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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]

We present cross sections for the formation of positronium chloride (PsCl) in its ground state from the charge exchange between positronium (Ps) and chloride (Cl−) in the range of 10 meV–100 eV Ps energy. We have used theoretical models based on the first Born approximation in its three-body formulation. We simulated the collisions between Ps and Cl− using ab initio binding energies and positronic wave functions at both the mean-field and correlated levels extrapolated to the complete basis set limit. The accuracy of these ab initio data was benchmarked on the PsF system with the existing highly accurate results, including the very recent quantum Monte Carlo results. We have investigated Ps excited states up to n = 4. The results suggest that the channel Ps(n = 2) is of particular interest for the production of PsCl in the ground state and shows that an accurate treatment of correlation effects (i.e., electron–electron and electron–positron correlations) leads to a significant change in the magnitude of the PsCl production cross section with respect to the mean-field level. [ABSTRACT FROM AUTHOR]

We present cross sections for the formation of positronium chloride (PsCl) in its ground state from the charge exchange between positronium (Ps) and chloride (Cl−) in the range of 10 meV–100 eV Ps energy. We have used theoretical models based on the first Born approximation in its three-body formulation. We simulated the collisions between Ps and Cl− using ab initio binding energies and positronic wave functions at both the mean-field and correlated levels extrapolated to the complete basis set limit. The accuracy of these ab initio data was benchmarked on the PsF system with the existing highly accurate results, including the very recent quantum Monte Carlo results. We have investigated Ps excited states up to n = 4. The results suggest that the channel Ps(n = 2) is of particular interest for the production of PsCl in the ground state and shows that an accurate treatment of correlation effects (i.e., electron–electron and electron–positron correlations) leads to a significant change in the magnitude of the PsCl production cross section with respect to the mean-field level. [ABSTRACT FROM AUTHOR]

The energetic stability of positron–dianion systems [A−; e+; A−] is studied via many-body theory, where A− includes H−, F−, Cl−, and the molecular anions (CN)− and (NCO)−. Specifically, the energy of the system as a function of ionic separation is determined by solving the Dyson equation for the positron in the field of the two anions using a positron–anion self-energy as constructed in Hofierka et al. [Nature 606, 688 (2022)] that accounts for correlations, including polarization, screening, and virtual-positronium formation. Calculations are performed for a positron interacting with H 2 2 − , F 2 2 − , and Cl 2 2 − and are found to be in good agreement with previous theory. In particular, we confirm the presence of two minima in the potential energy of the [H−; e+; H−] system with respect to ionic separation: a positronically bonded [H−; e+; H−] local minimum at ionic separations r ∼ 3.4 Å and a global minimum at smaller ionic separations r ≲ 1.6 Å that gives overall instability of the system with respect to dissociation into a H2 molecule and a positronium negative ion, Ps−. The first predictions are made for positronic bonding in dianions consisting of molecular anionic fragments, specifically for (CN) 2 2 − and (NCO) 2 2 − . In all cases, we find that the molecules formed by the creation of a positronic bond are stable relative to dissociation into A− and e+A− (positron bound to a single anion), with bond energies on the order of 1 eV and bond lengths on the order of several ångstroms. [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]

The investigation focused on nanovoids (free-volume defects) in spinel MgAl2O4 ceramics, synthesized at various temperatures, both before and after saturation (water-immersion), utilizing the positron annihilation spectroscopy method. The findings were analyzed within the framework of a multi-channel model. It was demonstrated that this model integrates channels capturing positrons by volumetric defects and the decay of ortho-positronium atoms. Within this refined approach, the first component reflects the microstructural characteristics of the spinel structure, the second component corresponds to volumetric defects near grain boundaries, while the third pertains to the "pick-off" process of ortho-positronium annihilation within water-filled nanopores of the ceramics. [ABSTRACT FROM AUTHOR]

The positron annihilation lifetime (PAL) spectroscopy characteristics of ethylene-propylene-diene monomer rubber (EPDM) composites reinforced with treated wheat husk fibers (WHFs) were investigated for the first time. PAL spectroscopy is employed to study the free volume of polymers. The use of lignocellulosic materials as reinforcement in polymeric composites has gained attention due to their low cost, availability, and eco-friendliness. In this study, the impact of the loading concentration on the interfacial adhesion between the EPDM matrix and WHFs is quantified, along with the evaluation of swelling measurement and tensile properties. Additionally, the nanoscopic properties derived from PAL spectroscopy correlate with the composites' macroscopic properties. In addition, the dielectric properties of the investigated samples have been studied, and their conductivity has been calculated. To determine the conduction mechanism within these samples and how it is affected by the addition of WHF, the change in electrical conductivity with the frequency of the external electric field applied to the samples was studied, and from this, the conduction mechanism was determined, and the barrier height value was calculated. The experimental results provide insights into the relationship between the structure and properties of EPDM-WHF biocomposites, offering valuable knowledge for developing sustainable and high-performance materials. [ABSTRACT FROM AUTHOR]

