An 18-state calculation of positron-hydrogen scattering

Coupled-state calculations in a K(4s,4p,5s,5p,3d) + Ps(1s,2s,2p,3s,3p,3d) approximation are reported for positron scattering by ground-state potassium in the energy range 0.5 - 60.0 eV. Comparison is made with the earlier work of Hewitt and co-workers in a K(4s,4p,5s,5p) + Ps(1s,2s,2p) approximation. For the first time cross sections for positronium formation in n = 3 states are obtained. We support the conclusion of Hewitt et al that above about 4 eV positronium formation is mainly into excited states, although we disagree significantly on the distribution of the excited state population. Fairly good agreement is obtained with the measurements of Zhou and co-workers for total positronium formation when scaling is used to estimate the amount of positronium in states with . Our cross sections for elastic scattering, 4s - 4p, 4s - 5s and 4s - 5p excitation of potassium are in relatively good agreement with those of Hewitt and co-workers but our calculations also emphasize the importance of the 4s - 3d excitation which is omitted in their approximation. Atom excitation to the n = 5 level is found to be a relatively minor process. Good agreement is obtained with the total cross section measurements of Kwan and co-workers and Parikh and co-workers when these are normalized upwards by 10% and corrections made for near forward elastic scattering. This upward normalization lies within the quoted overall 21% error on the measurements. Below 4 eV the calculated total cross section is dominated by elastic scattering while at high energies 4s - 4p excitation is the main contributor. In the vicinity of 6 eV, and consistent with the measurements of Parikh and co-workers, the total cross section displays a pronounced peak which derives largely, although not totally, from positronium formation. Positronium formation is not important above about 30 eV.

New evaluations of the neutron cross sections for 14N and 16O have been made for ENDF/B-VI. The evaluations are based at low energies on R-matrix analyses of reactions in the 15N and 17O systems, and at higher energies on GNASH calculations and experimental data evaluations, including covariance analyses. The 15N system R-matrix analysis includes data from reactions among the channels n+14N, p+14C, and α+11B at energies corresponding to excitations in 15N below Ex=13 MeV. The resonance structure of all cross sections in this energy range is fairly well reproduced. New data indicate a different J-value for the first resonance, however. Sub-threshold S-wave levels required to explain the large n+14N total and elastic cross sections near zero energy give scattering lengths that differ significantly from the previous values. The R-matrix analysis of the 17O system includes many new measurements of the n+16O total cross section, done primarily at Oak Ridge and at Karlsruhe. The resonance structure of all the cross sections [total, (n,n), (n,α), and (α,α)] is well represented by the fit in the region below En = 6.5 MeV. The new total cross section information gives different positions for some of the resonances and implies a different normalization for the (n,α) cross sections than that obtained in the ENDF/B-IV analysis. The evaluations at energies above the ranges of the R-matrix analyses incorporate results from a number of experiments performed since the previous ENDF/B evaluations. Especially important are new measurements of the total cross sections and differential elastic, inelastic, and gamma-ray production cross sections.

We report positron scattering cross sections of fluorine and fluorine-containing molecules across an energy range from 1 to 5000 eV. Targets considered in the present study are F, F2, HF, CF4, C2F6, C6F6, CHF3, C2H3F3, and C2H2F2. Elastic, inelastic, total, and positronium formation cross sections were calculated using the optical potential approach, and the ionisation cross sections are estimated employing the complex scattering potential-ionisation contribution method. The computed results are in satisfactory agreement with available theoretical data. Comparison with experiments shows excellent agreement in the high-energy range; however, discrepancies are observed at lower energies, attributed to deficiencies in the polarisation potential model used and, in part, to the difficulty in resolving forward-angle scattering effects in experimental measurements. Positron interactions with these molecules are important in plasma, astrophysics, and radiation physics, where ionisation, energy losses, and the formation and destruction of positronium are significant. The positron scattering cross-sections are primary inputs for modelling in astrophysics, materials science, and medical imaging, such as positron emission tomography.

