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Diamond is a semiconductor material with remarkable structural, thermal, and electronic properties that has garnered significant interest in the field of electronics. Although hydrogen (H) and oxygen (O) terminations are conventionally favored in transistor designs, alternative options, such as silicon (Si) and germanium (Ge), are being explored because of their resilience to harsh processing conditions during fabrication. Density-functional theory was used to examine the non-oxidized and oxidized group-IV (Si and Ge)-terminated diamond (100) surfaces. The (3 × 1) reconstructed surfaces feature an ether configuration and show relative stability compared with the bare surface. Hybrid-functional calculations of the electronic properties revealed reduced fundamental bandgaps (<1 eV) and lower negative electron affinities (NEAs) than those of H-terminated diamond surfaces, which is attributed to the introduction of unoccupied Si (Ge) states and the depletion of negative charges. Furthermore, oxidation of these surfaces enhanced the stability of the diamond surfaces but resulted in two structural configurations: ether and ketone. Oxidized ether configurations displayed insulating properties with energy gaps of ∼4.3 ± 0.3 eV, similar to H-terminated diamond (100) surfaces, whereas bridged ether configurations exhibited metallic properties. Oxidization of the metallic ketone configurations leads to the opening of relatively smaller gaps in the range of 1.1–1.7 eV. Overall, oxidation induced a shift from NEAs to positive electron affinities, except for the reverse-ordered ketone surface with an NEA of −0.94 eV, a value comparable to the H-terminated diamond (100) surfaces. In conclusion, oxidized group-IV-terminated diamond surfaces offer enhanced stability compared to H-terminated surfaces and display unique structural and electronic properties that are influenced by surface bonding. [ABSTRACT FROM AUTHOR]

We have applied positron annihilation spectroscopy to study the vacancy-type defects in β-Ga2O3 single crystals. The three different types of crystals were prepared by Czochralski and edge-defined film-fed growth and doped with Fe, Mg, and Sn for semi-insulating and n-type characteristics. The crystals were also subjected to 6-MeV proton irradiation for controlled introduction of mono-vacancy defects. Positron lifetime and the details of the anisotropy of the Doppler broadening signals were measured as a function of temperature, and the results were compared with the annihilation signals predicted by theoretical calculations. We find Ga vacancies in all three basic split Ga vacancy configurations to dominate the positron data in the as-grown crystals. In contrast, unrelaxed Ga vacancies are found as the main defect introduced by the irradiation. [ABSTRACT FROM AUTHOR]

Cu(In,Ga) Se 2 (CIGS) and Cu 2 ZnSnS 4 (CZTS) are potential absorber materials for solar cell applications. We report an investigation of these materials using muon spin spectroscopy. In these experiments, positive muons produced at accelerator facilities (here the ISIS Facility, Rutherford Appleton Laboratory, U.K.) are implanted into the material and come to rest at interstitial sites in the host lattice. The muon is a sensitive local probe to study materials properties on an atomistic level. An advantage of the method is that interface properties can be studied by placing the probe particles at different depths in the sample. Muonium, the positive muon with an electron, can be considered as a light isotope of hydrogen (mass ratio 1/9) with almost identical electronic properties to hydrogen. Thus, muon spectroscopy provides also information about hydrogen in the host material. The aim of the present experiment is to obtain information about the muonium/hydrogen states formed in CIGS and CZTS solar cell materials. A major goal of the experiment is to obtain information about the physical embedding process of the implanted particle into the host lattice. The present study combines experimental measurements with total energy calculations in the framework of density functional theory. We obtain the final configurations of muonium in CZTS, that we discuss in parallel to those in CIGS. This allows us to deepen our understanding of the influence of the hydrogen impurity on the properties of these materials. We also discuss the final steps in the process of muon implantation in these materials. [ABSTRACT FROM AUTHOR]

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]

A proposal has been made for the construction of a new generation of high-luminosity electron-positron colliders known as the Super Tau-Charm Facility (STCF) in China. The STCF aims to achieve a luminosity exceeding 0.5×1035cm−2s−1 and will operate within a center-of-mass energy range of 2 to 7GeV. Given the design challenges of the STCF collider ring, swap-out injection has been proposed as an alternative method for achieving the desired luminosity. Consequently, the STCF injector will explore both off-axis injection and swap-out injection methods concurrently. This paper will detail the ongoing research progress on these two injection techniques. [ABSTRACT FROM AUTHOR]

