Quality Assurance & Quality Control
Introduction to Radon Mitigation
Worker Health & Safety
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Continuous Radon Monitoring [aka Active Devices]

The CRM is a popular choice for its versatile application, tamper detection features, and convenience in time-sensitive measurements, e.g. real estate transactions.  While many types and brands are available for measurement practitioners, EPA says in The Protocols for Radon and Radon Decay Product Measurements in Homes, using instruments with greater efficiency, sensitivity, or signal-to-noise ratio can achieve results with a greater certainty than instruments with low efficiency, poor sensitivity or low signal-to-noise ratio.  EPA further highlights these instruments may facilitate tampering detection by being more sensitive to fluctuations in radon levels and recommend a minimum efficiency standard of 16 counts per hour per pCi/L.

As with any measurement device, routine performance checks prior to and after each measurement, annual calibrations, semi-annual instrument cross-checks, and quality control measurements, such as duplicates, spikes and background checks, be frequently employed.

Examples of Continuour Radon Monitors are as follows:

  1. Alpha Scintillation Chamber – CRMs
  2. Pulsed Ionization Chamber CRMs
  3. Solid State Silicon CRMs

Alpha Scintillation Chamber – CRMs

Alpha Scintillation Chamber consist of an alpha scintillation cell and photomultiplier tube counting system with timing circuitry and a control system. The Alpha Scintillation Chamber counts Alphas and sums the number of counts for a predetermined time period, stores the count in memory, then begins a new count for the next time period. (The microcomputer in some models will automatically convert the counts to pCi/L.)  This results in a series of short-term averages, reflecting the variations in radon concentration over time. Radon in the air can be sampled either by passive diffusion through a filter to remove RDPs in the cell or by continuously pumping filtered air through the cell. The sensitivity of these instruments typically ranges from 1 count per minute per picocurie per liter (1 cpm/pCi/L) to as high as 15 cpm/pCi/L, depending on the design.

The CRM runs continuously, recording the integrated radon concentration usually at hourly or half-hourly intervals. A CRM takes several hours to stabilize, so initial data points are normally discarded. This stabilization time is necessary because:

The time it takes the air in the chamber to be replaced with the room air.

  • The inherent delay in the RDP decay process.
  • The equilibration time between the instrument and the temperature and humidity of the room.

As with scintillation cells, the device must be calibrated in a radon chamber, usually semi-annually, and the background count rate should be checked regularly.  In addition, pump flow rates must be calibrated to ensure the volume of air sampled during the measurement is known.

Pulsed Ionization Chamber CRMs

Another type of commonly used CRM consists of a pulsed ionization chamber in conjunction with an electrometer and data logger.  The Pulsed Ionization Chamber also counts Alphas from the ambient air samples are delivered to the internal detector either by passive diffusion or active pumping.  RDPs are electrostatically removed and prevented from entering the internal pulsed ions sensing area. (As the initial Po-218 RDPs are created in air as positively charged ions, they are swept away from the sensing volume and collected on the negatively charged cabinet wall.  This prevents them and subsequent RDPs from contributing to the counts resulting from Radon-222 decay within the sensing volume.)

As radon gas atoms decay within the sensing volume of the chamber, a burst of ions is produced and converted to electrical pulses.  The pulses are counted, stored, displayed by the electrometer and the computerized data logger, and then converted to pCi/L using a calibration factor.  The data logger is programmed to subtract a fixed background, which is determined by performing periodic measurements of aged air. The sensitivity of this type of CRM is around 0.3 cpm/pCi/L.

Solid State Silicon CRMs

A third type of CRM relies on diffusion sampling, using a solid-state silicon detector to sense alpha decays. These units output continuous data to a printer and will display the current radon concentration. While not as sensitive as other CRMs (1 to 3 counts per hour/pCi/L), they are gaining acceptance because of their simple operation and comparatively low cost.  Care should be taken, however, to allow for exposure periods of sufficient length to obtain adequate counting statistics.

The CRM is a popular choice for its versatile application, tamper detection features, and convenience in time-sensitive measurements, e.g. real estate transactions.  While many types and brands are available for measurement practitioners, EPA says in The Protocols for Radon and Radon Decay Product Measurements in Homes, using instruments with greater efficiency, sensitivity, or signal-to-noise ratio can achieve results with a greater certainty than instruments with low efficiency, poor sensitivity or low signal-to-noise ratio.  EPA further highlights these instruments may facilitate tampering detection by being more sensitive to fluctuations in radon levels and recommend a minimum efficiency standard of 16 counts per hour per pCi/L.

As with any measurement device, routine performance checks prior to and after each measurement, annual calibrations, semi-annual instrument cross-checks, and quality control measurements, such as duplicates, which must be conducted 10% of the total number of measurements annually.

Chart 4-7
Continuous Radon Monitors – Chart of Advantages and Disadvantages