Annihilation Probability Research Articles

Optical analogs of pair production and annihilation in binary waveguide arrays with a linearly shifted straight section

We systematically investigate the optical analogs of the e−e+ pair production and annihilation phenomena which can take place simultaneously in a binary waveguide array having a linearly shifted straight section sandwiched between two straight sections. These two processes can be realized by the beam splitting in the linearly shifted straight section which performs the function of an external vector potential with a rectangular form in time in quantum electrodynamics (QED) which is different from two widely used concepts based on an external time-varying electromagnetic field, or an external ultrastrong time-independent potential barrier in space as in the Schwinger mechanism. We show that the numerical results for annihilation and pair production probabilities based on the simulations of beam propagation in binary waveguide arrays in the low-power regime are in very good agreement with the theoretical results. We also reveal that with certain requirements, the proposed discrete model quantitatively mimics the two processes as well as the continuous model of the genuine Dirac equation in free space in QED. We also show that the nonlinearity in the high-power regime can reduce these two effects.

Reduction of dislocation density in α-Ga2O3 epilayers via rapid growth at low temperatures by halide vapor phase epitaxy

We demonstrate that the dislocation density in α-Ga2O3 epilayers is markedly reduced via rapid growth at low temperatures by halide vapor-phase epitaxy. An α-Ga2O3 epilayer grown on (0001) sapphire at a high growth rate of 34 μm h−1 and a low temperature of 463 °C exhibited a dislocation density of 4 × 108 cm−2, which was approximately 1/100 of that in a conventional film. It is likely that the three-dimensional surface morphology developed during the growth enhanced the bending of the dislocations to increase the probability of pair annihilation. The combination of this technique with thick film growth and epitaxial lateral overgrowth resulted in a further low dislocation density of 1.1 × 107 cm−2.

Control of Host-Matrix Morphology Enables Efficient Deep-Blue Organic Light-Emitting Devices.

Mixing a sterically bulky, electron-transporting host material into a conventional single host-guest emissive layer isdemonstratedtosuppress phase separation of the host matrix while increasing the efficiency and operational lifetime of deep-blue phosphorescent organic light-emitting diodes (PHOLEDs) with chromaticity coordinates of (0.14, 0.15). The bulky host enables homogenous mixing of the molecules comprising the emissive layer while suppressing single host aggregation; a significant loss channel of nonradiative recombination. By controlling the amorphous phase of the host-matrix morphology, the mixed-host device achieves a significant reduction in nonradiative exciton decay, resulting in 120 ± 6% increase in external quantum efficiency relative to an analogous, single-host device. In contrast to single host PHOLEDs where electrons are transported by the host and holes by the dopants, both charge carriers are conducted by the mixed host, reducing the probability of exciton annihilation, thereby doubling of the deep-blue PHOLED operational lifetime. These findings demonstrate that the host matrix morphology affects almost every aspect of PHOLED performance.

Fast 3D kernel computation method for positron range correction in PET

Objective. The positron range is a fundamental, detector-independent physical limitation to spatial resolution in positron emission tomography (PET) as it causes a significant blurring of underlying activity distribution in the reconstructed images. A major challenge for positron range correction methods is to provide accurate range kernels that inherently incorporate the generally inhomogeneous stopping power, especially at tissue boundaries. In this work, we propose a novel approach to generate accurate three-dimensional (3D) blurring kernels both in homogenous and heterogeneous media to improve PET spatial resolution. Approach. In the proposed approach, positron energy deposition was approximately tracked along straight paths, depending on the positron stopping power of the underlying material. The positron stopping power was derived from the attenuation coefficient of 511 keV gamma photons according to the available PET attenuation maps. Thus, the history of energy deposition is taken into account within the range of kernels. Special emphasis was placed on facilitating the very fast computation of the positron annihilation probability in each voxel. Results. Positron path distributions of 18F in low-density polyurethane were in high agreement with Geant4 simulation at an annihilation probability larger than 10−2 ∼ 10−3 of the maximum annihilation probability. The Geant4 simulation was further validated with measured 18F depth profiles in these polyurethane phantoms. The tissue boundary of water with cortical bone and lung was correctly modeled. Residual artifacts from the numerical computations were in the range of 1%. The calculated annihilation probability in voxels shows an overall difference of less than 20% compared to the Geant4 simulation. Significance. The proposed method is expected to significantly improve spatial resolution for non-standard isotopes by providing sufficiently accurate range kernels, even in the case of significant tissue inhomogeneities.

