production yields of positron emitters (11C and 15O) with the gold‐standard CT‐only method. Results: The MRWC/CT method could predict 11C and 15O production yield maps that closely resembled the corresponding reference maps, while the CT‐only method failed. The structural similarity index measures between the reference and CT‐ or MRWC/CT‐derived maps were improved from 0.67 (CT‐only) to 0.87 (MRWC/CT) for 11C and 0.76 (CT‐only) to 0.93 (MRWC/CT) for 15O. Furthermore, applying post‐processing normalizations to account for elemental density variations in the production yields of positron emitters facilitated the determination of distal fall‐off positions in depth activity profiles. Conclusion: At least in the head area, the MRWC/CT method demonstrated potential for more precise predictions of proton‐induced positron emitter distributions via MC simulations than that of the CT‐only method. [ABSTRACT FROM AUTHOR]

A quadruple Beltrami (QB) equilibrium state for a four-component plasma that consists of inertial electrons, positrons, lighter positive (H+) ions and heavier negative ions O 2 − is derived and investigated. The QB relaxed state is a linear superposition of four distinct single Beltrami fields and provides the possibility of the formation of four self-organized vortices of different length scales. In addition, robust magnetofluid coupling characterizes this non-force-free state. The analysis of the QB state also shows that by adjusting the generalized helicities and densities of plasma species, the formation of multiscale structures as well as the paramagnetic and diamagnetic behavior of the relaxed state can be controlled. [ABSTRACT FROM AUTHOR]

We propose a positron acceleration scheme in which a laser-driven positron beam is injected into a solid target hit by a laser beam and accelerated in the sheath field on the target back side. The positron beam injection and acceleration in the target have been investigated with numerical simulation. The feasibility of such an acceleration scheme was proved according to the simulation results, which show that a 10 MeV positron beam can be accelerated up to 30 MeV. The dependency of the positron beam properties on the positron injection location, injection time, and target thickness was studied. Related acceleration details were obtained and analyzed. The acceleration scheme provides a method in positron energy controlling and its related applications. [ABSTRACT FROM AUTHOR]

In this paper, the oblique propagation of the ion-acoustic quantum soliton in polarized quantum plasma including relativistic degenerate electrons and positrons is studied. By using the reductive perturbation method, we derived the ZK equation for this model. This equation shows that in presence of the negative ion there are two slow and fast ion-acoustic modes. Our numerical results indicate that there are only compressive and refractive solitons for fast and slow modes, respectively. The effect of the negative ion parameters on the amplitude and width of the ion-acoustic quantum soliton are studied, as well. [ABSTRACT FROM AUTHOR]

Positron bound state properties in hydrogen cyanide are studied via many-body theory calculations that account for strong positron-electron correlations including positron-induced polarization, screening of the electron–positron Coulomb interaction, virtual-positronium formation and positron–hole repulsion. Specifically, the Dyson equation is solved using a Gaussian basis, with the positron self-energy in the field of the molecule calculated using the Bethe–Salpeter equations for the two-particle and particle–hole propagators. The present results suggest near cancellation of screening corrections to the bare polarization, and the non-negligible role of the positron–hole interaction. There are no existing measurements to compare to for HCN. Previous configuration interaction (CI) and fixed-node diffusion Monte Carlo (FN-DMC) calculations give positron binding energies in the range 35–44 meV, most of which used a single even-tempered basis centred near the nitrogen atom. Using a similar single-centre positron basis we calculate a positron binding energy of 41 meV, in good agreement. However, we find that including additional basis centres gives an improved description of the positron wave function near the nuclei, and results in a converged binding energy in the range 63–73 meV (depending on geometry and approximation to the positron–molecule correlation potential used). [ABSTRACT FROM AUTHOR]

The formulas obtained for deconvolution of positron mean lifetime results in the sequential etching technique for detecting defect depth profiles are presented. In this experiment, only the conventional positron lifetime measurement with radioisotope-based positrons is used. The important role of the positron implantation profile is discussed. These formulas were successfully used to describe the depth profile of defects formed in sliding contact in pure vanadium. Two different layers were detected below the surface. [ABSTRACT FROM AUTHOR]

