Download the NRPP Radon Chamber Accreditation Application here.

Protocol and Guidance for NRPP Accreditation of Radon Chambers
Rev.  Mar 2022

The Purpose of this Document

This document is a set of guidelines that will be used to approve radon chambers wishing to become certified by NRPP as either:

1. Radon Reference Chambers (secondary); or
2. Tertiary Radon Chambers

It is intended that the guidelines in this document set minimal standards, which are sufficient to ensure that chambers certified under its rubric will produce a product of high quality and have the confidence of the radon industry and related authorities.

Limitations:

This document is not intended to set guidelines for radon decay product chambers that are used exclusively for calibration of radon decay product measurement devices. This document is not intended to replace existing draft or completed criteria for the certification of radon decay product chambers. See Section 3.0 that follows (formerly Section 2.) Various documents exist and they should be used by NRPP’s technical committees in the future, but these documents (for example, the IEC 45B/203/CDV “STAR” protocol, “Design Criteria and Performance Characteristics of Radon Chambers” by Andreas George and Nancy Bredhoff, and “Manual for the Radon Chamber Qualification (RCQ) Proficiency Program” by Jon A. Broadway) were not used, in whole or in part, in the writing of this protocol.

1.0 Radon Reference Chambers (secondary)

Radon Reference Chambers (secondary) are facilities that, at a minimum, intercompare directly with the national reference. They are commonly referred to as “secondary” chambers since they represent facilities that are one generation away, in terms of traceability, to the national or international reference. The NRPP recognizes the national reference for radon gas (radon-222) to be that as established by the USEPA radon chambers.  However, the NRPP retains the right to approve other reference sources in the event EPA’s facilities are unavailable to allow chamber activities that are required by this protocol.

Use of certified Radon Reference Chambers (secondary):

• Performance Testing: The quality associated with an NRPP certified Radon Reference Chamber is required for performance testing of NRPP certified labs and service providers (with analytical capabilities) that seek to obtain or maintain certification.
• Device Evaluations: This level of quality is also required by NRPP during final performance evaluations that can lead to NRPP approval or listing of a measurement system.
• Assessing Tertiary Chamber Quality (see Section 2): The quality associated with an NRPP certified Radon Reference Chamber is furthermore required by the NRPP during comparisons for assessing the quality of Tertiary Chambers that seek to obtain or maintain certification.
• This level of quality is often employed for other services that normally include, but are not limited to, spiking and calibration for quality control needs and evaluations of devices by manufacturers during product development.

1.1 Compliance with Regulations

The chamber facility must provide the NRPP evidence that it complies with all applicable Federal, State and local regulations concerning the ownership of radioactive materials and the discharging of radon gas into the environment. A worker safety document must be available upon request to the NRPP if it is required by any of the regulating agencies.

1.2 Quality Assurance/Quality Control Plan and SOP

The chamber facility must make available to the NRPP a Quality Assurance/Quality Control Plan which details the way in which the chamber routinely checks its own performance, the types of control charts or statistics it employs, and the mechanisms that are in place in the event that control charts or statistics indicate that corrective measures need to be taken. A list of the people and their responsibilities is also required.

The chamber facility must make available to the NRPP standard operating procedures (SOPs) detailing its current procedures related to conducting performance tests.

1.3 Radon Source

The radon source must be a shielded commercial radium-226 source from which the radon is bled off in a controlled fashion. Sources of radon consisting of soil, uranium mill tailings, phosphate tailings or similar materials are not considered acceptable for the purpose of these protocols.

1.4 Chamber Size and Quality of Design

Although a few radon chambers are large enough to walk inside, smaller chambers have proven reliable as long as the number and type of devices being spiked is commensurate with the volume of radon available for spiking. A minimum size of 50 cubic feet (1.4 m³) is strongly recommended because of the propensity for large radon fluctuations when devices are put into or taken out of the chamber, and the very real possibility of some depletion of the radon inside the chamber because of passive diffusion or forced convection of radon from the chamber and into the device being spiked. There is no maximum size limit to the chamber volume.

