Passive integrating radon monitor using an electret ionisation chamber

A passive integrating measuring system for the detection of the mean radon concentration (222Rn) in dwellings is described. The monitor consists of a radon diffusion chamber forming a passive ion chamber which is operated without any current integrator or voltage supply during the exposure period. This chamber encloses a charged electret made of Teflon which exhibits a strong permanent electric field. Radiation caused by alpha decay of radon and its short-lived progeny leads to ionisation of the air inside the chamber. Ions of one polarity are attracted to the electret surface and reduce its surface charge. The decay of the electret surface potential is thus a function of the integral of the radon level. The advantages of this system are the wide variety of possible exposure times, ranging from one day to a year or longer, the simple readout, which takes only a few seconds, and the rechargeability of the electrets.