• The electron density in the D region obtained by Kunming (25.6N,103.8E) MF radar is used in this study. • The strong positive correlation of electron density in the D region with solar activity. • A empirical model of D region is established based on the correlation between D ionosphere and F107. The electron density (N e) in the D region, although its value is much smaller than that in the F region, has a highly important effect on electromagnetic wave propagation by energy absorption due to high-frequency collisions between electron-molecular, electron-ionic, molecular-ionic and so on. Solar activity is the main source of ionization in the D region of the ionosphere. The observed N e during 2008–2015 by the Kunming station (25.6°N, 103.8°E) medium radar (MF radar) is utilized to analyze its strong relationship with solar activity. Then, the relationship between them is employed to establish an empirical model in this study. The N e in the D region is enhanced with increasing height and time towards noon, and the solar zenith angle is increasingly smaller towards noon. The averages of N e at several heights below and above 76 km show that the D region ionosphere up to 76 km is more strongly controlled by solar radiation and less strongly controlled below 76 km. According to the above, an empirical model is established based on the relationship between the D-region ionosphere and solar activity index F 107. [ABSTRACT FROM AUTHOR]

It is found that the polarity reversal phenomenon occurs in the nanosecond pulse breakdown experiment. Two-dimensional axisymmetric particle-in-cell/Monte Carlo collisions' calculation is used to observe energetic electrons at positive nanosecond pulse voltage in atmospheric air and compared with previous calculation results under negative polarity in an attempt to explain the polarity reversal phenomenon. During the evolution of the positive polarity discharge, the difference in spatial net charge distribution leads to distortion of the electric field, which makes the electric field in the area of the ionization channel head very high, exceeding the threshold at which electrons can enter the high-energy state. The simulation results show that although electrons travel in the opposite direction to the ionization channel, energetic electrons can also be generated during the positive polarity discharge's evolution, which may differ from what some researchers expected. However, it is also found that the maximum energy of energetic electrons under positive nanosecond pulse voltage is lower than that under negative nanosecond pulse voltage (only about 1/4). This may be mainly because in the case of positive polarity, the energetic electrons in the head of the ionization channel will move to the low-field intensity region inside the ionization channel and cannot be accelerated continuously. However, it must be pointed out that in the case of positive polarity, energetic electrons still contribute significantly to the rapid breakdown of the air gap. This study provides a deeper understanding of the physics of nanosecond pulse discharge. [ABSTRACT FROM AUTHOR]

The positronium chemistry of a Fe2+/3+ solution is studied under full electrochemical control. For this novel approach to positronium electrochemistry, a suitable cell setup is used, which allows simultaneously both electrochemical measurements and positron annihilation spectroscopy. For the Fe2+/3+ redox couple, positronium serves as an ideally suited atomic probe owing to the rather different positronium chemistry of Fe2+ (spin conversion) and Fe3+ (total positronium inhibition and oxidation). This enabled the precise in situ monitoring of oxidation and reduction by means of positron lifetime upon slow cycling voltammetry or galvanostatic charging. The variation of the mean positron lifetime with the Fe2+/3+ concentration ratio could be quantitatively described by a reaction rate model for positronium formation and annihilation. An asymmetric behavior of the variation of the mean positron lifetime with applied potential, as compared to the simultaneously recorded symmetric current–potential curve, could be explained by the stronger influence of Fe3+ on the characteristics of positronium formation and annihilation. The highly reversible galvanostatic charging behavior monitored by positron lifetime underlines the attractive application potentials of positronium electrochemistry for in situ studies of iron-based redox-flow battery electrolytes. [ABSTRACT FROM AUTHOR]