1.4.1 As part of the facility’s quality control plan, a method of verifying that the radon concentrations are homogeneous throughout the chamber is required. For assessing homogeneous conditions, the radon variations as measured by devices at locations within the chamber should not be larger than the variance otherwise found for the devices under ideal circumstances. For the purposes for measuring homogeneity, a 48-hour exposure is recommended.

1.4.2 As part of the facility’s quality control plan, gamma radiation within the chamber must be assessed throughout the chamber and maintained at a minimum. The facility’s quality control plan must ensure that gamma radiation does not interfere with measurement systems that monitor the chamber or the devices being analyzed.  

1.5 Required Intercomparisons

1.5.1 Regardless of any in-house calibrations or standards employed by the chamber to ascertain chamber values (NIST traceable or otherwise), all chambers shall conduct an intercomparison at least annually with an EPA facility or other facility approved by the NRPP. It is the results of this intercomparison that will be used by NRPP to ascertain the viability of the chamber, as described below.

1.5.2 At this time, it is expected that at least one EPA facility will be available for the purposes of the intercomparisons. In the event EPA’s facilities are unavailable, the NRPP retains the right to approve other reference sources.

1.5.2.1 To conduct the actual intercomparison, four scintillation or Lucas cells (with a minimum volume of 200 ml) are delivered simultaneously to the EPA chamber or other approved reference source. A complete set of instructions on how to conduct the sampling shall accompany the cells. Also, any tubing, pumps, valves, connectors, etc., must be supplied by the private radon chamber.

1.5.2.1.1 For Radon Reference Chambers (secondary), the only devices allowed to be used for intercomparison purposes will be scintillation or Lucas cells as described above. Passive devices (such as detectors containing charcoal, electret ion chambers and alpha-track detectors) do not have sufficient precision for this purpose. Some continuous radon monitors do have sufficient precision to be used as intercomparison devices, but the decision on which monitors could be used requires lengthy discussions by the technical board of NRPP, which have not yet occurred.

1.5.2.2 The four cells are exposed as simultaneously as possible to an identical radon concentration at the EPA facility. The cells are returned to the private chamber as quickly as possible and the EPA chamber informs the NRPP of the target radon concentration, including its estimate of total uncertainty at the 95% confidence level.

1.5.2.3 The private chamber personnel analyze the four cells and report the average and the standard deviation of each radon measurement to the NRPP. The NRPP compares the reported values to the EPA target value. A private chamber facility passes the intercomparison if the average of the four cells is within 10% of the EPA target value and every report measurement is within +/- 25% from the EPA target value.

1.6 Reference Systems and Monitoring Practices

1.6.1 The radon in the chamber must be monitored and recorded continuously with a continuous radon monitor which has a precision which is as good, or better, than the precision of the device which is being exposed within the chamber at the time and is approved by the NRPP for this use. The continuous monitor must be able to provide hourly readings at the minimum.

1.6.1.1 Maintaining Traceability:  

As part of the facility’s quality control plan, the continuous radon monitor (and other supplementary measurement systems) must be periodically analyzed compared to the measurement system used and values obtained from required intercomparisons (see 1.5). This periodic analysis is required to normalize and/or verify the continuous monitor’s readings.

• It is preferred, but not required, that the analysis is conducted daily (e.g. at least one grab sample be taken daily with the results compared to the continuous monitor’s output for the equivalent time period).
• If the radon chamber is being maintained at a constant value over a long time period, then a weekly analysis may be sufficient (e.g. one grab sample taken once a week as long as the chamber value is not varied during that week).
• As supplementary systems are employed, the minimum frequency that an analysis must be made for comparing the continuous monitor to the measurement system(s) employed for intercomparisons is monthly.  

Information gained from the facility’s quality control plan must support the minimum frequency that grab samples are conducted.

