Catherine Ann Dukes

Surface modification of olivine by H+ and He+ bombardment

Laboratory simulations of solar wind irradiation of olivine, a constituent of ordinary chondritic meteorites and S-type asteroids, show a dramatic chemical reduction of surface iron and, to a lesser extent, silicon. Earths atmosphere oxidizes any reduced iron instantaneously, whereas irradiated material on the surface of an asteroid would remain reduced. Changes in optical reflectance, due to metallization of the surface, provide a possible explanation for differences in the near-infrared spectra of ordinary chondrite meteorites and their likely parent bodies, the S(IV) asteroids.

ENERGETIC PROCESSING OF INTERSTELLAR SILICATE GRAINS BY COSMIC RAYS

While a significant fraction of silicate dust in stellar winds has a crystalline structure, in the interstellar medium nearly all of it is amorphous. One possible explanation for this observation is the amorphization of crystalline silicates by relatively ‘‘low’’ energy, heavy-ion cosmic rays. Here we present the results of multiple laboratory experiments showing that single-crystal synthetic forsterite (Mg2SiO4) amorphizes when irradiated by 10 MeV Xe ions at large enoughfluences.Usingmodeling,weextrapolatetheseresultstoshowthat0.1Y5.0GeVheavy-ioncosmicrayscan rapidly (� 70 Myr) amorphize crystalline silicate grains ejected by stars into the interstellar medium. Subject headingg cosmic rays — dust, extinction Online material: color figures

Plasmon excitation in ion–solid interactions

Abstract We discuss recent advances in the area of plasmon excitations in ion–solid interactions. The basic aspects of plasmons are described, including surface modes. Kinetic plasmon excitation, a major contribution to the energy loss of fast charges in matter, should occur above a threshold ion velocity of ∼1.3 times the Fermi velocity. The unexpected observation of sub-threshold excitation is analyzed with the conclusion that the usually dominant mechanism is indirect excitation by fast secondary electrons. Slow ions with sufficiently high potential energy can excite multipole surface plasmons, as evidenced by the plasmon decay structure in the electron emission spectra.

Atomic collisions in solids: Astronomical applications

Airless bodies in space are subject to irradiation with energetic atomic particles, which generate atmospheres by sputtering and alter the surface composition. Astronomical observations with telescopes and space probes continuously provide new data that require new laboratory experiments for their interpretation. Many of these experiments also serve to expand the current frontier of atomic collisions in solids by discovering previously unknown phenomena. Some of the experimental techniques used in these experiments could find applications in other areas of atomic collisions in solids. We present results from our current experimental research program on sputtering and surface modification of ices and minerals and point out opportunities for research in this area.

Extraction from TeV-range accelerators using bent crystal channeling

Abstract Plans and first results from Fermilab Experiment 853 are presented. E853 is an experiment to test the feasibility and efficiency of extracting a low-intensity beam from the halo of the Tevatron using channeling in a bent silicon crystal. The motivation of the experiment is to apply crystal extraction to trans-TeV accelerators like the LHC. Channeling developments related to crystal extraction and some early results from accelerator studies at the Tevatron are presented.

Secondary electron emission spectra from clean and cesiated Al surfaces: the role of plasmon decay and data analysis for applications

We report measurements of energy spectra of secondary electrons emitted from clean and cesiated aluminum surfaces under the impact of 130 eV electrons. Measurements show that the decay of bulk and surface plasmons dominates the electron emission. In contrast with theoretical calculations, our experiments indicate that the electron collision cascade inside the solid produced by electrons excited by plasmon decay do not contribute significantly to electron emission. A simple analysis of electron energy distributions measured as a function of Cs surface coverage allows separation of rediffused incident electrons from the continuum background of true secondary electrons. The result shows that yields of rediffused electrons used in several applications may have been significantly overestimated.