Fast radio bursts (FRBs) are bright extragalactic transients likely produced by magnetars. We study the propagation of FRBs in magnetar winds, assuming that the wind is strongly magnetized and composed of electron-positron pairs. We focused on the regime where the strength parameter of the radio wave, a0, is larger than unity and the wave frequency, ω0, is larger than the Larmor frequency in the background magnetic field, ωL. We show that strong radio waves with a0 > 1 are able to propagate when ω0 > a0ωL, as the plasma current is a linear function of the wave electric field. The dispersion relation is independent of the wave strength parameter when ω0 > a0ωL. Radio waves could instead be damped when ω0 < a0ωL, as a significant fraction of the wave energy is used to compress the plasma and amplify the background magnetic field. Our results suggest that FRBs should be produced at large distances from the magnetar (i.e., R > 1012 cm, where the condition ω0 > a0ωL is satisfied). Alternatively, the structure of the magnetar wind should be strongly modified during a flare to allow for the escape of FRBs produced at radii R < 1012 cm. [ABSTRACT FROM AUTHOR]

Context. Relativistic magnetic reconnection studies have so far focused on symmetric configurations, where the upstream plasma has identical properties on the two sides of the layer. Yet, just like nonrelativistic reconnection on the dayside of the Earth's magnetosphere, relativistic reconnection can also operate at the interface between highly asymmetric environments. The boundary layer between a relativistic jet and an accretion flow forming around a supermassive black hole can present asymmetric configurations in terms of plasma composition, bulk velocity, temperature, and magnetization. Aims. We conducted the first study of relativistic magnetic reconnection where the upstream plasma is composed of electron-positron pairs on one side, and electrons and ions on the other. We also investigated the impact of a relativistic symmetric shear flow applied along the reconnecting field lines. Methods. We simulated magnetic reconnection using 2D particle-in-cell simulations. The initial setup was adapted from a classic Harris layer without a guide field, modified to accommodate plasma-composition and shear asymmetries in the upstream medium. Results. For a composition-asymmetric setup, we find that the reconnection dynamics is driven by the electron-ion side, which is the plasma with the lowest magnetization. The energy partition favors accelerating ions at the expense of electrons even more than in a corresponding symmetric setup. With respect to shear, a super-Alfvénic upstream decreases the laboratory-frame reconnection rate, but, unlike in nonrelativistic studies, does not shut off reconnection completely. Conclusions. The asymmetries examined in this work lower the overall efficiency of electron acceleration relative to corresponding symmetric configurations. In the context of a black hole jet-disk boundary, asymmetric reconnection alone is probably not efficient at accelerating electrons to very high energies, but it might facilitate plasma mixing and particle injection for other acceleration channels at the interface. [ABSTRACT FROM AUTHOR]

Context. There is growing evidence from gamma-ray observations at high and very high energies that particle escape is a key aspect shaping the morphological properties of pulsar wind nebulae (PWNe) at various evolutionary stages. Aims. We aim to provide a simple model for the gamma-ray emission from these objects including the transport of particles across the different components of the system. We applied it to sources HESS J1809−193 and HESS J1825−137. Methods. We developed a multi-zone framework applicable to dynamically young PWNe, taking into account the diffusive escape of relativistic electron-positron pairs out of the nebula into the parent supernova remnant (SNR) and their confinement downstream of the magnetic barrier of the forward shock until an eventual release into the surrounding interstellar medium (ISM). Results. For a wide range of turbulence properties in the nebula, the GeV–TeV inverse-Compton radiation from pairs that escaped into the remnant can be a significant if not dominant contribution to the emission from the system. It may dominate the pion-decay radiation from cosmic rays accelerated at the forward shock and advected downstream of it. In the TeV–PeV range, the contribution from particles escaped into the ISM can exceed by far that of the SNR+PWN components. Applied to HESS J1809−193 and HESS J1825−137, we found that spatially extended GeV–TeV emission components can be accounted for mostly from particles escaped into the ISM, while morphologically more compact components above 50 − 100 TeV are ascribed to the PWNe. In these two cases, the model suggests high turbulence in the nebula and a forward shock accelerating cosmic rays up to ∼100 TeV at most. Conclusions. The model provides the temporal and spectral properties of the flux of particles originally energized by the pulsar wind and ultimately released in the ISM. It can be used to constrain the transport of particles in the vicinity of pulsar-PWN-SNR systems from broadband gamma-ray observations, or in studies of the contribution of pulsar-related systems to the local positron flux. [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 objective of this work is to showcase the ortho-positronium lifetime as a probe for soft-tissue characterization. We employed positron annihilation lifetime spectroscopy to experimentally measure the three components of the positron annihilation lifetime—para-positronium (p-Ps), positron, and ortho-positronium (o-Ps)—for three types of porcine, non-fixated soft tissues ex vivo: adipose, hepatic, and muscle. Then, we benchmarked our measurements with X-ray phase-contrast imaging, which is the current state-of-the-art for soft-tissue analysis. We found that the o-Ps lifetime in adipose tissues (2.54 ± 0.12 ns) was approximately 20% longer than in hepatic (2.04 ± 0.09 ns) and muscle (2.03 ± 0.12 ns) tissues. In addition, the separation between the measurements for adipose tissue and the other tissues was better from o-Ps lifetime measurement than from X-ray phase-contrast imaging. This experimental study proved that the o-Ps lifetime is a viable non-invasive probe for characterizing and classifying the different soft tissues. Specifically, o-Ps lifetime as a soft-tissue characterization probe had a strong sensitivity to the lipid content that can be potentially implemented in commercial positron emission tomography scanners that feature list-mode data acquisition. [ABSTRACT FROM AUTHOR]

