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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]

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]

Particle beams focused to micrometer-sized spots play a crucial role in forefront research using low-energy positrons. Their expedient and wide application, however, requires highly-resolved, fast beam diagnostics. We have developed two different methods to modify a commercial imaging sensor to make it sensitive to low-energy positrons. The first method consists in removing the micro-lens array and Bayer filter from the sensor surface and depositing a phosphor layer in their place. This procedure results in a detector capable of imaging positron beams with energies down to a few tens of eV, or an intensity as low as 35 e + / s/mm 2 when the beam energy exceeds 10 keV . The second approach omits the phosphor deposition; with the resulting device we succeeded in detecting single positrons with energies upwards of 6 keV and efficiency up to 93%. The achieved spatial resolution of 0.97 μ m is unprecedented for real-time positron detectors. [ABSTRACT FROM AUTHOR]

The characterization of an electron–positron beam generated from the interaction of a multi-GeV electron beam with a lead plate is performed using GEANT4 simulations. The dependence of the positron beam size on driver electron beam energy and lead converter thickness is investigated in detail. A pancake-like positron beam structure is generated with a monoenergetic multi-GeV driver electron beam, with the results indicating that a 5 GeV driver electron beam with 1 nC charge can generate a positron beam with a density of 1015–1016 cm−3 at one radiation length of lead. In addition, we find that electron–positron beams generated using above-GeV electron beams have neutralities greater than 0.3 at one radiation length of lead, whereas neutralities of 0.2 are observed when using a 200 MeV electron beam. The possibility of observing plasma instabilities in experiments is also examined by comparing the plasma skin depth with the electron–positron beam size. A quasi-neutral electron–positron plasma can be produced in the interaction between a 1 nC, 5 GeV electron beam and lead with a thickness of five radiation lengths. Our findings will aid in analyzing and interpreting laser-produced electron–positron plasma for laboratory astrophysics research. [ABSTRACT FROM AUTHOR]

Currently, diamonds are widely used in science and technology. However, the properties of diamonds due to their defects are not fully understood. In addition to optical methods, positron annihilation spectroscopy (PAS) can be successfully used to study defects in diamonds. Positrons are capable of detecting vacancies, and small and large clusters of vacancies induced by irradiation, by providing information about their size, concentration, and chemical environment. By mapping in the infrared (IR) range, it is possible to consider the admixture composition of the main inclusions of the whole plate. This article presents the results of a study of defects in synthetic diamond plates, one of which was irradiated by electrons. It presents data about the distribution of the defect concentration obtained by Infrared spectroscopy. PAS with a monochromatic positron beam can be used as a non-destructive technique of detecting defects (vacancy) distribution over the depth of diamond plates. [ABSTRACT FROM AUTHOR]

Generation of energetic electron-positron pairs using multi-petawatt (PW) lasers has recently attracted increasing interest. However, some previous laser-driven positron beams have severe limitations in terms of energy spread, beam duration, density, and collimation. Here we propose a scheme for the generation of dense ultra-short quasi-monoenergetic positron bunches by colliding a twisted laser pulse with a Gaussian laser pulse. In this scheme, abundant γ-photons are first generated via nonlinear Compton scattering and positrons are subsequently generated during the head-on collision of γ-photons with the Gaussian laser pulse. Due to the unique structure of the twisted laser pulse, the positrons are confined by the radial electric fields and experience phase-locked-acceleration by the longitudinal electric field. Three-dimensional simulations demonstrate the generation of dense sub-femtosecond quasi-monoenergetic GeV positron bunches with tens of picocoulomb (pC) charge and extremely high brilliance above 1014 s−1 mm−2 mrad−2 eV−1, making them promising for applications in laboratory physics and high energy physics. The advent of petawatt lasers has reignited interest in the generation of positrons, since they could provide enough energy for practical applications. Here, an all-optical method for the generation of short, energetic positron beams are described numerically. [ABSTRACT FROM AUTHOR]

The positron source is one of the most important parts of the circular electron–positron collider (CEPC) linear accelerator. The design goal is to obtain a 3 nC bunch charge positron beam from a 4 GeV primary electron beam with an intensity of 10 nC/bunch. In this study, an all-solid-state, high-current pulse power system was developed to drive a flux concentrator to capture positrons by generating a magnetic field with a peak value of 6 T. Based on the inductive load topology circuit, the optimized parameters were designed to generate a pulse current with a high peak value and a high rise rate pulse current. In addition, a non-reverse voltage was realized to protect the solid-state switch components. For the characteristics of the long-distance transmission between the pulse power supply and the load in the system, the influences of the cable distribution parameters on the pulse were analyzed, and the design of the damping circuit was proposed to suppress the high-frequency ripples of the peak pulse current. An optimized coaxial discharge structure was designed to reduce the stray parameters in the main circuit. The design was verified through simulations and experiments, and the results demonstrated that the developed prototype successfully generated an ideal half-sinusoidal pulse current with a peak value of 15 kA, and a front pulsewidth of $2.5 ~\mu \text{s}$ at a repetition rate of 50 Hz. The pulse repetition stability is better than 0.1%. The final output was consistent with the theoretical analysis. The system can be applied in the CEPC positron source, which also provides a reference for the subsequent development of a positron source system at research facilities. [ABSTRACT FROM AUTHOR]

