An E-PERM®, also known as 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 referred to as the SPER-1 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 E-PERM® 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 measurements. They are known as "true integrators" because they are constantly collecting and "registering" the ions generated by the radon decaying inside the chamber.