Positronium is abundantly produced within the molecular voids of a patient's body during positron emission tomography (PET). Its properties dynamically respond to the submolecular architecture of the tissue and the partial pressure of oxygen. Current PET systems record only two annihilation photons and cannot provide information about the positronium lifetime. This study presents the in vivo images of positronium lifetime in a human, for a patient with a glioblastoma brain tumor, by using the dedicated Jagiellonian PET system enabling simultaneous detection of annihilation photons and prompt gamma emitted by a radionuclide. The prompt gamma provides information on the time of positronium formation. The photons from positronium annihilation are used to reconstruct the place and time of its decay. In the presented case study, the determined positron and positronium lifetimes in glioblastoma cells are shorter than those in salivary glands and those in healthy brain tissues, indicating that positronium imaging could be used to diagnose disease in vivo. [ABSTRACT FROM AUTHOR]

The morphology is a crucial indicator for diagnosing a low-energy, low-brightness particle beam. However, conventional positron beam diagnosis, based on the pixel scanning principle, is limited by physical constraints, such as the resolution of detector pixels. Here, we have presented a novel slow positron diagnosis method using compressive sampling. With a 100 × 100 pixel-sized mask, for example, the positron beam morphology can be significantly reconstructed with a peak signal-to-noise ratio of ∼40 dB, even at half the sampling rate compared to pixel scanning. It explores a promising approach for positron beam diagnosis with an ultra-high resolution and fast sampling rates. [ABSTRACT FROM AUTHOR]

Nuclear reaction cross sections for the formation of 72As and 71As in proton-induced reactions on enriched 72Ge targets were measured up to 45 MeV utilizing three different cyclotrons at the Forschungszentrum Jülich. The stacked-thin sample activation technique in combination with high-resolution γ-ray spectrometry was used. The major γ-ray peaks of 72As and 71As formed via the 72Ge(p,n)72As and 72Ge(p,2n)71As reactions, respectively, were analyzed. The incident proton energy and flux on a foil were determined using several monitor reactions. Based on integrated counts, irradiation data and the nuclear decay data, the reaction cross sections were measured. All data describe the first measurements. Theoretical nuclear model calculations were then carried out by using the codes TALYS 1.96, EMPIRE 3.2 and ALICE-IPPE. A very good agreement between the measured data and calculated values was found. The new data enabled us to calculate the thick target yields and estimate the radionuclidic impurities for a given energy range. Over the optimum energy range Ep = 14 → 7 MeV, the calculated thick target yield of 72As amounts to 272 MBq/μAh with no 71As impurity at all. The 72Ge(p,n)72As reaction on the enriched 72Ge is thus very suitable for clinical scale production of 72As at a medical cyclotron. [ABSTRACT FROM AUTHOR]

It is shown that electromagnetic interactions of cosmic ray protons provide a noticeable contribution to positrons production. This is due to a combination of low energy threshold of electron–positron pair creation compared to the thresholds of pion creation in strong interactions and rapid decrease of the cosmic rays energy spectrum. Moreover, the electromagnetically produced positrons are very soft, therefore their annihilation with background electrons directly produces the observed 511 keV gamma-line. [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]

