R. D. DuBois

Differential ionization studies for positron impact

Recent studies of differential ionization by positron impact being performed at the University of Missouri-Rolla are described. Recoil ion-scattered positron and recoil ion-scattered positron-ejected electron coincidences are measured to provide doubly and triply differential ionization data (DDCS and TDCS) for e+ - Ar collisions. The DDCS and TDCS studies are for 750 and 200 eV impact respectively. For the TDCS studies the positron scattering angles are between approximately ± 5o and electrons ejected with energies less than 15 eV and angles between 90o ± 50o with respect to the beam direction are detected. Examples of TDCS data for the scattered positron and the ejected electron are presented.

Electron- and Proton-Impact Excitation in Stored Hydrogenlike Uranium Ions

The projectile excitation of high-Z ions has been investigated in relativistic ion-atoms collisions by observing the subsequent x-ray emission. Within this experiment information about the population of the magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. In addition, we have observed for the first time the effect of electron-impact excitation in a heavy highly charged ion undergoing a collision with a neutral atom.

Differential Measurements of Ionization and Excitation of H2 by 250 eV Positrons

Ejected electron-recoil ion and scattered projectile-recoil ion coincidences are measured for 250 eV positron impact on molecular hydrogen. The electron detector is also sensitive to photon emission. Various combinations of these coincidence spectra are used to extract differential information about ionization and excitation of H2.

Discharge properties of macroscopic glass capillaries after irradiation by low intensity low energy single charged Ar ions

The discharge properties of a macroscopic cylindrically shaped glass capillary, initially charged by a 1 keV Ar+ beam tilted by 5° whith respect to the capillary axis is studied. Our experimental observations were compared to our Monte Carlo simulation, higlighting the importance of the surface conductivity.

Electron- and proton-impact excitation of He-like uranium

In this contribution, we present an experimental and theoretical study of the proton- and electron- impact excitation effects in helium-like uranium ions in relativistic collisions with different gaseous targets. The experiment was carried out at the experimental storage ring at GSI Darmstadt. Our experimental results are in agreement with predictions of the state-of-the art theory which treats both processes within the relativistic framework.

Differential Cross Sections for Ionization of Argon by 1 keV Positron and Electron Impact

Differential information was generated by establishing coincidences and imposing conditions on data recorded for target ions, scattered projectiles, and ejected electrons, as a function of projectile energy loss and scattering angles; in order to describe the interaction between a positron (electron) 1 keV beam and a simple Ar jet. Single ionization triply differential cross section (TDCS) results exhibit two distinct regions (lobes) for which binary (events arising from 2-body interaction) and recoil (events which can only be produced by many-body interactions) interactions are associated. Results indicate that binary events are significantly larger for positron impact, in accordance with theoretical predictions. A similar feature is found for different energy losses and scattering angles. Intensity of the recoil lobe for both projectiles, positron and electron, is observed to depend on the energy loss and scattering angle. Also, it can be noticed that for positron impact the recoil interactions intensity is larger than that observed for electron impact.

Triply Differential Measurements for Single Ionization of Molecular Nitrogen by 250 eV Positron Impact

Triply differential cross sections are presented for 250 eV positron impact single ionization of molecular nitrogen. Data are presented for binary and recoil interactions resulting from projectiles being scattered between 0 and 5 degrees and energy losses up to approximately 50 eV.

Triply differential measurements for 200 eV e+ – and e- – Ar collisions

Triply differential single ionization yields for projectile scattering and electron emission are presented for 200 eV positron and electron impact ionization of argon. The projectile scattering and the electron emission are observed to be different for positron and electron impact.

1st and 2nd order contributions to the double ionization of argon resulting from positron and electron impact

Angular dependences for electron emission resulting from single and double ionization of argon are presented for 200, 500, and 1000 eV positron and electron impact. From these data, double to single ionization ratios are determined as a function of the emission direction. At each impact energy, these ratios have different angular dependences positive and negative particle impact. The positron and electron induced ratios are analyzed to provide information about how first- and second-order double ionization mechanisms interfere and contribute to the differential electron emission.

Triply differential ionization of Ar by 500 eV positron and electron impact

Coincidences between recoil ions-ejected electrons and recoil ions-scattered projectiles have been used to study the kinematics of electron and positron impact ionization. Triply Differential (TDCS) data for 500 eV positron and electron impact on Ar are presented here as function of scattering angle for a given range of energy losses. Binary and recoil interactions can be distinguished allowing us to determine the relative intensity between those interactions. Preliminary integration of the data indicate an enhancement of the binary region for positron interaction while for electron impact the intensity of the recoil and binary interactions is comparable.