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Abstract

The resistivity of insulating materials for electrostatic, ion chamber dosimeters must be very high and must remain so after exposure to ionizing radiation. Low dielectric polarization and good fabrication characteristics make hydrocarbon polymers most suitable, but both the conductivity during irradiation and its decay afterward vary greatly even for the same type of polymer, probably depending on impurities. Amorphous, styrene-based polymers and copolymers polymerized in aqueous emulsion and initiated with K2S2O8 were found to have much more rapid decay of conductivity after irradiation than pure polystyrene or other nonpolar polymers. The synthesis method incorporates sulfate groups on the polymer chain ends and leaves emulsifier residues distributed throughout the polymer as was demonstrated by various analytical procedures. To identify the trapping species a synthesis program was carried out varying selected ingredients in the polymerization recipe. It was found that the postirradiation conductivity was not dependent on the chemical nature of the emulsifier residues. On the other hand, the decay time of conductivity after irradiation did depend on the polar groups incorporated in the polymer chain. It was concluded that effective charge carrier traps were constituted of a polar second phase highly dispersed through the polymer by association with polar groups incorporated on the polymer chain.