The equations for positron-hydrogen scattering are written as a set of coupled momentum-space Lippmann-Schwinger equations. An explicit expression for the positronium formation matrix element that is suitable for large-scale computation is derived. Elastic and positronium formation cross sections are obtained in the coupled static model as a validation of the method. Calculations at energies below the hydrogen n = 2 threshold are used to study the convergence of observable quantities with increasing numbers of physical Hand Ps states. At higher energies, a series of calculations including multiple hydrogen and positronium states are performed over the energy range from 1�0 to 4�0 Ryd. Cross sections for elastic scattering, excitation of the 2s and 2p hydrogen levels and positronium formation in the Is, 2s and 2p levels are reported.

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.

Positronium Formation by Positron Impact on Atoms at Intermediate Energies

The relativistic optical potential method has been used to calculate a complete set of cross sections for positron scattering from neon and argon in the energy range from zero to 1 keV. Elastic, positronium formation, excitation, direct ionization, and grand total cross sections are presented and compared to the recommended results of Ratnavelu et al. (J Phys Chem Ref Data 48:023102, 2019). Here, positronium formation is modeled as an ionization process by successively lowering the ionization thresholds. This process was carried out for the four lowest energy levels of positronium.Graphical abstract

Total cross sections of electron scattering by several sulfur-containing molecules OCS, SO2, SF4, SF6, SF5CF3, SO2Cl2 and SO2ClF are calculated at the Hartree-Fork level employing the modified additivity rule approach. The modified additivity rule approach, which was proposed by Shi et al. [Eur. Phys. J. D 45, 253 (2007); Nucl. Instrum. Meth. B 254, 205 (2007)], takes into consideration that the contributions of the geometric shielding effect vary as the energy of incident electrons, the target’s molecular dimension and the atomic and electronic numbers in the molecule. The present investigations cover the impact energies ranging from 30 to 5000 eV. The quantitative total cross sections are compared with those obtained by experiments and other theories. Good agreement is observed even at energies of several tens of eV. It shows that the modified additivity rule approach is applicable to carry out the total cross section calculations of electron scattering by these sulfur-containing molecules at intermediate and high energies, especially over the energy range above 100 eV or so. In the present calculations, the atoms are still represented by the spherical complex optical potential, which is composed of static, exchange, polarization and absorption terms.

Total cross sections for electron scattering on “quasi spherical” (CH4, SiH4, GeH4) molecules have been analyzed phenomenologically over a wide energy range. Regions, at low and high energies can be usefully represented by simple analytical formulae. Regularities associated with characteristic points such as the Ramsauer minima have been exposed. Comparison with other simple hydrides (NH3, H2O, H2S) allows the demonstration of a possible correlation between the maximum value of the total cross section and the bond length. Some points of contact with first-principles theory are noted and in particular the energy at which the maximum cross section occurs, is related to the occurrence of a partial wave resonance. In the absence of complete data for GeH4, prediction of characteristic points in the low energy cross section proves possible via the phenomenological analysis. Similarly, in the high energy regime, predictions of the cross section for SnH4 is made from data on the lighter molecules of the series, using non-relativistic Thomas-Fermi self consistent field scaling.

We report electron impact total cross sections, Q(T), for e-N(2)O scattering over an extensive range of impact energies approximately from 0.1 eV to 2000 eV. We employ an ab initio calculation using R-matrix formalism below the ionization threshold of the target and above it we use the well established spherical complex optical potential to compute the cross sections. Total cross section is obtained as a sum of total elastic and total electronic excitation cross sections below the ionization threshold and above the ionization threshold as a sum of total elastic and total inelastic cross sections. Ample cross section data for e-N(2)O scattering are available at low impact energies and hence meaningful comparisons are made. Good agreement is observed with the available theoretical as well as experimental results over the entire energy range studied here.

