The beam of very low energy antiprotons which can be created in the ASACUSA beam line offers a unique opportunity to investigate the processes which lead to ionization and excitation in collisions between charged particles and atoms/molecules in general. This is because antiprotons do not capture electrons, as positive particles do, and because they have still ample energy to excite/ionize even in collisions where their velocity is much slower than those of the target electrons.
This means that we can create benchmark data with which we can test atomic collision theory, especially for quasi-adiabatic collisions. The basic model of the theory is simple: We regard a few point-like, charged particles which interact via the Coulomb force. The complication comes from the facts that we have to treat a dynamically evolving system, and that the atomic electrons are subject to strong correlation. The resulting many-body problem is not yet solved to a satisfactory accuracy: Often, total ionization cross sections cannot be calculated to better than 30%.
We have measured total cross sections for single and multiple ionization for a number of targets, especially helium and molecular hydrogen, and much progress has been achieved. An example is shown in the second figure. Here we show the total single ionization cross section measured by our group at the LEAR (black points) and the AD (blue points) accelerators, compared with a few of the plethora of theoretical calculations which have been published during the last decade. Clearly, our data are able to distinguish between some of the calculations, but it would be nice with a more selective measurement.
We are now trying to move forward in that direction, in that we would like to be able to measure highly differential ionization cross sections. For this, we propose to build a -Y´Reaction Microscope¡ into the structure of the coming Extra Low ENergy Accelerator ELENA, at CERN’s AD complex: In the third figure, such a reaction microscope is shown in principle, and its suggested position in the ELENA structure.
We should then be able to measure differential cross sections like those shown in the fourth figure, which are calculated by McGowern et al PRA 79 042707 (2009) for single ionization of helium by 3 keV antiproton impact. Please note the two very different results which, however, when integrated give approximately the same total cross section. (Mesh: First Born, red: Advanced model).