Electron Yields of Lunar Regolith Simulant Layers

The charging of bulk lunar regolith has been recognized since the Apollo era as an immediate and critical issue facing our return to the moon. Accurate electron yield (EY) measurements of bulk highly insulating granular materials—which largely determine how such particles charge through interactions with space environments—are lacking due to many experimental complexities that have led to a critical knowledge gap. Such knowledge is essential for addressing fundamental science and myriad important lunar applications and simulations related to lunar dust and regolith electrostatic charging. The few prior EY studies of lunar dust were limited due to severe charging effects and showed yields trending toward unity both above and below crossover energies. Novel techniques have recently been developed to prepare bulk granular insulating samples and to conduct accurate EY measurements with minimal charging effects. Results for ranges of dust coverage and comparison to EY of bulk constituent materials confirm that the new methods work well. As a precursor to measurements of actual lunar regolith, secondary and backscattered EY measurements for incident energies from ~20 eV to 5 keV are presented here for standard lunar simulants and closely related very high-yield, highly insulating granular materials, including Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and SiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>. A “patch” model was applied to accurately predict composite yield curves in terms of the measured yields of bulk Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>, SiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>, and graphitic carbon constituents. The EY results for simulants are of immediate use to improve the reliability of current ground-based engineering tests for mitigation strategies of lunar applications and to provide better simulation models of charging dust.