Influence of multi-walled carbon nanotubes in polytetrafluoroethylene on the parameters of electronic structure and absorption of ultra-high-frequency radiation

Using the methods of angular correlation of annihilation radiation (ACAR), attenuation of electromagnetic radiation in 1.5–2.2 GHz frequency range, and optical ellipsometry, it was shown that in composites of polytetrafluoroethylene (PTFE) + multi-walled carbon nanotubes (MWCNTs), a 2% decrease in the probability of annihilation of positrons in free volumes in PTFE leads to changes in other parameters of electronic structure of composites by 8–29%. Polytetrafluoroethylene is transparent to electromagnetic radiation, but after the addition of 10 wt.% or more of MWCNTs, the composites demonstrate 200–410-fold decrease in the electromagnetic radiation intensity when the radiation passes through a specimen with a thickness of ≈2 mm. It was found that the average radius of the free volumes and the probability of annihilation of positrons are determined by the defect and electronic structures of the polymer matrix only. The Fermi angle and the probability of positrons annihilation with free electrons are determined by the analogous structures of MWCNTs only. Since the electronic characteristics of the atoms and defects in the polymer matrix (at least outside the interphase) do not change, the changes in the other ACAR parameters are mainly due to changes in the imperfect MWCNTs’ atomic and electronic structures. The average radius of free volumes reaches its maximum value in the composite with 10 wt.% MWCNTs. It was found that in a specimen with 10 wt.% MWCNTs, the highest density of free electrons is observed due to charge transfer from free volumes to MWCNTs, and the highest electron density is observed on defects. A disorder of MWCNTs and their branched conductive network can form the ‘tails’ of electronic density of states in a band gap. Thus, composite with 10 wt.% MWCNTs has the highest absorption coefficient for electromagnetic radiation.

Meron–antimeron annihilation induced by the electric field in a polar meron lattice

Polar topological domain structures may have potential applications in future memory devices. Exploring their dynamical behaviors is paramount to the device design. Here, the meron–antimeron annihilation processes induced by the electric field in ultrathin ferroelectric PbTiO3 films were studied by phase-field simulations. Two types of annihilation behaviors were identified: The merons with central polarizations parallel to the electric field will directly annihilate with their nearby antimerons; those antiparallel to the electric field will first be switched and then annihilate with antimerons. The annihilation ratio of meron–antimeron pairs (MAPs) decreases with the increase in the meron–antimeron distance. During annihilation processes, the positions of antimerons seldom change and the merons approach to antimerons at some critical electric fields. The annihilation probability of a meron is found to be closely related to its topological charge. Specifically, the restoration of an MAP from a partially annihilated state was observed under the decrease in the electric field. These results may have some indications to the development of polar-meron-based memory devices.

New model of intranuclear neutron-antineutron transformations in O816

There has been much work in recent years pertaining to viability studies for the intranuclear observation of neutron-antineutron transformations. These studies begin firstly with the design and implementation of an event generator for the simulation of this rare process, where one hopes to retain as much of the underlying nuclear physics as possible in the initial state, and then studying how these effects may perturb the final state observable particles for detector efficiency studies following simulated reconstruction. There have been several searches for intranuclear neutron-antineutron transformations, primarily utilizing the ${}^{16}_{8}$O nucleus, and completed within large underground water Cherenkov detectors such as Super-Kamiokande. The latest iteration of a generator is presented here for use in such an experiment. This generator includes several new features, including a new radial (position) annihilation probability distribution and related intranuclear suppression factor for ${}^{16}_{8}$O, as well as a highly general, modern nuclear multifragmentation model with photonic de-excitations. The latter of these may allow for improved identification of the signal using large underground detectors such as Super-Kamiokande and the future Hyper-Kamiokande, potentially increasing the overall signal efficiencies of these rare searches. However, it should be noted that certain fast photonic de-excitations may be washed out by $\pi^0$ decays to photons. These new features implemented in these $\bar{n}$\isotope[15][8]{O} simulations increase the overall physical realism of the model, and are easily portable to other future searches such as to extranuclear $\bar{n}{}^{12}_{6}$C for the ESS NNBAR experiment, as well as intranuclear $\bar{n}{}^{39}_{18}$Ar used in DUNE.