This article reports the first measurements of high-energy photons produced with the high-intensity PETawatt Aquitaine Laser (PETAL) laser. The experiments were performed during the commissioning of the laser. The laser had an energy of about 400 J, an intensity of 8 × 1018 W cm−2, and a pulse duration of 660 fs (FWHM). It was shot at a 2 mm-thick solid tungsten target. The high-energy photons were produced mainly from the bremsstrahlung process for relativistic electrons accelerated inside a plasma generated on the front side of the target. This paper reports measurements of electrons, protons and photons. Hot electrons up to ≈35 MeV with a few-MeV temperature were recorded by a spectrometer, called SESAME (Spectre ÉlectronS Angulaire Moyenne Énergie). K- and L-shells were clearly detected by a photon spectrometer called SPECTIX (Spectromètre Petal à Cristal en TransmIssion pour le rayonnnement X). High-energy photons were diagnosed by CRACC-X (Cassette de RAdiographie Centre Chambre-rayonnement X), a bremsstrahlung cannon. Bremsstrahlung cannon analysis is strongly dependent on the hypothesis adopted for the spectral shape. Different shapes can exhibit similar reproductions of the experimental data. To eliminate dependence on the shape hypothesis and to facilitate analysis of the data, simulations of the interaction were performed. To model the mechanisms involved, a simulation chain including hydrodynamic, particle-in-cell, and Monte Carlo simulations was used. The simulations model the preplasma generated at the front of the target by the PETAL laser prepulse, the acceleration of electrons inside the plasma, the generation of MeV-range photons from these electrons, and the response of the detector impacted by the energetic photon beam. All this work enabled reproduction of the experimental data. The high-energy photons produced have a large emission angle and an exponential distribution shape. In addition to the analysis of the photon spectra, positron production was also investigated. Indeed, if high-energy photons are generated inside the solid target, some positron/electron pairs may be produced by the Bethe–Heitler process. Therefore, the positron production achievable within the PETAL laser facility was quantified. To conclude the study, the possibility of creating electron/positron pairs through the linear Breit–Wheeler process with PETAL was investigated. [ABSTRACT FROM AUTHOR]

Abstract The positron annihilation lifetime (PAL) spectroscopy characteristics of ethylene-propylene-diene monomer rubber (EPDM) composites reinforced with treated wheat husk fibers (WHFs) were investigated for the first time. PAL spectroscopy is employed to study the free volume of polymers. The use of lignocellulosic materials as reinforcement in polymeric composites has gained attention due to their low cost, availability, and eco-friendliness. In this study, the impact of the loading concentration on the interfacial adhesion between the EPDM matrix and WHFs is quantified, along with the evaluation of swelling measurement and tensile properties. Additionally, the nanoscopic properties derived from PAL spectroscopy correlate with the composites’ macroscopic properties. In addition, the dielectric properties of the investigated samples have been studied, and their conductivity has been calculated. To determine the conduction mechanism within these samples and how it is affected by the addition of WHF, the change in electrical conductivity with the frequency of the external electric field applied to the samples was studied, and from this, the conduction mechanism was determined, and the barrier height value was calculated. The experimental results provide insights into the relationship between the structure and properties of EPDM-WHF biocomposites, offering valuable knowledge for developing sustainable and high-performance materials.

We consider a one-dimensional effective quantum electrodynamics (QED) model of the relativistic hydrogen-like atom using delta-potential interactions. We discuss the general exact theory and the Hartree–Fock approximation. The present one-dimensional effective QED model shares the essential physical feature of the three-dimensional theory: the nuclear charge polarizes the vacuum state (creation of electron–positron pairs), which results in a QED Lamb-type shift of the bound-state energy. Yet, this 1D effective QED model eliminates some of the most serious technical difficulties of the three-dimensional theory coming from renormalization. We show how to calculate the vacuum-polarization density at zeroth order in the two-particle interaction and the QED Lamb-type shift of the bound-state energy at first order in the two-particle interaction. The present work may be considered a step toward the development of a quantum-chemistry effective QED theory of atoms and molecules. [ABSTRACT FROM AUTHOR]

Electron and positron chemical potentials, positron affinity and bulk lifetime, positronium work function and positron effective mass have been computed for ZnxCd1−xS using a pseudo-potential approach within the virtual crystal approximation (VCA) and the independent particle model. A correction to VCA is included taking into consideration the disorder effect. This has permitted the computation of the positron affinity to separate materials of interest. The performances indicated that the positron annihilates differently in CdS than in ZnS. The bulk lifetime of positron has been obtained as 241.17 ps for CdS and 215.47 ps for ZnS. Hence, it decreases when going from CdS to ZnS. The positronium work function increases from 2.245 to 3.08 eV when augmenting the composition x from 0 to 1, indicating that only fewer positronium atoms are figured in a specimen surface and ejected for the vacuum. The positron effective mass augments from 0.96 to 1.34 m0 when going from CdS to ZnS materials. The details collected from the assisted investigation are of a former significance for an ameliorate accordance of positron annihilation in ZnxCd1−xS. [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]

