Average charge and its structure dependence of fragment ions under irradiation of a thin carbon foil with a 1-MeV/atom C3+ cluster ion

The charge states of fragment ions were measured for 1-MeV/atom C-3(+) cluster ions penetrating a carbon foil of 2.4-mu g/cm(2) thickness. Those ions were detected by event-by-event mode with a fluorescent screen detector monitored by a charge-coupled device camera. The detected ion signals were traced by Coulomb explosion imaging technique, which enables us to classify charge states of each fragment ion and spatial arrangements of aligned cluster ions after emerging from the foil. Using this technique, we found the reduction of the average charge per ion. We also found that (i) the measured average charges of the aligned fragment ions in a linear-chain arrangement are larger than those in a triangular arrangement, and (ii) the edge-position ions in a linear-chain arrangement have the average charges greater than the middle-position ion. These experimental results are well supported by a recent refined theory on the cluster average charge.