Optical analogs of pair production and annihilation in binary waveguide arrays with a curved section

We study theoretically and numerically the optical analogs of the relativistic quantum electron-positron dynamic pair production (PP) and annihilation processes in a single binary waveguide array (BWA) consisting of a curved section located between two straight sections at the input and output. Both of these processes can be mimicked by the breakup of beams in the curved section, which plays the role of the external oscillating field. Thanks to our scheme, we are able to observe not only the PP, but also annihilation in a single BWA with a single input beam. Moreover, this scheme allows us to obtain the numerical results for PP and annihilation probabilities by directly simulating beam propagation in binary waveguide arrays, and then compare them to the theoretical results for the same discrete model. This kind of verification demonstrates the striking excellent agreement between them in wide ranges of all relevant parameters. We also show that under certain conditions the probabilities of these two processes based on the discrete model with binary waveguide arrays agree quite well with those based on the truly continuous Dirac equation in quantum field theory.

The preexisting edge dislocations as recombination center of point defects enhancing irradiation tolerance in CoCrCuFeNi high entropy alloy

Defect clusters including dislocations, grain boundaries, and precipitations can serve as sink sources for point defects during radiation damage. As a result, radiation tolerance can be tuned by engineering the defects in the structural materials, however, is a challenging topic due to the complexity. In this paper, the mechanism of interaction between the preexisting dislocation and the displacement cascade in CoCrCuFeNi high entropy alloy (HEA) is investigated using molecular dynamics simulations. The results show that the dislocation climbs, and promotes the diffusion of point defects generated by the cascade to the dislocation line. As a result, it increases the probability of defect annihilation and recombination and therefore strengthens radiation tolerance. The interaction of dislocations with void in CoCrCuFeNi and Ni shows that CoCrCuFeNi high entropy alloy has excellent resistance to irradiation hardening. Our atomic insights could be beneficial in the design of high-performance irradiation resistant high entropy alloys.

Study of large axial space rebinning algorithm in γ-photon industrial tomography image reconstruction

The accuracy of the existing single slice and Fourier rebinning algorithms depends on the projection angle of the line of response. The increase of such projection angle with the detector size, typical in the large axial space of γ-photon industrial detection, and the loss of some projection data after rebinning, result in the degradation of the image quality. In addition, those algorithms consider the probability of positron annihilation equally distributed along the line of response, which prevents to estimate accurately the positions of the annihilation point, and can originate artifacts and noise in the reconstructed image. In this work, we propose an alternative large axial space rebinning algorithm. In that algorithm, initially the line of response is divided into transverse and axial components. Then, each line of response is uniformly rebinned into all the 2D sinogram data intersecting with it. To improve the accuracy of the estimate of the annihilation point location and suppress the noise effectively, we assign a Gaussian weight coefficient to the projection data, and optimise the rebinning algorithm with it. Finally, we reconstruct the image on the basis of the 2D sinograms with the optimised weights. On the computational side, the algorithm is also accelerated by making use of parallel computing. Both simulation and experimental results show that the proposed method improves the contrast and spatial resolution of 2D reconstructed images. Furthermore, the reconstruction time is not affected by the new method, which is therefore expected to meet the demand of γ-photon industrial inspection imaging.

Influence of the temperature of the photosphere and deeper layers of the Sun on the spectra of gamma quanta with energies above 511 keV during solar flares

Gamma-ray quanta, which occur during solar flares due to the interaction of accelerated protons with the photosphere and deeper layers of the sun, enter interplanetary space from a thickness of several tens of g/cm2. In the presented work, gamma quanta with energies of more than 511 keV are considered. This makes it possible to exclude from consideration the dependence of the probability of ortho- and parapositronium formation on the temperature and density of the solar matter. And also do not consider the probability of annihilation by two or 3 gamma quanta. Thus, the reactions of thermal neutrons remain dependent on the temperature. As the ambient temperature increases, the average number of elastic neutron scattering before capture increases. This leads to a more likely penetration of neutrons to a greater depth or their departure into the interplanetary space. The high temperature of the Sun below the photosphere may be one of the reasons for the absence of the 2.223 MeV line in solar flares with registered protons in the PAMELA and AMS2 experiments. Using the GEANT4 package, the spectra of gamma-quanta arising in nuclear interactions are calculated. The temperature-dependent features of the gamma-ray spectra are discussed.

Improving Texture and Microstructure Homogeneity in High-Purity Ta Sheets by Warm Cross Rolling and Annealing

The evolution of texture and microstructure uniformity in high-purity tantalum (Ta) sheets during 135° warm cross rolling (WCR) was analyzed in detail. X-ray diffraction suggested that relatively uniform ‘ideal’ deformation texture distribution across the thickness could be obtained from WCR, since more potential slip systems could be activated. Electron backscatter diffraction (EBSD) results indicated that the change in strain path in warm rolling could enhance dislocations mobility and increase the probability of dislocations rearrangement and annihilation. Thus, the proportion of low-angle grain boundaries was significantly reduced, and more sub-grain boundaries or sub-grains were formed via WCR. The calculation of geometrically necessary dislocation density based on the strain gradient model supports this result. The analysis of relative Schmid factor combined with the strain contouring map indicated that inhomogeneous orientation-dependent grain subdivision could be effectively weakened, and relatively uniform strain distribution could be formed in the WCR sample. Upon annealing, uniform fine grain size and more randomly oriented grains were obtained in the WCR sample after the completion of recrystallization because of relatively uniform grain subdivision and stored energy distribution.