This work presents an extension of the projector operator embedding scheme of Manby et al. [J. Chem. Theory Comput. 8, 2564 (2012)] in a multicomponent (MC) framework. Here, a molecular system containing electrons and other types of quantum species is divided into a wavefunction (WF) subsystem of interest and a density functional theory (DFT) environment. The WF-in-DFT partition decreases computational costs by partially truncating the WF subsystem basis set at the cost of introducing a controllable embedding error. To explore the applicability of the MC extension, third-order propagator-in-DFT calculations were performed for positron–anion complexes for alkoxides and carboxylates with carbon chains of different sizes. For these systems, it was found that selecting a WF subsystem with the positron and only the oxygen atoms caused an error of 0.1 eV or lower in positron-binding energies, while reducing between 33% and 55% the basis set size. The reduction of computational costs achieved with the embedding scheme allowed us to improve molecular positron-binding energy predictions by performing complete basis set limit extrapolations. Combining the WF-in-DFT embedding and the complete basis set extrapolation, positronium aliphatic alkoxides were predicted to be energetically stable by 0.3 eV with respect to Ps emission. Similarly, positronium carboxylates, both aromatic and aliphatic, were predicted to be stable by 1.3 eV. [ABSTRACT FROM AUTHOR]

Background: Positron emission tomography (PET) traditionally uses coincident annihilation photons emitted from a positron interacting with an electron to localize cancer within the body. The formation of positronium (Ps), a bonded electron-positron pair, has not been utilized in clinical applications of PET due to the need to detect either the emission of a prompt gamma ray or the decay of higher-order coincident events. Assessment of the lifetime of the formed Ps, however, can potentially yield additional diagnostic information of the surrounding tissue because Ps properties vary due to void size and molecular composition. To assess the feasibility of measuring Ps lifetimes with a PET scanner, experiments were performed in a Biograph Vision Quadra (Siemens Healthineers). Quadra is a long-axial field-of-view (LA-FOV) PET scanner capable of producing list-mode data from single interaction events. Results: Ortho-Ps (o-Ps) lifetimes were measured for quartz-glass and polycarbonate samples using a 22 Na positron source. Results produced o-Ps lifetimes of 1.538 ± 0.036 ns for the quartz glass and 1.927 ± 0.042 ns for the polycarbonate. Both o-Ps lifetimes were determined using a double-exponential fit to the time-difference distribution between the emission of a prompt gamma ray and the annihilation of the correlated positron. The measured values match within a single standard deviation of previously published results. The quartz-glass samples were additional measured with 82 Rb , 68 Ga and 124 I to validate the lifetime using clinically available sources. A double-exponential fit was initially chosen as a similar methodology to previously published works, however, an exponentially-modified Gaussian distribution fit to each lifetime more-accurately models the data. A Bayesian method was used to estimate the variables of the fit and o-Ps lifetime results are reported using this methodology for the three clinical isotopes: 1.59 ± 0.03 ns for 82 Rb , 1.58 ± 0.07 ns for 68 Ga and 1.62 ± 0.01 ns for 124 I . The impact of scatter and attenuation on the o-Ps lifetime was also assessed by analyzing a water-filled uniform cylinder (20 ϕ × 30 cm 3 ) with an added 82 Rb solution. Lifetimes were extracted for various regions of the cylinder and while there is a shape difference in the lifetime due to scatter, the extracted o-Ps lifetime of the water, 1.815 ± 0.013 ns, agrees with previously published results. Conclusion: Overall, the methodology presented in this manuscript demonstrates the repeatability of Ps lifetime measurements with clinically available isotopes in a commercially-available LA-FOV PET scanner. This validation work lays the foundation for future in-vivo patient scans with Quadra. [ABSTRACT FROM AUTHOR]

