Forward–backward correlation in secondary electron emission from a thin carbon foil by frozen-charged and penetration

The statistical distributions of the number of simultaneously emitted secondary electrons (SE’s) from a thin carbon foil have been measured with frozen-charged H 0 and H + projectiles of 2.5–3.5 MeV. In this experiment, the forward- and backward-emitted electrons have been measured simultaneously together with foil-transmitted particles using a list-mode data taking system in order to investigate their correlation. From the two-dimensional spectra of the emitted SE’s, we have derived the probability distributions of the SE emission, P ( n f , n b ) , where n f and n b represent the number of SE’s emitted simultaneously in the forward and backward directions, respectively. Compared with those for the H + penetration, the emission probabilities for the H 0 penetration exhibit quite different n f - and n b -dependence. Firstly, P ( n f = 0 , n b = 0 ) takes a remarkably large value. Secondly, among P ( n f , n b ) ’s with n f + n b = ( constant ) , P ( n f ≠ 0 , n b = 0 ) and P ( n f = 0 , n b ≠ 0 ) become the largest and the second largest probabilities. In contrast, such events that SE’s are emitted in both of the forward and backward directions happen with significantly smaller probabilities. These trends are common to all projectile energies measured and seem to be explained by the predominance of the high energy internal SE production for the H 0 penetration due to the screening effect of its bound electron.