Strong and electromagnetic interactions in heavy exotic atoms

The level energies of the lower atomic orbits of negatively charged hadrons (e.g. pions) are shifted with respect to the values following from the electromagnetic interaction alone as a consequence of the fact that for orbits closer to the nucleus the strong interaction becomes progressively more important. Also the pions have a chance of being captured by the nucleus that drastically increases with decreasing quantum number, which results in a broadening of the levels. By inserting in the Klein-Gordon equation for the pion-nucleus system an optical potential representing the strong interaction, one attempts to describe these shifts and widths. Experimental results for the pionic 3d orbit in some heavy nuclei indicate that for them strong interaction widths are a factor of two smaller than predicted. Similar deviations have been observed for the strong interaction shifts. In order to check and investigate these discrepancies the pionic X-ray spectra of 181Ta, natRe, natPt, 197Au, 208Pb and 209Bi have been measured to obtain data which provide for a systematical investigation of the pionic 3d levels. Special attention was devoted to methods of obtaining clearly improved results: large volume high-purity Ge detectors in combination with Compton suppression BGO shields were used; good statistics were obtained by using the very intense pion beam; background radiation in the Ge detectors caused by neutrons produced in the targets has largely been reduced by using neutron TOF discrimination with respect to the incident stopped pion. By adding an isovector term to the optical potential the deviation from the theory can be explained. 100 refs.; 42 figs.; 37 tabs