We report measurements of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma‐ray Burst Monitor that was produced during a negative cloud‐to‐ground (CG) lightning leader. This is the first report of a downward directed TGF occurring during a CG flash but detected by a space‐based instrument. The gamma ray photons are produced 3 ms preceding a return stroke (−146 kA) and are essentially simultaneous with an isolated low frequency radio pulse. Based on timing, the pulse is estimated to initiate at approximately 6 km altitude, and its polarity indicates downward moving negative charge, the opposite of regular satellite‐detected upward TGFs. A likely scenario is that the runaway electrons accelerate into the upper, positively charged end of the leader in a high field region, with the reverse positron beam generating upward gamma rays detectable from space. A search for similar waveform features indicates that this type of downward CG‐TGF may occur prior to 1% of high peak current CG strokes. Extrapolating gives a global rate of 5–10% of previously known TGFs and potentially a significant fraction of global TGFs. Key Points: A TGF detected by Fermi GBM occurred 3 ms before a −CG stroke during the descending negative leader, coincident with a 120 μs radio pulseThe radio pulse polarity indicates that it is likely a downward TGF produced at ~6 km altitude with a reverse positron beam detected by Fermi GBMThis type of downward CG‐TGF may occur prior to 1% of high‐intensity CG strokes, constituting 5–10% of the previously known TGF population [ABSTRACT FROM AUTHOR]

Counterstreaming electron beams drifting along the magnetic field in electron–proton plasmas may lead to the generation of electrostatic (ES) waves and structures widely observed in planetary magnetospheres. In electron–positron plasmas, streaming electron/positron beams along the background magnetic field may first result in ES waves followed by the electromagnetic (EM) waves with substantial magnetic field perturbations, as shown by Jao and Hau [Phys. Rev. E 98, 013203 (2018)]. In this study, we examine the formation of ES and EM waves driven by current free counterstreaming electron beams in electron–ion background plasmas and current free electron–positron beams in electron–proton–positron background plasmas based on one-dimensional EM particle-in-cell simulations with the drift velocity being along the background magnetic field. For the former cases, the effects of ion-to-electron mass ratios are examined, which have implications for two- and three-dimensional particle in cell simulations with reduced mass ratios. For the latter cases, the effects of positrons are examined, which have applications to astrophysical plasma environments and laboratory experiments. It is shown that the presence of positrons or light ions may shorten the occurrence times between the ES and EM waves driven by current free electron beam instability. The EM waves have relatively longer wavelengths as compared to the ES waves and the magnetic field evolution resembles parallel proton firehose instability. The effects of positrons, background compositions, and ion-to-electron mass ratios on the occurrence of ES and EM waves are consistent with the linear fluid theory. [ABSTRACT FROM AUTHOR]

V/Ag nano-scaled multilayers were prepared by using a magnetron sputtering deposition method. Each layer of Ag and V has a thickness of about 6 nm, and the total thickness of the multilayer film is 350 nm. Doppler broadening of annihilation radiation was measured by using a slow positron beam to study the microstructure of the above samples. It was found that the Doppler broadening S and W parameters measured in the V/Ag multilayers were close to those measured in the Ag reference sample. Coincidence Doppler broadening measurements also showed that the electron momentum distributions in V/Ag multilayers and Ag monolayer were almost identical. This suggests that Ag has a strong affinity to positrons, and almost all the positrons ejected into the multilayers are confined to the Ag nanolayers. Theoretical calculations indicate that the positron wavefunction is well localized in the Ag nanolayer even for a layer thickness of only 1 nm. [ABSTRACT FROM AUTHOR]

Open spaces in amorphous Al2O3 films fabricated by atomic layer deposition and in AlONx deposited by a reactive sputtering technique were probed by using monoenergetic positron beams. In these films, open spaces with three different sizes were found to coexist. The mean size and the concentration of open spaces were decreased by annealing at 800 °C in N2 atmosphere, which was associated with rearrangements of the amorphous network in the films and their crystallization. Nitrogen incorporation into Al2O3 suppressed the shrinkage of network structures due to the annealing, which was attributed to the formation of a stable network through the introduction of Al-N bonding. For Al2O3 deposited on GaN, the temperature treatment above 800 °C led to the introduction of vacancytype defects in the GaN substrate. This fact suggests that the change in the network structure of Al2O3 enhances chemical instability at the Al2O3/GaN interface and the out-diffusion of atoms from the substrate. [ABSTRACT FROM AUTHOR]

