Surface charge measurement of gamma-rays irradiated polymer insulating materials

Polybutylene naphthalate (PBN) was employed as test sample which was previously irradiated in air up to 100 kGy and then up to 1000 kGy with dosage rate of 10 kGy / h by using a ⁶⁰Co gamma-source. The experiment was carried out on sample surface under a negative dc stress between two parallel electrodes. An electrostatic probe was designed to measure the surface charge density during charging period and decay process. Obtained results showed that both max charge density and total charge decreased with increasing the total dose of irradiation. It indicated that the capability of storing charges reduced by the increase of the total dose of irradiation. It is suggested that charge accumulation depends upon the density of surface states, which is varied by radiation induced cross-linking and degradation reaction of the molecule structure. Furthermore, the charge decay mainly relies on the recombination of surface charge with ions of opposite sign in surrounding media.