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We present the results of irradiation experiments on crystalline olivine with 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 implantation into interstellar grains in shocks in the ISM. Irradiated samples were analysed by transmission electron microscopy (TEM). The irradiation causes 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 olivine occurs in conjunction with an increase in the porosity of the material due to the formation of bubbles. In addition, the amorphized layer is decient in oxygen and magnesium. We nd 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 eciently alter the dust structure and composition. Our result are consistent with the lack of crystalline silicates in the interstellar medium and also with the compositional evolution observed from olivine-type silicates around evolved stars to pyroxene-type silicates around protostars.

Structural and chemical alteration of crystalline olivine under low energy He+ irradiation

Structural and chemical alteration of crystalline olivine under low energy He⁺ irradiation