Positrons and electrons in primary cosmic rays as measured in the PAMELA experiment

The PAMELA experiment is being carried out on board the Russian satellite Resurs DK1 placed in the near-earth near-polar orbit on June 15, 2006. The apparatus comprising a silicon-strip magnetic spectrometer and an electromagnetic calorimeter allows measurement of electron and positron fluxes in cosmic rays in a wide energy interval from ～100 MeV to hundreds of GeV. The high-energy electron and positron separation technique is discussed and the data on positron-to-electron ratio in primary cosmic rays up to E ≈ 10 GeV from the 2006–2007 measurements are reported in this work.     

Modulation of electrons and positrons in 2006–2015 in the PAMELA experiment

The PAMELA magnetic spectrometer was launched aboard the Resurs DK-1 satellite into a nearpolar circumterrestrial orbit with an altitude of 350–600 km to study fluxes of the particles and antiparticles of cosmic rays in the wide energy range of ~80 MeV to several hundred gigaelectronvolts. The results from observations of temporal variations in electron and positron fluxes in 2006–2015 are presented. The ratio of electron and positron fluxes measured in this time interval reveals a dependence on the rigidity of particles, the solar activity, and the polarity of the solar magnetic field.     

A positron remoderator for the high intensity positron source NEPOMUC

A remoderator for the high intensity positron source NEPOMUC was developed and installed at the beam facility. A beam of remoderated positrons could be produced with different energies and a diameter of less than 2 mm was obtained. The efficiency of the remoderation setup was determined to be 5%. Due to the brilliance of the remoderated beam, the measurements at the coincidence Doppler broadening spectrometer (CDBS) and at the positron annihilation induced Auger electron spectrometer (PAES) could be improved. The setup and functionality of the remoderation device is presented as well as the first measurements at the remoderator, CDBS and PAES.     

PAMELA spectrum of electrons and positrons of cosmic rays in the energy range of 0.05–1.2 TeV

A proprietary method is used to process measurement data from a high-energy particle (protons, electrons, and positrons with Е ≥ 50 GeV) spectrometer in a near-Earth orbit. The data from three detector systems are used: a tracker in a constant magnetic field (TRK), a calorimeter (CAL), and a neutron detector (ND). A relatively simple and efficient way of isolating electrons and positrons from the total charged particle flux entering the PAMELA spectrometer is proposed. A technique for determining the energy of isolated primary particles and retrieving their energy spectra is described. The composite electron and positron spectrum (below, the total electron and positron flux is referred to simply as the electron flux) for energies up to 1.5 TeV is presented.     

用正电子湮没多普勒增宽谱测量金属中电子的费米能级

高校中不少实验室具有半导体探头能谱仪,可扩展用作正电子湮没多普勒增宽谱仪,并测量金属中电子的费米能级     

Direct measurement of the parapositronium lifetime in alpha-SiO2

The mean lifetime of delocalized parapositronium in alpha-SiO2 has been determined directly for the first time using a newly developed positron lifetime spectrometer based on BaF2 scintillators and a fast digital oscilloscope. The lifetime is found to be 156+/-4 ps, which is much longer than its intrinsic lifetime of 125 ps. This indicates clearly that the primary many-body effect on positronium in alpha-SiO2 is the screening of the Coulomb interaction between the constituent particles by electrons of the medium and that the average distance between the electron and positron in positronium becomes larger than its vacuum value.     

Improved Measurement of γ-Energy from Parapositronium 2γ-Decay

Using a silica aerogel sample with oxygen gas filled as the positronium(Ps)source and a time selecting energy spectrometer,the measurement of the y-energy of parapositronium(p-Ps)2y-decay is further improved and the result of hv=510995. 34±0.69eV is obtained by a slithering comparison with the precisely known y-energies of ¹⁹²Ir.The rest mass of a pair of positron and electron is determined by the correction with the binding energy of Ps,which shows a mass equality within 1.4ppm between positron and electron by the comparison with the rest mass of electron.     

TeV Electrons and Positrons Measured by PAMELA Spectrometer

Abstract-The PAMELA magnetic spectrometer was launched onboard the Resurs-DK1 satellite to the Earth polar orbit with altitude of 350–600 km to study the fluxes of cosmic ray particles and antiparticles in a wide energy range. Starting science operation in July 2006, PAMELA spectrometer performed continuous observation without major interruption till January 2016 collected about 10 billion trigger events. New analysis included data for all period of observations in 2006–2016 with improved selection efficiency and it allows to extend measurements of total electron and positron flux in TeV region with better statistic accuracy. In this paper, the observed energy spectrum of electrons and positrons is presented and compared with the other modern measurements.

