Vicinage effect on secondary-electron emission in the forward direction from amorphous carbon foils induced by swift C ions

Takahashi, Yasuyuki; Narumi, Kazumasa; Chiba, Atsuya; Saito, Yuichi; Yamada, Keisuke; Ishikawa, Norito   ; Sugai, Hiroyuki; Maeda, Yoshihito

C and C ions with 62.5-250 keV/u were incident on self-supporting amorphous carbon foils of 1.4-150 g/cm (70-7500 ). The secondary electrons emitted in the forward direction from a carbon foil were detected by a microchannel-plate detector placed at the exit side of the target. The vicinage effect on the secondary-electron yield was evaluated with the ratio of the secondary-electron yield R = /2, where and are the yields induced by the C and C ion with the same velocity, respectively. For the first time the disappearance of the vicinage effect on the secondary-electron yield from amorphous carbon foils bombarded with 62.5 keV/u C ions was observed for thick foils of 61-150 g/cm. The internuclear distance between the fragment ions at the exit of the target was evaluated by calculating trajectories of the fragment ions considering the Coulomb explosion. For a 62.5 keV/u C ion, we have determined the threshold internuclear distance where the vicinage effect disappears exits between 6 and 23 . It is expected that the vicinage effect on the energy loss (production process) in this velocity region disappears at the internuclear distance of a few . This result means that the transport or transmission process is important for the appearance of the vicinage effect. Moreover, the threshold internuclear distance depends on the velocity of the ion and increases as the velocity increases. The average charge of the ion increases with increase of the velocity of the ion. These mean that there is a possibility that a charge state plays an important role in the origin of the vicinage effect. In order to account for the experimental results, we discussed two models taking account of two kinds of potentials induced in response to the charge of the fragment ion in the transport process.