Binary AsxSe100−x alloys from the border of a glass-forming region (65 < x < 70) subjected to nanomilling in dry and dry–wet modes are characterized by the XRPD, micro-Raman scattering (micro-RS) and revised positron annihilation lifetime (PAL) methods complemented by a disproportionality analysis using the quantum–chemical cluster modeling approach. These alloys are examined with respect to tetra-arsenic biselenide As4Se2 stoichiometry, realized in glassy g-As65Se35, glassy–crystalline g/c-As67Se33 and glassy–crystalline g/c-As70Se30. From the XRPD results, the number of rhombohedral As and cubic arsenolite As2O3 phases in As-Se alloys increases after nanomilling, especially in the wet mode realized in a PVP water solution. Nanomilling-driven amorphization and reamorphization transformations in these alloys are identified by an analysis of diffuse peak halos in their XRPD patterning, showing the interplay between the levels of a medium-range structure (disruption of the intermediate-range ordering at the cost of an extended-range one). From the micro-RS spectroscopy results, these alloys are stabilized by molecular thioarsenides As4Sen (n = 3, 4), regardless of their phase composition, remnants of thioarsenide molecules destructed under nanomilling being reincorporated into a glass network undergoing a polyamorphic transition. From the PAL spectroscopy results, volumetric changes in the wet-milled alloys with respect to the dry-milled ones are identified as resulting from a direct conversion of the bound positron–electron (Ps, positronium) states in the positron traps. Ps-hosting holes in the PVP medium appear instead of positron traps, with ~0.36–0.38 ns lifetimes ascribed to multivacancies in the As-Se matrix. The superposition of PAL spectrum peaks and tails for pelletized PVP, unmilled, dry-milled, and dry–wet-milled As-Se samples shows a spectacular smoothly decaying trend. The microstructure scenarios of the spontaneous (under quenching) and activated (under nanomilling) decomposition of principal network clusters in As4Se2-bearing arsenoselenides are recognized. Over-constrained As6·(2/3) ring-like network clusters acting as pre-cursors of the rhombohedral As phase are the main products of this decomposition. Two spontaneous processes for creating thioarsenides with crystalline counterparts explain the location of the glass-forming border in an As-Se system near the As4Se2 composition, while an activated decomposition process for creating layered As2Se3 structures is responsible for the nanomilling-driven molecular-to-network transition. [ABSTRACT FROM AUTHOR]

The Vlasov equations are used to determine the transverse and longitudinal dielectric permeabilities of a hot, rarefied ultrarelativistic electron-positron plasma. The Fermi distribution is used as the unperturbed distribution function of the electrons and positrons. It is shown that, owing to the abundant production of electron-positron pairs, the plasma frequency of the ultrarelativistic plasma increases linearly with rising temperature. Here the ratio of the frequency at which the spectral density of the equilibrium emission is maximal to the plasma frequency remains fixed and equals 28. It is shown that in this sort of plasma, in particular in the matter of the early universe in the ultrarelativistic electronpositron epoch, the spectrum of the radiation differs very little from black-body radiation. [ABSTRACT FROM AUTHOR]

A bulky organic monomer with a long alkyl chain, as, e.g., hexadecyltrimethylammonium (HDTMA+), is the structural unit prerequisite for ionic-liquid engineering as well as porous materials design; however, the detailed structure in the entangled agglomerate, particularly caused by moderate temperatures, has not yet been well studied. Here, the local molecular structures in the entangled HDTMA+ monomers stirred at different temperatures were studied by open space analysis using positronium coupled with molecular dynamics simulations. The agglomerate stirred at 323 K exhibits the small and large open spaces with the size distributions centered at ~ 0.27 nm and ~ 0.55 nm, respectively. Both open spaces are found to be dominantly surrounded by protons. Upon increasing temperature to 423 K, the enlargements of the small and large open spaces occur broadening their size distributions, whereas the proton-rich chemical environment is maintained. The enhanced porousness at high temperatures is explained by the efficiently shrunk monomers triggered off by the lowering of HDTMA+ diffusivity upon coexisting with propanol molecules. [ABSTRACT FROM AUTHOR]

Positronium lifetime imaging (PLI) is a newly demonstrated technique possible with time-of-flight (TOF) positron emission tomography (PET), capable of producing an image reflecting the lifetime of the positron, more precisely ortho-positronium (o-Ps), before annihilation, in addition to the traditional uptake image of the PET tracer. Due to the limited time resolution of TOF- PET systems and the added complexities in physics and statistics, lifetime image reconstruction presents a challenge. Recently, we described a maximum- likelihood approach for PLI by considering only o-Ps. In real-world scenarios, other populations of positrons that exhibit different lifetimes also exist. This paper introduces a novel two-component model aimed at enhancing the accuracy of o-Ps lifetime images. Through simulation studies, we compare this new model with the existing single-component model and demonstrate its superior performance in accurately capturing complex lifetime distributions. [ABSTRACT FROM AUTHOR]

A long gamma–ray burst (GRB) is observed when the collapse of a massive star produces an ultrarelativistic outflow pointed toward Earth. Gamma-ray spectra of long GRBs are smooth, typically modeled by joint power-law segments describing a continuum, with no detected spectral lines. We report a significant (>6σ) narrow emission feature at ~10 mega–electron volts (MeV) in the spectrum of the bright GRB 221009A. Over 80 seconds, it evolves in energy (~12 to ~6 MeV) and in luminosity (~1.1 to <0.43 × 1050 erg second−1) but has a constant width of ~1 MeV. We interpret this feature as a blueshifted spectral line produced by the annihilation of electron-positron pairs, potentially in the same location responsible for emitting the brightest GRB pulses. [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]

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]

The single-center convergent close-coupling (CCC) method is applied to calculate positron scattering from boron. A model potential approach is utilized to extract the positronium formation, direct ionization, and values between the positronium formation and ionization thresholds. We present results for total, electron loss, elastic, momentum transfer, total bound state excitation, positronium formation, direct ionization, stopping power, and mean excitation energy from 10 − 5 eV to 5000 eV. For boron, there is only one other set of theoretical positron calculations for elastic and momentum transfer above 500 eV, which is in excellent agreement with the current CCC results. Using the current results for boron atoms and previous CCC calculations for hydrogen and fluorine atoms, positron scattering from BF, BF 2 , BF 3 , and BH molecules is calculated for energies between 0.1 eV and 5000 eV with a modified independent atom approach. [ABSTRACT FROM AUTHOR]

