Uncertainty of 222Rn Concentrations in the Usepa Radiation and Indoor Environments National Laboratory Exposure Chamber

Abstract

The U.S. Environmental Protection Agency (EPA) operates an environmentally controlled chamber for purposes of exposing various radon and decay product measurement equipment to known (222)Rn concentrations. Exposure durations range from 1 h to several months, and (222)Rn concentrations vary between 37 and 4,440 Bq m(-3). Radon concentrations are generated from Ra sources mounted on the chamber, but concentrations are continuously measured using equipment calibrated using (222)Rn generated from Ra laboratory standards made from a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) and verified through an aliquot measured by the NIST. The (222)Rn concentrations that are used to calibrate the equipment in the chamber are produced from bubbling a measured volume of air through the (226)Ra laboratory standard solution. This paper describes the process to derive an average chamber concentration during the exposure period. This includes the generation of laboratory (226)Ra standards from the original NIST SRM, (222)Rn emanation into a measured volume of air and transfer to Rocky Mountain Glassworks stainless steel scintillation cells to generate individual cell calibration factors, the calibration of larger mounted flow-through scintillation cells, and the reduction of cumulative chamber scintillation cell counts over various time periods. Each step is associated with an uncertainty based on measured and estimated factors, and each step adds an additional uncertainty to the cumulative total. The methods used follow guidance in the 1993 ISO Guide to the Expression of Uncertainty in Measurement and NIST Technical Note 1297. Chamber exposures of 96 h and (222)Rn concentrations between 150 and 2,700 Bq m(-3) are associated with a combined standard uncertainty of 4.1% (1 s). Ninety-six hour (96 h) exposures of lower (222)Rn concentrations of between 150 - 370 Bq m(-3) are associated with a combined standard uncertainty of 5%. Results indicate that there are measures that can be taken to reduce the estimated uncertainty slightly. The calculations used in this analysis have been incorporated into the computer code used to track chamber concentrations and exposures so that estimated uncertainties can be associated with each exposure.