Charging of insulators by multiply-charged-ion impact probed by slowing down of fast binary-encounter electrons

The interaction of ion beams with insulators leads to charging-up phenomena, which at present are underinvestigation in connection with guiding phenomena in nanocapillaries with possible application in nanofocusedbeams. We studied the charging dynamics of insulating foil targets [Mylar, polypropylene (PP)] irradiated withswift ion beams (C, O, Ag, and Xe at 40, 23, 40, and 30 MeV/u, respectively) via the measurement of theslowing down of fast binary-encounter electrons. Also, sandwich targets (Mylar covered with a thin Au layeron both surfaces) and Mylar with Au on only one surface were used. Fast-electron spectra were measured bythe time-of-flight method at the superconducting cyclotron of Laboratori Nazionali del Sud (LNS) Catania. Thecharge buildup leads to target-material-dependent potentials of the order of 6.0 kV for Mylar and 2.8 kV forPP. The sandwich targets, surprisingly, show the same behavior as the insulating targets, whereas a single Aulayer on the electron and ion exit side strongly suppresses the charging phenomenon. The accumulated numberof projectiles needed for charging up is inversely proportional to electronic energy loss. Thus, the charging up isdirectly related to emission of secondary electrons.