We report the implementation of effective quantum electrodynamics (QED) potentials for all-electron four-component relativistic molecular calculations using the DIRAC code. The potentials are also available for two-component calculations, being properly picture-change transformed. The latter point is important; we demonstrate through atomic calculations that picture-change errors are sizable. Specifically, we have implemented the Uehling potential [E. A. Uehling, Phys. Rev. 48, 55 (1935)] for vacuum polarization and two effective potentials [P. Pyykkö and L.-B. Zhao, J. Phys. B: At., Mol. Opt. Phys. 36, 1469 (2003) and V. V. Flambaum and J. S. M. Ginges, Phys. Rev. A 72, 052115 (2005)] for electron self-energy. We provide extensive theoretical background for these potentials, hopefully reaching an audience beyond QED specialists. We report the following sample applications: (i) We first confirm the conjecture of P. Pyykkö that QED effects are observable for the AuCN molecule by directly calculating ground-state rotational constants B0 of the three isotopomers studied by microwave spectroscopy; QED brings the corresponding substitution Au–C bond length rs from 0.23 to 0.04 pm agreement with experiment. (ii) In regard to spectroscopic constants of van der Waals dimers M2 (M = Hg, Rn, Cn, Og), QED induces bond length expansions on the order of 0.15(0.30) pm for row 6(7) dimers. (iii) We confirm that there is a significant change of valence s population of Pb in the reaction PbH4 → PbH2 + H2, which is thereby a good candidate for observing QED effects in chemical reactions, as proposed in [K. G. Dyall et al., Chem. Phys. Lett. 348, 497 (2001)]. We also find that whereas in PbH4 the valence 6s1/2 population resides in bonding orbitals, it is mainly found in nonbonding orbitals in PbH2. QED contributes 0.32 kcal/mol to the reaction energy, thereby reducing its magnitude by −1.27%. For corresponding hydrides of superheavy flerovium, the electronic structures are quite similar. Interestingly, the QED contribution to the reaction energy is of quite similar magnitude (0.35 kcal/mol), whereas the relative change is significantly smaller (−0.50%). This curious observation can be explained by the faster increase in negative vacuum polarization over positive electron self-energy contributions as a function of nuclear charge. [ABSTRACT FROM AUTHOR]

The rapid progress that plasma wakefield accelerators are experiencing is now posing the question as to whether they could be included in the design of the next generation of high-energy electron-positron colliders. However, the typical structure of the accelerating wakefields presents challenging complications for positron acceleration. Despite seminal proof-of-principle experiments and theoretical proposals, experimental research in plasma-based acceleration of positrons is currently limited by the scarcity of positron beams suitable to seed a plasma accelerator. Here, we report on the first experimental demonstration of a laser-driven source of ultra-relativistic positrons with sufficient spectral and spatial quality to be injected in a plasma accelerator. Our results indicate, in agreement with numerical simulations, selection and transport of positron beamlets containing N e + ≥ 10 5 positrons in a 5% bandwidth around 600 MeV, with femtosecond-scale duration and micron-scale normalised emittance. Particle-in-cell simulations show that positron beams of this kind can be guided and accelerated in a laser-driven plasma accelerator, with favourable scalings to further increase overall charge and energy using PW-scale lasers. The results presented here demonstrate the possibility of performing experimental studies of positron acceleration in a laser-driven wakefield accelerator. [ABSTRACT FROM AUTHOR]

We present ionization cross sections of hydrogen molecules by electron and positron impact for impact energies between 20 and 1000 eV. A three-body Classical Trajectory Monte Carlo approximation is applied to mimic the collision system. In this approach, the H2 molecule is modeled by a hydrogen-type atom with one active electron bound to a central core of effective charge with an effective binding energy. Although this model is crude for describing a hydrogen molecule, we found that the total cross sections for positron impact agree reasonably well with the experimental data. For the electron impact, our calculated cross sections are in good agreement with the experimental data in impact energies between 80 eV and 400 eV but are smaller at higher impact energies and larger at lower impact energies. Our calculated cross sections are compared with the scaled cross sections obtained experimentally for an atomic hydrogen target. We also present single differential cross sections as a function of the energy and angle of the ejected electron and scattered projectiles for a 250 eV impact. These are shown to agree well with available data. Impact parameter distributions are also compared for several impact energies. [ABSTRACT FROM AUTHOR]

We provide in this work Stark broadening data (widths and shifts) for 120 ultraviolet spectral lines of Ni  v ion. Our calculations are performed using the semiclassical perturbation method. For energy levels and oscillator strength calculations, we use the multiconfiguration Hatree–Fock approach. Stark shifts and widths are calculated for collisions with electrons and with the positive ions: H+, He+, and He++, allowing us to take into account the important perturbers in stellar atmospheres. We compare our Stark widths with estimations obtained using the Cowley formula. Our electron impact Stark widths are also fitted with temperature using a logarithmic formula. Finally, our obtained Stark widths are used to investigate the influence of Stark broadening in the atmospheric conditions of hot DA white dwarfs. Despite the importance of ultraviolet lines of Ni  v ion for modelling white dwarf atmospheres and also for investigations of variation of fundamental constants with gravitational potential, we did not find Stark broadening data previously calculated or measured for Ni  v ion. The objective of this work is to give the missed data. [ABSTRACT FROM AUTHOR]