Background Purpose Methods Results Conclusions Positron emission tomography (PET) has been investigated for its ability to reconstruct proton‐induced positron activity distributions in proton therapy. This technique holds potential for range verification in clinical practice. Recently, deep learning‐based dose estimation from positron activity distributions shows promise for in vivo proton dose monitoring and guided proton therapy.This study evaluates the effectiveness of three classical neural network models, recurrent neural network (RNN), U‐Net, and Transformer, for proton dose estimating. It also investigates the characteristics of these models, providing valuable insights for selecting the appropriate model in clinical practice.Proton dose calculations for spot beams were simulated using Geant4. Computed tomography (CT) images from four head cases were utilized, with three for training neural networks and the remaining one for testing. The neural networks were trained with one‐dimensional (1D) positron activity distributions as inputs and generated 1D dose distributions as outputs. The impact of the number of training samples on the networks was examined, and their dose prediction performance in both homogeneous brain and heterogeneous nasopharynx sites was evaluated. Additionally, the effect of positron activity distribution uncertainty on dose prediction performance was investigated. To quantitatively evaluate the models, mean relative error (MRE) and absolute range error (ARE) were used as evaluation metrics.The U‐Net exhibited a notable advantage in range verification with a smaller number of training samples, achieving approximately 75% of AREs below 0.5 mm using only 500 training samples. The networks performed better in the homogeneous brain site compared to the heterogeneous nasopharyngeal site. In the homogeneous brain site, all networks exhibited small AREs, with approximately 90% of the AREs below 0.5 mm. The Transformer exhibited the best overall dose distribution prediction, with approximately 92% of MREs below 3%. In the heterogeneous nasopharyngeal site, all networks demonstrated acceptable AREs, with approximately 88% of AREs below 3 mm. The Transformer maintained the best overall dose distribution prediction, with approximately 85% of MREs below 5%. The performance of all three networks in dose prediction declined as the uncertainty of positron activity distribution increased, and the Transformer consistently outperformed the other networks in all cases.Both the U‐Net and the Transformer have certain advantages in the proton dose estimation task. The U‐Net proves well suited for range verification with a small training sample size, while the Transformer outperforms others at dose‐guided proton therapy. [ABSTRACT FROM AUTHOR]

The dissemination of positron-acoustic (PA) nonlinear structures, including the solitary waves (SWs) and cnoidal waves (CWs), is analyzed in an unmagnetized electron–positron–ion (e–p–i) plasma having inertial cold positrons and inertialess Cairns distributed electrons and Maxwellian positrons as well as immobile positive ions. The reductive perturbation method (RPM) is introduced to reduce the fluid equations to this model to the Korteweg–de Vries (KdV) type equation for studying small amplitude PA waves (PAWs). Moreover, the Kawahara (sometimes called the fifth-order KdV) equation is also obtained to investigate the characteristics of large amplitude PAWs. The effects of related parameters, such as nonthermal parameters, hot positron concentration, electron concentration, and temperature ratios, are numerically examined on the features of SWs and CWs. [ABSTRACT FROM AUTHOR]

The corrosion behavior of 310S austenitic stainless steel, subjected to different surface treatments machined (MA), sandblasting (SB), and polishing (PO), was exposed to a 550°C supercritical water (SCW) environment. The aged samples were analyzed using variable‐energy slow positron beam techniques. The obtained results revealed that the plastic deformation of the near‐surface region of the MA and SB samples was substantially recovered in the SCW conditions. At least two distinct oxide layers formed, and the oxidation process created a Fe/Cr depletion zone in the inner layer. Various surface treatments, however, led to different corrosion profiles. The depth profile of slow positron beam characterization suggests that significant residual stress and deformation zones on the surfaces of the sandblasted samples probably provided a diffusion path for the oxidation of the 310S. Only the SB samples exhibited a negative weight change after SCW exposure. At the same time, the SB samples showed the highest concentration of the positron traps in this region, which was explained by open‐volume defects associated with the microcracks introduced by sandblasting. [ABSTRACT FROM AUTHOR]

Energetic positrons have been observed in the interstellar medium, and high-energy positrons with relativistic energies up to approximately 1 TeV have been detected in Galactic cosmic rays. We conducted a study on the acceleration of particles, specifically positrons, in a nonrelativistic quasi-parallel collisionless shock induced by a plasma consisting of protons, electrons, and positrons. The positron-to-proton number density ratio in the plasma is 0.1. We focused on a representative shock with a sonic Mach number of 17.1 and an Alfvénic Mach number of 16.8 in the rest frame of the shock. To investigate the acceleration mechanisms of particles including positrons in the shock, we utilized 1D particle-in-cell simulations. It was found that all three species of particles in the shock can be accelerated and exhibit power-law spectra. At the shock front, a significant portion of incoming upstream particles are reflected and undergo significant energy increases, and these reflected particles can be efficiently injected into the process of diffusive shock acceleration (DSA). Moveover, the reflected positrons can be further accelerated by an electric field parallel to the magnetic field when they move along the magnetic field upstream of the shock. As a result, positrons can be preferentially accelerated to be injected in the DSA process compared to electrons. [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]

