Tracking failure of gamma-ray irradiated polybutylene polymers under magnetic field

Polymer insulating materials used in radioactive environments are subjected to a risk of surface degradation caused by tracking. The tracking behavior becomes more complex with the presence of magnetic field. This paper presents investigation of tracking failure of gamma-ray irradiated polybutylene naphthalate (PBN) and polybutylene terephthalate (PBT) under magnetic field. The samples were irradiated in air up to 100 kGy and then up to 1000 kGy with dose rate of 10 kGy/h by using a 60Co gamma-source. Magnetic field (B→) with flux density of 250 mT was assembled perpendicularly to electric field (E→), which gave rise to relative angle between E→×B→ and surface normal vector (n→) of 0°, 90° and 180°, respectively. The experiment was carried out by applying a negative dc pulse voltage between a pair of brass needle-semicircle plate electrodes, the time to tracking failure and the discharge quantity were measured. Obtained results show the dependence of the tracking characteristics upon the radiation dose as well as the direction of E→×B→. It is suggested that the tracking behavior is in related to the thermal durability of the material and the surface discharge manner which are altered by the radiation induced chemical reactions and the combined electric-magnetic force, respectively.