A comparative study of electron- and positron-polyatomic molecule scattering

Detailed measurements of the total ionization cross-sections for Ne, Ar, Kr and Xe by positron impact are presented in the energy range from threshold up to ~1 keV. From these and other available data, the cross-sections for positronium formation are extracted. Comparison is made with other results where available. Structure in the positronium formation cross-section has been found which appears to be associated with positronium formation in excited states. Upper and lower limits on the contribution from these are estimated.

Total positron-impact ionization and positronium formation from the noble gases

Measurements of the single (σi+) and the ratio of double to single (R2) ionization cross sections for positron impact on neon, krypton and xenon are presented from near threshold to 1000 eV By multiplying the R2 values of each target by the appropriate σi+, new data for the corresponding double (σi2+) ionization cross sections have also been obtained The ratio between positron and electron σi+ maxima is generally seen to be lower for higher target atomic number, as expected from the static interaction between the projectile and undistorted target Comparison with available data suggests that Ps formation accompanied by the production of singly or doubly charged ions is a significant channel in positron-atom scattering

Single and double ionization of neon, krypton and xenon by positron impact

We reexamine in detail the various processes undergone by positrons in the ISM from their birth to their annihilation using the most recent results of positron interaction cross sections with H, H2 and He. The positrons' lives are divided into two phases: the 'in-flight' phase and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

/pdf/the-lives-and-deaths-of-positrons-in-the-interstellar-medium-5f2y1hrbtz.pdf

The lives and deaths of positrons in the interstellar medium

Positronium formation (Ps) cross sections for positrons impinging on atomic hydrogen were measured in the impact energy range from 13 to 255 eV at the High Intensity Positron (HIP) beam at Brookhaven National Laboratory (BNL). The Ps-formation cross section was found to rise rapidly from the threshold at 6.8 eV to a maximum value of (2.98±0.18) ×10−16 cm2 for ∼15 eV positrons. By 75 eV it drops below the detection limit of 0.17×10−16 cm2 which is the present level of statistical uncertainty. A more elaborate data analysis was performed leading to somewhat smaller cross sections than originally published [1]. During the course of this experiment further relative data on impact-ionization cross sections were accumulated.

Results of the Bielefeld-Brookhaven e+-H experiment

Doubly differential ionization cross sections for positron impact on argon atoms were determined by energy- and angle-resolved measurements of ejected electrons in time correlation with the produced ions. Corresponding measurements with incident electrons were made for comparison. With primary particles (positrons or electrons) of 100 eV and ejected electrons of 15 eV, measurements were extended over electron-emission angles from 0\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{}. Lacking theoretical predictions for the ionization of argon we compared our data with the doubly differential ionization cross sections of Klar and Berakdar computed for positron and electron impact on hydrogen. The measured angular dependence of positron and electron cross sections for Ar agrees well with the theory for H. In particular, we find that at small angles of electron ejection the cross section for positron impact exceeds that for electron impact by an order of magnitude in accordance with the calculations of Klar and Berakdar (unpublished).

Positron-atom doubly differential ionization cross sections

New data on positron collisions with atomic hydrogen leading to ionization were taken by using the high-intensity positron beam at Brookhaven National Laboratory (BNL). The measured cross section values for positron-impact ionization, which supersede earlier ones, are in good agreement with other experimental and most theoretical results, while the new data for positronium formation, formerly normalized to impact-ionization data, now disagree with recent theoretical predictions.

Ionization of atomic hydrogen by positrons

Our previous differential cross section measurements on argon were extended to impact energies of 120 eV and krypton was used as an alternative target gas. Some distinctive features were observed in the cross sections of the transfer and transfer-ionization reaction channel which await a theoretical explanation. The narrow feature at , observed for the transfer-ionization reaction at 75 eV, does not persist at 90 eV and could not be observed with krypton. Therefore, a connection to Thomas scattering ( is the Thomas angle for Ps formation) is not supported.

Differential Ps-formation and impact-ionization cross sections for positron scattering on Ar and Kr atoms

For positron impact on xenon the authors have measured the ratios of double-to-single and triple-to-single ionization cross sections, R2 and R3, respectively, and compared them with the corresponding cross section ratios for electron impact. At high projectile energies, at about 600-800 eV, the cross-section ratios R2(e+) and R2(e-) merge. At lower projectile energies, the positron ratios lie below the electron ratios. Qualitatively, this agrees with the results of Charlton et al. (1988, 89) for He, Ne and Ar.

Multiple ionization of xenon by positron impact

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