ENERGY LOSS AND ENERGY STRAGGLING OF LIGHT IONS IN FULLERITE

Abstract

To determinate the stopping cross section in fullerite a feasible approach, taking into account the high radiation sensitivity and mechanical fragility of fullerite films, should be employed. In this work, the stopping cross sections of ¹H, ³H, ⁴He and ⁷Li ions for several selected energies were measured by Rutherford backscattering, neutron depth profiling and alpha spectroscopy using sandwich structures of samples composed of fullerite deposited on a firm substrate (Si, steel) with an intermediate marker (Au, N, Li, B, Pu). In addition, ion transmission through a thin C film supporting a fullerite layer was also utilized. The measured stopping cross sections follow the theoretical predictions calculated for carbon, but are systematically (10–35%) higher than the theoretical ones (with the exception of 5 — 5.5 MeV ⁴He). The observed deviation of the experimental data can partly be explained by the chemical state effects in fullerite, which accounts for about 20–50% of the difference. The measured energy straggling exceeds Bohr's value by a factor of about 2 for alpha spectroscopy and ion transmission, and 2.5 or 9.5 for Rutherford backscattering and neutron depth profiling, respectively. The discrepancy can be explained by a thickness variation, such as surface roughness of the fullerite films.

Keywords:

• Fullerite
• Stopping cross section
• Rutherford back-scattering
• Neutron depth profiling
• Ion transmission

ACKNOWLEDGMENTS

The work was supported by Grant Agency of the Czech Republic under the projects No. 202-96-0077 and 202-97-K038. One of the authors (J.V.) would like to express his thanks for support of the work also by Japan Atomic Energy Research Institute under the program of JAERI Research Fellowship.