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Center for Indoor Environments - Radon Laboratory
Radiation and Indoor Environments Laboratory
Environmental Radon Chamber
The EPA’s Radiation and Indoor Environments National Laboratory maintains and operates an environmental radon chamber that has an internal volume of 18.41 cubic meters (Figure 1). The chamber is a walk-in unit with multiple sampling ports enabling a variety of instruments to be exposed simultaneously by being placed in the chamber or by drawing air from the chamber through the sampling ports (Figure 2). A computer controlled measurement and feedback system is used for the regulation of temperature, humidity, and radon-222 gas concentration.
Temperature is controlled in the chamber by a suspended metal plate through which either refrigerant or hot gas is flowed. Humidity is controlled using ahumidifier and a dehumidifier. Both temperature and relative humidityare measured continuously and adjusted as necessary by a computerized monitoring system. A continuous recirculation system provides a constant flow of air allowing for uniform mixing within the chamber.
Radon is obtained through radioactive decay of radium-226 sources are integrated into the chamber’s air flow system. A constant volume of dry air is flowed through the sources and then regulated using a mass flow controller. The mass flow is adjusted externally as part of a custom designed computer monitoring and control system for the chamber (Figure 3). The computer software adjusts the flow of radon gas along with a series of shunt valves to maintain either a constant or variable concentration of radon within the chamber as needed.
Radium salt sources are used as our radon gas source. Various sizes of radium salt sources are used depending upon the radon gas concentration needed in the environmental chamber (Figure 4).
A Gas transfer/gas flow system is used in the calibration of radon gas measurement systems (Figure 5). Note the radium bubbler/gas flow system on the right that is used to flow known radon gas from our radium bubblers. The known radon gas is inserted into a Tedlar bag and is transferred to our scintillation cells using the gas transfer system.
Custom designed 0.36 liter stainless steel Lucas scintillation cells are the primary radon measurement system used by the EPA radon laboratory to determine radon concentrations within the chamber. The cells are evacuated, filled with chamber air, and then analyzed for radon gas activity. This measurement system has been intercompared with NIST through performance tests. Results of these performance tests has resulted in a calculated bias between -0.98% to +2.37%.
Routine internal checks of bias and precision of the scintillation cells are conducted at the EPA radon laboratory through the use of a series of radon emanation flasks. These flasks containing radium-226 in solution were produced by EPA’s radio chemistry laboratory from NIST radium-226 ampoules. The counting system for the scintillation cells is routinely checked with a permanently sealed scintillation cell containing a small amount of radon-226 and producing a known count rate. These procedures are used to assess any changes in calibration factors of the scintillation cells and associated counters.
Continuous radon measurements of the chamber environment are conducted using two custom made 1.4 liter stainless steel flow-through scintillation cells coated with zinc sulfide and continuously counted on 12.7cm diameter photomultiplier tubes. The continuous flow through cells are calibrated using the smaller 0.36 liter scintillation cells filled with air from the chamber. The two continuous flow through cells operate independently from each other and provide hourly results of the radon concentration within the chamber. The average of the two continuous radon monitor results serves as the reference value for the radon chamber during the period of interest.
Charcoal Canister System
The EPA radon laboratory currently operates an 8 station, computer controlled, charcoal canister analysis system. The system employs sodium iodide detectors to analyze charcoal canisters using gamma spectroscopy. The system is currently calibrated to analyze 4 inch diameter, diffusion barrier charcoal canisters.
An 8 station charcoal canister analytical system using sodium iodide detectors is used (Figure 6). This system uses known cesium sources and check sources as the backbone for our quality assurance system. Charcoal canisters are routinely exposed in the environmental radon chamber to further ensure accurate and precise radon measurements are obtained.
The quality assurance system for this system includes the following elements:
- - Daily background counting.
- Daily calibration checks using cesium-137 sources to adjust peak centroids.
- Daily efficiency checks using radium-226 laced canisters.
- Monthly efficiency calibrations using radium-226 laced canisters.
- Posted quality control checks based on daily and monthly checks and calibrations.
- Multi-point quality control exposures in the EPA Radon Chamber for all lots of charcoal canisters purchased.
- Computer controlled quality control checks of exposure time and analyses to avoid overly long transit times for analyzed canisters.
- Fixed canister handling method with duplicate entry of canister exposure data to assure properly matched measured values to canister number.
Gas Transfer System
The radon laboratory has several radon bubblers (Figure 7) that are used as a known source of radon used in calibrating various system in the radon laboratory. These radon bubblers are glass containers containing a NIST traceable amount of radium. The gas transfer system to bubble a known amount of air through our radon bubblers that allows for the emanation of a precisely known amount of radon gas into our radon detection systems.
