An update on the NIST radon-in-water standard generator: Its performance efficacy and long-term stability

A Ra-226-Rn-222 generator that could be used as a transfer standard for radon-in-water measurement calibrations was previously developed and described. The generator utilized a novel Rn-222 emanation source that was comprised of a Ra-226 solution encapsulated in polyethylene. The long-term performance of this standard generator has now been investigated and evaluated. The evaluation included exhaustive and more reliable measurement uncertainty analyses for the generator's performance and routine operation. Modifications to the original protocol for operation of the generator has also resulted in improved precision in the Rn-222 activity concentration in a dispensed aliquant. The evaluation results indicate that the generator performance has remained stable, and that the calibration parameters are still well within their given uncertainty intervals for the originally determined canonical values. Over a period of approximately six years, experimentally determined values of the Rn-222 emanation fraction, the most critical parameter, have remained constant and invariant of conditions within statistical variations of about 0.3% (corresponding to a relative standard deviation of the mean). All evaluation measurements were performed by 4 pi-alpha beta liquid scintillation spectrometry of gravimetrically determined aliquants dispensed from the generator.