Radiation from matter-antimatter annihilation in the quark nugget model of dark matter

We revisit the properties of positron cloud in quark nugget (QN) model of dark matter (DM). In this model, dark matter particles are represented by compact composite objects composed of a large number of quarks or antiquarks with total baryon number $B\sim 10^{24}$. These particles have a very small number density in our galaxy which makes them "dark" to all DM detection experiments and cosmological observations. In this scenario, anti-quark nuggets play special role because they may manifest themselves in annihilation with visible matter. We study electron-positron annihilation in collisions of free electrons, hydrogen and helium gases with the positron cloud of anti-quark nuggets. We show that a strong electric field of anti-quark nuggets destroys positronium, hydrogen and helium atoms and prevents electrons from penetrating deeply in positron cloud, thus reducing the probability of the electron-positron annihilation by nearly five orders of magnitude. Therefore, electron annihilation in the positron cloud of QNs cannot explain the observed by SPI/INTEGRAL detector photons with energy 511 keV in the center of our galaxy. These photons may be explained by a different mechanism in which QN captures protons which annihilate with anti-quarks in the quark core or transform to neutrons thus reducing the QN core charge and increasing QN temperature. As a result QN loses positrons to space which annihilate with electrons there. Even more positrons are produced from charged pions resulting from the proton annihilation. Another manifestation may be emission of photons from $\pi^0$ decays.

Geminate exciton fusion fluorescence as a probe of triplet exciton transport after singlet fission

The geminate annihilation of two triplet excitons created by singlet exciton fission is affected by the dimensionality of transport as determined by typically anisotropic triplet exciton mobilities in organic molecular crystals. We analyze this process using a random-walk model where the time dynamics of the geminate annihilation probability is determined by the average exciton hopping times along the crystallographic directions. The model is then applied to the geminate fluorescence dynamics in rubrene, where the main channel for triplet-triplet annihilation is via triplet fusion and subsequent photon emission, and we identify the transitions between transport in one, two, and three dimensions.

Influence of helium on the evolution of irradiation-induced defects in tungsten: An object kinetic Monte Carlo simulation* *Project supported by the Science Challenge Project (Grant No. TZ2018002), the National Natural Science Foundation of China (Grant No. 11905135), and the National MCF Energy R&D Program of China (Grant No. 2018YFE0308103).

Understanding the evolution of irradiation-induced defects is of critical importance for the performance estimation of nuclear materials under irradiation. Hereby, we systematically investigate the influence of He on the evolution of Frenkel pairs and collision cascades in tungsten (W) via using the object kinetic Monte Carlo (OKMC) method. Our findings suggest that the presence of He has significant effect on the evolution of irradiation-induced defects. On the one hand, the presence of He can facilitate the recombination of vacancies and self-interstitial atoms (SIAs) in W. This can be attributed to the formation of immobile He-SIA complexes, which increases the annihilation probability of vacancies and SIAs. On the other hand, due to the high stability and low mobility of He-vacancy complexes, the growth of large vacancy clusters in W is kinetically suppressed by He addition. Specially, in comparison with the injection of collision cascades and He in sequential way at 1223 K, the average sizes of surviving vacancy clusters in W via simultaneous way are smaller, which is in good agreement with previous experimental observations. These results advocate that the impurity with low concentration has significant effect on the evolution of irradiation-induced defects in materials, and contributes to our understanding of W performance under irradiation.

