Backward- and forward-electron-emission measurements for MeVH0projectiles incident on thin carbon foils: Correlation with the charge state of the emergent projectile

Abstract

We have performed projectile-by-projectile measurements of backward and forward secondary-electron emission of thin carbon foils under impact of MeV ${\mathrm{H}}^{0}$ projectiles. The emitted electrons were detected in coincidence with the protons or the neutrals emerging from the target. We have used a very thin target for which we know that, at energies above 2 MeV, the emergent neutrals are essentially transmitted, i.e., they have kept their electron throughout the target. In these conditions the emission yields measured in coincidence with emergent neutrals are found lower than for protons of the same velocity, but the reduction factor is not the same for backward and for forward emission. We show that this can be explained by the screening of the proton charge by the electron during the ${\mathrm{H}}^{0}$-target interaction. We have observed other effects related to forward electron emission: if the ${\mathrm{H}}^{0}$ projectile emerging from the foils results from an electron-capture event taking place close to the exit surface, the forward emission is enhanced (at energies above 500 keV) by the contribution of Auger electrons resulting from the rearrangement of the carbon atoms ionized in the capture events. For ${\mathrm{H}}^{0}$ projectiles ionized in the target we have used the statistics of the number of forward-emitted electrons to deduce the probability for an incident electron to be transmitted through a very thin target and to produce cascade electrons.