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Mesoporous silicon (PS) samples were processed by anodising p + Si wafers in (1:1) HF-ethanol solution. Different current densities were used to obtain three different porosities (41%, 56% and 75%). In all cases the morphology of the PS layer is columnar with a mean crystallite size between 12 nm (75% porosity) and 19 nm (41% porosity). These targets were irradiated at the GANIL accelerator, using different projectiles ( 130 Xe ions of 91 MeV and 29 MeV, 238 U ions of 110 MeV and 850 MeV) in order to vary the incident electronic stopping power S e . The fluences ranged between 10 11 and 7 Â 10 13 cm À2 . Raman spectroscopy and cross sectional SEM observations evidenced damage creation in the irradiated nanocrystallites, without any degradation of the PS layer morphology at fluences below 3 Â 10 12 cm À2 . For higher doses, the columnar morphology transforms into a spongy-like structure. The damage cross sections, extracted from Raman results, increase with the electronic stopping power and with the sample porosity. At the highest S e (>10 keV nm À1 ) and the highest porosity (75%), the track diameter coincides with the crystallite diameter, indicating that a single projectile impact induces the crystallite amorphization along the major part of the ion path. These results were interpreted in the framework of the thermal spike model, taking into account the low thermal conductivity of the PS samples in comparison with that of bulk silicon.

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