Analysis of the projectile track charging by H+ secondary ion velocity distribution measurements

Secondary ions, emitted from LiF, Al (covered with layers of Al2O3) and Au bombarded by a MeV argon (25 keV/u) beam, are analyzed by a XY-TOF detection system. This new method allows, for each emitted ion, the simultaneous measurement of its time-of flight (TOF) and its impact coordinates (XY) on the detector surface, after acceleration by a homogenous electric field. Angular distributions and initial velocities for atomic H+ secondary ions are determined as a function of the projectile impact angle θp (37° up to 78°, with respect to the surface normal) and the electrical surface target properties. Emission of low velocity H+ ions (v0<30 km/s) occurs preferentially close to the line of incidence of the projectile. Their initial velocity distribution depends clearly on the target electrical conductivity. At constant θp, the maximum of the velocity distribution, v0max, is found to be 26.5, 27.9 and 32.5 km/s for Au, Al and LiF targets, respectively. For Au target, no clear dependence on the impact angle θp was found. However, increasing θp from 37° to 78° v0max shifts from 27.9 to 30 km/s for Al and 32.5–35.5 km/s for LiF. This shift is explained by a transient positive charging up of the nuclear track and is an indication of the track potential decay. The dependence of v0 on θp is explained by the variation of the length of the track inside the insulator layer, once neutralization time is assumed to be dependent on the electron current inside the track of the insulator layers from a conducting substrate.