Low-energy positron beams are applied in a number of experiments in solid state physics and materials science as well as in atomic and particle physics. These experiments comprise of surface studies, depth dependent defect spectroscopy, and fundamental research with leptonic systems. The neutron induced positron source in Munich (NEPOMUC) is operated as a user facility providing a positron beam with an intensity of about 109 moderated positrons per second. In this paper, a review of the status of the positron beam facility with its instruments is given and several experiments are exemplarily highlighted. Recent and future developments for further improving the performance of positron beam experiments are discussed. [ABSTRACT FROM AUTHOR]

Relativistic positron beams were generated by laser wakefield electrons bombarding on solid target. Very stable positron beams were generated in our experiments. The total yield of positrons is about 4.4×108/shot. The energy spectra of positrons and electrons obey quasi-Maxwell distribution. Compared with the direct method, the indirect method produces positrons (38.5 MeV) and electrons (50.5 MeV) with much higher slope temperature. [ABSTRACT FROM AUTHOR]

We report here positron annihilation-induced Auger electron spectroscopy measurements of an as received TiO2(110) surface. We observe peaks at ~ 250 eV and ~ 500 eV due to the Auger decay of annihilation-induced 1s holes in carbon (C KVV), and oxygen (O KVV) respectively. In addition to these KVV Auger peaks we observe a broad, low energy peak extending up to 15 eV. Since the incident positron beam energy of 1.5 eV is well below the energy threshold required to directly knock out electrons with kinetic energies up to 15 eV, this peak must be Auger-related. Based on energy conservation requirements and its considerable spectral weight, we argue that this low energy peak is principally due to the Auger decay of both annihilation induced 2s core holes in oxygen and annihilation induced valence holes in carbon. [ABSTRACT FROM AUTHOR]

Positron beam measurements carried out on high temperature Ni+ implanted FeCrCoNi show no void formation but only presence of monovacancies. With thermal treatment, evidence for the formation of nano sized σ-CoCr are obtained from positron based ratio curve analysis in the temperature range 973–1123 K. Corroborative evidence for the presence of these precipitates are obtained from EBSD and GIXRD measurements. Further annealing beyond 1150 K leads to coarsening of these precipitates and they no longer trap the positrons. • First time study on the defect microstructure and its evolution in high temperature Ni+ implanted FeCrCoNi using the variable low energy positron beam. • Defect sensitive S-parameter reveals presence of monovacancies for the high temperature implanted condition. No presence of voids observed. • Positron beam based ratio curve analysis show formation of nano sized CoCr precipitates after annealing beyond 1023 K. [ABSTRACT FROM AUTHOR]

This paper shows results of conventional positron lifetime and slow positron beam studies of the long range effect. These techniques are not commonly used for this topic. The experimental investigations of pure metals exposed to ion irradiation are reported and discussed on the basis of the literature data. The positron lifetimes measured in the etching experiment give information about kind of defects, their distribution, and existence of the long range effect. The above can be achieved using the positron beam for the case of low energy ion implantation. [ABSTRACT FROM AUTHOR]

The neutron-induced positron source Munich (NEPOMUC) provides a monochromatic low-energy positron beam with an intensity of > 1 × 109e+ s-1 and a full width at half maximum (FWHM) diameter of about 10 mm. To create a small beam focus or sharp positron pulses of 100 ps FWHM the beam brightness needs to be enhanced by remoderation. This is achieved by focusing the primary beam magnetically onto a tungsten single crystal W(100) in reflection geometry. Afterwards the beam exhibits an intensity of > 5 × 107e+s-1 and a diameter of < 2 mm FWHM. To further optimize the beam quality of NEPOMUC we redesigned the remoderation unit. The new setup allows a precise positioning of the remoderator crystal within the focus of the magnetic lens. Additionally, a replacement of the crystal within several minutes and without breaking the beamline vacuum is possible that offers the opportunity for systematic tests of different remoderator materials. [ABSTRACT FROM AUTHOR]

• Formation and growth of helium bubbles in FeCrCoNi studied using positron beam based Doppler broadening spectroscopy. • Positron lifetime and electron momentum distribution calculations in FeCrCoNi containing He m -V n complexes using MIKA code. • Positron beam results point out to higher bubble growth resistance in FeCrCoNi when compared to nickel. Positron beam-based Doppler broadening results on He ion-implanted FeCrCoNi show distinct changes in defect-sensitive S-parameter corresponding to the formation of helium-vacancy complexes, nucleation/ growth of helium bubbles, and finally the helium bubble rupture, with isochronal annealing. Data analyses of the positron beam-based ratio curves provide clear information on the kinetics of bubble growth and rupture. The difference data in S-parameter (ΔS), which corresponds to the bubble growth, displays a maximum at 1123 K, with annealing temperature. Comparing the ΔS value to that reported for nickel, it is concluded that FeCrCoNi has higher bubble growth resistance than nickel. [ABSTRACT FROM AUTHOR]

