Bremsstrahlung radiation and induced current in polyimide

Investigations were made of current transients induced in polyimide by pulses of bremsstrahlung radiation. Test specimens, 50 μm thick, were mounted in a vacuum chamber that was positioned as the target in the output beam of an electron accelerator. The electron beam was converted to bremsstrahlung via its interaction with a 4 mm thick lead shield placed in front of the test chamber. The test specimens were stressed by direct voltages to give applied fields approaching 10⁷ V/m and the current following a bremsstrahlung pulse was recorded on a fast oscillograph. Experiments were carried out with both fresh material and material irradiated by electrons to various doses, with a maximum of 100 kGy. It was found that there were marked differences in behaviour between fresh and irradiated material. With positive dc polarisation and an unirradiated sample the output current signal showed an initial negative peak followed by a positive one, the amplitudes of the peaks increasing with increasing polarising voltage. The negative peak decreased in amplitude with increasing radiation dose and at 5 kGy had practically disappeared completely. The positive one remained and increased in amplitude with dose and field. The material also demonstrated a diode effect, in that with a negative bias no output signal was observed. Upon reversal of the field, however, a very large positive pulse was evident