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Energetic neutral atom (ENA) imaging techniques have become a powerful tool for remotely probing plasma environments in space. ENA imagers cover energies from 0.01 keV up to a few MeV, and they use different techniques to cover such a broad energy range. Most of them convert the ENA into a charged particle to remove the converted ENA from the initial neutral direction. In the >∼0.2 keV/nuc to 10’s of keV/nuc range, the conversion subsystem is usually an ultra-thin carbon foil. The sensitivity of ENA imagers based on charge conversion by carbon foils is driven by the ability of these foils to convert a neutral atom into an ion. The charge state distribution after the carbon foils is a strong function of the chemical and physical properties of the exit surface. In this study, we analyze the composition and structure of the surface using X-ray photoelectron spectroscopy. The surface is roughly 88% carbon and 12% oxygen, forming strong C O bonds. Annealing the foil lowers the oxygen content to about 9%. We coat the surface of the foils with Au, Al 2 O 3 , or MgO. We compare the exit charge state distributions of hydrogen prior to and post coatings. While no significant difference is observed in the exit charge state for the Au and Al 2 O 3 coatings, there is a slight decrease of the positive fraction after MgO. The annealing of the foil has the benefit of reducing the angular scattering of hydrogen by a factor of ∼1.2. This is a significant improvement that has the potential to increase sensitivity of ENA imagers.

Thin carbon foils are used in many fields of physics, including for the detection of keV particles in space plasma instrumentation. The interaction of ions or neutral atoms with these foils results in different effects: electron emission, charge exchange, angular scattering, and energy loss and straggling. We report measurements of the energy loss and straggling and angular scattering of 1–50 keV protons for different foil thicknesses (nominal 0.5–2.0 μg/cm2). We derive simple semi-empirical relationships for the energy loss and straggling as a function of incident energy and foil thickness. We also derive a semi-empirical relationship between the derived thickness and the scattering constant. This provides a simple and straightforward tool to estimate the energy loss by measuring the angular scattering of protons for a particular foil.

Graphene could be an alternative to amorphous carbon foils, in particular in space plasma instrumentation. The interaction of ions or neutral atoms with these foils results in different effects: electron emission, charge exchange, angular scattering, and energy straggling. We showed in previous studies that (1) the charge exchange properties are similar for graphene and regular carbon foils, and (2) the scattering at low energies (few keVs) is less for graphene than for one of our thinnest practical carbon foils. In this study, we report measurements of the energy loss and straggling of ∼1–50 keV H, He, C, N, and O ions in graphene. We compare graphene and a carbon foil for hydrogen. We provide simple power law fits to the average energy loss, energy straggling, and skewness of the energy distributions. We find the energy loss for ions transiting through graphene to be reduced compared to thin carbon foils but the energy straggling to be larger, which we attribute to the non-uniformity of the graphene foils used in this study.