1.7 Control of Radon Concentrations

1.7.1 The chamber must be able to be controlled in such a way as to allow the change in radon concentration in a controlled manner. The radon concentration must be able to be set as low as 4.0 pCi/L and as high as 30 pCi/L to allow for a variety of exposures. It is preferred, but not necessary, that the chamber be able to be set as high as one hundred pCi/L to allow for relatively rapid exposure of long-term devices.

1.7.2 Once the radon concentration is set to a particular value, there shall be sufficient control of the radon source and/or chamber parameters to be able to maintain the new radon level consistently (defined as meaning that the coefficient of variation of the hourly measurements must be 10%, or less) over the exposure period for the particular device being tested. This does not preclude allowing the chamber operator to vary concentrations during exposures, as deemed appropriate, as long as the radon concentrations are monitored and accounted for or reconciled when reporting chamber exposures.

1.8 Environmental Controls

1.8.1 The temperature of the chamber must be able to be maintained at room temperature (68 degrees to 72 degrees Fahrenheit) over the entire exposure period for the particular device being tested. The temperature of the chamber must be monitored continuously with an hourly recording device that must be calibrated in accordance with national standards or manufacturers’ recommendations and traceable to NIST standards. It is preferred, but not necessary, that the temperature of the chamber be controlled in such a manner as to allow a change in temperature when desired. This does not preclude allowing the chamber operator to vary temperature during exposures as deemed appropriate as long as the temperatures are monitored and the hourly data is available and accounted for or reconciled when reporting chamber exposures.

 1.8.2 The humidity of the chamber must be constantly monitored and recorded at least hourly with a device that is calibrated in accordance with national standards or manufacturers’ recommendations and traceable to NIST standards. It is necessary that the chamber be able to maintain humidity conditions that reflect realistic (real-world) parameters. It is preferred, but not necessary, that the humidity inside the chamber be controlled in such a way as to allow the humidity to be varied at will and maintained at any new level set for the duration of an exposure period (dependent upon the device being tested). This does not preclude allowing the chamber operator to vary humidity during exposures, as deemed appropriate, as long as humidity is monitored and the hourly data is available and accounted for or reconciled when reporting chamber exposures.

1.8.3 It is preferred, but not necessary, that the barometric pressure within the chamber be monitored and recorded at least daily. This information may be useful for correcting calculations involving measurement devices that are sensitive to atmospheric pressure. Barometric pressure data available from a nearby airport, corrected for the station pressure at the elevation of the chamber facility, may be used for this purpose.

1.8.4 Elevation of the chamber shall be included in reports for chamber exposures. 

1.9 Business Practices

1.9.1 Because of the potential conflict of interest, a company or individual which runs a private radon chamber shall not performance test any lab or device associated with that company. For example, a laboratory which sells charcoal canisters and also runs a radon chamber shall not use the radon chamber, even if it is certified by NRPP as a performance testing chamber, to performance test its own charcoal canisters. This does not preclude, of course, the chamber from running performance tests on test devices marketed by other companies or from using its chamber to calibrate its own devices.

1.9.2 Companies or individuals that run private radon chambers (which are certified by NRPP as performance testing chambers) and also manufacture, market or sell radon measurement devices, or have any financial interest in radon measurement devices, shall notify potential performance test customers (laboratories and/or measurement professionals with analytical capabilities) of the types of devices which the company manufactures, markets or sells or has a financial interest in. In this way, the potential customer (laboratory and/or measurement professionals with analytical capabilities) can decide whether it wishes to have its device performance tested by a possible business competitor.

