Interaction Of Low-energy Positrons Research Articles

Modelling the line shape of very low energy peaks of positron beam induced secondary electrons measured using a time of flight spectrometer

In this paper, we present results of numerical modelling of the University of Texas at Arlington’s time of flight positron annihilation induced Auger electron spectrometer (UTA TOF-PAES) using SIMION® 8.1 Ion and Electron Optics Simulator. The time of flight (TOF) spectrometer measures the energy of electrons emitted from the surface of a sample as a result of the interaction of low energy positrons with the sample surface. We have used SIMION® 8.1 to calculate the times of flight spectra of electrons leaving the sample surface with energies and angles dispersed according to distribution functions chosen to model the positron induced electron emission process and have thus obtained an estimate of the true electron energy distribution. The simulated TOF distribution was convolved with a Gaussian timing resolution function and compared to the experimental distribution. The broadening observed in the simulated TOF spectra was found to be consistent with that observed in the experimental secondary electron spectra of Cu generated as a result of positrons incident with energy 1.5 eV to 901 eV, when a timing resolution of 2.3 ns was assumed.

Low energy positron interactions with uracil--total scattering, positronium formation, and differential elastic scattering cross sections.

Measurements of the grand total and total positronium formation cross sections for positron scattering from uracil have been performed for energies between 1 and 180eV, using a trap-based beam apparatus. Angular, quasi-elastic differential cross section measurements at 1, 3, 5, 10, and 20 eV are also presented and discussed. These measurements are compared to existing experimental results and theoretical calculations, including our own calculations using a variant of the independent atom approach.

Low-energy positron scattering from gas-phase uracil

AbstractQuantum scattering calculations are presented for the interaction of low energy positrons with the uracil molecule, an important component of biological systems. The rotational elastic and inelastic cross sections and vibrational inelastic cross sections are reported and compared with existing experiments, indicating a general trend of the cross sections different from the experimental findings and in line with what should be expected from the behavior of the total cross sections in similar polar targets. Some specific considerations can be drawn on the reliability of existing experiments, as to their size vis-à-vis to the computed integral cross sections over the same range of energies.

Low-energy positron interactions with xenon

Low-energy interactions of positrons with xenon have been studied both experimentally and theoretically. The experimental measurements were carried out using a trap-based positron beam with an energy resolution of ∼80 meV, while the theoretical calculations were carried out using the convergent close-coupling method and the relativistic optical potential approach. Absolute values of the grand total, positronium formation and grand total minus positronium formation cross sections are presented over the energy range of 1–60 eV. Elastic differential cross sections (DCS), for selected energies, are also presented both below and above the positronium formation threshold. Fine energy-step measurements of the positronium formation cross section over the energy range of 4.4–8.4 eV, and measurements of the elastic DCS at the energies of 5.33 and 6.64 eV, have been carried out to investigate the ionization threshold regions corresponding to the 2P3/2 and 2P1/2 states of the Xe+ ion. The present results are compared with both experimental and theoretical values from the literature where available.

Low-energy positron interactions with krypton

Cross sections for positron scattering from krypton have been measured with an energy resolution of \ensuremath{\sim}60 meV over the energy range 0.5--60 eV. Absolute values of the grand total (${\ensuremath{\sigma}}_{\mathrm{GT}}$), positronium formation (${\ensuremath{\sigma}}_{\mathrm{Ps}}$), and grand total minus positronium formation (${\ensuremath{\sigma}}_{\mathrm{GT}}\ensuremath{-}{\ensuremath{\sigma}}_{\mathrm{Ps}}$,) cross sections are presented. Theoretical estimations of ${\ensuremath{\sigma}}_{\mathrm{GT}}$ and ${\ensuremath{\sigma}}_{\mathrm{GT}}\ensuremath{-}{\ensuremath{\sigma}}_{\mathrm{Ps}}$ are also performed for this target using the convergent close-coupling method and the relativistic optical potential approach. We also provide experimental and theoretical results for elastic differential cross sections, for selected energies both below and above the Ps threshold. Where available, the present results are compared to both experimental and theoretical values from the literature.

Low energy positron interactions with rare gas atoms: threshold features and benchmark cross sections

Using a high resolution (~60 meV FWHM) positron beam from a Surko buffer-gas trap, we have measured grand total cross sections as well as total elastic, positronium formation and electronic excitation cross sections in the rare gases. Strong threshold (Wigner) cusps are observed in the elastic scattering channel in all the rare gases at the opening of the positronium formation channel. We present low energy elastic scattering measurements for neon which show excellent agreement with recent theory and potentially establish this system as a 'benchmark'.

Positron-molecule interactions: Resonant attachment, annihilation, and bound states

This article presents an overview of current understanding of the interaction of low-energy positrons with molecules with emphasis on resonances, positron attachment and annihilation. Annihilation rates measured as a function of positron energy reveal the presence of vibrational Feshbach resonances (VFR) for many polyatomic molecules. These resonances lead to strong enhancement of the annihilation rates. They also provide evidence that positrons bind to many molecular species. A quantitative theory of VFR-mediated attachment to small molecules is presented. It is tested successfully for selected molecules (e.g., methyl halides and methanol) where all modes couple to the positron continuum. Combination and overtone resonances are observed and their role is elucidated. In larger molecules, annihilation rates from VFR far exceed those explicable on the basis of single-mode resonances. These enhancements increase rapidly with the number of vibrational degrees of freedom. While the details are as yet unclear, intramolecular vibrational energy redistribution to states that do not couple directly to the positron continuum appears to be responsible for these enhanced annihilation rates. Downshifts of the VFR from the vibrational mode energies have provided binding energies for thirty species. Their dependence upon molecular parameters and their relationship to positron-atom and positron-molecule binding energy calculations are discussed. Feshbach resonances and positron binding to molecules are compared with the analogous electron-molecule (negative ion) cases. The relationship of VFR-mediated annihilation to other phenomena such as Doppler-broadening of the gamma-ray annihilation spectra, annihilation of thermalized positrons in gases, and annihilation-induced fragmentation of molecules is discussed.

