Device Protocols 3
3.1 Protocol for Using Continuous Working Level Monitors (CW) to Measure Indoor Radon Decay Product Concentrations
3.1.1 Purpose
This protocol provides guidance for using continuous working level monitors (CW) to obtain accurate and reproducible measurements of indoor radon decay product concentrations. Adherence to this protocol will help ensure uniformity among measurement programs and allow valid intercomparison of results. Measurements made in accordance with this protocol will produce results representative of closed-building conditions. Measurements made under closed-building conditions have a smaller variability and are more reproducible than measurements made when the building conditions are not controlled. The investigator should also follow guidance provided by the EPA in "Protocols for Radon and Radon Decay Product Measurements in Homes" (U.S. EPA 1992c) or other appropriate EPA measurement guidance documents.
3.1.2 Scope
This protocol covers, in general terms, the sample collection and analysis method, the equipment needed, and the quality control objectives of measurements made with CW. It is not meant to replace an instrument manual but, rather, provides guidelines to be incorporated into standard operating procedures by anyone providing measurement services. Questions about these guidelines should be directed to the U.S. Environmental Protection Agency.
3.1.3 Method
The CW method samples the ambient air by filtering airborne particles as the air is drawn through a filter cartridge at a low flow rate of about 0.1 to one liter per minute. An alpha detector such as a diffused-junction or surface-barrier detector counts the alpha particles produced by the radon decay products as they decay on the filter. The detector is set normally to detect alpha particles with energies between two and eight MeV. The alpha particles emitted from the radon decay products radium A (Po-218) and radium C' (Po-214) are the significant contributors to the events that are measured by the detector. All CW detectors are capable of measuring individual radon and thoron decay products, while some can be adapted to measure the percentage of thoron decay products. The event count is directly proportional to the number of alpha particles emitted by the radon decay products on the filter. The unit contains typically a microprocessor that stores the number of counts and elapsed time. The CW detector can be set to record the total counts registered over specified time periods. The unit must be calibrated in a calibration facility to convert count rate to Working Level (WL) values. This may be done initially by the manufacturer, and should be done periodically thereafter by the operator.
3.1.4 Equipment
In addition to the CW detector, equipment needed includes replacement filters, a readout or programming device (if not part of the detector), an alpha-emitting check source, and an air flow rate meter.
3.1.5 Predeployment Considerations
The plans of the occupant during the proposed measurement period should be considered before deployment. The CW measurement should not be made if the occupant will be moving during the measurement period. Deployment should be delayed until the new occupant is settled in the house.
The CW detector should not be deployed if the user's schedule prohibits terminating the measurement at the appropriate time.
3.1.5.1 Pre-Sampling Testing. The CW detector should be tested carefully before and after each measurement in order to:
Verify that a new filter has been installed and the input parameters and clock are set properly;
Measure the detector's efficiency with a check source such as Am-241 or Th-230 and ascertain that it compares well with the technical specifications for the unit; and
Verify the operation of the pump.
When feasible, the unit should be checked after every fourth 48-hour measurement or week of operation to measure the background count rate using the procedures that are in the operating manual for the instrument.
In addition, participation in a laboratory intercomparison program should be conducted initially and at least once every 12 months thereafter, and after equipment repair, to verify that the conversion factor used by the microprocessor is accurate. This is done by comparing the unit's response to a known radon decay product concentration. At this time, the correct operation of the pump also should be verified by measuring the flow rate.
3.1.6 Measurement Criteria
The reader should refer to Section 1.2.2 for the list of general conditions that must be met to ensure standardization of measurement conditions.
3.1.7 Deployment and Operation
3.1.7.1 Location Selection. The reader should refer to Section 1.2.3 for standard criteria that must be considered when choosing a measurement device location.
