Monte Carlo simulation of the charge distribution induced in an insulator by a low energy electron beam

In an insulating target bombarded by a primary electron beam, a space charge develops due to the stabilisation in traps of electrons and holes. For the primary energies considered (a few hundreds of eV), the global charge of the target is positive. The secondary electrons emitted at low energies are attracted back to the surface. Monte Carlo simulations show that a self-regulation effect takes place, where the total secondary yield tends to the unit and the surface potential stabilises at a low positive value. Calculations performed for different sets of physical parameters indicate that this behaviour is quite general. Moreover, one observes the formation of a ring of negative charges around the primary beam impact zone, caused by the electrons driven back to the surface.