Positronium Formation Cross Sections Research Articles

P-Wave Positron-Hydrogen Scattering, Annihilation, and Positronium Formation

In a previous paper (Bhatia A.K. 2016), a hybrid theory for the scattering of positrons from hydrogen atoms was applied to calculate S-wave phase shifts, annihilation, and positronium formation cross sections. This approach is now being applied to calculate P-wave positron-hydrogen scattering. The present results, obtained using short-range correlation functions along with long-range correlations in the Schrödinger equation at the same time, agree very well with the results obtained in an earlier calculation by Bhatia et al. (1974), using the Feshbach projection operator formalism. In these earlier calculations, the correction due to the long-range correlations was applied to the variational results. In spite of the fact that this ad hoc correction destroyed the variational bound, the final results have been considered accurate. Annihilation cross-sections, positronium formation cross-sections, calculated in the distorted-wave approximation, are also presented.

Positron-Hydrogen Scattering, Annihilation, and Positronium Formation

In previous papers (Bhatia A.K. 2007, 2012) a hybrid theory for the scattering of electrons from a hydrogenic system was developed and applied to calculate scattering phase shifts, Feshbach resonances, and photoabsorption processes. This approach is now being applied to the scattering of positrons from hydrogen atoms. Very accurate phase shifts, using the Feshbach projection operator formalism, were calculated previously (Bhatia A.K. et al. 1971 and Bhatia et al. 1974a). The present results, obtained using shorter expansions in the correlation function, along with long-range correlations in the Schrödinger equation, agree very well with the results obtained earlier. The scattering length is also calculated and the present results are compared with the previous results. Annihilation cross-sections, and positronium formation cross-sections, calculated in the distorted-wave approximation, are also presented.

Positronium formation in collisions between positrons and alkali-metal atoms (Li, Na, K, Rb and Cs) in Debye plasma environments

Positronium (Ps) formation processes in collisions between positrons and alkali atoms (Li, Na, K, Rb and Cs) in Debye plasma environments have been investigated using the classical trajectory Monte Carlo (CTMC) method for the energy range 1–500 keV. Interactions between the active electron in an alkali atom with the positron have been described using a model potential along with the Debye–Hückel potential. In such a process, positronium formation cross sections have been calculated in unscreened, as well as in Debye plasma, conditions. The results show that the cross sections for positronium formation are significantly affected by Debye screening lengths. Also, positronium formation depends on collision energy and plasma conditions. The effect of Debye plasma environments on such a formation process is explained in terms of the classical trajectory framework.

Internal consistency in positron-hydrogen-scattering calculations

Internal consistency in a close-coupling approach to positron-hydrogen scattering is investigated with a particular focus on the potential overlap between the atomic and positronium continua. We present results for total, total ionization, and $1s$ positronium-formation cross sections for projectile energies up to 100 eV. We show that, irrespective of whether the continuum is treated by one center, or the other, or both, the same cross sections are generally obtained. This is true only if sufficiently large orbital angular momentum is taken in the close-coupling expansion. Furthermore, unitarity of the close-coupling approach ensures convergence of the physically observable cross sections even if the individual components are not convergent.

Positron scattering from vinyl acetate

Using a Beer–Lambert attenuation approach, we report measured total cross sections (TCSs) for positron scattering from vinyl acetate (C4H6O2) in the incident positron energy range 0.15–50 eV. In addition, we also report an independent atom model with screening corrected additivity rule computation results for the TCSs, differential and integral elastic cross sections, the positronium formation cross section and inelastic integral cross sections. The energy range of these calculations is 1–1000 eV. While there is a reasonable qualitative correspondence between measurement and calculation for the TCSs, in terms of the energy dependence of those cross sections, the theory was found to be a factor of ∼2 larger in magnitude at the lower energies, even after the measured data were corrected for the forward angle scattering effect.

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.

Cross Sections for Positron Impact with 2,2,4-Trimethylpentane

2,2,4-Trimethylpentane (C8H18), a hydrocarbon produced all over the world on a large scale in the processing of crude oil, has long been known and used in the energy sector. It has also recently attracted the attention of the radiation physics and chemistry community, owing to its applications in medical imaging techniques. Charged-particle interactions with this species unfortunately remain mostly unknown. In this study, we report on measured total cross sections for positron scattering from 2,2,4-trimethylpentane in the energy range from 0.12 to 50 eV. We also present calculations of the total cross sections, elastic integral and differential cross sections, positronium formation cross sections, and inelastic integral cross sections at energies from 1 to 1000 eV using the independent atom model with screening corrected additivity rule. A knowledge of those scattering cross sections might, through simulation models, help to improve the accuracy of current radiation detection devices and hence provide better estimates of the extent of any charged-particle-induced damage in biomolecular systems.

