Calibrations of Continuous Radon Monitors
The U.S. EPA has indicated in its protocols (Indoor Radon and Radon Decay Product Measurement Device Protocols, EPA 402-R-92-004 and Protocols for Radon and Radon Decay Product Measurements in Homes, EPA 402-R-92-003) and in Guidance on Quality Assurance (EPA 402-R-95-012) that continuous radon monitors should be calibrated at least annually. Users of continuous radon monitors must perform sufficient background measurements to establish a reliable instrument background. A background measurement should be performed as part of the calibration procedure. Further, a continuous radon monitor should be cross-checked with a monitor that produces results in the same unit (e.g., pCi/L) and that has been calibrated within the last three months. See Section 8.3.2 of Guidance on Quality Assurance for more details regarding calibrations and cross-checks.
We perform calibrations of continuous radon monitors by exposing them in the Bowser-Morner radon calibration chamber for at least 48 hours. The radon concentration in the chamber is typically held as constant as possible to a value of approximately 25 pCi/L although we have performed calibrations for specific applications at concentrations as small as 10pCi/L and as large as 100 pCi/L. The relative humidity and temperature are typically held at values of approximately 50% and 70° F, respectively; however, calibrations or evaluations of monitors can be performed at other humidities and temperatures.
A background measurement is performed by exposing the monitor to an atmosphere of aged nitrogen for at least 24 hours. For a monitor that has a photomultiplier tube, a background measurement and measurements to determine the plateau and proper operating voltage are included as part of the calibration procedure. Plateau measurements are made either by placing a standard scintillation cell in contact with the photomultiplier tube, or exposing the monitor to the atmosphere in the radon calibration chamber, whichever is appropriate, and making measurements with different voltages applied to the photomultiplier tube. The operating voltage is selected near the center of the plateau region for the photomultiplier tube, and subsequent background and calibration exposures are performed using that voltage.
For some types of continuous radon monitors, maintenance operations such as replacing batteries are included as part of the calibration procedure. Bowser-Morner is not a repair facility. If in our judgment, a monitor requires repair before it is calibrated, we advise the client of this situation.
Bowser-Morner personnel have received training from femto-TECH, Inc., Sun Nuclear Corporation, and Pylon Electronic Development Company to perform calibrations of their continuous radon monitors. We also have experience in calibrating other types of monitors, including custom-built instruments.
Call Jim Short at (937) 236-8805, Ext. 249, to schedule calibrations of continuous radon monitors. Calling ahead will help minimize the time that your monitors are out of service.
Spiking of Passive Radon Detectors
A spiked sample is a detector that has been exposed to a known concentration of radon in a calibration chamber. The U.S. EPA has indicated in its protocols (Indoor Radon and Radon Decay Product Measurement Device Protocols, EPA 402-R-92-004 and Protocols for Radon and Radon Decay Product Measurements in Homes, EPA 402-R-92-003) and in Guidance on Quality Assurance (EPA 402-R-95-012) that all organizations should arrange for the exposure of passive detectors in a radon calibration chamber on a regular basis. This is true for both "Analytical Service Providers," who analyze the detectors and produce the measurement results and "Residential Service Providers," who place and retrieve the detectors but do not analyze them.
According to Guidance on Quality Assurance (p. 8-11), "The EPA recommends that all organizations using passive devices expose, record and interpret the results of three spikes per 100 measurements (as average over the anticipated number of measurements during several months), with a minimum of three per year and a maximum (although more may be conducted) of six per month." Our interpretation of this requirement is that an organization should arrange for three spiked samples sometime during the period that 100 detectors are used in the field. Some organizations use detectors in large volumes and participate in a monthly or quarterly spiking program with Bowser-Morner. Organizations using detectors in smaller quantities arrange for spike samples once or twice per year. It is logical to assume that an organization that uses 100 or fewer detectors per year should spike one sample every four months to comply with the minimum requirement of three spikes per year. However, it is not possible to assess precision from one sample. Furthermore, to identify a possible outlier, at least three samples must be in the group. For these reasons, our recommendation is always to spike samples in groups of three or more. For an organization using 100 or fewer detectors per year, therefore, we recommend that one set of three samples be spiked sometime during the year.
Short-term passive devices (charcoal and electret ion chamber devices) are exposed in the Bowser-Morner chamber for a period of time that is consistent with how they are deployed in the field, usually two to seven days. The conditions in the chamber are held as constant as practicable with radon concentration in the range of 5 to 30 pCi/L, relative humidity in the range of 20% to 80%, and temperature at approximately 70° F.
Because of their lower sensitivity, long-term passive devices (alpha-track and electret ion chamber devices) must be given a larger radon exposure than short-term devices. They are exposed in the chamber by one of two methods. With the first method, they may be exposed for up to two weeks with relatively constant chamber conditions, usually with a radon concentration of approximately 110 pCi/L. The other method is to expose the detectors for three to four weeks, while the radon concentration and relative humidity may vary over the ranges stated for short-term devices.
If you'd like more information about how we can help you with radon monitor calibration and spiking, please let us know.
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