The influence of anharmonic potentials in calculations of planar channeling radiation from GeV positrons

The frequency of radiation from planar channeled positrons in the GeV region is discussed for realistic planar potentials when the dipolarity condition is not satisfied. It is found that there is a critical beam momentum at which the maximum radiated frequency is practically the same for all channeled positrons. For this momentum a sharp peak in the calculated composite spectrum is to be expected and this could explain the sharp peaking observed in a recent CERn experiment for 5 GeV/c positrons.     

Evidence for zero-point motion of positrons in metals

The Doppler-broadened photon spectra arising from positrons annihilating in a single crystal of cadmium have been studied over the temperature range from 4.2 K to its melting point. Line shape analysis provides evidence for zero-point motion of positrons trapped in vacancies.     

Monte Carlo calculation of electron spectra produced in water phantoms by 20 keV-1 GeV photons

Abstract

A Monte Carlo calculation for the energy spectra of electrons and positrons produced in infinite and semiinfinite water phantoms by photons ranging in energies from 20 keV to 1 GeV are presented. The dominant processes considered are the photoelectric effect, Auger effect, Compton effect, pair, and triplet production. Bremsstrahlung produced by electrons and positrons with energies greater than 1 MeV is also included. The effect of multiple Compton scattering, not considered in the earlier calculation, for the infinite phantom for photon energies higher than 2 MeV has been incorporated. For a semi-infinite phantom, multiple Compton scattering and backscattering through the top are considered. The results are compared with the earlier calculations for the first-collision spectrum. It is found that the inclusion of multiple Compton scattering significantly increases the average number of electrons/cm³ per photon/cm² at all energies considered whereas bremsstrahlung reduces the number of high energy electrons and produces more low energy electrons in the spectrum.     

The emission of slow positrons from magnesium oxide powder

From Monte Carlo analyses of measured time-of-flight spectra for slow positrons traveling down a tube which is first evacuated and then filled with low-density helium gas, it is shown that the positrons leave the MgO-coated source grid at angles close to the normal direction. This result is important for low-energy positron collision studies. An interpretation of the result based on the surface charge held by the oxide is proposed.     

正负电子三光子湮灭特性的计算机模拟研究

给出了正负电子三光子湮灭模拟实验中描述三个光子发射角的随机变量α₁、α₂、α₃和描述三个光子能量的随机变量w₁、w₂、w₃的抽样方法.根据该抽样方法所产生的子样,得到了关于α₁、α₂、α₃和w₁、w₂、w₃的边缘分布和联合分布。有关结论与已知的量子电动力学(QED)理论预言及实验数据作了比较.     

Orientation dependence of relativistic-positron annihilation in single crystals

An effect of the orientation dependence of the cross section for the single-photon annihilation of relativistic positrons with atomic electrons in a crystal is predicted. It is shown that the probability for the single-photon annihilation of a channeled positron in a crystal may be either suppressed in a crystal in relation to a homogeneous medium or, on the contrary, enhanced. The reason is that, depending on their incidence angle, the positrons may be either in the vicinity of ion planes of the crystal, where the electron density is higher, or far away from them, where the electron density is lower.     

New approximation for the interaction potential of light channeled particles with crystals

A new approximation for the continuous interaction potentials of atomic planes and strings with channeled electrons and positrons in crystals is proposed. A model in which atomic electron distributions within a certain atomic subshell are assumed to be exponentially decaying is used. Contrary to the Thomas-Fermi (TF) and Thomas-Fermi-Dirac (TFD) models this approach takes into account the shell structure of atoms in reasonably good agreement with the Hartree-Fock method (HF). Simple analytical expressions for the continuous plane (string) potentials are given. Calculations of several planar potentials for positrons channeled in silicon and tungsten are presented. Results are compared with widely known Lindhard and Molière approximations. Some estimates of the Kumakhov spontaneous radiation intensity under positron planar channeling conditions in the classical approximation are also given.     

高能电子和正电子在晶体沟道中的辐射

介绍了高能电子和正电子在晶体中的沟道辐射,对超相对论电子和正电子在周期弯曲晶体中的相干辐射进行了分析,并提出了初步的实验设想. The channeling radiation of high energy electrons and positrons in crystals was introduced. According to the new idea proposed by A.V.Korol, the coherent radiation of ultra relativistic electrons and positrons channeled in periodically bent crystals was analyzed. The characteristics of the radiation were obtained by using classic electromagnetic theory and a tentative experimental plan for testing was suggested.     

Emission of photons by positrons channeled in single crystals near an energy of 100 GeV

Emission by 120-GeV positrons in the channeling regime in the (011) plane of a silicon single crystal has been considered. Trajectories of positrons under different initial conditions have been calculated within the theory of nonlinear oscillations. The amplitude distribution function of channeled particles has been determined taking into account the nonlinearity of their motion. The intensity of radiation under various initial conditions has been calculated by two different methods. These results can be useful for comparison with experimental data at energies of positrons beginning with 100 GeV and higher.     

Can the observed structures in positron spectra be caused by internal conversion?

