Forward-backward correlation and its incident energy dependence in secondary-electron emission from a thin carbon foil upon proton penetration

Abstract

The statistical distributions of the number of simultaneously emitted secondary electrons (SEs) from a carbon foil have been measured with proton beams of $0.5\ensuremath{-}3.5$ MeV. In this experiment, the forward- and backward-emitted SEs have been measured simultaneously with foil-transmitted protons using a digitizer. As a method to examine how the forward and backward SE emissions correlate to each other, the forward (backward) SE yields ${\ensuremath{\gamma}}_{\mathrm{F}}$ (${\ensuremath{\gamma}}_{\mathrm{B}}$), that is, the mean number of the forward-emitted (backward-emitted) electrons per projectile, have been evaluated as a function of the number of the backward-emitted (forward-emitted) SEs, ${n}_{\mathrm{B}}$ (${n}_{\mathrm{F}}$). At higher incident energies, ${\ensuremath{\gamma}}_{\mathrm{F}}$ (${\ensuremath{\gamma}}_{\mathrm{B}}$) increases with increasing ${n}_{\mathrm{B}}$ (${n}_{\mathrm{F}}$). With decreasing incident energy, this so-called positive correlation becomes weaker and then changes to negative at the lowest incident energy. Although measurements using a slightly thicker foil exhibit just the same trend, the correlation changes from positive to negative at the higher incident energy. For a given foil thickness, the range of the produced binary electron and hence the incident proton energy seems to determine the sign of the correlation. A simple Monte Carlo simulation for the forward and backward SE emission in the present experimental condition can qualitatively reproduce the observed incident-energy dependence of the positive correlation but cannot reproduce the negative one observed at the lower incident energies.