New results from the GSI REACTION-MICROSCOPE

After many years of intense research the dynamical response of quantum-mechanical many-particle systems perturbed by an external time dependent force still remains as one the most fundamental and challenging problems in physics. Among them the three-body Coulomb problem represents the most prominent example. It can be studied in great detail in atomic collisions and ionization of simple atoms by charged particle impact can be viewed as a prototype of a three-particle problem. In the case of electron-impact ionization the first exact theoretical treatment was published just recently [1] for the simplest system, namely ionization of atomic hydrogen, more than thirty years after the pioneering experiments of Ehrhardt [2]. The possibility to obtain fully differential ionization cross section data with these kinematically complete so called (e,2e) experiments is indispensable for the progress achieved in theory. In the case of ion-impact ionization a large data set was collected in numerous experimental investigations since the first measurements by Rudd and coworkers in the sixties [3]. But, and in contrast to (e,2e) measurements, only total cross sections or those differential in the ejected electron energies and angles have been measured. The measurement of projectile scattering angles, and thus of the transferred momentum during the collision, were possible only for the very specific situation of unlikely large projectile deflections. Based on this limited experimental information the continuum distorted wave (CDW) theories have proven to be remarkably successful representing the theoretical basis for charge transfer and for ionization in fast ion-atom collisions [4]. In this report we summarize recent experimental results obtained for single ionization of helium and for simultaneous ionization of both, projectile and target in fast ion-atom collisions. Both experiments have been performed at the UNILAC using the GSI REACTION-MICROSCOPE [5] to collect for the first time kinematically complete data sets for ion impact ionization with sufficient statistics to allow an unsurpassed test of ionization theories.