We suggest a new alternative model of positron origin in the Galaxy. It is shown in our model that interactions of the electromagnetic fields of colliding ions (ultraperipheral ion collisions) can contribute to the total production of Galactic positrons. The corresponding cross-section is estimated by using the Born approximation and the equivalent photon method. This process of ion collisions dominates in the range of subrelativistic energies and produces positrons with energies of several MeV. Despite its low efficiency, as it requires more than 0.1 erg to produce a single positron, this process may be an effective source of positrons in the Galactic medium. [ABSTRACT FROM AUTHOR]

Nonlinear ion-acoustic solitary and shock waves were analysed in a system of collisionless, dissipative and magnetised multicomponent plasma. The system was made up of a fluid of inertial ions, electrons that were distributed superthermally and positrons. Korteweg–de Vries–Burgers (KdVB) equation was produced when the reductive perturbation approach was used to evaluate the plasma system nonlinearly. The tanh method was used to find the analytical solution of the derived KdVB equation. The study demonstrated that the superthermal parameters, magnetic field direction, ion kinematic viscosity and positron and electron concentration all affect the oscillatory shock wave features such as speed, amplitude and width. On the other hand, the bifurcation analysis showed that the equilibrium point produces an unstable spiral source as the Burgers term increases and that it transforms into a nodal source with any further increase in the Burgers term. The present results are helpful for understanding collective phenomena and the stability criterion of ion-acoustic (IA) solitary and shock waves in magnetoplasmas with superthermal electrons and positrons, which are observed in some astrophysical environments, like the interstellar medium. [ABSTRACT FROM AUTHOR]

We consider a self-consistent axially symmetric system supported by a classical nonlinear spinor field minimally coupled to electric and magnetic Maxwell fields. The presence of the nonlinearity of the spinor field ensures the existence of a minimum positive energy of the system (a mass gap), of a minimum charge (a charge gap), and of a minimum magnetic moment. In turn, the presence of the electric charge results in qualitative changes in the behavior of physical characteristics of the systems under consideration as compared with the case of an electrically neutral spinor field. It is shown that, with a suitable choice of free system parameters, there exists a regular finite-energy particlelike solution describing a localized spinning object whose physical parameters correspond to the main characteristics of an electron/positron (including the spin equal to 1/2), but with the characteristic size comparable to the corresponding Compton wavelength. Also, we show that four local Dirac equations are equivalent to two nonlocal equations. [ABSTRACT FROM AUTHOR]

This paper comprehensively investigates the oblique propagation of ion-acoustic solitary waves (IASWs) with arbitrary amplitude in a magnetoplasma consisting of inertialess non-Maxwellian (nonthermal) electrons, inertialess Maxwellian positrons, and inertial adiabatically heated ions. It is postulated that the positive ions demonstrate adiabatic behavior that is distinguished by anisotropic thermal pressure. The study utilizes Sagdeev's pseudopotential theory to analyze the fluid equations of the plasma model and reduce them to the energy equation. Different plasma configuration factors, such as nonthermal parameters, positron concentration, and parallel and perpendicular ion pressure, are being studied to see how they affect the properties of solitary waves with large amplitudes. The findings demonstrate the simultaneous coexistence of compressive and rarefactive IASWs, significantly influenced by positrons and nonthermal electron parameters. The study provides valuable insights into wave phenomena in magnetized plasmas and presents possible applications in both Space and laboratory plasma environments. [ABSTRACT FROM AUTHOR]

Conducting polymers like polyaniline (PANI) are promising pseudocapacitive electrode materials, yet experience instability in cycling performance. Since polymers often degrade into oligomers, short chain length anilines have been developed to improve the cycling stability of PANI‐based supercapacitors. However, the capacitance degradation mechanisms of aniline oligomer‐based materials have not been systematically investigated and are little understood. Herein, two composite electrodes based on aniline trimers (AT) and carbon nanotubes (CNTs) are studied as model systems and evaluated at both pre‐cycling and post‐cycling states through physicochemical and electrochemical characterizations. The favorable effect of covalent bonding between AT and CNTs is confirmed to enhance cycling stability by preventing the detachment of aniline trimer and preserving the electrode microstructure throughout the charge/discharge cycling process. In addition, higher porosity has a positive effect on electron/ion transfer and the adaptation to volumetric changes, resulting in higher conductivity and extended cycle life. This work provides insights into the mechanism of enhanced cycling stability of aniline oligomers, indicating design features for aniline oligomer electrode materials to improve their electrochemical performance. [ABSTRACT FROM AUTHOR]