1.9.3 As long as the EPA action level remains at 4.0 pCi/L, it is expected that radon chambers certified by the NRPP will maintain radon concentrations of 4.0 pCi/L or higher during performance tests. Except for performance tests of grab devices, the minimum exposure time for the performance testing of all devices will be 48 hours. The minimum exposure (in pCi days/L) for a specific device is found by multiplying 4.0 pCi/L times the minimum number of days the device can be deployed (as determined from the advertising or manufacturer’s brochure accompanying the device). For example, for a device which is advertised as having a 91-day minimum exposure time, the minimum exposure in the chamber will be 4.0 pCi/L multiplied by 91 days, or 364 pCi days/L. However, in no case will the exposure time be less than 48 hours, regardless of the radon concentration used. Further, if the device participant specifies a minimum exposure duration greater than 48 hours, the chamber facility should comply with that request if possible.  For example, if the participant requests an exposure of no less than 12 days, the chamber facility may be able to comply.  However, if the participant requests an exposure of no less than 6 months, the chamber facility likely is not able to comply.

1.9.4 Minimal educational and/or training standards should include a BS in Physical Science and applicable experience in radiation measurement for a chamber designer and for a chamber operator. The designer and operator shall have at least the training equivalent to that of a two day Radon Measurement Operator course as approved by NRPP.

2.0 Tertiary Radon Chambers

Tertiary Radon Chambers are facilities that, at a minimum, intercompare with an accredited Radon Reference Chamber (secondary) as described in Section 1.0.   The term “tertiary” therefore indicates an additional or third generation away, in terms of traceability, to the national reference.

Use of certified Tertiary Radon Chambers:

• NRPP certified Tertiary Radon Chambers are used for many purposes including, but not limited to, initial evaluations of devices by manufacturers, spiking and calibration.

2.1 Compliance with Regulations (tertiary chambers): 

Tertiary chambers must comply with all provisions stipulated in Section 1.1.

2.2 Quality Assurance/Quality Control Plan and SOP (tertiary chambers): 

Tertiary chambers must comply with all provisions stipulated in Section 1.2.

2.3 Radon Source (tertiary chambers): 

For Tertiary chambers, a shielded commercial radium-226 source is recommended but other sources can be acceptable so long as the radon is bled off in a controlled fashion to ensure only radon-222 is present in the chamber.

2.4 Chamber Size and Quality of Design (tertiary chambers):   

Tertiary chambers must comply with all provisions stipulated in Section 1.4.

2.5 Required Intercomparisons (tertiary chambers): 

Note:  For the purpose of clarity, the term “Radon Reference Chamber” will be used below to indicate NRPP certified Radon Reference Chambers (secondary).

2.5.1 Regardless of any in-house calibrations or standards employed by the chamber to ascertain chamber values (NIST traceable or otherwise), all chambers shall conduct an intercomparison with a Radon Reference Chamber at least annually. It is the results of this intercomparison that will be used by NRPP to ascertain the viability of the chamber, as described below.

2.5.2.2 The measurement system used to monitor concentrations within the Tertiary Chamber is exposed in the Radon Reference Chamber. The Radon Reference Chamber rccords the target radon concentration, including it’s estimate of total uncertainty at the 95% confidence level.

2.5.2.3 The Tertiary Chamber personnel analyze the measurements taken and report the average and the standard deviation of each radon measurement to the Radon Reference Chamber. The Radon Reference Chamber renders a report that compares the Tertiary Chamber’s reported values to the target values of the Radon Reference Chamber.  Tertiary Chamber provides such reports to the NRPP as evidence of required intercomparisons.  The report should include an estimate of total uncertainty for the Chamber’s monitor or system, at the 95% confidence level.  A Tertiary Chamber facility passes the intercomparison if the average of the measurements they report are within 15% of the Radon Reference Chamber target value.

2.6 Reference Systems and Monitoring Practices (tertiary chambers): 

2.6.1 The radon in the chamber must be monitored and recorded continuously with a continuous radon monitor which has a precision which is as good, or better, than the precision of the device which is being exposed within the chamber at the time and is approved by the NRPP for this use. The continuous monitor must be able to provide hourly readings at the minimum.

2.7 Control of Radon Concentrations (tertiary chambers):  

The tertiary chamber must be able to control the range of radon concentrations in accordance with Section 1.7

2.8 Environmental Controls (tertiary chambers): 

2.8.1 Temperature: The temperature of the chamber must be able to be maintained at room temperature (68 degrees to 72 degrees Fahrenheit) over the entire exposure period for the particular device being tested. Tertiary chambers used to test or calibrate devices that are affected by temperature should monitor and record air temperature within the chamber in accordance with Section 1.8.1.