Relativistic electron-positron plasmas are ubiquitous in extreme astrophysical environments such as black-hole and neutron-star magnetospheres, where accretion-powered jets and pulsar winds are expected to be enriched with electron-positron pairs. Their role in the dynamics of such environments is in many cases believed to be fundamental, but their behavior differs significantly from typical electron-ion plasmas due to the matter-antimatter symmetry of the charged components. So far, our experimental inability to produce large yields of positrons in quasi-neutral beams has restricted the understanding of electron-positron pair plasmas to simple numerical and analytical studies, which are rather limited. We present the first experimental results confirming the generation of high-density, quasi-neutral, relativistic electron-positron pair beams using the 440 GeV/c beam at CERN's Super Proton Synchrotron (SPS) accelerator. Monte Carlo simulations agree well with the experimental data and show that the characteristic scales necessary for collective plasma behavior, such as the Debye length and the collisionless skin depth, are exceeded by the measured size of the produced pair beams. Our work opens up the possibility of directly probing the microphysics of pair plasmas beyond quasi-linear evolution into regimes that are challenging to simulate or measure via astronomical observations. Relativistic electron-positron (pair) plasmas play a fundamental role in the magnetospheres, jets, and winds of black holes and neutron stars, but existing studies have been purely theoretical. Here, the authors open up the exciting possibility to probe relativistic pair-plasmas in the laboratory. [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]

A theoretical study is performed for the single-photon creation of an electron–positron pair in an external magnetic field. We present general expressions for process rates that are valid for arbitrarily high field strengths. Simple analytical expressions are obtained in the limiting cases of weak and strong fields. The emphasis is placed on the investigation of the spin alignment of the created particles as a function of the incident photon polarization and magnetic field strength. The obtained results show that the maximum value of the polarization degree of electrons (positrons) decreases in supercritical magnetic field. [ABSTRACT FROM AUTHOR]

The theory of heavy ion double charge exchange (DCE) reactions proceeding by effective rank-2 isotensor interactions is presented. Virtual pion–nucleon charge exchange interactions are investigated as the source for induced isotensor interactions, giving rise to the Majorana DCE (MDCE) reaction mechanism. MDCE is of a generic character, proceeding through pairs of complementary ( π ± , π ∓ ) reactions in the projectile and target nucleus. The dynamics of the elementary processes is discussed, where the excitation of pion–nucleon resonances are of central importance. Investigations of initial and final state ion–ion interactions show that these effects are acting as vertex renormalizations. In closure approximation, well justified by the finite pion mass, the second-order transition matrix elements reduce to pion potentials and effective two-body isotensor DCE interactions, giving rise also to two-body correlations in either of the participating nuclei. Connections to neutrinoless Majorana double beta decay (MDBD) are elucidated at various levels of the dynamics, from the underlying fundamental electro-weak and QCD scales to the physical scales of nuclear MDBD and MDCE physics. It is pointed out that heavy ion MDCE reactions may also proceed by competing electro-weak charge exchange processes, leading to lepton MDCE by electrons, positrons, and neutrinos. [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]

The design and parameters of scintillation detectors of Compton polarimeters for measuring photons formed during the annihilation of electron-positron pairs at rest are considered. The discussed setup makes it possible to measure and compare the polarization correlations of scattered annihilation photons in these two quantum states. The amplitude parameters of scatterers and scattered photon detectors obtained from preliminary tests of these detectors using radioactive gamma sources, as well as in the process of collecting experimental data, are given. The possibility of measuring and comparing the polarization correlations of scattered annihilation photons is shown. [ABSTRACT FROM AUTHOR]

Positronium (Ps) is a valuable probe to investigate nanometric or sub-nanometric cavities in non-metallic materials, where Ps can be confined. Accessible experimental measurements concern the lifetime of trapped Ps, which is largely influenced by pick-off processes, depending on the size of the cavity as well as on the density of the electrons belonging to the surface of the host trap. Another relevant physical quantity is the contact density, that is the electron density at the positron position, which is usually found to be well below the vacuum value. Here, we review the principal models that have been formulated to account and explain for these physical properties of confined Ps. Starting with models, treating Ps as a single particle formulated essentially to study pick-off, we go on to describe more refined two-particle models because a two-body model is the simplest approach able to describe any change in the contact density, observed in many materials. Finally, we consider a theory of Ps annihilation in nanometric voids in which the exchange correlations between the electron of Ps and the outer electrons play a fundamental role. This theory is not usually taken into account in the literature, but it has to be considered for a correct theory of pick-off annihilation processes. [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]

