The statistical distributions of the number of simultaneously emitted secondary electrons (SE’s) from a thin carbon foil induced by the frozen-charged ${\mathrm{H}}^0$ and ${\mathrm{H}}^{+}$ 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 ${\mathrm{H}}^0,$ the average SE yields per projectile at the forward direction, ${\gamma }_{\mathrm{F}},$ and at the backward direction, ${\gamma }_{\mathrm{B}},$ are significantly smaller than the corresponding ones for ${\mathrm{H}}^{+}$ due to the screening effect of its bound electron. In addition to the suppression of low-energy electron production for ${\mathrm{H}}^0,$ the preferential forward emission of high-energy electrons makes the proton-hydrogen difference in the ratio of ${\gamma }_{\mathrm{B}}$ to the stopping power more striking. Although the probability of simultaneous n electron emission per unit projectile, $W_n,$ for ${\mathrm{H}}^0$ is significantly smaller than that for ${\mathrm{H}}^{+}$ 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 ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^0$ can be well reproduced by a Poisson distribution with a mean equal to the difference of their ${\gamma }_{\mathrm{F}}$ or ${\gamma }_{\mathrm{B}}$ values and the validity of the above-mentioned interpretation on the proton-hydrogen difference of the SE emission is quantitatively confirmed.

• Received 17 January 2002

DOI:https://doi.org/10.1103/PhysRevA.65.052902