Low-Fluence Electron Yields of Highly Insulating Materials

Electron-induced electron yields of highresistivity, high-yield materials - ceramic polycrystalline aluminum oxide and the 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 energydependent total yield curves and yield decay curves were observed, even for incident electron fluences of \u3c 3 fC/mm2. The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron re-capture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain anomalies measured in highly insulating, high-yield materials, and to provide a method for determining the limiting yield spectra of uncharged dielectrics. Relevance of these results to spacecraft charging is also discussed