Ionisation of Atomic Hydrogen and Helium by Low Energy Antiprotons

Lead Research Organisation: Swansea University
Department Name: College of Science


Experimental studies of collision processes, in which fundamental charged particles ionize simple atoms and molecules, provide the data necessary for our understanding of the many-body interaction and the development a theoretical description of this fundamental yet unsolved problem in collision dynamics. The ionization process plays an important role in astrophysical and technological plasmas and in the dissociation of molecules in the Earth's atmosphere. A comparison of single and multiple ionization of simple atomic and molecular targets by equi-velocity protons, electrons and their corresponding antiproton and positron antiparticles makes it possible to determine the effects of projectile mass and charge. The simplest collision system for testing theory is the single ionization of atomic hydrogen by antiprotons. The system has only one active electron, a precisely known wave function and, unlike in the electron impact case, there are no exchange effects. In addition, the antiproton cannot capture the hydrogen electron and straight-line constant velocity projectile paths may be used. The cross section for single ionization of atomic deuterium (no isotope effects expected) was measured in the energy range 30 keV / 1 MeV at the Low Energy Antiproton Ring (LEAR) at CERN by the PS194 collaborative grouping lead by Knudsen and co-workers from the University of Aarhus and including Prof. M Charlton (one of the present applicants). It was found that for antiproton velocities larger than the target electron velocity (2.2 x 108 cm s-1 or 25 keV antiproton energy) the experimental data and all theories are in agreement, as might be expected. However, at lower energies there are significant discrepancies. Most of the theories show an almost energy independent trend, as predicted by the Fermi-Teller mechanism, in which the collision is of a quasi-molecular nature and the target electron binding energy vanishes at a certain critical distance. Experimental measurements are required urgently in the energy regime below 30 keV to determine the magnitude and energy dependence of the cross section for this most fundamental of collision systems


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Capra A (2016) Limit on the electric charge of antihydrogen in Hyperfine Interactions

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Indelicato P (2014) The Gbar project, or how does antimatter fall? in Hyperfine Interactions

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Jonsell S (2016) The role of antihydrogen formation in the radial transport of antiprotons in positron plasmas in Journal of Physics B: Atomic, Molecular and Optical Physics

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Knudsen H (2008) Ionization of helium and argon by very slow antiproton impact. in Physical review letters

Description The total cross sections for single ionization of helium and single and double ionization of argon by antiproton impact have been measured in the kinetic energy range from 3 to 25 keV using a new technique for the creation of intense slow antiproton beams.
Exploitation Route The new data provide benchmark results for the development of advanced descriptions of atomic collisions and we show that they can be used to judge, for the first time, the validity of the many recent theories.
Sectors Other

Description Our findings have been used by many scientists to justify theoretical and experimental work in number of areas of basic physics. Organisations such as CERN and our institutions have used our work to promote their respective missions, and in the public understanding of science.
First Year Of Impact 2007
Sector Other
Impact Types Cultural

Description University of Arhus 
Organisation Aarhus University
Country Denmark 
Sector Academic/University 
Start Year 2009
Description University of Tokyo 
Organisation University of Tokyo
Country Japan 
Sector Academic/University 
Start Year 2009