Günther Sinapius

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.

Positron sources for atomic physics experiments

The authors have demonstrated the possibility of producing strong and point-like positron sources by collection of the ion beams from an on-line mass separator. At ISOLDE beta + emission rates of 3.7*109 Bq were obtained from 81Rb sources collected on an area smaller than 4.5 mm2. When the activity is deposited on a transmission moderator, low-energy positron beams with high brightness can be produced. With a 6 mu m thick polycrystalline tungsten transmission moderator a brightness per energy of better than 800 s-1 mm-2 rad-2 eV-1 was achieved. This value could still be considerably increased by optimisation of the ion beam parameters and by means of a more efficient moderator.

Multiple ionization of xenon by positron impact

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.

Positron‐impact ionization of atomic hydrogen

In a crossed‐beam experiment the impact‐ionization cross section, integrated over all angles and energy partitions but excluding positronium formation, has been measured for positrons as well as electrons in the energy region of 30 to 600 eV. Below 400 eV the positron cross section is larger than the electron one, at 40 eV by about a factor of 2. Absolute cross sections are obtained by comparing the electron data with literature values. Our cross sections for positron‐impact ionization lie above all theoretical predictions; within the errors they barely agree with the distorted‐wave polarized‐orbital calculations of Ghosh et al. for the model in which the scattered positron is described by a plane wave.

Ionisation of Atomic Hydrogen by Positron Impact

We have built a crossed beam apparatus to measure the relative ionisation cross sections of atomic hydrogen by positron impact. A schematic diagram is given in Fig. 1.

Measurement of the Impact Ionisation and Positronium Formation Cross Sections for Positron Scattering on Molecular Hydrogen

We measured the impact ionisation and positronium-formation cross sections for positron scattering on molecular hydrogen with the same experimental set-up used for our measurements on helium.1 As in the case of helium the positron impact-ionisation cross section (not including positronium formation) exceeds the corresponding electron cross section2 in the energy range around the respective maxima (Fig. 1). It is lower than the electron cross section right above ionisation threshold. This is in accordance with the Wannier threshold law, as described earlier.3

The Brookhaven Positron-Hydrogen Scattering Experiment: Experimental Set-Up

The aim of this experiment is to study the positron impact ionisation of atomic hydrogen with and without positronium formation. It will be run by a collaboration of members of the Brookhaven Consortium, the City College of CUNY, St. Patrick’s College at Maynooth and the University of Bielefeld.1 The Bielefeld positron group will provide the scattering chamber and the experimental set-up to measure the cross sections.

Production of Short-Lived Positron Sources

In all fields where low-energy positrons are employed, high intensities are desirable. The low-energy positrons originate from high-energy positrons that are moderated in solids. During the last decade the intensity of moderated positron beams has risen from less than one to 109 e+/s. There are two ways to increase the intensity, either by having a higher flux of high-energy positrons hit the moderator or by using more efficient moderators. In this text we will only deal with the first option.

Comparison of Impact-Ionisation and Charge-Transfer Cross Sections for Positron and Proton Scattering on Helium

Cross sections for impact ionisation of helium by positrons and protons as well as charge transfer cross sections for positron-helium and proton-helium scattering are usually compared at equal velocities of the incident projectiles (vi) because the size of the cross sections depends on the duration of the interaction between projectile and target particle. The left parts of Figs. 1 and 2 show our measured values1 of cross sections for impact ionisation and charge transfer and literature values2-9 for the corresponding proton scattering processes. The charge transfer cross sections for positron and proton scattering on Helium intersect at vi = 4 x 106 m/s; at high velocities σPs is significantly greater than σH.

APPLICATIONS OF INTENSE POSITRON BEAMS IN ATOMIC PHYSICS EXPERIMENTS

The aim of this talk is to summarize the experiences with different positron sources and to point out which experiments will be possible in the near future. In this context I shall restrict myself to atomic physics experiments.