2.8.2 Humidity: Tertiary chambers used to test or calibrate devices that are affected by humidity should monitor and record air humidity within the chamber in accordance with Section 1.8.2.

2.8.3 Other Environmental Factors: Tertiary chambers used to test or calibrate devices that are affected by barometric pressure, gamma radiation or elevation should monitor and record those environmental factors and provided, accounted for or reconciled when reporting chamber exposures.

3.0 Radon Decay Product Chambers

3.1 Compliance with Regulations

The chamber must provide NRPP evidence that it complies with all applicable Federal, State and local regulations concerning the ownership of radioactive materials and the discharging of radon gas into the environment. A worker safety document must be provided if it is required by any of the above agencies. This document can be combined with the document required in section 1.1 if the chamber wishes to be certified in both radon and radon decay products.

3.2 Quality Assurance/Quality Control Plan and SOP

The chamber must provide NRPP a Quality Assurance/Quality Control Plan which details the way in which the chamber routinely checks its own performance, the types of control charts or statistics it employs, and the mechanisms that are in place in the event that control charts or statistics indicate corrective measures need to be taken. A list of the people and their responsibilities is also required. This document can be combined with the document required in section 1.2 if the chamber wishes to be certified in both radon and radon decay products.

The chamber must provide NRPP a standard operating plan (SOP) detailing its normal operating procedures. Any updates that the chamber makes to its SOP shall be forwarded to NRPP in a timely fashion.

3.3 Radon Decay Product Source

The radon decay product source will be a shielded commercial radium source from which the radon is bled off in a controlled fashion. In no case may the radon source be soil gas because of the inherent difficulties with soil gas (it is difficult to control and has widely varying amounts of thoron (radon 220)).

3.4 Chamber Size

Because of the propensity for radon decay products to plate out on the interior chamber walls, a minimum chamber volume of 125 cubic feet is required.

3.4 Environmental Controls

3.4.1 The chamber must be able to be controlled in such a way as to allow the change in radon decay product concentration in a controlled manner. A particle generator is required. The particle generator must have some method of being controlled so that the secular equilibrium of the chamber can be set and maintained.

3.4.2 Once the radon decay product concentration is set to a particular value, there shall be sufficient control of the radon and particle source and/or chamber parameters to be able to maintain the new radon decay product concentration at a constant value (defined as meaning that the coefficient of variation of the hourly measurements must be 10% or less) over the exposure period for the particular device being tested.

3.4.3 It must be possible to maintain the temperature of the chamber at room temperature (68 degrees to 72 degrees Fahrenheit) over the entire exposure period for the particular device being tested. The temperature of the chamber must be monitored continuously with an hourly recording device. It is preferred, but not necessary, that the temperature of the chamber be controlled in such a manner as to allow a change in temperature when desired.

3.4.4 The humidity of the chamber must be constantly monitored and recorded at least hourly. It is necessary that the chamber be able to maintain humidity conditions that reflect realistic (real-world) parameters. It is preferred, but not necessary, that the humidity inside the chamber be controlled in such a way as to allow the humidity to be varied at will and maintained at any new level set for the duration of an exposure period (dependent upon the device being tested).

3.4.5 It is preferred, but not necessary, that the barometric pressure within the chamber be monitored and recorded at least daily.

3.4.6 The radon decay product concentration in the chamber must be monitored continuously with a continuous working level monitor which has a precision comparable to the precision of the device which is being exposed within the chamber at the time.  The continuous monitor must be able to provide hourly readings at the minimum.

3.4.6.1 Periodic grab samples must be taken to normalize or verify the continuous monitor’s readings. It is required, that at least one grab sample be taken daily with the results compared to the continuous monitor’s output for the equivalent time period.

