Amorphization of Forsterite Grains Due to High Energy Heavy Ion Irradiation--Implications for Grain Processing in ISM

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Amorphization of Forsterite Grains Due to High Energy Heavy Ion Irradiation--Implications for Grain Processing in ISM
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   UCRL-ABS-209107  AMORPHIZATION OF FORSTERITE GRAINS DUE TO HIGH ENERGY HEAVY ION IRRADIATION – IMPLICATIONS FOR GRAIN PROCESSING IN ISM.  S. Bajt 1 , R. A. Baragiola 2 , E. M. Bringa 1 , J. P. Brad-ley 1 , Z. R. Dai 1 , C. A. Dukes 2 , T. Felter 1 , G. A. Graham 1 , S. O. Kucheyev 1 , M. J. Loeffler 2 , M. C. Martin 4 , A. Tiel-ens 3 , D. Torres 1  and W. van Breugel 1 , 1 Lawrence Livermore National Laboratory, 7000 East Avenue, L-210, Liv-ermore, CA 94550, USA (, 2 University of Virginia, Charlottesville, VA 22903, 3 SRON Laboratory for Space Research Groningen, NL-9700 AV, Groningen, The Netherlands, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. Introduction:  Only ~1% of the interstellar me-dium (ISM) is composed of interstellar dust. How-ever, dust plays a key role in the evolution of stars and planets. The dust consists of various materials formed in different environments by different mechanisms. Interstellar dust particles are irregulary shaped and are composed mostly of amorphous silicates and carbona-ceous grains. ISO (Infrared Space Observatory) spec-tra indicate crystalline silicates in the envelopes around evolved stars [1], while the silicates in ISM show in-frared features characteristic of amorphous material [2, 3]. The absence of crystalline silicates in diffuse ISM is intriguing, not well understood, and potentially in conflict with the abundance of crystalline presolar silicates recently identified in IDPs and polar micro-meteorites [4, 5]. Different processes leading to amor-phization of silicates in ISM have been proposed, such as grain-grain collision [6, 7, 8], formation by recon-densation [7], amorphization by heating and quenching processes, and amorphization by ion irradiation [9]. In this abstract we focus on ion irradiation processes. One can discriminate between two energy regimes for the stopping power of energetic ions [10]. At keV ener-gies, the energy deposition of ions is dominated by nuclear collisions, while at MeV energies and above, corresponding to cosmic rays, the energy deposition is due to electronic excitations. While keV ions seem to be efficient in amorphizing silicate dust in laboratory experiments [9, 11, 12] their fluences are relatively low in ISM. Laboratory experiments using high energy (MeV-GeV) light ions (e.g. protons and helium ions) irradiation were inefficient at amorphizing crystalline silicates [13]. Another possible mechanism for amor-phization of silicate grains is heavy ion cosmic ray damage. Heavy ions (e.g. Fe) fluences are low in ISM but due to their large stopping power the cumulative damage is ~1000x larger than damage due to light ions, such as H, over the lifetime of interstellar dust grains [14]. Experiment:  To test the hypothesis that high en-ergy heavy ions are more damaging than high energy light ions we performed a series of irradiation experi-ments. All the experiments were done with forsterite single crystals (ISM dust analogs) using an ion beam accelerator. Characterization of bulk samples included mid-infrared spectroscopy and X-ray Photoelectron Spectroscopy (XPS) at University of Virginia and Rutherford Backscattering/chanelling (RBS/C) spec-trometry at Lawrence Livermore National Laboratory (LLNL). The bombarded surface layers were also stud-ied using a transmission electron microscope (TEM) at LLNL and mid-infrared spectral microscope at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. This infrared microspectroscopy beamline 1.4.3. is equipped with a ThermoNicolet Magna 760 FTIR bench and a SpectraTech Nic-Plan IR microscope. An MCT-A detector and KBr beams-plitter are used for mid-IR microspectroscopy. The synchrotron source can be focused to a diffraction-limited spot size onto a sample, and then the reflected or transmitted light is measured. For the mid-IR this spot size is 3 to 10 microns in diameter. A sychrotron-based infrared microscope is also ~100 times more sensitive than conventional source microscope. The samples were studied with TEM after being thinned to electron transparency using a focused ion beam in-strument. Radiation damage simulations performed on parallel computeres at LLNL were used to compare experimental data with different empirical models. Results:  Forsterite (Mg 2 SiO 4 ) grains have been ir-radiated with energetic, 10 MeV Xe ions using flu-ences ranging from 5x10 11 ions/cm 2  to 3x10 13 ions/cm 2 . The changes in forsterite were measured using He +  RBS. The damage buildup curve shown in Figure 1 gives an effective amorphous track of 5.6 nm and an amorphization fluence of ~2 x 10 13  /cm 2 . Using high resolution TEM imaging, we were able to see an ap-proximately 1 µ m thick surface layer that had trans-formed from crystalline to amorphous, as well as indi-vidual ion tracks in samples irradiated to relatively low fluences. Crystalline and amorphous areas have other-wise identical elemental composition as measured by EDX and XPS. The effect of ion fluence on amorphi-zation can be clearly seen in mid-infrared spectra ob-tained independently using a reflectance attachment at the University of Virginia (Figure 2) and a synchro- Lunar and Planetary Science XXXVI (2005)2342.pdf  tron-based infrared spectrometer at the ALS (Figure 3) on bulk samples. Figure 1: Damage buildup curve as a function of fluence obtained from RBS/C measure-ments. Figure 2: Reflectance data from a mid-infrared spectrometer using an ATR reflec-tance attachment. Using a focused ion beam instrument we prepared electron transparent samples to selectively study only the top layer of irradiated forsterites with a mid-infrared spectroscopy microscope and TEM. Our data support the hypothesis that high energy heavy cosmic rays can efficiently amorphize silicate grains in the ISM and this might provide an explanation for the ob-served absence of crystalline dust in the ISM clouds. Figure 3: Reflectance data from synchrotron-based mid-infrared spectrometer. References:  [1] Waters L. et al., (1996) A & A, 315, L361. [2] Lutz D. et al. (1996) A & A, 315, L269. [3] Chiar et al., (2000) ApJ, 537, 749. [4] Messenger S. et al. (2003), Science 300, 105. [5] Yada et al. (2005) LPSC XXXVI, this volume. [6] Tielens A. et al. (1987) ApJ, 319, L100. [7] Jones A. et al. (1996) ApJ, 469, 740. [6] Jones A. (1997) in From Stardust to Planetesimals, ed. Y. Pendleton & A. Tielens (San Francisco: Astron. Soc. Pacific), ASP Conf. Ser., 122, 97. [8] Jäger C. et al. (2003), A & A, 401, 57. [9] Bringa E. M. and Johnson R. E. (2003) , in Solid State Astrochemistry, eds. Pirronello V. et al., Dordrecht, Kluwer, 357. [10] Brucato J. R. et al. (2004), A & A, 413, 395. [11Demyk K. et al. (2004) A & A, 420, 547. [12 Day K. (1977) MNRAS, 178, 49P. [13] van Breugel W. et al. (2004) in The Dusty and Molecular Universe, ESA Special Publication, to be published. Acknowledgments. This work was performed in part under the auspices of the US Department of En-ergy by the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC03-76F00098 at Lawerence Berkeley National Laboratory. The work by R. A. Baragiola, C. A. Dukes and M. J. Loeffler at the Uni-versity of Virginia was funded by the NASA Office of Space Research, Cosmochemistry Program. Lunar and Planetary Science XXXVI (2005)2342.pdf
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