3.1.7.2 Operation. The CW detector should be programmed to run continuously, recording the periodic integrated WL and, when possible, the total integrated average WL. The sampling period should be 48 hours, with a grace period of two hours (i.e., a sampling period of 46 hours is acceptable if conditions prohibit terminating sampling after exactly 48 hours). The longer the operating time, the smaller the uncertainty associated with using the measurement result to estimate a longer-term average concentration. The integrated average WL over the measurement period should be reported as the measurement result. If results are also reported in pCi/L, it should be stated that this approximate conversion is based on a 50 percent equilibrium ratio, which is typical of the home environment, and any individual environment may have a different relationship between radon and decay products.
3.1.8 Retrieval of Detectors
When the measurement is terminated, the operator should note the stop-date and -time and whether the standardized conditions are still in effect.
3.1.9 Documentation
The reader should refer to Section 1.2.4 for the list of standard information that must be documented so that data interpretation and comparison can be made.
In addition, the serial number of the CW detector and calibration factor used should be recorded.
3.1.10 Analysis Requirements
3.1.10.1 Sensitivity. All known commercially available CW detectors are capable of a lower limit of detection (LLD [calculated using methods described by Altshuler and Pasternack 1963]) of 0.01 WL or less.
3.1.10.2. Precision. Precision should be monitored and recorded using the results of side-by-side measurements described in Section 3.1.11.3 of this protocol. This method can produce duplicate measurements with a coefficient of variation of 10 percent or less at 0.02 WL or greater. An alternate measure of precision is a relative percent difference, defined as the difference between two duplicate measurements divided by their mean; note that these two measures of precision are not identical quantities. It is important that precision be monitored frequently over a range of radon concentrations and that a systematic and documented method for evaluating changes in precision be part of the operating procedures.
3.1.11 Quality Assurance
The quality assurance program for a CW system includes four parts: (1) calibration and known exposures, (2) background measurements, (3) duplicate measurements, and (4) routine instrument checks. The purpose of a quality assurance program is to identify the accuracy and precision of the measurements and to ensure that the measurements are not influenced by exposure from sources outside the environment to be measured. The quality assurance program should include the maintenance of control charts (Goldin 1984); general information is also available (Taylor 1987, U.S. EPA 1984).
3.1.11.1 Calibration and Known Exposures. Every CW detector should be calibrated in a radon calibration chamber before being put into service, and after any repairs or modifications. Subsequent recalibrations should be done once every 12 months, with cross-checks to a recently calibrated instrument at least semiannually.
3.1.11.2 Background Measurements. Background count rate checks must be conducted after at least every 168 hours (fourth 48-hour measurement) of operation and whenever the unit is calibrated. The CW should be purged with clean, aged air or nitrogen in accordance with the procedures given in the instrument's operating manual. In addition, the background count rate may be monitored more frequently by operating the CW in a low radon environment.
3.1.11.3 Duplicate Measurements. When two or more CW detectors are available, the precision of the measurements can be estimated by operating the detectors side-by-side. The analysis of duplicate results should follow the methodology described by Goldin (section 5.3 in Goldin 1984), by Taylor (Taylor 1987), or by the EPA (U.S. EPA 1984). Whatever procedures are used must be documented prior to beginning measurements. Consistent failure in duplicate agreement may indicate a problem in the measurement process and should be investigated.
3.1.11.4 Routine Instrument Checks. Checks using an Am-241 or similar-energy alpha check source must be performed before and after each measurement. In addition, it is important to check regularly all components of the equipment that affect the result.
Pump and flow meters should be checked routinely to ensure accuracy of volume measurements. This may be performed using a dry-gas meter or other flow measurement device of traceable accuracy.
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3.2 Protocol for Using Radon Progeny Integrating Sampling Units (RPISU or RP) to Measure Indoor Radon Decay Product Concentrations
3.2.1 Purpose
This protocol provides guidance for using radon progeny integrating sampling units (RPISU or RP) to produce accurate and reproducible measurements of indoor radon decay product concentrations. Adherence to this procedure will help ensure uniformity in measurement programs and allow valid intercomparison of results. Measurements made in accordance with this protocol will produce results representative of closed-building conditions. Measurements made under closed-building conditions have a smaller variability and are more reproducible than measurements made when the building conditions are not controlled. The investigator should also follow guidance provided by the EPA in >Protocols for Radon and Radon Decay Product Measurements in Homes (U.S. EPA 1992c) or other appropriate EPA measurement guidance documents.
