Analysis of electret ion chamber radon detector response to 222Rn and interference from background gamma radiation

Environmental radon (222Rn) monitors that incorporate electret detectors are confounded by background gamma radiation, which may cause the electret to discharge by as much as 7.5 volts per mR. Although background gamma corrections were formerly made by multiplying the known background gamma exposure rate with a constant conversion factor, this research demonstrates that doing so introduces an error ranging up to about 20%, especially in high gamma background areas. A new, more accurate method of background gamma correction has been developed that uses an average, voltage-dependent discharge factor, Dgamma (V Kg C(-1)). This factor and its coefficients were experimentally determined by separately exposing groups of electret radon detectors to photons from 60Co and 137Cs. Statistical analysis shows that Dgamma is independent of the orientation of the electret during irradiation but that some dependency on dose rate or energy of the irradiating photons may be expected. The discharge of the electret due only to gamma irradiation, Vgamma is determined by multiplying the total integrated gamma exposure by Dgamma. The discharge of the electret during a radon measurement can then be corrected for background gamma radiation by subtracting Vgamma from the total discharge of the electret resulting in a net discharge due solely to radon. A new equation has also been developed in this study for the radon discharge factor, D(Rn)(V m3 Bq(-1) s(-1)), that is entirely consistent with the gamma discharge factor and was thoroughly evaluated by exposing the electret radon detectors to known concentrations of radon.