Mechanical size effect of eutectic high entropy alloy: Effect of lamellar orientation

The effect of lamellar orientation on the deformation behavior of eutectic high entropy alloy at the micrometer scale, and the roles of two rarely explored laminate orientations (i.e., the lamellar orientation at ∼ 0° and 45° angles with the loading direction) in regulating size-dependent plasticity were investigated using in-situ micropillar compression tests. The alloy, CoCrFeNiTa0.395, consists of alternating layers of Laves and FCC phases. It was found that the yield stress of the 0° pillars scaled inversely with the pillar diameters, in which the underlying deformation mode was observed to transform from pillar kinking or buckling to shear banding as the diameter decreased. In the case of the 45° pillars with diameters ranging from 0.4 to 3 μm, there exists a ‘weakest’ diameter of ∼ 1 μm, at which both constraint effect and dislocation starvation are ineffective. Irrespective of the lamellar orientations, the strain hardening rate decreased with decreasing pillar diameter due to the diminishing dislocation accumulation that originated from the softening nature of large shear bands in the 0° pillars, and the enhanced probability of dislocation annihilation at the increased free surfaces in the 45° pillars. The findings expand and deepen the understanding of the mechanical size effect in small-scale crystalline materials and, in so doing, provide a critical dimension for the development of high-performing materials used for nano- or microelectromechanical systems.

Luminescence of self-trapped excitons in alkali halide crystals at low temperature uniaxial deformation

The processes of temperature quenching of the luminescence of the self-trapped excitons (STEs) from 93 to 200 K in the conditions of applied elastic low-temperature deformation for several alkali-halide crystals (AHCs) are presented in the paper. The theoretical explanation of the influence of low-temperature uniaxial deformation on the formation of the luminescence of the STEs was done. The increase in activation energy of the temperature quenching of the luminescence of the STEs, narrowing of the emission bands, increase in the frequency of the active oscillations of the STEs and the decrease in the Huang-Rhys parameter are shown as the results. These changes indicate a weakening of the exciton-phonon interaction in elastically deformed AHCs, which, in turn, leads to an increase in the probability of radiative annihilation of excitons.

One-photon pair annihilation in pulsed plane-wave backgrounds

We study the $2\rightarrow1$ process of electron-positron pair annihilation to a single photon in a plane-wave background. The probability of the process in a pulsed plane wave is presented, and a locally constant field approximation is derived and benchmarked against exact results. The stricter kinematics of annihilation (compared to the $1\rightarrow2$ processes usually studied) leads to a stronger dependence on the incoming particle states. We demonstrate this by studying the effect that initial state wavepackets have on the annihilation probability. The effect of annihilation in a distribution of particles is studied by incorporating the process into Monte Carlo simulations.

Effect of Gamma irradiation on the nanofree volume and electrical properties of PVA/PEG/reduced graphene oxide nanocomposites

Reduced graphene oxide (RGO) doped polyvinyl alcohol/ Polyethylene glycol (PVA/PEG) films were prepared by solution casting technique and exposed to 100 kGy gamma radiation. The structural modifications in the samples before and after irradiation were studied using X-ray diffraction XRD, Fourier transform infrared spectroscopyFTIR anddifferential scanning calorimetry DSC. The free volume holes Vf determined from the o-Ps lifetime τ3measured by the positron annihilation spectroscopy (PALS) is found to decrease from (84.52 to 67.24A3) and from (85.67 to 78.21 A3) for nonirradiated and irradiated samples respectively with increasing the nanoscale RGO fillercontentto the polymer matrix. The resulting decrease of the free volume holes size due to this incorporation could be explained byboth the formation of hydrogen-bonding and crosslinking between RGO and PVA/PEG blend as confirmed byXRD and FTIR measurements. This also caused a decrease in the concentration of free volumes Fr for both nonirradiated and irradiated samples. The increased D.C. electrical conductivity with increasing RGOvolume fraction(0–0.265) suggests a conductivity chain formation through the nanoparticle aggregation. The deduced Ln σis found to be positively correlated with the probability of free annihilation of positrons I2, trapped at crystalline-amorphous interfaces. The D.C. conductivity results could be successfully fitted using the model suggested by Miyamoto and Shibayama. The critical free volumes for the charge carriers’ transport has been deduced to be 80.34 and 82.16A3 for nonirradiated and irradiated samples respectively.

Positron diffusion enhances the accumulation effect in small particles embedded in a low-density matrix

This report considers the mathematical model of positron’s behavior in a system consisting of small particles embedded in a matrix. It takes into account their initial distribution after the slowing down process, random walk, and finally annihilation. The theoretical results are compared with the results obtained from an experiment in which positron lifetime measurements were carried out for samples consisting of nickel particles of various diameters embedded in an epoxy resin. The model well describes the measurements extracted from the probability of positron annihilation in particles, as a function of their volume fraction. Based on these results, it was possible to determine the parameter characterizing the accumulation effect in this system and to estimate the positron diffusion length in the matrix. The model can be useful in studying, using positron annihilation techniques, polymer composites with particles and nanoparticles, as well as shedding light on the behavior of swift positrons in composites.