Dependence of secondary electron emission on the emergent angle of 1 MeV/u H+, He2+ and Li3+ penetrating a thin carbon foil

Abstract

With 1MeV/u H+, He2+ and Li3+ beams incident on a carbon foil of 2.2μg/cm2 in thickness, the statistical distributions of the number of simultaneously emitted secondary electrons (SE’s) have been measured as a function of the emergent angle of foil-transmitted ions in the range from 0.0mrad to 4.0mrad for every 0.5mrad step. The measurement of SE’s was carried out in the forward and backward directions of the incident beam independently. The SE yields, that is, the mean number of emitted electrons per incident ion, increase with increasing the emergent angle and saturate at larger angles. This trend is common to the projectiles used and also to the forward and backward directions. As a result of a Monte Carlo simulation taking account of the impact parameter dependent energy loss in a single collision of a proton with a carbon atom, it is found that the calculated energy losses exhibit an emergent angular dependence quite similar to that of the measured SE yields. However the measured SE yields divided by the square of the projectile atomic number, ZP, becomes smaller with increasing ZP, especially for the backward emission. A similar observation has been reported by other authors and this trend seems to originate from the preferencial fast internal SE production by heavier projectiles followed by their escape from a thin target foil.