Wilhelm Raith

Total cross section measurements for positron and electron scattering on hydrocarbons between 5 and 400 eV

The total cross sections of methane, ethane, ethene, propane, propene, cyclopropane, n-butane, isobutane and 1-butene were measured in a transmission experiment utilising positrons from a 22Na source and a tungsten moderator and electrons from a thermionic cathode. As expected the cross sections for positrons are smaller than those for electrons and both grow with the size of the molecules. Above 30 eV the positron and electron cross sections decrease with increasing energy and approach each other. All measured positron and electron cross sections between 100 and 400 eV can be described by a formula containing the energy, the number of molecular electrons and only three fit parameters.

A source of highly polarized electrons at the stanford linear accelerator center

Abstract A polarized electron source based upon photoionization of a state-selected 6 Li atomic beam has been developed as an injection gun for the Standford two-mile electron linear accelerator. The source (PEGGY) produces a pulsed beam of electrons with a maximum intensity of 2.6×10 9 electrons per pulse, a polarization of 0.85, a pulse length of 1.6 μs, and a repitition rate of 180 pulses/s. Since its installation at SLAC in July 1974, PEGGY has been used in several energy electron-nucleon scattering experiments.

Total cross section measurements for electron scattering from molecular hydrogen at very low energies

Absolute cross sections were determined from transmission measurements made with an electron time-of-flight spectrometer in the energy range of 0.02-2 eV. The total cross section for molecular hydrogen was found to increase monotonically with energy; no resonance-like structures were observed. The isotopes H2, D2 and HD have identical cross sections within the accuracy of the measurements (+or-2.5%). Helium cross section measurements were also made in order to check the spectrometer.

The Ramsauer minimum of methane

With a time-of-flight spectrometer the authors measured the electron total cross section of methane in the energy range 0.085-12 eV with particular attention to the Ramsauer minimum for which they determined a minimum cross section of 1.36(4)*10-16 cm2 at an energy of 0.40(2) eV. Both the cross section and the energy location are in excellent agreement with the early measurements of Ramsauer and Kollath but in significant disagreement with more recent measurements of Barbarito et al. (1979). By applying the formulae of the modified effective-range theory to the low-energy data up to 0.5 eV. The authors obtain momentum transfer cross sections in good agreement with the most recent analysis of swarm experiment data by Haddad (1985).

Total cross section measurement for e-SF6 scattering down to 0.036 eV

The authors report transmission measurements made with an electron time-of-flight spectrometer. The gas pressure in the target cell was determined by means of a capacitance manometer. From 2 eV down to 0.07 eV the results agree extremely well with the predictions of Morrison et al. (1977) obtained from a coupled-channel theory.

Surface magnetic properties of oxygen on ferromagnetic films: a spin resolved MDS study

Using spin polarized metastable deexcitation spectroscopy (SPMDS) we determine spin asymmetries for remanently magnetized iron(110) and cobalt(0001) films clean and oxygen covered. For clean surfaces a dominance of minority spins is observed at the Fermi edge. At the Fermi edge oxygen exposure leads to a reversal of the dominance of the spin for iron, but not for cobalt, being in agreement with theoretical calculations. An oxygen induced structure at a kinetic energy of 5.5 eV shows a dominance of minority spins, but with different dependence on coverage for iron and cobalt.

Production of a spin-polarized, metastable He(23S) beam for studies in atomic and surface physics

This beam was developed as a target for a crossed-beam electron-atom scattering experiment on the interaction of a polarized spin-1/2 electron with a polarized spin-1 atom. In the future this beam will be used in “Spin-Polarized Metastable Atom Deexcitation Spectroscopy” (SPMDS) for studying ferromagnetic surfaces without and with adsorbate layers. We use a discharge source for producing a beam of metastable helium atoms, a permanent sextupole magnet with a central stop at its exit for selecting He(23S) atoms in the Zeeman substatems=+1, a zero-field spin flipper for reversing the atomic beam polarization with respect to a magnetic guiding field, and a Stern-Gerlach magnet for analyzing the atomic polarization. At a distance of 90 cm beyond the exit of the sextupole, in the “interaction region” of an experiment, the polarized beam has a circular cross section of about 6 mm FWHM and a particle density of 1 · 107 atoms/cm3. The reversible spin polarization was determined asP=0.90±0.02. A possible contamination of the beam with metastable singlet atoms is included within this value; the ground-state He atoms are not considered to be part of the polarized beam. An observed contamination with long-lived Rydberg atoms can easily be destroyed by applying a high electric field.

Ionization of atomic hydrogen by positrons

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.

Measurement of spin asymmetries in the electron impact ionisation of alkali atoms

Using a GaAsP polarised electron source and state-selected atomic beams of lithium, sodium and potassium the authors investigated the spin dependence of the total ionisation cross section in the energy range from threshold to ten times the threshold value. For Li and Na the measured spin asymmetries are very similar, having as a function of energy E a maximum of about 0.5 at twice the threshold energy I and a fall off towards zero at higher energies proportional to In(I/E). The potassium data are considerably lower in the region from threshold up to five times threshold energy, showing an asymmetry of about 0.25 there. With lithium the near-threshold range was probed with a narrow electron energy distribution ( Delta E=0.17 eV). It was found that the spin asymmetry stays well below unity at threshold (AI approximately=0.45) and has little dependence on energy in this region.

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

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.