Single and Double Ionization of Helium by Fast Antiproton and Proton ImpactLars H. Andersen, P. Hvelplund, H. Knudsen et al.|Physical Review Letters|1986 The first ion-atom-collision data obtained with antiprotons are presented. We measured the single- and double-ionization cross section for 0.5-5-MeV antiprotons and protons colliding with helium. For ion energies above \ensuremath{\sim} 2 MeV, the single-ionization cross section is the same for protons and antiprotons. However, surprisingly, the double-ionization cross section for antiprotons is approximately a factor of 2 larger than that for protons. The present data constitute a challenge for future theoretical models of charged-particle-atom collisions.
Single-electron-capture cross section for medium- and high-velocity, highly charged ions colliding with atomsH. Knudsen, H. K. Haugen, P. Hvelplund|Physical review. A, General physics|1981 Experimental cross sections for single-electron capture for medium- to high-energy, highly charged oxygen and gold ions colliding with helium atoms are presented. Simple estimates based on the classical cross sections introduced by Bohr and Lindhard for multiply charged ions are shown to lead to good first approximations of the absolute magnitude of the cross sections for any target atom. Furthermore, these estimates suggest a universal scaling of the capture cross sections with ion and target-atom parameters. The experimental results of this work as well as those obtained by other groups are shown to be in accordance with the suggested scaling laws, which are also supported by the results of the few more-refined theoretical treatments that have been published.
Ionization of atoms by particle and antiparticle impactH. Knudsen, J. F. Reading|Physics Reports|1992 Ionization of helium and molecular hydrogen by slow antiprotonsP. Hvelplund, H. Knudsen, U. Mikkelsen et al.|Journal of Physics B Atomic Molecular and Optical Physics|1994 Measurements of the cross sections for single and double ionization or helium as well as for the creation of H<sup>+</sup><sub>2</sub> and H<sup>+</sup> ions from H<sub>2</sub> for impact of antiprotons in the energy range 13-500 keV are presented. The results are compared with our earlier, less accurate data and with data for equivelocity proton impact. The single ionization cross section of helium agrees remarkably well with a continuum distorted wave (CDW-EIS) calculation. The ratio between the double and the single ionization cross sections of helium increases dramatically with decreasing projectile energy. New theoretical calculations are called for.
Experimental evidence of quantum radiation reaction in aligned crystalsQuantum radiation reaction is the influence of multiple photon emissions from a charged particle on the particle's dynamics, characterized by a significant energy-momentum loss per emission. Here we report experimental radiation emission spectra from ultrarelativistic positrons in silicon in a regime where quantum radiation reaction effects dominate the positron's dynamics. Our analysis shows that while the widely used quantum approach is overall the best model, it does not completely describe all the data in this regime. Thus, these experimental findings may prompt seeking more generally valid methods to describe quantum radiation reaction. This experiment is a fundamental test of quantum electrodynamics in a regime where the dynamics of charged particles is strongly influenced not only by the external electromagnetic fields but also by the radiation field generated by the charges themselves and where each photon emission may significantly reduce the energy of the charge.