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Electrical Breakdown of Polypropylene Film Under Electron Irradiation

This report deals with dc breakdown phenomena of polypropylene film under electron irradiation in vacuum. Electrons emitted from a hot cathode irradiate and penetrate some depth into a polypropylene specimen. It is expected that deposited electrons produce an electric field in the specimen and may give a source of electron avalanches. In this investigation we studied a relation between the breakdown probability of the specimen and the electron acceleration voltage. During electron irradiation the specimen was stressed by a dc bias voltage that was low enough not to lead to any breakdown without the electron irradiation. Even if the bias voltage was lowenough, breakdown occurred under electron irradiation. The results show that the breakdown probability varies with change of the electron penetration depth. Using a 7.5 pm polypropylene film as a specimen, the highest breakdown probability was obtained when the distance between the back electrode of the specimen and the layer where the electrons were trapped was about 3 to 4 ¿m. For deeper electron penetration as well as for shallower penetration, a lower breakdown probability was obtained. It is inferred from these results that the breakdown was electronic under electron irradiation and that the length of avalanches necessary for a breakdown to take place was about 3 ¿m.