Positron scattering from molecular hydrogen

We present results for total and partial cross sections for positron scattering from H${}_{2}$. The total scattering and positronium formation cross sections are reported between 0.5 and 200 eV. Total quasielastic and inelastic scattering cross sections are reported for energies between the positronium formation threshold and 50 eV, with quasielastic differential scattering cross sections reported at 1, 3, 5, 7, and 10 eV. Our results are compared with previous work, both experimental and theoretical, with particular attention paid to the region below the positronium formation threshold, where there are apparent discrepancies in previous work. A discussion of possible reasons for discrepancies between this and previous work is presented, including a focus on known systematic effects in the experimental results.

Positronium formation in molecular oxygen

New measurements of the positronium (Ps) formation cross section in molecular oxygen have been made using the 'positron loss' technique used in earlier measurements in the noble gases – the reduction in beam intensity being attributed to Ps formation. The results are generally in agreement, in terms of dependence on incident positron energy, with earlier measurements, but are lower in magnitude than those deduced from total ionization measurements below the first threshold for direct ionization, which employed ion detection. QPs for molecular oxygen exhibits an interesting peak-like dependence on incident positron energy just above threshold, and possible reasons for this structure are discussed. These include the possibility of a dissociative attachment process; a unique peak at similar projectile energies has long been observed in electron-oxygen scattering. The most likely explanation, however, lies in coupling between Ps formation and excitation to the continuum, the latter having previously been shown also to have a peaked energy dependence above threshold.

Positronium formation in the noble gases

We present results based upon our ab initio relativistic optical potential method for the simulation of positronium formation in the noble gases neon, argon, krypton and xenon. The method used in this simulation is based upon an approach, first suggested by Reid and Wadehra, whereby the ionization threshold of the atom is effectively reduced by 6.8 eV, the binding energy of positronium in its ground state. This procedure is then modified in order to account for the fact that positronium formation cross sections tend to zero much more rapidly than the corresponding direct ionization cross sections as the energy of the incident positron increases. The agreement of our positronium formation cross sections with experiment is quite good for argon, krypton and xenon over most of the energy region while for neon our cross sections are generally too small at all energies. Nonetheless, our method predicts positronium formation cross sections in better agreement with experiment than the much more sophisticated theoretical approaches used in the past. We also show that adding positronium formation in this way improves agreement with the measured elastic differential cross section at small scattering angles.

Total, elastic, and inelastic cross sections for positron and electron collisions with tetrahydrofuran

We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahydrofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies, have been measured for positron collisions with THF. The positron beam used to carry out these experiments had an energy resolution in the range 40-100 meV (full-width at half-maximum). We also present TCS results for positron and electron scattering from THF computed within the independent atom model using the screening corrected additivity rule approach. In addition, we calculated positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations). While our integral and differential positron cross sections are the first of their kind, we compare our TCS with previous literature values for this species. We also provide a comparison between positron and electron-impact cross sections, in order to uncover any differences or similarities in the scattering dynamics with these two different projectiles.

Experimental and theoretical cross sections for positron collisions with 3-hydroxy-tetrahydrofuran

Cross section results from a joint experimental and theoretical investigation into positron scattering from 3-hydroxy-tetrahydrofuran (3H-THF) are presented. Total and positronium (Ps) formation cross sections have been measured from 1 to 190 eV using the positron beamline at the Australian National University, which has an energy resolution between 60 and 100 meV. The total cross section (TCS) and the elastic and total inelastic integral cross sections in the energy range between 1 and 1000 eV have been computed within the Independent Atom Model using the Screening Corrected Additivity Rule approach. In addition, we have calculated elastic differential cross sections at selected incident energies. Our computations represent the first theoretical results reported for this target species, while our measured Ps formation cross sections are also novel. Comparison of the present TCS with the previous results from the University of Trento shows a good level of agreement at the lowest energies. We also provide a comparison between the present cross sections for 3H-THF and those from our earlier study on the parent molecule tetrahydrofuran.