Observing collective effects originating from the interplay between quantum electrodynamics and plasma physics might be achieved in upcoming experiments. In particular, the generation of electron–positron pairs and the observation of their collective dynamics could be simultaneously achieved in a collision between an intense laser and a highly relativistic electron beam through a laser frequency shift driven by an increase in the plasma density increase. In this collision, the radiation of high-energy photons will serve a dual purpose: first, in seeding the cascade of pair generation; and, second, in decelerating the created pairs for detection. The deceleration results in a detectable shift in the plasma frequency. This deceleration was previously studied considering only a small sample of individual pair particles. However, the highly stochastic nature of the quantum radiation reaction in the strong-field regime limits the descriptive power of the average behavior to the dynamics of pair particles. Here, we examine the full kinetic evolution of generated pairs in order to more accurately model the relativistically adjusted plasma density. As we show, the most effective pair energy for creating observable signatures occurs at a local minimum, obtained at finite laser field strength due to the trade-off between pair deceleration and the relativistic particle oscillation at increasing laser intensity. For a small number of laser cycles, the quantum radiation reaction may re-arrange the generated pairs into anisotropic distributions in momentum space, although, in the one-dimensional simulations considered here, this anisotropy quickly decreases. [ABSTRACT FROM AUTHOR]

Three P-wave doubly excited autodissociating resonant states have been unveiled in positronium hydride. The investigation of these states was facilitated through the application of a projection operator method, supported by the stochastic variational method. Precise computations of the resonant positions and widths were executed using the complex coordinate rotation method, employing explicitly correlated Gaussians as basis functions. Our analysis has exposed the intrinsic structural characteristics of these profoundly excited states. The lowest P-wave doubly excited states manifest a distinctive trait, encompassing both the H - +e + and Ps - +H + configurations. [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]

A theoretical method to calculate Delbrück scattering amplitudes for photon energies above the electron‐positron (e+e−${e}^{+}{e}^{-}$) pair production threshold is presented. The method is based on the application of the relativistic Dirac–Coulomb Green function and describes the interaction of the virtual e+e−$e^+e^-$ pair with the Coulomb field of a target to all orders in the coupling strength parameter αZ$\alpha Z$. To illustrate the application of the developed approach, detailed calculations have been performed for the scattering of 2.754 MeV photons off bare ions with a wide range of nuclear charge numbers. Results of these calculations clearly indicate that the higher‐order terms beyond the Born approximation lead to a strong enhancement of the imaginary part of the Delbrück amplitude and have to be taken into account for the analysis and guidance of gamma‐ray scattering experiments. [ABSTRACT FROM AUTHOR]

Very-high-energy (VHE) photons around TeV energies from a gamma-ray burst (GRB) jet will play an essential role in the multimessenger era, with a fair fraction of the events being observed off-axis to the jet. We show that different energy photons (MeV and TeV photons in particular) arrive from different emission zones for off-axis observers even if the emission radius is the same. The location of the emission region depends on the jet structure of the surface brightness, and the structures are generally different at different energies, mainly due to the attenuation of VHE photons by electron–positron pair creation. This off-axis zone-shift effect does not justify the usual assumption of the one emission zone at a certain radius and also produces a time delay of VHE photons comparable to the GRB duration, which is crucial for future VHE observations, such as by the Cherenkov Telescope Array. [ABSTRACT FROM AUTHOR]

Understanding the production mechanism of exotic hadrons with the structures beyond naive parton model can be helpful to find out their inner configuration and also to check the theory of quantum chromodynamics (QCD). Theoretically, fragmentation is known as the dominant production mechanism of conventional heavy hadrons with large transverse momentum so as this mechanism can also be considered for heavy pentaquarks production. In the present work, we first study the fragmentation function of S-wave fully heavy pentaquarks in leading-order perturbative QCD and then we will estimate their production cross section in pair annihilation process at lowest order of perturbation theory, relevant for the present and future high energy electron–positron colliders. [ABSTRACT FROM AUTHOR]

Measurements of the energy distribution of the positron annihilation radiation in Y zeolites are carried out to elucidate the various ortho-positronium annihilation processes that take place in these microporous materials and relate them to the ortho-positronium lifetime components observed by positron annihilation lifetime spectroscopy in earlier studies. Y zeolites with various Si/Al ratios in the range from 2.6 to 40 are investigated under a controlled atmosphere of N2 gas and after exposure to the atmosphere. The ortho-positronium annihilation modes are characterized by analyzing the γ-ray spectra through the use of the f3γ index, which represents the fraction of 3γ-annihilation, and the S parameter. The proportion of ortho-positronium self-annihilation is found to be no less than 15% in N2 gas, but drastically decreases by the physisorption of water molecules and possibly positronium spin exchange interaction with oxygen in the atmosphere. The presence of water molecules suppresses ortho-positronium intrinsic annihilation and, at the same time, promotes ortho-positronium pick-off annihilation in the pores. Similarly, the S parameter rises due to an increased contribution from para-positronium annihilation and ortho-positronium pick-off annihilation, which is also consistent with the physisorption of water and positronium spin exchange. In conjunction with the previously measured ortho-positronium lifetime components, the specific ortho-positronium annihilation sites and processes in these porous materials under different environments are determined and a complete picture of the ortho-positronium behavior in Y zeolites is obtained. [ABSTRACT FROM AUTHOR]

