Backward electron emission from carbon and gold foils under impact of MeV Au+n clusters

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Gold cluster beams delivered by the 2.5 MV Van de Graaff accelerator of the Institut de Physique Nucléaire de Lyon, equipped with a liquid metal ion source (LMIS), were used for secondary electron emission measurements. We investigated backward emission from thin carbon and gold targets bombarded by Au+n projectiles (1⩽n⩽9) of energies between 150 and 500 keV/atom in a standard vacuum of 10−7 mbar. For Au+ ions incident on carbon and gold targets it is observed that the electron yield follows the velocity dependence of the energy deposited at the surface of the target in ionization processes induced both by the incident projectile and by the recoiling target atoms. For Au+n clusters of a given velocity the electron yield per incident gold atom is observed to decrease when the cluster size increases. Given the large contribution of the recoil atoms to the electron emission, we studied the dependence of the cluster effect on the target thickness, for thicknesses of the order of the size of the atomic displacement cascades. The comparison of the electron yields of thin carbon targets of various thicknesses at the impact of Au+ and Au+n projectiles of the same velocity has effectively evidenced a thickness effect for cluster induced electron emission.