Research Interests

Exploratory studies of multicharged ion interactions with surfaces have been ongoing at the Multicharged Ion Research Facility (MIRF) in the Physics Division of Oak Ridge National Laboratory (ORNL) for a number of years now. The most recent research work has been focused on ion-surface interactions involving the following topics:

Measurements are performed in a floating potential ultra-high vacuum chamber with base pressures in the 10-10 Torr range, shown schematically in Figure 1, into which decelerated ion beams from an ECR ion source are directed, as described in the article:
Rev. Sci. Inst.

Figure 1: Schematic diagram of the decelerated beam surface scatttering apparatus

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Charge-state distributions of multicharged ions

In the study of the interactions of slow highly charged ions with metal, semiconductor, and insulator surfaces in a binary collision, backscattering geometry, the goal is to improve fundamental understanding of neutralization and energy dissipation occurring in such interactions, and then to apply the knowledge gained to probe and modify the surfaces of single crystals, thin films, and nanostructures.

The large-angle backscattering technique has seen increased use in studies of multicharged ion (MCI) neutralization during interactions with solid surfaces (see several related articles). The use of MCI projectiles has been shown to significantly enhance the surface sensitivity of the backscattering technique (Phys. Rev. Letter article). Also, in contrast to grazing incidence studies where a large number of lattice sites are involved, large-angle backscattering measurements allow the resolution of interactions occurring with just one or two atoms located on the target surface (see Figure 2).

Figure 2: Comparison between grazing scattering and large-angle backscattering

Simultaneous energy and charge-state analysis of the backscattered projectiles (see typical spectra) have provided insights into energy loss mechanisms accompanying projectile neutralization leading to particular final charge states (Rev. Sci. Inst. article). More recently, the large-angle backscattering technique has provided information on site-specific MCI neutralization at a Au(110) surface. In a latter work, a strong target azimuth dependence was observed in the scattered projectile charge-state distributions. Extensive trajectory simulations performed in conjunction with the measurements were able to reproduce the observed variations with target azimuth (Phys. Rev. Letter article), and provided a framework for demonstrating differences in MCI neutralization at the different scattering sites on the corrugated Au(110) metal surface.

Studies were extended to insulator targets by measuring projectile backscattering from RbI(100). Goals of this work were to identify and understand differences in final charge-state distributions and neutralization mechanisms that occur on ordered insulator and metal surfaces. RbI(100) was chosen in the hope of obtaining site-resolved projectile neutralization information at Rb and I lattice sites (see Figure 3).

Figure 3: Site-resolved projectile neutralization information at Rb and I lattice sites.
At normal incidence, interaction with second and deeper layers is blocked:
predominantly quasi-binary scattering from surface layer.

Due to their significant mass difference, large-angle backscattering from the two lattice sites should lead to sufficiently different elastic binary collision energy losses to be resolvable using our time-of-flight (TOF) energy analysis technique, as can be seen in these spectra (NIMB #1, NIMB #2, Surf. Coat. Tech.).

For extended and detailed information about the different performed analysis, please refer to the articles mentioned above. We have to point out, however, that in spite of all the work already done, many aspects of the site-specific projectile neutralization still need interpretation and explanation, and the basis for the interesting trends found in the different studies will be explored in greater detail in upcoming experiments and simulations.

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Chemical sputtering of graphite

There is significant technological interest in using graphite as a plasma-facing component on present and future fusion devices, and in using different types of graphite or carbon fiber composites (CFC?s), together with tungsten, beryllium, or other refractory metals, in the ITER divertor. Motivated in part by this interest, an experimental research program was recently started at MIRF to investigate chemical sputtering of graphite surfaces in the limit of very low impact energies (i.e. below 10 eV/D), where there is currently little experimental data (Phys. Scripta, J. Nucl. Mat. article #3).

Our experimental approach is based on the use of a sensitive quadrupole mass spectrometer (QMS) which samples the partial pressures of selected mass species in the scattering chamber resulting from the incident ion beam. A schematic diagram of the experimental apparatus can be seen in Figure 1.

In Figure 2, a typical background-subtracted mass spectrum showing chemical sputtering production of methane (CD4) is shown. In particular, this spectrum corresponds to the result obtained for the lowest beam energy reached up to date.

Figure 4: Background-subtracted mass spectrum for a ~ 13 eV D3 ion beam.

One of our goals was to determine chemical sputtering yields of CD4 for a D2 ion beam incident on ATJ graphite at room temperature and 800 K. The results are shown in Figure 3, as a function of the beam energy (J. Nucl. Mat. article #1, J. Nucl. Mat. article #2 ).