The energy distribution of positrons emitted in quasimolecular collisions of Uranium on Uranium and Uranium on Curium has been measured by several groups. Peak structures in the positron spectra were observed. We discuss the possibility that these structures originate from internal conversion processes following nuclear Coulomb excitation or transfer reactions. Consequences for the nuclear photon spectra and theδ-ray distribution are pointed out and experimental procedures for an unambiguous determination of the significance of conversion processes are discussed.     

Effect of the band structure of energy levels on the angular distribution of diffracted X-ray radiation for positron plane channeling in silicon

It is shown that the inclusion of the band structure of energy levels of channeled positrons (as in the case of electrons) leads to a qualitative change in the angular distribution of X-ray radiation.     

Characteristics of the energy spectra of cosmic rays and a single source model

As the accuracy of measuring the energy spectra of different nuclei in the primary cosmic ray flux and their ratios grows, more evidence appears for the nonpower character of these spectra at energies below the knee at 3–4 PeV. Irregularities in the spectra are the natural consequence of the nonuniformity of the cosmic ray source distributions: their types, ages and distances to the Earth; the nonuniformity of the interstellar medium; and the different densities, temperatures, and natures of magnetic fields. In particular, the flattening of the proton and helium energy spectra, the growth of the fraction of positrons in the total flux of positrons and electrons, and the constancy of the ratio of antiprotons to protons at sub-PeV energies could be due to the contribution from nearby and young sources emitting harder energy spectra of particles. It is shown that the recent measurements of the ratio of the boron and carbon nuclei performed in the AMS-02 experiment could also indicate that there is a contribution from a single comparatively young and nearby source.     

Angular-dependent XPS valence-band spectra from single-crystal copper

High-resolution angular-dependent X-ray photoemission (XPS) valence-band spectra from a copper single crystal with (001) orientation are presented and analyzed in terms of a direct-transition model of photoemission. This model includes the effects of photon momentum, spectrometer geometry, and, for very low angles of emission, refraction of electrons in leaving the surface, but neglects matrix-element variation. Theoretical calculations are found to be in good agreement with experiment for six distinct emission directions. For low (grazing) angles of emission, a sharpening of the d-band peak is observed that may be associated with surface-specific band-structure changes.     

Radiation from particles in crystal undulators with allowance for a polarization of the medium

The radiation from relativistic particles channeled in a microscopic undulator created by applying transverse ultrasonic vibrations to a single crystal is investigated. The results of numerical investigations are presented for real crystal undulators. The calculations show the radiation spectra in the particle and photon energy ranges where narrowing of the spectrum occurs when the particle energy is close to a definite threshold energy. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 437–441 (25 September 1998)     

Large-angle channeling radiation from relativistic electrons in optically transparent crystals

In the work within the frame of quantum electrodynamics are obtained new formulae describing the large-angle photon emission from channeled electrons with taking into account of the dispersion of refractive index. Calculations based on these formulae show that the spectral and angular distributions of large-angle optical and ultraviolet radiation from planar channeled sub-GeV electrons in optically transparent crystal reflect the band structure of transverse energy levels of channeled electrons. Comparison with ordinary Cherenkov radiation spectrum reveals that channeling (depending on the beam energy) leads to sufficient change of the large-angle emission spectrum.     

Differential electron emission for single and multiple ionization of argon by 500 eV positrons

Triply differential electron emission cross sections are measured for single ionization of argon by 500 eV positrons. Data are presented for coincidences between projectiles scattered into angles of 3 degrees and electrons with emission energies less than 10 eV that are observed between 45 and 135 degrees along the beam direction. For interpretation, these are compared to cosine squared representations of the binary and recoil lobes which are convoluted over experimental parameters. Singly differential electron emission data for double and triple ionization by positrons are also presented.     

Photon emission from channeled heavy ions excited Resonant-coherently

Abstract

The possibility of the radiative deexcitation from channeled heavy ions excited by the Okorokov Effect (Resonant coherent excitation) is discussed and the experiments on the photon emission due to the deexcitation from hydrogen-like Ne⁹⁺ and Fe⁸⁺ ions are reported.     

Bremsstrahlung in the nuclear medium: Search for coherence phenomena

Photon angular distributions and energy spectra up to the kinematical limit have been measured in proton reactions with light and heavy targets. For the first time the dependence of photon spectra on scattered protons has been measured to investigate the influence of multiple-scattering processes on the photon production. A strong suppression of bremsstrahlung relative to a quasi-free production model using the free pn bremsstrahlung cross section is observed in the low-energy regime of the photon spectrum in p+nucleus reactions. We attribute this effect to modifications of the bremsstrahlung amplitude in the nuclear medium and to interference of photon amplitudes due to multiple scattering of nucleons.     

Slow positrons and thermal Ps production using thin foils

A one-dimensional diffusion model for positrons implanted in Cu and Al single crystal thin foils was solved. The fraction of thermalized positrons reaching the surface was obtained for various film thicknesses as a function of the incident positron energy in a transmission-mode geometry. The results indicate that for foil thicknesses of the order of the diffusion length (1000 Å) the reemission fraction is roughly half the fraction of a semi-infinite crystal. For thicker foils the annihilation of the positrons in the bulk and smearing effects due to a higher implantation energy effectively reduce the surface reemission. In this paper it is shown that thin foils can also be used to efficiently produce low-energy (from thermal to a few eV) Ps beams, physically separated from the primary positron beam.