We performed a multi-component configuration interaction (MC_CI) study on positron binding with polar and non-polar molecules. The present MC_CI calculation with large basis sets provides results in agreement with experimentally measured positron affinities for polar molecules. It shows improvements in accuracy compared to the previously reported MC_CI calculations for positron binding in non-polar molecules. We show the electronic and positronic features in the positron-bound benzene compound by analyzing these leptonic density properties obtained by the present MC_CI calculations. [ABSTRACT FROM AUTHOR]

In this work, we report elastic cross sections for the scattering of low-energy positrons by para-benzoquinone, for energies up to 6 eV. The cross sections were calculated with the Schwinger multichannel method in the static plus polarization approximation at two different polarization schemes. Our computed elastic integral cross section indicates the presence of a bound state in the A g scattering symmetry. In the absence of results in the literature on collisions of positrons with para-benzoquinone, the present integral cross sections results were compared with the data available in the literature for the benzene and pyrazine molecules. [ABSTRACT FROM AUTHOR]

Minute examination of local lattice structures in matter affected by impurity doping is of special importance for the development of functional materials. In order to obtain microscopic information on spinel ferrites, in the present work, we introduced nonmagnetic In3+ ions in Fe3O4 and probed their site selectivity and the doping effect on the local lattice structures and bulk magnetism by means of 57Fe Mössbauer spectroscopy and positron annihilation spectroscopies. The Mössbauer parameters of the area intensity and isomer shift (IS) show that In3+ ions predominantly reside in the tetrahedral A site, especially at low doping level. With increasing concentration of In ions, however, they gradually occupy the octahedral B site replacing Fe3+ ions. Along with the site information, the IS values confirmed that the introduced In ions squeeze the B-site Fe ions at their nearest neighbors. Supporting results were obtained from positron annihilation lifetime spectroscopy; positron lifetimes become shorter with increasing In concentration, signifying that the oxygen ions are pressed by the introduced In ions resulting in lowering the volume of the adjacent lattice vacancies. The results of Doppler broadening spectroscopy also support the squeezing effect; the positrons in the vacancies adjacent to In ions are more likely to annihilate with the inner shell electrons of the surrounding oxygen ions as a result of a reduction in the vacancy volume. [ABSTRACT FROM AUTHOR]

We present our ab initio study of UXO 2 (X = La, Ce, Pu, Am) compounds. Depending on the ionization of the associated element, trivalent or tetravalent, either uranium stays tetravalent as in UO 2 or it is pentavalent to compensate the charge. We study the evolution of the lattice parameter as a function of X content and in the presence of oxygen vacancies. We also calculate the formation energies of several vacancy defects and show that depending of the ionic state of X the most stable charge state can be either positive (tetravalent: Ce, Pu) or negative (trivalent: La, Am). We also consider the positron lifetime in the presence of these vacancies and we show that it is almost independent of the doping element and its amount. For uranium vacancies, the positron is always trapped, while for oxygen vacancies, the trapping seems to depend on the surrounding cation oxidation states. Finally, we show that different defects have similar positron lifetimes, resulting in extremely difficult defect identification with dedicated experimental studies as the positron annihilation spectroscopy. [ABSTRACT FROM AUTHOR]

Two new positronic molecules have been recently discovered: e + H − 2 [Charry et al., Angew. Chem., Int. Ed. 57, 8859 (2018)] and (PsH)2 [D. Bressanini, J. Chem. Phys. 155, 054306 (2021)]. These molecules seem to be stabilized by a new type of bond, the positronic bond, where one or two positrons are directly responsible for the bonding of two otherwise repelling negative ions. We show that an additional positron can attach to (PsH)2 to form a locally stable species with three positrons whose potential energy curve shows an equilibrium structure at about 8 bohr and a binding energy of 11.5(5) mhartree with respect to the dissociation into PsH + e+PsH. This molecule, tentatively called e+(PsH)2, is the first system with three positrons discovered. [ABSTRACT FROM AUTHOR]

