Long term stability of electrets used in electret ion chambers

Commercially available electret ion chambers have been using electrets for more than 15 years. As a quality control procedure, a few electrets from each production batch are set aside for studying their long term stability. These are measured after storage of 1–13 years, to calculate the decay rates for different storage periods. Two types of electrets are studied. The first type is the Teflon® electret made of PTFE Teflon® (polytetrafluoroethylene) with a thickness of 1.524 mm and the second type is Teflon® electret made of FEP Teflon® (fluorinated ethylene propylene) with a thickness of 0.127 mm. In both cases, one side is carbon coated. The first type shows an average decay of about 4% per year. The second type shows an average decay rate of about 1% per year. The decay rate does not change significantly over the years of study. After accounting for decay due to ions in storage conditions, the half life (time required to decay by 50%) of 1.524 mm and 0.127 mm thick electrets is 14 and 68 years, respectively. Such unusually long half lives make the electrets useful for a number of practical applications where a high electrostatic field is needed over an extended period, without having to use batteries or high voltage sources.