Experimental studies of the interaction of low energy positrons with atoms and molecules

Experimental studies of the interaction of low energy positrons with atoms and molecules

Experimental studies of the interaction of low energy positrons with atoms and molecules

This paper presents recent data on positron annihilation below and above the threshold for positronium formation in atoms and molecules. Use of a trap-based beam allows experimentation down to 50 meV and up to tens of electron volts with an energy resolution of 25 meV (FWHM). Above positronium formation, results are presented for absolute cross-section measurements in argon. Below positronium formation, absolute Zeff spectrum in argon, xenon and the alkane molecules are presented. Data are compared to other experimental and theoretical results where available. The effect of fluorination of hydrocarbons on Zeff below the threshold for Ps formation as well as other outstanding questions are discussed.

Positron Scattering and Annihilation Studies Using a Trap-Based Beam

An overview is presented of recent studies of the interaction of low-energy positrons with atoms and molecules using a trap-based positron beam. The beam is tunable from ~100 meV to many electron volts, with a typical energy resolution of 25 meV (FWHM). A variety of scattering processes are investigated using the properties of positron orbits in a spatially varying magnetic field. Recent scattering measurements are reviewed, and new data for electronic excitation, ionization, and positronium formation in noble gases are presented. In a separate set of experiments, positron annihilation below the threshold for positronium formation is studied in molecules as a function of positron energy. The data show large Feshbach resonances in the range of energies of the molecular vibrations. Within the framework of this model, these results provide the first experimental evidence that positrons bind to hydrocarbon molecules.

Measurement of positronium emission energy from the surface of a semiconductor

A new Monte Carlo (MC) simulation program based on the GEANT code has been developed to study the most effective position of the target for the measurement of work functions using positronium (Ps) and thermal Ps with the time-of-flight (TOF) technique. We find that distances of <1.5 cm and >2 cm are suitable for the measurement of thermal and work function Ps, respectively. The kinetic energy of Ps emitted from the front surface of the micro-channel plate (MCP) is measured by slow positron bombardment. The kinetic energy of the emitted Ps is equivalent to the magnitude of the Ps work function, which was determined by measuring the TOF of ortho-Ps ( o-Ps). Ps emission energy is measured for the first time to be 0.79±0.30 eV. This measurement is consistent with a general description of the production of work function Ps resulting from a common interaction of low-energy positrons with the electrons of the traversed material.

γ-ray spectra from positron annihilation on atoms and molecules

Positron annihilation on a wide variety of atoms and molecules is studied. Room-temperature positrons confined in a Penning trap are allowed to interact with molecules in the form of low-pressure gases so that the interaction is restricted to binary encounters between a positron and a molecule. Data are presented for the \ensuremath{\gamma}-ray spectra resulting from positrons annihilating in such interactions. The Doppler broadening of these spectra is a measure of the momentum distribution of the annihilating electron-positron pairs. Consequently, these spectra provide information about the electron and positron wave functions. Systematic studies of annihilation line shapes are discussed for noble gases, a variety of inorganic molecules, alkanes, alkenes, aromatics, and perfluorinated and partially fluorinated hydrocarbons. In the case of molecules, the measurements are used to determine the probability of positrons annihilating at specific locations in the molecule. For example, in the case of partially fluorinated hydrocarbons, we have been able to determine the relative probability of annihilation on the fluorine atoms and on the C-H bonds. Insights that these studies provide in understanding the interaction of low-energy positrons with atoms and molecules are discussed.

Many-body calculations of positron scattering and annihilation from noble-gas atoms

We have applied many-body theory methods to study the interaction of low-energy positrons with noble-gas atoms. The positron - atom correlation potential includes explicitly the contribution from the target polarization by the positron and that from the virtual positronium (Ps) formation. It is demonstrated that the correlations and Ps formation (or tunnelling of electrons from the atom to the positron) create virtual levels in the positron - atom system. The existence of the virtual levels strongly influences the scattering and increases the positron annihilation rate by up to 400 times. The calculated elastic cross sections are in good agreement with experimental data. The inclusion of virtual Ps formation greatly improves the agreement with experimental annihilation rates with respect to calculations taking only the polarization into account. Our calculations have highlighted the difference between the calculation of positron - atom scattering and the calculation of corresponding annihilation rates. The annihilation rate is very sensitive to the behaviour of the wavefunction at small positron - electron separations. A simple approximate formula based on the Sommerfeld factor is suggested to account for the effect of electron - positron Coulomb attraction on the annihilation rates.

Interaction of low-energy positrons with molecular nitrogen

Low energy collision and annihilation parameters for positrons scattered by molecular nitrogen have been calculated in the adiabatic (fixed nuclei) approximation. The interaction of the positron with the nitrogen molecule is represented as a superposition of static and polarization potentials. The elastic scattering cross section, the annihilation rate and the rotational excitation cross section are presented.