Characterization and Improvement of
Ultra-Thin Carbon Foils for
Inclusive Dates: 07/01/03 06/30/05
Background - Ultra-thin carbon foils are a key nanotechnology component of many time-of-flight (TOF) ion and neutral mass spectrometers and several energetic neutral atom (ENA) imagers currently being flown on a variety of space missions. One or both of two fundamental properties of the foils are generally employed in these sensors secondary electron emission or charge-state conversion. While foils are critical components of such devices, they also straggle the energy and scatter the direction of the primary particles as they pass through. Because these undesirable effects scale with the thickness of the foils, a significant effort has been made to develop means for handling and supporting extremely thin (less than 10 nm) foils. So far, the technical literature in the keV range is often not consistent because there is no systematical study on both energy loss and energy straggling for a broad range of ion energies and species and foil thicknesses. Therefore, empirical scaling laws for different foil thicknesses are difficult to determine.
Approach - In this project, we propose to quantify all these effects, both the wanted and the unwanted, as functions of the various foil and projectile parameters. These measurements will allow us to better understand the physics of the interaction of particles with carbon foils and to develop significantly improved space instruments in the future. We also propose to examine enhancements to simple carbon foils by depositing extremely thin layers of different materials that have significantly different electronic interactions with the projectiles. The main goal is to fully characterize and find improvements to the carbon foils used for space instruments. The objectives have been divided into two categories:
Accomplishments - The team set up an apparatus for measurements of the interaction of particles with carbon foils. The apparatus works in a high-vacuum chamber in the space instrument calibration facility at SwRI and performed measurements of the interaction of ions with carbon foils for a broad range of ion energies and species and foil thicknesses. We now have at our disposal a useful and consistent set of data from which systematic understanding and quantitative relationships between the ion beam parameters and foil thickness can be developed. The set of data and the empirical laws are useful tools for accurate modeling and development of space plasma instrumentation. The goal of our second objective was to increase the charged fraction exiting the foils so as to increase the sensitivity of neutral atom imagers used for space applications. The surface treatments applied on the carbon foils reduced the charged fraction of the ions exiting the foil. However, we could significantly modify the properties of charge exchange of carbon foils for the first time to our knowledge. Moreover, the treated foils keep their properties even after having been exposed to air.