A design and topological solution for a tunnel field-effect transistor of a new type is proposed and the simulation of the transistor is performed. The device is a vertical ballistic field-effect transistor with a cylindrical metallic gate based on a cylindrical undoped AlxGa1–xAs quantum nanowire located in an Al2O3 matrix. For the given geometry of the device structure, the optimum of the fraction of aluminum in the semiconductor composition varying along the transistor channel is found, at which, unlike a conventional tunnel field-effect transistor, not only is the complete suppression of the quantum barrier for electrons by a positive gate voltage ensured but also the minimum possible electrical resistance of the transistor channel is achieved. The current-voltage characteristics of the transistor are calculated within the framework of a rigorous quantum-mechanical description of the electron transport in its channel, taking into account the nonparabolic nature of the band structure of the semiconductor. [ABSTRACT FROM AUTHOR]

This study identified the cause of the minor lifetime component and the effect of the polymer adhesive in the positron source on positron annihilation lifetime spectroscopy. Three types of positron sources with different concentrations of the polymer adhesive were compared using nickel (Ni) and adhesive specimens. The unnecessary components of the positron lifetime due to the polymer adhesive could be eliminated by a source correction of approximately 20.8%. The results showed that minimizing the amount of polymer adhesive in the positron source is recommended, or a detailed source correction is required, to analyze the positron lifetimes accurately. [ABSTRACT FROM AUTHOR]

In this work, the impact of extending π -bridge and the adsorption of dyes on the nanocrystalline surface of TiO2 on photovoltaic parameters in dye-sensitized solar cells (DSSCs) has been explored. The dyes comprise phenothiazine (donor), along with benzene, furan, thiophene and selenophene (π -spacers), and cyanoacrylic acid anchoring group (acceptor). The dye geometries, charge transference, and electronic characteristics were investigated using density functional theory (DFT). The impact of extended π -bridge in conjugated systems was explored in dyes along with dyes adsorbed on TiO2 nanocrystalline surface (dye@TiO 2). For the architecture D– π –A in this work, the dyes are effectively adsorbed on the TiO2 nanocrystalline surface. The energy gap between HOMO and LUMO (HLG), ionization potential (IP), reorganization energies (λ) , electron affinity (EA), photovoltaic parameters, adsorption energy (AE), and total density of state (TDOS) are simulated theoretically. Additionally, our investigation illustrates that the studied organic photosensitizer dyes may have improved photovoltaic characteristics and are suitable applicants for efficient charge transportation in organic electronic materials. All the reported dyes are with negative ΔGinj and positive ΔGreg for electron injection and have higher values of JSC, LHE and VOC. The absorption energies range from 358 to 482 nm. Extending π-bridge within the D-π-A arrangement results in bathochromic shift in absorption and narrowing the HLG. Due to presence of H-bonding between oxygen of carboxyl group and a proton on TiO2 surface stabilizes the dye-TiO2 combination. [ABSTRACT FROM AUTHOR]

The antenna subtraction method has achieved remarkable success in various processes relevant to the Large Hadron Collider. In Reference [1], an algorithm was proposed for constructing real-radiation antenna functions for electron-positron annihilation, directly from specified unresolved limits, accommodating any number of real emissions. Here, we extend this algorithm to build antennae involving partons in the initial state, specifically the initial-final and initial-initial antennae. Using this extended algorithm, we explicitly construct all NLO QCD antenna functions and compare them with previously extracted antenna functions derived from matrix elements. Additionally, we rigorously match the integration of the antenna functions over the initial-final and initial-initial unresolved phase space with the previous approach, providing an independent validation of our results. The improved antenna functions are more compact and reduced in number, making them more readily applicable for higher-order calculations. [ABSTRACT FROM AUTHOR]

An analysis of reductive perturbation method (RPM) is presented to show why the solitary structures of non-linear ion acoustic waves (IAWs) cannot be obtained in magnetized electron ion plasma by employing this technique. In RPM, the non-linear Korteweg–de Vries equation is derived using stretched co-ordinates in the reference frame of the wave phase speed, considering the dispersion to be a higher-order effect that balances the non-linearity to produce a solitary structure. The maximum amplitude | Φ m | of the non-linear solitary wave turns out to be larger than one that contradicts the small amplitude approximation. In the presence of negative ions, the maximum amplitude satisfies the condition | Φ m | < 1. To elaborate these points, the results have been applied to an experimental plasma consisting of positive ions of xenon (Xe +) and negative ions of fluorene (F −) along with electrons. The amplitude and width of the solitary structures depend upon the ratio of the electron to positive ion density ( n e 0 n i 0 ). Since the non-linear coefficient turns out to be negative, rarefied (dip) solitons are formed in the magnetized Xe + − F − − e plasma. [ABSTRACT FROM AUTHOR]

