Total-cross-section measurements for positron and electron scattering byO2,CH4

Abstract

Total cross sections (Qr's) have been measured for I -500-eV positrons and electrons scattered by 02, CH4, and SF6 using a beam-transmission technique. The positron Qr s are compared with the corresponding electron Qr's for each target gas. It is found that the positron Qr's are, in general, lower than the electron results. There are no prominent structures observed for positron scattering at low energies that would be comparable to the narrow shape resonances observed for electrons scattering from various molecules, such as SF6. The positron Qr curve for CH4 reveals a significant increase in the vicinity of its positronium formation threshold, while the 02 and SF6 curves are monotonically increasing in this vicinity. At the highest energies investigated, there are indications of a tendency toward merging of the positron and electron Qr s for these gases. Interesting similarities are found in a comparison of the present positron and electron Qr curves for CH4 with prior comparison measurements for argon.

Do positrons measure atomic and molecular diameters?

“…Experiments with weak positron sources usually require the use of strong magnetic fields, what increases the effective path length of projectiles in the gas cell, compare reference [76][77][78], so measured cross sections can be overestimated. On the other hand, wider apertures in scattering cell worsen the angular resolution, what underestimates cross sections in an energy-dependent manner.…”

“…In Table 2 we compare different determinations of atomic and molecular "radii" -experimental from viscosity, X-ray diffraction and self-diffusion measurements together with theoretical from self-consistent field calculations (for atoms) and present -from positron to- [79] (parameter-free model potential) and Zecca et al [74] (Schwinger multichannel). Experiments: Dababneh et al [76], Floeder et al [77], Sueoka and Mori [78] and Zecca et al [74].…”

A rigid sphere approach to positron elastic scattering by noble gases, molecular hydrogen, nitrogen and methane

“…Figure 7c shows calculations of elastic cross-sections for methane (CH 4 ) using two different values of rigid sphere radius: R 0 = 1.4632a 0 and R 0 = 1.5445a 0 . The use of smaller R 0 yields cross-sections consistent with DFT results using Boronski-Nieminen functional by Franz et al [10], while larger R 0 gives better agreement with old experimental datasets by Sueoka and Mori [79] and Dababneh et al [80]. On the other hand it is impossible with present approach to find R 0 allowing to reproduce TCS by Zecca et al [72].…”

“…To best [65], Zammit et al [66] and Fedus et al [3]; theories for N2: Elza et al [67], Carvalho et al [68], Tenfen et al [69], Mukherjee and Mukherjee [70] and Franz et al [10]; theories for CH4: Jain and Gianturco [71], Zecca et al [72] and Franz et al [10]. Experiments for H2: Fedus et al [3], Karwasz et al [51], Zecca et al [73] and Machacek et al [74]; experiments for N2: Hoffman et al [75], Sueoka and Hamada [76], Karwasz et al [51] and Zecca et al [77]; experiments for CH4: Floeder et al [78], Sueoka and Mori [79], Dababneh et al [80] and Zecca et al [72].…”

Monte Carlo model for electron degradation in SO2 gas: Cross sections, yield spectra, and efficiencies

“…The total electron impact collision cross sections have been measured by Sokolov and Sokolova [1981] and Zubek et al [1981] for energies up to 10 eV, by Dababneh et al [1986] up to 50 eV, and by Szmytkowski and Maciag [1986] up to 100 eV. Recently, Zecca et al [1995] reported total cross section for electron scattering on SO2 in the energy range 90-4000 eV, which is in very good agreement with Szmytkowski and Maciag [1986] in the overlapping energy Table 1 The total e-SO2 collision cross section profile is presented in Figure 1, and the values at a few energies are given in The total elastic cross sections are fitted using the semiempirical formula [Zecca et al, 1995] a ( Table 1 at a few energy points.…”