Positronium Formation Cross Sections Research Articles

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.

State selective electron capture in low energy positron and argon collisions

A three-body classical trajectory Monte Carlo method is applied in the study of positron and argon atom collisions at impact energies between 10 and 60eV. The calculations are based on the independent particle model. We present state selective and total positronium formation cross sections as a function of the impact energy. Our results are compared with recent experimental data.

Oscillatory quantum screening effects on the positronium (Ps) formation in quantum plasmas

The oscillatory quantum screening effects on the positronium formation by the positron are investigated in quantum plasmas. It is found that the oscillatory screening effects suppress the positronium formation radius as well as the positronium formation cross section. It is also found that the positronium formation radius decreases with increasing quantum wave number. It is found that the peak position of the differential cross section is receded from the projectile with decreasing quantum wave number. In addition, the oscillatory screening effects on the positronium formation cross section are found to be decreased with an increase of the collision energy.

Energy dependence of the positronium formation cross-section in argon

The positronium (Ps) formation cross-section, QPs, for positron–argon interactions has been measured for incident positron energies from threshold to 60 eV, in an attempt to resolve the apparent discrepancy between earlier experimental results. QPs was found to vary smoothly with positron energy between 15 and 30 eV, in qualitative agreement with earlier results using methods involving the measurement of positron neutralization (as in the current experiment) and in disagreement with the double-peak energy dependence reported by Laricchia et al (2002 J. Phys. B: At. Mol. Opt. Phys. 35 2525–40), who used a positron-ion coincidence detection method. Possible reasons for this discrepancy, including Ps fragmentation and excited-state Ps formation, are discussed.

Convergent close-coupling calculations of positron-magnesium scattering

The single-center convergent close-coupling method has been applied to positron-magnesium scattering at incident energies from 0.01 to 100 eV. Cross sections are presented for elastic scattering and excitation of $3 {}^{1}P$, as well as for the total ionization and total scattering processes. We also provide an estimate of the positronium formation cross section. The results agree very well with the measurements of the total cross section by Stein et al. [Nucl. Instrum. Methods Phys. Res. Sect. B 143, 68 (1998)], and consistent with the positronium formation measurements of Surdutovich et al. [Phys. Rev. A 68, 022709 (2003)] for positron energies above the ionization threshold. For energies below the positronium formation threshold (0.8 eV) we find a large $P$-wave resonance at 0.17 eV. A similar resonance behavior was found by Mitroy and Bromley [Phys. Rev. Lett. 98, 173001 (2007)] at an energy of 0.1 eV.

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.

Positron scattering from neon and argon

High-resolution measurements of positron interactions with Ne and Ar are presented, as well as theoretical treatments. The data extend over a range of 0.3 to 60 eV and comprise measurements of the grand total, positonium formation, and grand total minus positronium formation cross sections. Theoretical treatments of scattering from Ne and Ar are performed under the relativistic optical potential approach, as well as calculations using the convergent close-coupling method. Comparisons of the present measurements and theories are made with previous theoretical and experimental work.

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'.

Comment on ‘Total and positronium formation cross sections for positron scattering from H2O and HCOOH’

We would like to clarify some misunderstandings we noticed in a recent work by Casten Makochekanwa et al (2009 New J. Phys. 11 103036) with regard to our theoretical results on positronium (Ps) formation cross sections in collisions of positrons impacting H2O molecules in the gas phase. Thus, here we clearly discuss again the threshold for Ps formation as well as the domain of validity of our theoretical predictions.

Positronium formation cross-sections for Xe, CO2 and N2

The positronium formation cross-sections for Xe, CO2 and N2 have been measured using coincidences between γ-rays from positronium self-annihilation and the resultant ion. In the case of Xe, there is excellent agreement with previous experimental determinations. For CO2 there is broad agreement in magnitude with previous measurements in contrast with N2 where good shape agreement at low energies (< 40 eV) is found though the magnitude of the present cross-section is significantly higher.

Positron-helium collisions: Positronium formation using the distorted-wave approximation

Accurate Hylleraas-type correlated helium wave functions are used to predict positronium (Ps) formation cross sections in positron-helium collisions within the frame work of the distorted-wave approximation at intermediate and high energies of positron impact. Exponential correlated atomic target wave functions taking into account up to $N=30$ basis terms are utilized. Reliable total cross sections for the ground and excited $2s$-state Ps formation are reported at intermediate and high energies. The present distorted-wave results are in conformity with the existing theoretical and experimental values available in the literature for intermediate and high-energy positrons. Surface plots of the DWA differential cross section reveal rich structures due to constructive and destructive interference between angular momentum states of the moving Ps.

Total and positronium formation cross sections for positron scattering from H2O and HCOOH

Total and positronium formation cross sections have been measured for positron scattering from H2O and HCOOH using a positron beam with an energy resolution of 60 meV (full-width at half-maximum (FWHM)). The energy range covered is 0.5–60 eV, including an investigation of the behavior of the onset of the positronium formation channel using measurements with a 50 meV energy step, the result of which shows no evidence of any channel coupling effects or scattering resonances for either molecule.