     

Exclusive meson production at HERMES

The exclusive electroproduction of pseudoscalar and vector mesons was studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized hydrogen target.The results are compared to calculations based on generalized parton distributions,some of which are sensitive to the contribution of the total angular momentum of the quarks to the proton spin.     

Surface and bulk investigations at the high intensity positron beam facility NEPOMUC

The NEutron-induced POsitron source MUniCh (NEPOMUC) at the research reactor FRM II delivers a low-energy positron beam (E = 15-1000 eV) of high intensity in the range between 4 × 10⁷ and 5 × 10⁸ moderated positrons per second. At present four experimental facilities are in operation at NEPOMUC: a coincident Doppler-broadening spectrometer (CDBS) for defect spectroscopy and investigations of the chemical vicinity of defects, a positron annihilation-induced Auger-electron spectrometer (PAES) for surface studies and an apparatus for the production of the negatively charged positronium ion Ps⁻. Recently, the pulsed low-energy positron system (PLEPS) has been connected to the NEPOMUC beam line, and first positron lifetime spectra were recorded within short measurement times. A positron remoderation unit which is operated with a tungsten single crystal in back reflection geometry has been implemented in order to improve the beam brilliance. An overview of NEPOMUC's status, experimental results and recent developments at the running spectrometers are presented.     

A novel design for a variable energy positron lifetime spectrometer

We present computer simulations of a new design of a variable energy positron lifetime beam that uses for a start signal the secondary electron emission from a 25-nm thick carbon foil (C-foil) located in front of the sample. A needle of ∼30 μm diameter is positioned on-axis right behind the foil, creating a radial electric field that deflects the secondary electrons radially outward so as to miss the sample and to hit the micro-channel plate (MCP) detector placed down beam. The MCP signal provides the start signal for the positron lifetime spectrometer. A grid can be further introduced between the sample holder and the MCP to yield a cleaner signal by preventing the positrons with large transmitted scattering angle from hitting the MCP. The cylindrical symmetry of this design reduces the experimental complexity and offers good timing resolution. We show that the design is robust against the transmitted energy and angle of the secondary electrons and positrons.     

Triple differential cross-sections for the fast electron/positron impact ionization of hydrogen and helium in the Glauber approximation

Triple differential cross-section for electron and positron impact ionization of hydrogen and helium are calculated by using the Glauber approximation along with post-collision interaction effects which are estimated classically. The present results are compared with the recent absolute data of Ehrhardt, Jung and coworkers for the electron impact case. The positron impact case is found to lead to a larger binary to recoil peak maxima ratio (compared to the electron impact case) which further increases when post-collision interaction effects are included.     

Effects of the working conditions of retarding field spectrometers on the energy distribution of secondary electrons

Retarding field spectrometers designed for Auger electron spectroscopy show spurious peaks in the low energy range which are connected with secondary electron emission from the electrodes of the spectrometer. We present a general explanation for all the spurious effects reported to date.The understanding of these phenomena has led us to the building of a new spectrometer which works with optimal experimental conditions.We present the results and experimental curves of secondary electron energy distribution for an evaporated aluminium target.     

An application of the electron mirror in the time-of-flight spectrometer

We suggest applying of the spherical electron mirror in the time-of-flight electron spectrometer with a position sensitive detector in order to increase the effective acceptance solid angle of the spectrometer. The spherical electron mirror is placed near the specimen and it focuses electrons on a position sensitive detector as a converging electron flux. The electron mirror increases the acceptance angle of the spectrometer by a factor of 20. The electron mirror of the spectrometer consists of an inner spherical electrode of the radius R and an outer spherical electrode of the radius 1.1R. The central segment of the inner electrode inside the linear angle of 80° is made of a grid. The detector plate radius is about 0.23R. The acceptance solid angle of the spectrometer with this electron mirror is about 1.1sr, the range of the polar angle of emission is 20°–90° relative to the surface normal, and the range of the azimuth angle of emission at its maximum is ±36° relative to the basic plane of the spectrometer. The design of the spectrometer allows to recover the electron trajectory for each detected electron and to calculate the energy and the emission angle of the electron. The energy resolution of the spectrometer is about 0.2 eV/ns for the electron energy of 10 eV. The energy range is from E_min0.1eU_r up to eU_r, where U_r is the retarding potential of the electron mirror. The perturbing influence of the grid of the electron mirror limits mainly the angular resolution of the spectrometer and affects relatively slightly the energy resolution. The electron spectrometer with two detectors and two electron mirrors symmetric about the spectrometer axis allows to measure electron pairs in coincidence in a wide range of emission angles and energies with k-resolutions.     

