Energy loss ofC12projectiles in different carbon modifications

The stopping cross sections of the three carbon modifications diamond, graphite, and glassy carbon are investigated for carbon projectiles of intermediate velocity. The inverted Doppler-shift attenuation method was used as the experimental technique, and it enabled us to measure the ratios of the three stopping cross sections precisely over a wide energy range. For velocities between 3 and 4 times Bohr’s velocity the stopping cross sections of graphite and glassy carbon are found to be 1.036 and 1.072 times larger than that of diamond, respectively. These differences are attributed to binding effects. To understand these effects, we have evaluated the mean ionization potentials utilizing the local-plasma approximation for the inner-shell electrons and the dielectric response function for the valence electrons. The theoretical ratios calculated by inserting these potentials into the Bethe-Bloch stopping-power formula agree well with our experimental results. Furthermore, we have obtained a value of 53.3±4.1 fs for the lifetime of the first excited state of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ nucleus.