Secondary-electron emission from thin carbon foils by H⁰ and H⁺ in frozen-charge states

The statistical distributions of the number of simultaneously emitted secondary electrons (SE’s) from a thin carbon foil induced by the frozen-charged H⁰ and H⁺ projectiles of 2.5–3.5 MeV have been measured by using the coincidence technique with the foil-transmitted particles. The measurement was carried out at the forward and backward directions of the incident beam separately. For frozen-charged H⁰, the average SE yields per projectile at the forward direction, γ_F, and at the backward direction, γ_B, are significantly smaller than the corresponding ones for H⁺ due to the screening effect of its bound electron. In addition to the suppression of low-energy electron production for H⁰, the preferential forward emission of high-energy electrons makes the proton-hydrogen difference in the ratio of γ_B to the stopping power more striking. Although the probability of simultaneous n electron emission per unit projectile, W_n, for H⁰ is significantly smaller than that for H⁺ at small n, their difference decreases with increasing n both at the forward and backward directions. This behavior of W_n also suggests that there is not a large proton-hydrogen difference in the production of high-energy electrons. As a result of a simple model calculation, the difference between the emission statistics by H⁺ and H⁰ can be well reproduced by a Poisson distribution with a mean equal to the difference of their γ_F or γ_B values and the validity of the above-mentioned interpretation on the proton-hydrogen difference of the SE emission is quantitatively confirmed.