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]

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 present a theoretical and computational study of positron supercritical resonances in systems consisting of two highly-charged bare nuclei. The resonance positions and widths depending on the internuclear separation are calculated with the help of the complex-scaling generalized pseudospectral method in modified prolate spheroidal coordinates. The results are applied to estimate the probability of spontaneous positron creation in slow U –U and Cm –Cm collisions. [ABSTRACT FROM AUTHOR]

The High Energy Cosmic Ray Observatory (HERO) is an experimental design for the direct study of cosmic rays based on the use of an ultraheavy ionization calorimeter. The effective geometric factor of the facility will be at least 12 m sr for cosmic-ray protons and at least 16–20 m sr for nuclei and electrons. During 5–7 years of exposure, this mission will allow measuring the energy spectra of all common and rare cosmic-ray nuclei in the energy range of eV/particle with element-by-element resolution of the charge of nuclei and with sufficiently high energy resolution. It is planned to study not only the cosmic ray nuclei with charges , but also the superheavy nuclei beyond the iron peak, as well as high-energy electrons, positrons, and gamma rays. The main objectives, design appearance, and characteristics of the space mission are discussed. [ABSTRACT FROM AUTHOR]

The research of the electron and positron spectrum of galactic cosmic rays is an important task of high-energy astrophysics. The necessity of using new methods of electron and positron registration is caused by insufficient statistical reliability of data from modern experiments in the TeV energy range. In the present work, one of the possible registration methods based on the use of synchrotron radiation of electrons and positrons in the Earth's magnetic field is investigated. Using modeling of trajectories of high-energy electrons, positrons, and synchrotrons photons emitted by them, the detector count rate for the ISS and ROS orbits has been estimated. The possibility of separation of electrons and positrons using this method is shown. [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]

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]

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]

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]

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]

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]

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]

The melt‐quenched glassy‐crystalline As70Se30 alloys subjected to nanomilling in dry and combined dry‐wet modes are probed by positron annihilation lifetime (PAL) spectroscopy. Modified Positronics approach parameterizing conversion of Ps‐hosting holes in positron trapping sites is applied to identify volumetric changes in the polyvinylpyrrolidone (PVP) stabilized alloys in respect to dry‐milled ones. Under transition from wet‐ to dry‐milled arsenoselenide probes, positron traps with defect‐specific lifetime ascribed to multiatomic vacancies in As–Se network appear instead of Ps‐decaying sites preferentially in PVP environment, these changes occurring in the oxidized arsenoselenide matrix. [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]

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]

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]

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]

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 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 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]

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]

The origin of the diffuse gamma-ray background in the range from hundreds keV to several MeV is not known conclusively. From current models and observations it is believed that, at least partially, this background is formed by blazars and remnants of supernovae (SN) of type Ia in distant galaxies. However, these contributions are not sufficient to reproduce the observed level of the signal. In this work we propose another source which could contribute to this background, namely the jets of active galactic nuclei (AGN). The composition of jets is not known, but there are observational hints that the fraction of positrons there is substantial. Positrons are partially evacuated to the intergalactic medium and partially mix with the circumgalactic medium and annihilate there comparatively quickly. Using the AGN luminosity function, we estimated the positron production rate and the contribution of the positron annihilation to the cosmic background below 511 keV. We also estimated the analogous contribution from positron annihilation within SN Ia remnants in distant galaxies. The contribution of AGNs is estimated to be a factor of 5–10 smaller than the observed background intensity, and the contribution from SNe is yet smaller by one order of magnitude. Nevertheless, the contribution of AGNs appeared to be larger than the contribution of blazars estimated from Swift-BAT and Fermi-LAT observations. The main uncertainty in our model is the fraction of positrons remaining in the circumgalactic medium which makes our estimation an upper limit. [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]