Structural and chemical alteration of crystalline olivine under low energyHe+ irradiation

We present the results of irradiation experiments on crystalline olivine wi th He+ ions at, energies of 4 and 10 keV and fluences varying from 5 10(16) to 10(18) ions/cm(2). The aim of these experiments is to simulate ion impl antation into interstellar grains in shocks in the ISM. Irradiated samples were analysed by transmission electron microscopy (TEM). The irradiation ca uses the amorphization of the olivine, at all He+ fluences considered. The thickness of the amorphized region is 40 +/- 15 nm and 90 +/- 10 nm for the 4 keV and 10 keV experiments, respectively The amorphization of the olivin e occurs in conjunction with an increase in the porosity of the material du e to the formation of bubbles. In addition, the amorphized layer is deficie nt in oxygen and magnesium. We find that the O/Si and Mg/Si ratios decrease as the He+ fluence increases. These experiments show that the irradiation of dust in supernova shocks can efficiently alter the dust structure and co mposition. Our result are consistent with the lack of crystalline silicates in the interstellar medium and also with the compositional evolution obser ved from olivine-type silicates around evolved stars to pyroxene-type silic ates around protostars.