Bombarding insulating foils with highly energetic ions

Insulating (MYLAR), semi-insulating (MYLAR-Au) and conducting foils have been bombarded by very energetic 64 MeV u-178Kr32+ ions. The velocity spectra of fast electrons emitted in the backward and forward directions have been measured and analyzed as a function of the elapsed time in the run. A shift of binary encounter and convoy electrons emitted in the forward direction toward lower velocities has been observed with insulating targets. No such shift occurs with metallic targets. The surface potential evolves with time (i.e. ion fluence) both at forward and backward emission angle. It is shown that strong bulk charging of insulating targets leads to a positive potential as high as 9 kV before charge breakdown. (authors)