Dependence of electron and positron backscattering coefficients on Al film thickness

The backscattering coefficient of 1-4 keV electron and positron beams normally incident impinging on Al thin film targets is stochastically modeled within a Monte Carlo frame work. The aim of the present paper is to study the behavior of the backscattering coefficient as a function of the Al film thickness. To the authors’ knowledge, no theoretical or experimental work on the dependence of the positron backscattering coefficient on film thickness targets has been reported so far. It is found that the backscattering coefficient for both electron and positron beams presents different behaviors when the Al film thickness belongs to the nano-scale. Beyond this scale, the behavior becomes qualitatively similar.     

Gamma-ray spectra in the positron-annihilation process of molecules at room temperature

In this study, a fully self-consistent method was developed to obtain the wave functions of the positron and electrons in molecules simultaneously. The wave function of a positron at room temperature, with a characteristic energy of approximately 0.04 eV [1], was used to analyse the experimental results of its annihilation in helium, neon, hydrogen, and methane molecules. The interactions between the positron and molecule provide a significant correction in the gamma-ray spectra of the annihilating electron–positron pairs. It was also observed that high-order correlations offered almost no correction in the spectra, as the interaction between the low-energy positron and electrons cannot drive the electrons into excited electronic states. More accurate studies, which consider the coupling of the positron–electron pair states and vibration states of nuclei, must be undertaken.     

Hydrodynamic analysis of electrostatic counter-streaming instability in a spin-polarized electron–positron–ion plasma

In this study, we present linear analysis of electrostatic counter-streaming instability in spin-polarized electron–positron–ion (e-p-i) plasma. With the aid of the separate spin evolution-quantum hydrodynamic (SSE-QHD) model, we derive the dispersion relation of counter-streaming instability. We numerically solve the dispersion and find four wave solutions: Langmuir wave, positron acoustic mode, and two electron and positron spin-dependent waves. It is noted that coupling of streaming and spin effects excites Langmuir instability and positron acoustic mode instability. However, in the absence of spin effect, only Langmuir instability will survive in e-p-i plasma. We have also discussed the effects of positron concentration, streaming speed, and spin polarization on the real frequency of waves and the growth rate. The present study may be helpful for understanding longitudinal wave propagation and instabilities in dense magnetized environments.     

Energy scale calibration of the KEDR tagging system

The KEDR detector tagging system is a symmetric focusing magnetic spectrometer of small-angle electrons born in the interaction region of the VEPP-4M collider. Using this system, which is designed for studying the two-photon processes, the scattered electrons and positrons energy is measured with a resolution of ΔE/E₀= 0.03–0.6% where E₀ is the beam energy. Two different methods are employed for calibrating the tagging system energy scale: the first one involves the electron/positron tagging by the energy of the singlebremsstrahlung photon measured in the BGO calorimeter, and the second one is based on the determination of the recoil electrons spectrum edge in the Compton backscattering of monochromatic laser radiation. The technical implementation and the present status of the calibration system are discussed in this paper.     

On the instability of electrostatic waves in a nonuniform electron–positron magnetoplasma

The dispersion properties of three-dimensional electrostatic waves in a nonuniform electron–positron (EP) magnetoplasma are analyzed. A new dispersion relation is derived by use of the electron and positron density responses arising from the electron and positron continuity and Poisson equations. In the local approximation, the dispersion relation admits two wave modes with different velocities. The growth rates of various modes are illustrated both analytically and numerically. Considering the temperature gradients produces a linearly stable transverse mode. The growth rate of the slow mode instability due to the density inhomogeneity only is the highest one, though it appears at higher thermal energy. The angle of the wave propagation affects drastically on the instability features in each case. The applications of the present analysis are briefly discussed.     

Positron Characteristics in Cadmium and Zinc Chalcogenides

Electron energy levels and positron states have been calculated for cadmium and zinc chalcogenide compounds within the pseudo-potential approach and the independent particle model.Furthermore,the present contribution deals with the electron and positron chemical potentials allowing the calculation of the positron affinity to different materials of interest and hetero-structures formed by these materials.Besides,we here determine the positron diffusion constant by means of the positron deformation potential.An attempt has been made to scale positron affinity and diffusion constant with the lattice constant and the band gap energy,respectively.Such scaling is found to be not possible.The information gathered by the present study is of prime importance for a better understanding of positron trapping at interfaces and precipitates and should be useful in slow positron beam experiments.