3.2.2 Scope
This protocol covers, in general terms, the equipment, procedures, analysis, and quality control objectives for measurements made with RPs. It is not meant to replace an instrument manual but, rather, provides guidelines to be incorporated into standard operating procedures by anyone providing measurement services. Questions about these guidelines should be directed to the U.S. Environmental Protection Agency.
3.2.3 Method
3.2.3.1 Thermoluminescent Dosimeter (TLD) RP. There are three types of RPs. The TLD type contains an air sampling pump that draws a continuous, uniform flow of air through a detector assembly. The detector assembly includes a filter and at least two TLDs. One TLD measures the radiation emitted from radon decay products collected on the filter, and the other TLD is used for a background gamma correction. This RP is intended for a sampling period of 48 hours to a few weeks.
Analysis of the detector TLDs is performed in a laboratory using a TLD reader. Interpretation of the results of this measurement requires a calibration for the detector and the analysis system based on exposures to known concentrations of radon decay products.
3.2.3.2 Alpha Track Detector (ATD) RP. A second type of RP consists of an air sampling pump and an ATD assembly. The air sampling pump draws a continuous, uniform flow of air through a filter in the detector assembly where the radon decay products are deposited. Opposite to the side of the filter where the radon decay products are deposited is a cylinder with three collimating cylindrical holes. Alpha particles emitted from the radon decay products on the filter pass through the collimating holes and through different thicknesses of energy-absorbing film before impinging on a disc of alpha track detecting plastic film (LR-115 or CR-39). Analysis of the number of alpha particle tracks in each of the three sectors of the film allows the determination of the number of alpha particles derived from radium A (Po-218) and radium C' (Po-214). This feature allows the determination of the equilibrium factor for the radon decay products. This type of RP is intended for a sampling period of about 48 hours to a few weeks.
Etching and counting of the alpha track assembly is carried out by mailing the detector film to the analysis laboratory. Interpretation of the results of this measurement requires a calibration for the detector and the analysis system based on exposure to known concentrations of radon decay products.
3.2.3.3 Electret RP. The electret RP is similar in operation to the TLD-type RP, except that the TLD is replaced with an electret. The current model of this device contains a one-liter-per-minute constant air flow pump and collects the decay products on a 11.4 cm2 filter. As the radon decay products that are collected on the filter decay, negatively charged ions generated by alpha particle radiation are collected on a positively-charged electret, thereby reducing its surface voltage. This reduction has been demonstrated to be proportional to the radon decay product concentration. For more general information on electrets, the reader should refer to Section 2.3.
RPs are true integrating instruments if the pump flow rate is uniform throughout the sampling period. The electret must be removed from the chamber and the electret voltage measured with a special surface voltmeter both before and after exposure. To determine the average radon concentration during the exposure period, the difference between the initial and final voltages is divided first by a calibration factor and then by the number of exposure days. A background radon concentration equivalent of ambient gamma radiation is subtracted to compute radon concentration. Electret voltage measurements can be made in a laboratory or in the field.
3.2.4 Equipment
The three types of RP sampling systems include a sampling pump and the detector assembly. Sampling with the TLD-type RP requires either a fresh detector assembly or fresh TLD chips to be inserted in the detector assembly. Using the electret-type RP requires a sufficient charge on the electret. Sampling with the ATD-type RP requires a fresh detector disc (LR-115 or CR-39). An air flow rate meter should be available for checking flow rates with the RP, and spare filters should be available as replacements as needed.
3.2.5 Predeployment Considerations
The plans of the occupant during the proposed measurement period should be considered before deployment. The RP measurement should not be made if the occupant will be moving during the measurement period. Deployment should be delayed until the new occupant is settled in the house.
