210Po implanted in glass surfaces:: Calibration and improved performance for retrospective radon reconstruction

We studied the implanted Po-210-Pb-210 activities and cumulative radon gas exposure of glass surfaces in two groups of homes to establish an authentic calibration for retrospective radon gas reconstruction and to identify factors that affect the calibration. Sixty-seven glass objects from 24 Minnesota homes were analyzed. The radon gas concentrations in these homes were measured repeatedly with annual, yearlong detectors over a two-decade span while the glass was being exposed to radon progeny. The surface activity-radon exposure correlation was better (R-2 similar to 0.8) than in studies where only contemporary radon concentrations had been measured. For example, in our study of a large group of Iowa homes, cumulative radon exposure based on contemporary radon gas measurements showed an R-2 similar to 0.5 for 2147 glass surfaces. The lower correlation probably results from the inaccuracy of the contemporary radon gas based exposure estimates and the more diverse set of deposition conditions, including many homes with a smoker. The correlation between implanted activity and radon gas exposure was improved by 10 to 20% when the measured contemporary radon progeny deposition rates were included in the regression as a variable. We investigated new techniques to reduce the significant interference in our detectors from intrinsic alpha activity in glass. We did not observe any significant change in the activity-exposure ratio with the age of the glass, suggesting that chronic losses over time are small. Neither data set showed any significant effect due to the size, or type of the glass; washing frequency, existence of surface films, or air movement. Implanted surface activity measurement proved to be as good a predictor for long-term radon concentrations as yearlong radon gas measurements in the Minnesota homes.