E-PERMs are ion chambers fabricated from electrically conductive plastic which use an electret as both the high voltage source for ion collection and the integrating ion sensor.  They are passive devices requiring no power to function. They employ an electret to collect ions generated by radon.  The loss of surface voltage on the electret and the exposure period determines the average radon concentration in the place where they were located during the exposure.  The electret, which is a charged disk of Teflon, collects the negative ions (the electret is charged with a positive polarity) generated by the radiation emitted by the radon and radon decay products as they undergo radioactive decay in the chamber.  These ions are drawn to the surface of the electret where they cause a reduction in its surface voltage.  The amount of that voltage reduction, which is measured with a special voltmeter is directly related to the time integrated average radon concentration, which the electret was exposed to during the measurement period.  E-PERMs provide a true integrated radon concentration reading, i.e., the sensor retains all of the radon signal they "see" from the beginning to the end of any exposure period. We believe this to be the most accurate short term test available.                                                               

         Last modified: December 03, 2003