Coincidence Doppler broadening spectroscopy (CDBS), known for its exceptional precision, is extensively employed in examining the electronic states of polymers. In this study, polyvinyl chloride (PVC)–ethyl vinyl acetate (EVA) polymers blends were prepared using conventional techniques, following specific ratios (75:25, 50:50, and 25:75, respectively). Spectroscopic analysis reveals that the longitudinal component of linear momentum surpasses the momentum unit (i.e., 10−3 m0c) when considering the ratio curve associated with the participation of oxygen core electrons. When compared to non‐polymer samples, the PVA polymer and PVA–EVA polymer blend exhibit greater free volume voids in contrast to non‐polymer structural defects. Furthermore, the annihilation of ortho‐positronium with core electrons, likely EVA oxygen core electrons, contributes to the measured free volume voids. An increased annihilation of positrons or positroniums by oxygen in the CDBS spectrum leads to higher calculated W parameter values. Conversely, as the PVC content increases, the W parameter decreases, suggesting reduced ortho‐positronium annihilation and consequently, enhanced formation of para‐positronium in the overall annihilation process. In summary, this investigation demonstrates the utility of CDBS in elucidating the electronic states and free volume voids in PVC and EVA polymers. [ABSTRACT FROM AUTHOR]

We study the near threshold behavior of cross sections of low-energy antiproton scattering off the ground and excited states of positronium with zero total orbital momentum L = 0. In our computational experiment, the existence of singularities called the Gailitis–Damburg oscillations above the thresholds of excited states of positronium and antihydrogen atoms is confirmed. In the future the obtained results can be useful for developing proposals for improving the conditions of experiments with antimatter. [ABSTRACT FROM AUTHOR]

This paper gives high-precision theoretical predictions for cross sections of the process e+e– → γZ for future electron–positron colliders. The calculations are performed using the SANC system. They include complete one-loop electroweak radiative corrections, as well as longitudinal polarization of the initial state. Numerical results are given for the center-of-mass system energy range = 250–1000 GeV with various polarization degrees in the α(0) and Gμ EW schemes. This study is a contribution to the research program of the CEPC project being under development in China. [ABSTRACT FROM AUTHOR]

TRIPOLI-4® is a general-purpose Monte Carlo radiation transport code developed by the Service d'Études des Réacteurs et de Mathématiques Appliquées at CEA-Saclay. It uses continuous-energy nuclear data to simulate neutron, photon, electron, and positron transport in fields like radiation shielding, reactor physics, and nuclear criticality safety. To study radiation protection dosimetry in human tissues and organs, male and female adult computational phantoms from the Medical Internal Radiation Dose–Oak Ridge National Laboratory phantoms family and the International Commission on Radiological Protection (ICRP) publication 110 were recently modeled and calculated using the geometry options of TRIPOLI-4. To easily use the ICRP 110 voxel-based phantoms in different exposure scenarios, a newly developed phantom option is available in TRIPOLI-4 and its display tool T4G. This new phantom option is helpful for modeling one or more phantoms and for improving calculation performance in real irradiation environments. The 2020 published pediatric computational reference phantoms are accessible from ICRP publication 143. Male and female pediatric phantoms are also verified with the new T4G tool and TRIPOLI-4 code. This paper reports on recent works using TRIPOLI-4 on adult and pediatric computational phantoms. The modeling methods of stylized and voxel-based phantoms, the graphic displays of modeled phantoms with T4G, and the verification procedures for single-phantom and two-phantom application cases are presented. Validation for external and internal dosimetry calculations has been performed. Calculation results on organ dose S values for nuclear medicine applications are presented for single female and single male voxel phantoms using 131I and 177Lu radiation sources. Effective dose calculations for two-phantom cases using 99mTc and 18F sources are compared with traditional H*(10) calculations from nuclear medicine patient to patient caregiver. [ABSTRACT FROM AUTHOR]

Black hole and neutron star environments often comprise collisionless plasmas immersed in strong magnetic fields and intense baths of low-frequency radiation. In such conditions, relativistic magnetic reconnection can tap the magnetic field energy, accelerating high-energy particles that rapidly cool by inverse Compton (IC) scattering the dense photon background. At the highest particle energies reached in bright gamma-ray sources, IC scattering can stray into the Klein–Nishina regime. Here, the Comptonized photons exceed pair-production threshold with the radiation background and may thus return their energy to the reconnecting plasma as fresh electron–positron pairs. To reliably characterize observable signatures of such Klein–Nishina reconnection, in this work, we present first-principles particle-in-cell simulations of pair-plasma relativistic reconnection coupled to Klein–Nishina and pair-production physics. The simulations show substantial differences between the observable signatures of Klein–Nishina reconnection and reconnection coupled only to low-energy Thomson IC cooling (without pair production). The latter regime exhibits strong harder-when-brighter behaviour; the former involves a stable spectral shape independent of overall brightness. This spectral stability is reminiscent of flat-spectrum radio quasar (FSRQ) GeV high states, furnishing evidence that Klein–Nishina radiative physics operates in FSRQs. The simulated Klein–Nishina reconnection pair yield spans from low to order-unity and follows an exponential scaling law in a single governing parameter. Pushing this parameter beyond its range studied here might give way to a copious pair-creation regime. Besides FSRQs, we discuss potential applications to accreting black hole X-ray binaries, the M87* magnetosphere, and gamma-ray binaries. [ABSTRACT FROM AUTHOR]