Positron-Excited Lithium Atom Collisions

The inelastic scattering of positrons by excited lithium alkali atoms Li*(2p) have been investigated within the frame work of the coupled-static and frozen-core approximations with the assumption that the elastic and rearrangement channels are open. In the present work, a rather complicated computer code is developed based on the coupled-static, frozen-core and Green’s function partial wave expansion technique. The partial and total elastic and positronium (Ps) formation cross sections of e+-Li*(2p) are calculated through a wide range of incident energy of positrons ranging from 0.3 eV to 1000 eV. Also, we have calculated the partial and total elastic and rearrangement (reversal of the Ps formation) cross sections of Ps-Li+ collisions through the low, intermediate and high energy regions. The effect of polarization potential of the Ps atom is taken into our consideration. The total cross sections which corresponding to twelve partial cross sections (calculated at twelve values of the total angular momentum l = 0 to l = 11) are calculated for each channel. Our calculated total positronium formation cross sections are compared with experimental results and those calculated by other authors. The present calculations encourage the experimental physicists to carry out positron-lithium experiments by taking the excited lithium target into accounts in order to obtain more positronium especially in the low and intermediate energy regions.

Low energy positron scattering from krypton and xenon

Absolute grand total, positronium formation, and grand total minus positronium formation cross sections for positron scattering from krypton and xenon have been measured with an energy resolution of ~60 meV over the energy range 0.5-60 eV. Experimental results for elastic differential cross sections measured using the same apparatus, for selected energies both below and above the Ps threshold, are also presented. Theoretical estimations of these cross sections are also performed using the convergent close coupling method (CCC) and the relativistic optical potential (ROP) approach.

Trends in Positron Scattering from Biomolecules

We present recent measurements of the total scattering and positronium formation cross sections for Uracil, THF, 3H-THF and make comparisons with previous work and recent results; H2O and Pyrimidine.

Positronium formation with excitation

Cross-sections for positron-impact ionization leaving either the resultant ion or positronium atom in an excited state are presented. For atomic targets, excited-state positronium contributes significantly to the positronium formation cross-section, while for CO2 and N2 it appears suppressed. Instead, for these targets large cross-sections for ionization-excitation are noted.

Positronium formation in positron-hydrogen collisions with Debye potentials

Positronium (Ps) formation cross sections (n = 1, 2) in positron-hydrogen collisions in Debye plasma environment are calculated using the screening approximation model for various Debye screening lengths from the Ps formation thresholds to 50 eV. The effect of the screened Coulomb potential on Ps formation process is investigated by using the Debye-Hückel potential. The present results are compared with available theoretical calculations.

Coupled-channel optical calculation for positron-lithium scattering

Positron scattering with atomic lithium is investigated by using a coupled-channel optical method. The ionization continuum and positronium formation channels are taken into account via a complex equivalent-local optical potential. The positronium formation cross sections and the ionization cross sections, as well as the total scattering cross sections, are reported at energies above 3 eV and compared with available experimental and theoretical data.

Scaling behaviour of positronium formation cross sections for positron scattering from hydrogen-like ions

Positronium (Ps) formation processes of hydrogen (H)-like ions by positron impact are investigated by employing the two-centre two-channel eikonal final state-continuum initial distorted wave model. The differential and integrated cross sections for Ps formation into the ground state are calculated for ions A(Z − 1) + with nuclear charge Z = 1–9. Comparisons with the first-order Coulomb–Born and the continuum distorted-wave final-state models are made. The differential cross sections and the Thomas double-scattering processes for H-like ions are investigated. For the integrated Ps formation cross sections, a scaling law is introduced and it is reasonably followed by H-like ions with Z ⩾ 3 in the entire energy range. The maximum positions of the ground Ps formation cross sections for the H-like isoelectronic sequences are found to be in good agreement with the wave vector matching model of Charlton (2006 J. Phys. B: At. Mol. Opt. Phys. 39 4575).

Positron-rubidium atom scattering at intermediate and high energies

In this work, the coupled-channel optical method (CCOM) is implemented in a study of positron-rubidium (Rb) scattering at intermediate and high energies. The CCOM provides a realistic calculation for positron-atom scattering where the ab-initio optical potential accounts for the omitted continuum effect in the scattering system. The integral, positronium (Ps) formation and total cross sections are compared to experimental and other theoretical data where available.