MECHANISMS FOR ION-INDUCED PLASMON EXCITATION IN METALS

Abstract We have studied the excitation of plasmons produced by 100 eV He + , Ne + and Ar + and by 5–100 keV H + and He + projectiles in Al and Mg through the observation of electrons from plasmon decay, ejected from clean and cesiated surfaces. At low velocities, plasmon excitation occurs only for ions of high potential energy and is independent of velocity. The effect of Cs adsorption on this potential plasmon-excitation mechanism on Al surfaces suggests that the excited plasmons are not bulk plasmons, as was assumed previously, but short-wavelength surface plasmons. For ions moving faster than a threshold velocity v th ∼1.3 v Fermi predicted by electron gas theories, kinetic plasmon excitation can occur because the valence electrons cannot respond instantaneously to screen the moving charge. We found that, contrary to theoretical expectations, plasmon excitation by H + and He + projectiles occurs below v th . With the aid of a simple model, we suggest that this sub-threshold excitation results from energetic secondary electrons.

Ozone generation by rock fracture: Earthquake early warning?

We report the production of up to 10 ppm ozone during crushing and grinding of typical terrestrial crust rocks in air, O2 and CO2 at atmospheric pressure, but not in helium or nitrogen. Ozone is formed by exoelectrons emitted by high electric fields, resulting from charge separation during fracture. The results suggest that ground level ozone produced by rock fracture, besides its potential health hazard, can be used for early warning in earthquakes and other catastrophes, such as landslides or land shifts in excavation tunnels and underground mines.

Solar wind sputtering rates of small bodies and ion mass spectrometry detection of secondary ions

Solar wind interactions with the surfaces of asteroids and small moons eject atoms and molecules from the uppermost several nm of regolith grains through a process called sputtering. A small fraction of the sputtered species, called secondary ions, leave the surface in an ionized state, and these are diagnostic of the surface composition. Detection of secondary ions using ion mass spectrometry (IMS) provides a useful method of analysis due to low backgrounds and high instrument sensitivities. However, the sputtered secondary ion yield and the atomic composition of the surface are not 1-to-1 correlated. Thus, relative yield fractions based on experimental measurements are needed to convert measured spectra to surface composition. Here, available experimental results are combined with computationally derived solar wind sputtering yields to estimate secondary ion fluxes from asteroid-sized bodies in the Solar System. The Monte Carlo simulation code SDTrimSP is used to estimate the total sputtering yield due to solar wind ion bombardment for a diverse suite of meteorite and lunar soil compositions. Experimentally measured relative secondary ion yields are then used to determine the abundance of refractory species (Mg+,Al+,Ca+,Fe+) relative to Si+, and it is shown that relative abundances can be used to distinguish whether a body is primitive or has undergone significant geologic reprocessing. Estimates of the sputtered secondary ion fluxes are used to determine the IMS sensitivity required to adequately resolve major element ratios for nominal orbital geometries.

THE EFFECTS OF CRACKING ON THE SURFACE POTENTIAL OF ICY GRAINS IN SATURN’S E-RING: LABORATORY STUDIES

Within Saturns E-ring, dust grains are coated by water vapor co-released with ice grains from the geyser-like eruptions of Enceladus. These ice-coated grains have intrinsic surface potential and interact synergistically with the ions and electrons of Saturns magnetospheric plasmas. We perform laboratory experiments to investigate the effects of water-ice growth on the surface potential, using amorphous solid water (ASW) films. We estimate the growth of the surface potential to be ~−2.5 mV (Earth) yr−1 and −112 mV yr−1 for E-ring grains at ~4.5R s and 3.95R s outside Enceladuss plume, respectively. In addition, our measurements show that the linear relationship between the surface potential and the film thickness, as described in previous studies, has an upper limit, where the film spontaneously cracks above a porosity-dependent critical thickness. Heating of the cracked films with (and without) deposited charge shows that significant positive (and negative) surface potentials are retained at temperatures above 110 K, contrary to the minimal values (roughly zero) for thin, transparent ASW films. The significant surface potentials observed on micron-scale cracked ice films after thermal cycling, −(5–20) V, are consistent with Cassini measurements, which indicate a negative charge of up to −5 V for E-ring dust particles at ~5R s. Therefore, the native grain surface potential resulting from water-vapor coating must be included in modeling studies of interactions between E-ring icy surfaces and Saturns magnetospheric plasma.