Low-fluence electron yields of highly insulating materials

Electron-induced electron yields of high-resistivity high-yield materials--ceramic polycrystalline aluminum oxide and polymer polyimide (Kapton HN)--were made by using a low-fluence pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in the energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of < 3 fC/[mm.sup.2]. The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron recapture based on an extended Chung--Everhart model of the electron emission spectrum. This model is used to explain the anomalies measured in highly insulating high-yield materials and to provide a method for determining the limiting yield spectra of uncharged dielectrics. The relevance of these results to spacecraft charging is also discussed. Index Terms--Charging, dielectrics, electron, emission.