3.4.6.2 The grab samples shall be taken with a minimum flow rate of 10 L/minute and a minimum filter size of 1” diameter.  The flow rate needs to be measured accurately. The minimum sampling time for the grab sample is five minutes. Various counting regimes are allowed.

3.4.6.3 In order to calculate the counting efficiency of the detector used to analyze the grab sample, some kind of alpha-emitting standard using the same geometry as the filter is required.

3.5 Required Intercomparisons

3.5.1 Regardless of any in-house calibrations or standards (NIST traceable or otherwise) employed by the chamber to ascertain chamber values, all chambers shall have an intercomparison with an EPA chamber at least annually. It is the results of this intercomparison that will be used by NRPP to ascertain the viability of the chamber, as described below.

3.5.2 At this time, it is expected that at least one chamber at the Radiation and Indoor Environment National Laboratory at Las Vegas will be available for the purposes of the intercomparisons. Any costs involved will have to be borne by the private chambers using the Las Vegas facility.

3.5.2.1 To conduct the actual intercomparison, the chamber operator must take the radon decay product (RDP) grab sampling equipment to the EPA chamber and do the intercomparison in person. The same minimum characteristics which described an appropriate grab device in sections 2.4.6.2 and 2.4.6.3 apply.

3.5.2.2 Intercomparisons using RDP grabs require that four grabs are taken one after the other, while the target chamber (the EPA chamber) varies the working level concentration.  Specifically, two RDP samples (grabs) are taken at one chamber value and two RDP samples (grabs) are taken at a second value. The EPA chamber will then inform NRPP of the two chamber concentrations, including their standard deviations.

3.5.2.3 The private chamber will analyze the four samples and report the four individual results to the group at NRPP which will be assigned the responsibility of comparing these four values to the two EPA chamber results. A private chamber will have passed the intercomparison if (a) the average of each pair of samples is within 10% of the corresponding target chamber value and (b) no individual sample is more than 25% from the corresponding chamber value. By EPA chamber value it is meant the radon decay product concentration, plus and minus 2 standard deviations, in WL, as determined by the EPA using whatever methodology for producing the radon decay products and for determining the concentrations they feel are appropriate.

Note: the NRPP Technical committees may want to consider averaging the performance ratios (called “Bias factors”) as discussed in the U.S. EPA’s “Manual for the Radon Chamber Qualification (RCQ) Proficiency Program”, pages 3-17and 3-18, as an alternative to the passing indicators discussed above.

3.6 Business Practices

3.6.1 Because of the potential conflict of interest, a company or an individual which runs a private radon decay product chamber shall not performance test any lab or device associated with that company. For example, a laboratory which sells continuous working level monitors and also runs a performance testing radon decay product chamber shall not use the chamber, even if it is certified by NRPP as a performance testing chamber, to performance test its own continuous working level monitor. This does not preclude, of course, the chamber from running performance tests on test devices manufactured by other companies or from using its chamber to calibrate its own equipment.

3.6.2 Companies which run private radon decay product chambers (which are certified by NRPP as performance testing chambers) and also manufacture, market or sell radon decay product measurement devices or have any financial interest in the selling or marketing of such devices, shall notify potential performance test customers (laboratories and/or measurement professionals with analytical capabilities) of the types of devices which the company manufactures, markets or sells or has a financial interest in. In this way, the potential customer (laboratory and/or measurement professional with analytical capabilities) can decide if it wishes to have its device performance tested by a possible business competitor.

3.6.3 As long as the EPA action level remains at 4.0 pCi/L, it is expected that EHA certified radon decay product chambers will maintain radon decay product concentrations of .02 WL or higher during performance tests. Except for performance tests of grab devices, the minimum exposure time for the performance testing of all devices will be 48 hours.

3.6.4 Minimal educational and/or training standards must be set by the NRPP Technical board for a chamber designer and for a chamber operator. In the interim, it is necessary that the designer and operator have at least the training equivalent to that of a two day Radon Measurement Operator course as offered by the National Radon Training Centers.