Stored Charge Effects on Electron Dose-Depth Profiles in Insulators

The dose-depth profiles for electrons extracted from a pulsed electron beam machine have been studied using a castable radiation-sensitive plastic. These profiles exhibit a pronounced range shortening at charge fluences greater than 0.5 μC/cm2 which reaches a limiting value, corresponding to a limiting internal field, at fluences greater than 1 μC/cm2. This behavior is interpreted in terms of charge build-up and relaxation by radiation-induced conductivity in the host material, poly(vinyl toluene). Positive charge-trapping additives reduce this conductivity and allow the internal field to build up at lower charge fluences. The ratio of radiation-induced conductivity to dose rate derived from these measurements 1.6 × 10−18 sec/Ω-cm rad compares favorably with the value found at low dose by direct measurement of 3.0 × 10−18 sec/Ω-cm rad.