A D Stauffer

Elastic scattering of electrons from neon and argon

The authors continue their earlier study on the effects of polarisation and exchange in low-energy elastic scattering of electrons from noble gases. They present detailed results for argon and conclude that including only the dipole part of the polarisation potential and treating exchange exactly produces the most reliable results. The behaviour for neon is also very similar to that for helium. Values for phaseshifts, differential, total elastic and momentum transfer cross sections are given for neon and argon and compared with recent experimental and theoretical results.

Positron scattering from argon

The authors have extended their work on the polarised-orbital approximation to the calculation of scattering and annihilation processes for the positron-argon system in the elastic-scattering region. They present results for the phaseshifts; the elastic, diffusion and annihilation cross sections; and the angular correlation of photons produced during annihilation. As in previous calculations the results are in very good agreement with various experimental measurements. The authors have also investigated the angular distribution of the scattered positron and the possibility of forming a bound state of the system. The model predicts that there is no bound state of the positron-argon system.

Positron scattering from krypton and xenon

The elastic scattering of positrons from noble gases has been formulated in a frozen-core version of the polarised-orbital approximation. This approximation has been applied to the scattering of positrons from krypton and xenon. The authors present results for phaseshifts and elastic, diffusion and annihilation cross sections in the low-energy (elastic scattering) region, as well as the angular correlation of the photons produced during annihilation. Good agreement is obtained between the elastic scattering cross section and recent experimental results for xenon below the positronium formation threshold. The agreement is less satisfactory in the case of krypton.

The transmission of photoelectrons emitted from CsI photocathodes into Xe, Ar, Ne and their mixtures: a Monte Carlo study of the dependence on E/N and incident VUV photon energy

Although ultraviolet photosensor devices offer many advantages when used in radiation detectors, there is often a significant reduction in pulse amplitude when the photosensor operates in a detector filled with a noble gas. This is due to the backscattering of electrons by the noble gas atoms. In this study, we investigate the problem of the backscattering of the photoelectrons emitted from a CsI photocathode into Xe, Ar, and Ne and the binary mixtures Xe–Ar, Ar–Ne and Xe–Ne using a detailed Monte Carlo simulation. Results for the photoelectron transmission efficiencies are presented and discussed for the case of a CsI photocathode irradiated with photons with energies in the range Eph = 6.8–9.8 eV (183–127 nm) and for applied reduced electric fields in the range E/N = 1–40 Td. The dependence on incident photon energy, nature of the gas and applied electric field are examined, and the results are explained in terms of electron scattering in the different noble gases.

Elastic scattering of electrons from krypton and xenon

Results are given for the low-energy elastic scattering of electrons from krypton and xenon. The calculations were carried out by treating exchange exactly and including only the dipole part of the polarisation potential. Values are given for the phaseshifts, differential, total elastic and momentum transfer cross sections. Comparison are given with recent experimental and theoretical work for these processes.

Electron impact excitation of the argon 3p54s configuration: differential cross-sections and cross-section ratios

New electron-impact differential cross-section (DCS) and DCS ratio measurements for the excitation of the four levels making up the 3p54s configuration of argon are reported at incident electron energies of 14, 15, 17.5, 20, 30, 50 and 100 eV. These cross-sections were obtained using a conventional high resolution electron spectrometer. Elastic electron scattering from argon was used as a calibration standard. Electron–helium DCSs were used to determine the instrumental transmission of the spectrometer. Further checks of the relative shape of these DCS measurements were made using the method of gas mixtures (Ne mixed with Ar). We also present results from new calculations of these DCSs using the R-matrix method, the unitarized first-order many-body theory, the semi-relativistic distorted-wave Born approximation, and the relativistic distorted-wave method. Comparison with available experimental DCSs and DCS ratios is also presented.

Positron scattering from neon

The authors have used the polarised-orbital approximation for positron scattering from noble gases as formulated recently by McEachran et al. (1977) to calculate scattering and annihilation processes in neon. They present results for phaseshifts, elastic, diffusion and annihilation cross sections as well as the angular correlation of the photons produced during annihilation in the low-energy (elastic scattering) region. Very good agreement is obtained between these elastic scattering cross sections and recent experimental measurements.

A Monte Carlo study of backscattering effects in the photoelectron emission from CsI into CH4 and Ar-CH4 mixtures

Monte Carlo simulation is used to investigate photoelectron backscattering effects in the emission from a CsI photocathode into CH4 and Ar-CH4 mixtures for incident monochromatic photons with energies Eph in the range 6.8 eV to 9.8 eV (182 nm to 127 nm), and photons from a continuous VUV Hg(Ar) lamp with a spectral distribution peaked at Eph = 6.7 eV (185 nm), considering reduced applied electric fields E/N in the 0.1 Td to 40 Td range. The addition of CH4 to a noble gas efficiently increases electron transmission and drift velocity, due to vibrational excitation of the molecules at low electron energies. Results are presented for the photoelectron transmission efficiencies f, where f is the fraction of the number of photoelectrons emitted from CsI which are transmitted through the gas as compared to vacuum. The dependence of f on Eph, E/N, and mixture composition is analyzed and explained in terms of electron scattering in the different gas media, and results are compared with available measurements. Electron drift parameters are also calculated and compared with experimental data, confirming the choice of electron scattering cross-sections used in the simulations.

Polarisabilities and shielding factors for He, Ne and Ar

Multipole polarisabilities and shielding factors for helium, neon and argon have been calculated accurately in the coupled Hartree-Fock approximation. A shell by shell analysis is given for the first four multipoles.

Screening and distortion effects in positron impact ionisation of helium

Total cross sections for positron impact ionisation of helium are calculated within a distorted-wave formalism. By using various physical models the authors found that those which include complete screening of the residual ion are capable of reproducing the experimental data. The inclusion of distortion of the outgoing waves brings the authors CPT and CPE model results into very good agreement with the measurements of Fromme et al (1986) up to 300 eV.