Figure 5: Energy dependence of chemical sputtering yields for the production of methane (CD4) for two different ATJ graphite sample temperatures.

In addition to the exploration of the chemical sputtering produced by very low impact energies, our research has been focused on comparisons of atomic and molecular ion impact, to better determine the range where atomic and molecular species at the same velocity behave in an equivalent manner with respect to the chemical sputtering yields (J. Nucl. Mat. article #3).

For extended and detailed information about the different performed analysis, please refer to the articles mentioned above.

In the near future, we plan to implement a time-of-flight approach to complement the quadrupole mass spectrometer measurements; approach that will allow us to detect product radicals and greatly reduce or eliminate the need for wall corrections (see J. Nucl. Mat. article #2) presently needed to deduce the chemical sputtering yield.

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A novel Device for Quantitative Detection of 14C

14C labeled compounds are widely used in the pharmaceutical industry, e.g., as tracers to determine the fate of these compounds in vivo. The sensitivities of most present methods are inadequate to permit utilization of sufficiently small quantities of 14C to avoid the issues of radioactive waste and contamination, both of which are unacceptable for environmental, health and safety, and financial reasons. For these reasons, the pharmaceutical industry is currently exploring other high-sensitivity 14C detection methodologies that permit reduction of 14C labeling to slightly above background levels.

Conventional accelerator mass spectrometry (AMS) is currently the only approach that offers sufficiently high sensitivity to avoid the above radiological issues, but it requires large-scale facilities that are usually not dedicated to a single task, with correspondingly high cost. The AMS technique further entails time consuming sample preparation prior to the actual measurements, and so is not suited to quasi-real time monitoring of 14C levels.

The natural abundance of 14C in "modern" samples is about 1.18 10-12 per 12C atom, and this determines the background level from which the levels of tracers used must be distinguishable. The main difficulty in single atom detection of 14C arises from the isobaric interferences due to atomic ions (e.g., -N) and molecular ions (e.g. -CH2 and 13CH). In conventional AMS, the approach consists of using a negative ion source to eliminate the 14N contamination, since it does not support a stable negative ion, accelerating the negative ion beam in a tandem accelerator to high energy (few 100's keV to few MeV), and then dissociating molecular ions isobaric with 14C-, also present in the ion beam, either in a foil or gas target. Subsequent stages of electrostatic and magnetic analysis are then used to isolate the 14C ions before their detection.

Figure 6: Schematic drawing of the 14C detection ion-surface scattering experiment at MIRF

To demonstrate proof-of-principle of a new technique[1] utilizing energies in the tens of keV range, the ORNL Multicharged Ion Research Facility (MIRF) electron cyclotron resonance (ECR) ion source is used for the production of a multicharged carbon beam with charge state of +3 or higher to eliminate molecular isobar interference at mass 14. After magnetic selection of the desired charge state, the ion beam, which will still be dominated by 14N multicharged ions of the same charge state, is directed at grazing incidence to an insulator singlecrystal surface, where efficient negative ion formation [2] takes place without appreciable energy loss of the scattered ions. The different scattered charge states are separated with a low-resolution, small insertion length, permanent magnet dipole. Two subsequent stages of electro-static analysis further spatially separate the desired 14C- ions from scattered neutrals and other background prior to their detection on a two-dimensional position-sensitive detector (2-D PSD). The neutral 14N scattered beam produces at most an energy independent background on the 2-D PSD, and thus may be eliminated by suitable subtraction techniques. Unique characteristics of the apparatus are its small size, low cost, high efficiency (i.e., through-put), and ease of sample preparation, in compari-son to conventional AMS hardware. As a result, if the proof-of-principle demonstration is successful, this apparatus should find great utility in such applications as quasi-real time monitoring of 14C- based chemical tracer uptake in biological systems, atmospheric pollution studies, cancer research, medical diagnostics, and other biomedical studies.

[1] U.S. patent 6,455,844 issued 9/24/2002
[2] e.g., A.G. Borisov and V.A. Esaulov, J. Phys. Condens. Matter 12, R177-206 (2000)

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This research was sponsored by the Office of Fusion Energy Sciences and the Office of Basic Energy Sciences of the U.S. Department of Energy under contract No. DE-AC05-00OR22725 with UT-Battelle, LLC. Postdocs were appointed through the ORNL Postdoctoral Research Associates Program administered jointly by Oak Ridge Institute of Science and Education (ORISE) and Oak Ridge National Laboratory.

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Rev: 04-May-2012 by MEB