Surface Potentials Near the UV Light/Dark Boundary

Summary form only given. Study of surface potentials near the lunar terminator is important for understanding dust dynamics in this region. The Lunar Ejecta and Meteorite Experiment deployed during the Apollo 17 mission observed an increased frequency of dust impacts at lunar sunrise and sunset. A laboratory model of the lunar surface is being developed for measuring the surface potential distribution near the UV light/dark boundary and for developing instruments to diagnose the lunar photoelectron layer. In initial experiments, a metal sheet is illuminated by a 1000 W Hg-Xe UV source. An adjacent metal surface is cut into stripes insulated from each other. A shadow of the UV light is made by a light stopper outside the vacuum chamber. The surface potential of the illuminated metal sheet and surface potentials of the metal stripes are measured by high impedance probes. Surface potentials are comparable in magnitude to the difference between the work function (~4.1 eV) and the most energetic photons (~6 eV). Experiments will continue with materials of different work function and with simulated lunar regolith.