Forward-backward correlated secondary electron emission depending on the emergence angle of 1Â MeV/u light ions

The statistical distributions of the number of forward- and backward-emitted secondary electrons (SE's) from a thin carbon foil have been measured simultaneously in coincidence with foil-transmitted 1H+, 4He2+ and 6Li3+ at a fixed velocity of 1 MeV/u. The emergence angles of the projectiles, θem, were varied from 0 to 2 [mrad] for H+ and from 0 to 1 [mrad] for He2+ and Li3+ ions. Irrespective of ion species, measured total SE yields, that is, the average number of simultaneous emitted electrons per projectile, increase with increasing θem both in the forward and backward directions. This trend is consistent with the calculated θem-dependent energy losses of protons penetrating a thin carbon foil. As for the forward- or backward-emitted SE yields, we have obtained a negative forward-backward(F-B) correlation, except for the θem=0.0 [mrad] emergence of H+ projectiles where a positive correlation has been observed. The negative F-B correlation becomes prominent with increasing the atomic number of the projectile, Zp. In addition, this correlation appears to become slightly stronger with increasing θem. The observed F-B correlation and its Zp- and θem-dependences are interpreted from the point of view of production of high energy internal SE's.