An Electret Ion Chamber (EIC), is a passive integrating ionization monitor consisting of a very stable electret mounted inside a small chamber made of electrically conducting plastic. The electret, a charged Teflon® disk, serves as both the source for ion collection and as the integrating ion sensor. Negative ions produced inside the chamber are collected on the positively charged electret, causing a reduction of its surface charge. The measurement of the depleted charge during the exposure period is a measure of integrated ionization during the measurement period. The electret charge is read before and after the exposure using a specially built non-contact electret voltage reader. Using this data as input to the appropriate formula, one can determine the radon activity present over the duration of the test.
The basic components of the EIC System consist of the electret reader, chambers, and electrets. There are chambers of different sizes and electrets of different sensitivities to meet a wide range of monitoring situations. Typically, a more sensitive electret, referred to as an ST Electret, is used for short-term measurements, and an LT, or less sensitive electret, is used for longer term testing. They are known as “true integrators” because they are constantly collecting and “registering” the ions generated by the radon decaying inside the chamber.
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.
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