Described here is a proposed experiment to use laser-assisted photorecombination of positrons from a trap-based beam and metal atoms in the gas phase to measure positron-atom binding energies. Signal rates are estimated, based in part upon experience studying resonant annihilation spectra using a trapbased positron beam. [ABSTRACT FROM AUTHOR]

We report results from total cross section measurements for positron-acetone scattering. The energy range of these experiments was 0.2-23 eV, while the energy resolution of our positron beam was ∼260 meV. The present data clearly highlight the important role played by the strong permanent dipole moment and significant dipole polarisability of the acetone molecule on the low-energy scattering dynamics of this system. For positron energies above about 6 eV the present data is found to be in quite good agreement with the only other total cross section results available in the literature from the Yamaguchi group, however, at lower energies the level of agreement is rather poor. To the best of our knowledge, no theoretical calculations are currently available for positron-acetone scattering. PACS Codes: 34.80.Uv [ABSTRACT FROM AUTHOR]

Beam-based positron annihilation spectroscopy (PAS) is a powerful porosimetry technique with broad applicability in the characterization of nanoporous thin films, especially insulators. Pore sizes and distributions in the 0.3-30 nm range are nondestructively determined with only the implantation of low-energy positrons from a table-top beam. Depth-profiling with PAS has proven to be an ideal way to measure the interconnection length of pores, search for depth-dependent inhomogeneities or damage in the pore structure, and explore porosity hidden beneath dense layers or diffusion barriers. The capability of PAS is rapidly maturing as new intense positron beams around the globe spawn more accessible PAS facilities. After a short primer on the physics of positrons in insulators, the various probe techniques of PAS are briefly summarized, followed by a more detailed discussion of the wide range of nanoporous film parameters that PAS can characterize. [ABSTRACT FROM AUTHOR]

Positron acceleration in plasma wakefield faces significant challenges, as the positron beam must be pre-generated and precisely coupled into the wakefield and, most critically, suffers from defocusing issues. Here we propose a scheme that utilizes laser-driven electrons to produce, inject, and accelerate positrons in a single setup. The high-charge electron beam from wakefield acceleration creates copious electron–positron pairs via the Bethe–Heitler process, followed by enormous coherent transition radiation due to the electrons' exiting from the metallic foil. Simulation results show that the coherent transition radiation field reaches up to tens of GV m−1, which captures and accelerates the positrons to cut-off energy of 1.5 GeV with energy peak of 500 MeV (energy spread ~ 24.3%). An external longitudinal magnetic field of 30 T is also applied to guide the electrons and positrons during the acceleration process. This proposed method offers a promising way to obtain GeV fast positron sources. Generation of high-energy positron beams is more complex than for electrons, requiring positrons to be pre-generated and compensation of defocusing effects. Here, a system allowing positron creation, injection and acceleration in a single setup is considered theoretically. [ABSTRACT FROM AUTHOR]

Slow positrons are generated from a 22Na source and cone-shaped solid neon moderator and extracted as a magnetically guided beam. Measurements are presented for the mean parallel and perpendicular energies and the radial distribution of the beam particles. Over a distance of 7 m, where the magnetic field B varies from 0.005 to 0.12 T, the beam transport is found to be adiabatic for mean energies up to 50 eV. Non-adiabatic effects, evidenced by an increase in energy in motion perpendicular to B, are observed at larger transport energies. The implications of these observations for buffer-gas positron traps and other positron-transport beamlines are discussed. [ABSTRACT FROM AUTHOR]

Relativistic spin-polarized positron beams are indispensable for future electron-positron colliders to test modern high-energy physics theory with high precision. However, present techniques require very large scale facilities for those experiments. We put forward a novel efficient method for generating ultrarelativistic polarized positron beams employing currently available laser fields. For this purpose, the generation of polarized positrons via multiphoton Breit-Wheeler pair production and the associated spin dynamics in single-shot interaction of an ultraintense laser pulse with an ultrarelativistic electron beam is investigated in the quantum radiation-dominated regime. The pair production spin asymmetry in strong fields, significantly exceeding the asymmetry of the radiative polarization, produces locally highly polarized particles, which are split by a specifically tailored small ellipticity of the laser field into two oppositely polarized beams along the minor axis of laser polarization. In spite of radiative de-polarization, a dense positron beam with up to about 90% polarization can be generated in tens of femtoseconds. The method may eventually usher high-energy physics studies into smaller-scale laser laboratories. [ABSTRACT FROM AUTHOR]