The formation of positronium (Ps) in different states by the impact of intermediate energy (20--500 eV) positrons on atomic hydrogen is calculated using the distorted-wave Born approximation along with the first Born approximation. Differential and integrated cross sections for the formation of Ps(1{ital s}), Ps(2{ital s}), Ps(2{ital p}{sub 0}), and Ps(2{ital p}{sub 1}) have been evaluated individually and then 1/{ital n}{sup 3} behavior for charge-transfer cross sections is used for {ital n}{gt}=3 to obtain the total cross sections for positronium formation. The present results for positronium formation cross sections are in agreement with the results of other available investigations.

The LHC has brought much new information on total, elastic and diffractive cross sections, which is not always in agreement with extrapolations from lower energies. The default framework in the Pythia event generator is one case in point. In this article we study and implement two recent models, as more realistic alternatives. Both describe total and elastic cross sections, whereas one also includes single diffraction. Noting some issues at high energies, a variant of the latter is proposed, and extended also to double and central diffraction. Further, the experimental definition of diffraction is based on the presence of rapidity gaps, which however also could be caused by colour reconnection in nondiffractive events, a phenomenon that is studied in the context of a specific model. Throughout comparisons with LHC and other data are presented.

Calculated total, differential, and momentum-transfer cross sections are reported for the vibrationally elastic scattering of electrons from ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$ molecules in the range of energy 0.5--20 eV. The total cross sections are approximated by a sum of the rotationally elastic ones and the rotational transition from ground to first excited state (00\ensuremath{\rightarrow}10). The first-order Born approximation with the rotating-molecule model is used in the calculation of the first rotational excitation cross section. The experimental value of the dipole moments of ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$ are used in this step of the calculation. Spherically approximate molecular wave functions are applied to calculate the spherical part of the interactions of the incident electron with the molecule and the rotationally elastic-scattering cross section, in which the static and exchange interactions are calculated exactly within the accuracy of the molecular wave function and a parameter-free correlation-polarization potential is used to account for the polarization effect. The agreement of the differential cross sections with the experimental data is good at small angles and reasonably good at intermediate and large angles. Good agreement with measured values is obtained for the momentum-transfer cross sections. Total cross sections are reported with theoretical results on the e-${\mathrm{NH}}_{3}$ system below 10 eV. The possible sources of the discrepancies between the current theoretical and experimental results are analyzed for the total cross sections at very low energies by a quantitative consideration of the contributions of very strong forward scattering at near-zero angles to the total cross sections.

The absolute total cross section (TCS) for electron scattering from hexafluoro-1,3-butadiene(1,3-C4F6) was obtained in a transmission experiment for electron impact energies from0.6 to 370 eV. The TCS energy function has two pronounced enhancementsseparated by a deep minimum located near 2.6 eV: a low-energy hump around1 eV and a dominant very broad enhancement centred near 30 eV. The1,3-C4F6 TCS results are compared with the data for hexafluoro-2-butyne(2-C4F6) and the influence of the structural differences on the electron-scattering TCS for isomers of theC4F6 molecule (isomeric effect) is explicitly indicated. The most pronounced role of the moleculargeometry on the magnitude and shape of the TCS energy dependence is observed atthe lowest energies studied. Above 40 eV the TCSs for both isomers are closeto each other. Some conformities of TCSs within a series of perfluorocarbons(C2F4,1,3-C4F6, C6F6) as well as in a series of their hydrocarbon counterparts(C2H4,1,3-C4H6, C6H6) are also pointed out and discussed.

The elastic and total cross sections for formaldehyde, acetaldehyde, acetone, 2-butanone and formamide are calculated using the spherical complex optical potential formalism in the intermediate energy range from 50 eV to 10 keV. These cross sections find application to various fields like radiation damage and biological sciences. The present results are compared with the available experimental and theoretical data and are found to give excellent agreement. The elastic cross sections reported for most of the targets in the present energy range are done for the first time. The energy dependence of the contribution of ionization and elastic cross section with respect to the total cross section and the correlation of total cross section with polarizability of the molecules are also studied.