On the double ionization of helium by very slow antiproton impact

Here we present experimental information on the cross section for double ionization of helium atoms by slow antiproton impact. It is used to discern between many advanced theoretical calculations. Earlier measurements of the ratio R between the double and single ionization cross sections for antiproton impact on helium show a persistent increase for the projectile energy decreasing from 10 MeV to 10 keV. The present data show that below 10 keV this increase stops and we give an upper limit to R.