U. Brinkmann

MULTIPLE IONIZATION OF ARGON, KRYPTON AND XENON ATOMS BY POSITRON-IMPACT.

Cross section ratios sigma 2+/ sigma + for double and sigma 3+/ sigma + for triple relative to single ionization of argon, krypton and xenon atoms by positron impact were measured from threshold up to 1000 eV. All sigma 2+/ sigma + ratios exhibit double peak structures of which the high-energy structure is attributed to inner-shell processes, in agreement with theoretical estimates.

Double ionisation of rare gas atoms by positron impact

The ratio of the double to single ionisation of the rare gases Ne, Ar, Kr and Xe by positron impact was measured for projectile energies from close to threshold up to 600 eV.

Multiple and inner-shell ionization by positron and electron impact

Recent results for the K- and L-shell ionization of silver and gold, respectively, and the multiple ionization of argon, krypton, and xenon atoms by positron and electron impact will be discussed. The measured cross section ratios for triple relative to single ionization by positron impact decrease more rapidly towards threshold compared to electron impact. This behavior is similar compared to results obtained for the inner shell ionization of silver and gold, and attributed to the acceleration or deceleration which incident electrons or positrons, respectively, undergo in the target nuclear field.

Inner shell contributions to multiple ionization of argon by positron impact

Relative cross sections for double and triple ionization of argon by positron impact are reported. The measured cross section ratios for double and triple relative to single ionization display multiple peak structures of which the high-energy maxima are attributed to inner-shell processes.

Recent experiments at the Giessen slow positron source TEPOS

Abstract Recent results for the K- and L-shell ionization of silver and gold, respectively, by positron and electron impact and for the energy loss of 6–20 keV positrons in thin foils obtained at the Giessen slow positron source TEPOS will be discussed. The measured cross section for inner-shell ionization by positron impact decreases more rapidly towards threshold compared to electron impact. This behaviour is confirmed by theoretical calculations performed in the plane wave Born approximation which include an electron exchange term and take the deceleration or acceleration of the incident positron or electron, respectively, in the target nuclear field into account.

Observation of convoy electrons induced by Rutherford-scattered protons

Abstract We have measured the spectrum of convoy electrons emitted from a 200 μg/cm 2 Au foil in the 54° direction with respect to the 200 keV p + incoming beam. The electrons were detected in coincidence with the projectile scattered in the same direction. In the coincident 54° electron spectrum the convoy-electron peak appeared at 83 ± 1eV electron energy, while in the 0° single spectrum it appeared at 96 eV. If we attribute this effect to the larger energy loss of the projectile scattered in the foil at large angles compared with those penetrating through the foil without any deflection, a 5 eV shift in the electron spectrum can be explained by the longer path of the protons in the foil. A further 0.2 eV shift is due to energy transfer to the target atom. It is an open question, however, how to explain the remainder of the energy shift in the coincident 54° electron spectrum.

Experiments at the Giessen source TEPOS with low energy positrons

The TEPOS facility at the Giessen LINAC delivers intense positron beams in the energy range between some eV and 6 keV; with postacceleration up to 80 keV. Results for remoderation and positron storage will be discussed. Further the energy-loss of positrons in thin aluminium foils at incident energies of 6–20 keV was measured. Cross sections for K- and L-shell ionization of thin silver and gold targets by positron and electron impact were determined at projectile energies of 30–70 keV. The experimental results are presented in detail; they are confirmed by calculations in plane wave Born approximation (PWBA) which include an electron exchange term and take into account the deceleration or acceleration of the incident projectile in the nuclear field of the target atom.