The International Atomic Energy Agency (IAEA) in Vienna and the European Union (EU) in Bruxelles formed the "International Radon Metrology Programme" (IRMP, scientific secretary: F. Steinhäusler, University of Salzburg, Austria). The IRMP is designed to assess and foster the improvement of radon and decay product measurements that are made around the world. Within the framework of the IRMP, the U.S. Environmental Protection Agency Radiation and Indoor Environments National Laboratory (EPA) in Las Vegas, Nevada, organized jointly with the U.S. Bureau of Mines an international intercomparison exercise at a former uranium mine (Twilight Mine, Colorado) and the EPA Radon Laboratory. The main objective of this exercise was to compare radon and radon decay product instruments under both well-controlled as well as widely fluctuating exposure conditions. The laboratory exposures occurred under relatively steady radon and decay product conditions, with a moderate equilibrium ratio, while the conditions in the mine fluctuated greatly and the equilibrium ratio was low. An additional purpose of the exercise was to provide a forum for manufacturers and measurement organizations worldwide to exchange information and plan improvements in their operations and calibration programs. Altogether 19 organizations from seven countries intercomparing 32 different radon and radon decay product instruments participated in this exercise. This paper summarizes the results from the analysis of the experimental data obtained in the Bureau of Mines Twilight Mine in July of 1994, as well as the results from the EPA Radon laboratory in August of 1994.
A sample of commercially available, charcoal adsorption type, short-term radon detectors was blind tested under controlled laboratory conditions in order to obtain a "snapshot" of the accuracy and precision of the detectors. The results of the controlled exposures were then compared to a previous field study of the same type of commercially available radon detectors. Radon detectors, purchased from seven different commercial vendors, were exposed to a reference (222)Rn gas concentration at the U.S. Environmental Protection Agency's (EPA) Radon Chamber located at the Radiation and Indoor Environments National Laboratory in Las Vegas, Nevada. EPA Test 1 was performed under a controlled simulated field exposure paralleling, to the extent possible, the previous actual field exposure conditions. A second controlled exposure, EPA Test 2, was performed under a relatively steady state of (222)Rn gas concentration, at the same temperature, but a more moderate relative humidity. In the previous field setting evaluation of detectors, five out of six companies tested did not pass the accuracy guideline (all individual relative errors < or =25%) established during the EPA's former Radon Measurement Proficiency Program (EPA-RMPP). As compared to the field test, the detectors in this study generally exhibited better accuracy and precision. Not surprisingly, it appeared temporal fluctuations in radon concentrations and increased humidity had a negative influence on the accuracy and precision of detectors for some companies. The inability of three out of seven companies to meet former EPA-RMPP guidelines for accuracy, even under ideal exposure conditions (constant temperature, humidity, and radon concentration), highlights the importance of blind testing commercially available radon detectors. Furthermore, the consistent over-reporting or under-reporting trends in the overall results for all three tests suggest a potentially widespread systematic bias for the individual companies that merits further investigation. It is unknown if this one-time "snapshot" represents the overall reliability of commercially available charcoal-based radon detectors. Nonetheless, the findings suggest the need for improved vigilance to assure that the public can rely on commercially available radon detectors to make an informed decision whether or not to perform additional testing or to mitigate.
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.
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