Instability features of steady states of the plasma diode with electron and positron counter flows are studied. There are several types of such states for each value of the inter-electrode distance. The case when charged particles moving in the diode plasma are not reflected from potential extrema is considered. We have solved an equation for the amplitude of the electric field perturbation for steady states with an inhomogeneous field distribution. Studying the dispersion equation has shown that all considered solutions are unstable. We have also confirmed this result when simulating small perturbation evolution of a steady-state solution. [ABSTRACT FROM AUTHOR]

In the present work, we used the recently parallelized version of the Schwinger multichannel method to calculate positron scattering cross sections by methane, acetylene, ethylene, and ethane in the static plus polarization approximation. These calculations, marked by heightened computational demands compared to our previous work, exhibit significantly improved agreement with most recent experimental data. Furthermore, we investigate the potential existence of bound states in our calculations through the analysis of scattering lengths. Our findings demonstrate a superior agreement with both theoretical predictions and experimental measurements when compared with previous computational approaches. [ABSTRACT FROM AUTHOR]

N 157B located in the Large Magellanic Cloud is the first pulsar wind nebula detected outside of the Galaxy in γ-rays. In this paper, we analyse the emission above 30 GeV from N 157B using ∼14.3 yr of Fermi -LAT data. The γ-ray spectrum between 30 and 500 GeV is well described by a single power-law function with a photon index of 1.83 ± 0.26, and its integral photon flux is (4.10 ± 0.83) × 10−11 photons cm−2 s−1. We adopt a one-zone leptonic model to investigate whether the multiband non-thermal emission of the target source can be generated by synchrotron radiation and inverse-Compton scattering of the electrons/positrons. Assuming the electrons/positrons in the nebula have a broken power-law spectrum with two breaks, the model can reproduce the observed fluxes in the radio, X-ray, and γ-ray bands. This result indicates that the γ-ray emission from N 157B can be explained by the leptonic process of electrons/positrons via inverse-Compton scattering. [ABSTRACT FROM AUTHOR]

Background: PET imaging after yttrium-90 (Y-90) radioembolization is challenging because of the low positron fraction of Y-90 (32 × 10−6). The resulting low number of events can be compensated by the high sensitivity of long axial field-of-view (LAFOV) PET/CT scanners. Nevertheless, the reduced event statistics require optimization of the imaging protocol to achieve high image quality (IQ) and quantification accuracy sufficient for post-treatment dosimetry. Methods: Two phantoms (NEMA IEC and AbdoMan phantoms, mimicking human liver) filled with Y-90 and a 4:1 sphere (tumor)-to-background ratio were scanned for 24 h with the Biograph Vision Quadra (Siemens Healthineers). Eight patients were scanned after Y-90 radioembolization (1.3–4.7 GBq) using the optimized protocol (obtained by phantom studies). The IQ, contrast recovery coefficients (CRCs) and noise were evaluated for their limited and full acceptance angles, different rebinned scan durations, numbers of iterations and post-reconstruction filters. The s-value-based absorbed doses were calculated to assess their suitability for dosimetry. Results: The phantom studies demonstrate that two iterations, five subsets and a 4 mm Gaussian filter provide a reasonable compromise between a high CRC and low noise. For a 20 min scan duration, an adequate CRC of 56% (vs. 24 h: 62%, 20 mm sphere) was obtained, and the noise was reduced by a factor of 1.4, from 40% to 29%, using the full acceptance angle. The patient scan results were consistent with those from the phantom studies, and the impacts on the absorbed doses were negligible for all of the studied parameter sets, as the maximum percentage difference was −3.89%. Conclusions: With 2i5s, a 4 mm filter and a scan duration of 20 min, IQ and quantification accuracy that are suitable for post-treatment dosimetry of Y-90 radioembolization can be achieved. [ABSTRACT FROM AUTHOR]