High-resolution positron scattering from helium: Grand total and positronium-formation cross sections

Measurements of the grand total and positronium-formation cross sections for positrons scattered by helium within the impact energy range from 10 to 60 eV are presented. All measurements presented here use a high-resolution $(\ensuremath{\sim}70\text{ }\text{meV})$ trap-based pulsed positron beam. Scattering is studied using a high-magnetic field, and absolute measurements of the scattering cross sections are obtained without the need for normalization to other cross sections. We also present single center, convergent close coupling calculations of the total cross section. A detailed study of the cross section to investigate the possibility of scattering resonances and channel coupling has been made. Comparisons with previous cross-section measurements and theoretical calculations are also included.

Positronium formation for e+-Na scattering at low and intermediate energies

There have been various theoretical calculations for positron-sodium scattering that attempt to calculate accurate positronium(Ps) formation cross sections at low and intermediate energies. The recent hyperspherical close-coupling (HSCC) calculations (Le et al. 2005 Phys. Rev. A 71, 032713) has handled the contribution from the positronic bound states to a great extent. Their calculations have further strengthened the argument that the theoretical Ps cross sections at low energies deviate from the predictions of the experimental measurements (Surdutovich et al. 2002 Phys. Rev. A 65, 032713). Here we examine whether the inclusion or the neglect of the positronic bound states in the calculation have any significant effects on the low energy Ps formation cross sections. Thus, we attempt to investigate if large-scale CC calculations are deficient in this respect.

POSSIBLE POSITRONIUM FORMATION IN THE COLLISIONS OF POSITRONS WITH CALCIUM AND STRONTIUM POSITIVE IONS

The inelastic collisions of positrons with calcium and strontium positive ions are treated for the first time as a three-channel problem within the framework of the improved coupled-static and frozen-core approximations with the assumption that the elastic, ground positronium and excited-positronium formation channels are open. The calculations of the partial and total cross sections are carried out for eight values of the total angular momentum ℓ (0 ≤ ℓ ≤7), at 20 values of the incident energy lying above the excited positronium formation threshold and extending to a wide region above the ionization threshold of the target ion. The total collisional positronium formation cross sections of e+– Sr+scattering show a peak around the ionization threshold of Sr+but display a peak at 30 eV for e+– Ca+scattering. In both scattering problems the total excited Ps formation cross sections have considerable values in the energy range [Formula: see text].

Application of exterior complex scaling to positron-hydrogen collisions including rearrangement

The application of an exterior complex scaling method to an atomic scattering problem with distinct rearrangement channels is reported. Calculations are performed for positron-hydrogen collisions in an s -wave model employing an electron-positron potential of V12 =- [8+ (r1 - r2) 2] -1/2, using the time-independent propagating exterior complex scaling method. This potential has the correct long-range Coulomb tail of the full problem and the results demonstrate that exterior-complex-scaling-based methods can accurately calculate scattering, ionization,and positronium formation cross sections in this three-body rearrangement collision.

Differential cross sections for positronium formation in positron–hydrogen scattering

Positron scattering by hydrogen atom is an interesting system to test theoretical methods due to its simplicity. Recently, theoretical calculations have reported differential cross sections (DCS) for positronium (Ps) formation for this system. The present work utilises the coupled-channel optical method (CCOM) that allows simultaneous treatment of the target channels and the Ps channels in the close-coupling method and the incorporation of the continuum effects via an optical potential to provide a comparative view of the DCS for Ps(1s) formation and Ps(2s) formation at energies ranging from 20 to 100eV. A large 12-states and 15-states CCOM calculations have been undertaken and the results compared with other available data.

Transport coefficients for positron swarms in nitrogen

Monte Carlo simulations of positron transport in nitrogen have been performed based on a complete set of cross sections that has been updated as a result of both experimental and theoretical advances. As the positronium formation cross section becomes dominant at larger mean energies in nitrogen than in argon and hydrogen the effect is much smaller in the case of nitrogen. Nevertheless, the differences between bulk and flux transport coefficients, even in nitrogen, exceed those for electrons, providing another example of the very special case of transport of charged particles under the influence of strong non-conservative collisions.

Optical Potential Calculation for Positron Collision with Helium

The positron--helium system is studied with the momentum-space coupled-channel optical method. The positronium formation cross sections from the positronium formation threshold to 400 eV and total cross sections from 17 to 3000 eV are calculated. Ionization continuum and positronium formation channels are included via a complex equivalent-local optical potential in the present calculations. The results are compared with the available experimental and theoretical data.

Electron- and positron-impact atomic scattering calculations using propagating exterior complex scaling

Calculations are reported for four-body electron-helium collisions and positron-hydrogen collisions, in the S-wave model, using the time-independent propagating exterior complex scaling (PECS) method. The PECS S-wave calculations for three-body processes in electron-helium collisions compare favourably with previous convergent close-coupling (CCC) and time-dependent exterior complex scaling (ECS) calculations, and exhibit smooth cross section profiles. The PECS four-body double-excitation cross sections are significantly different from CCC calculations and highlight the need for an accurate representation of the resonant helium final-state wave functions when undertaking these calculations. Results are also presented for positron-hydrogen collisions in an S-wave model using an electron-positron potential of V12 = − (8 + (r1 − r2)2)−1/2. This model is representative of the full problem, and the results demonstrate that ECS-based methods can accurately calculate scattering, ionization and positronium formation cross sections in this three-body rearrangement collision.