The RPISU should not be deployed if the user's schedule prohibits terminating the measurement at the appropriate time.
Prior to installation in the building, the pump should be checked to ensure that it is operable and capable of maintaining a uniform flow through the detector assembly. Extra pump assemblies should be available during deployment in case a problem is encountered.
Arrangements should be made with the occupant of the building to ensure that entry into the building is possible at the time of installation, and to determine availability of a suitable electrical outlet near the sampling area in the selected room.
3.2.6 Measurement Criteria
The reader should refer to Section 1.2.2 for the list of general conditions that must be met to ensure standardization of measurement conditions.
3.2.7 Deployment and Operation
3.2.7.1 Location Selection. The reader should refer to Section 1.2.3 for standard criteria that must be considered when choosing a measurement device location.
In addition, the air intake (sampling head) should be placed at least 50 centimeters (20 inches) above the floor and at least 10 centimeters (four inches) from surfaces that may obstruct flow.
3.2.7.2 Operation. The RP should be installed and, if possible, the air flow rate checked with a calibrated flow meter. The location, date, starting time, running-time meter reading, and flow rate should be recorded on the detector assembly envelope and in a log. The RP should be observed for a few minutes after initiating measurements to ensure continued operation. The occupants should also be informed about the RP and requested that they report any problems or pump shut-down. The occupants should be aware of the length of time the RP will be operated, and an appointment should be arranged to retrieve the unit. The criteria for the standardized measurement conditions (Section 1.2.2) should also be told to the occupants.
The sampling period should be at least 48 hours, and may need to be longer, depending on the type of RP head. A longer operating time decreases the uncertainty associated with the measurement result.
3.2.8 Retrieval of Devices
Prior to pump shut-down, the flow rate should be measured with a calibrated flow meter (if possible) and the unit should be observed briefly to ensure that it is operating properly. The detector assembly or detector film should be removed for processing and the date, time, running-time meter reading, and flow rate should be recorded both on the envelope and in a log book. The filter should be checked for holes or dust loading and any other observed conditions that might affect the measurement. If TLDs or film discs are to be removed from the detector assembly, removal should be delayed for at least three hours after sampling is completed to allow for decay and registration of radon decay products on the filter.
3.2.9 Documentation
The reader should refer to Section 1.2.4 for the list of standard information that must be documented so that data interpretation and comparison can be made.
In addition, the serial numbers of the RPs, TLDs, film discs, or electrets must be recorded.
3.2.10 Analysis Requirements
Analysis of the film from the ATD-type RPs requires an analysis laboratory equipped to etch and count alpha track film.
Analysis of TLD-type RPs requires a TLD reader. The TLD reader is an instrument that heats the TLDs at a uniform and reproducible rate and measures simultaneously the light emitted by the thermoluminescent material. The readout process is controlled carefully, with the detector purged with nitrogen to prevent spurious emissions. Prior to analyzing the RPISU dosimeters, the TLD reader should be tested periodically using dosimeters exposed to a known level of alpha or gamma radiation. TLDs are prepared for reuse by cleaning and annealing at the prescribed temperature in an oven.
Analysis of the electret-type RPs requires a specially-built surface voltmeter for measuring electret voltages before and after exposure. For more information on analysis requirements, the reader should refer to Section 2.3.10 (Electret Ion Chamber Radon Detectors) of the Radon Measurement Device Protocols.
3.2.10.1 Sensitivity. The lower limit of detection (LLD [calculated using methods described by Altshuler and Pasternack 1963]) should be specified by individual suppliers for RP detectors exposed according to their directions. The LLD will depend upon the length of the exposure and the background of the detector for materials used. The LLD should be calculated using the results of the laboratory control devices.
3.2.10.2 Precision. Precision should be monitored and recorded using the results of the duplicate detector analyses described in Section 3.2.11.3. This method may achieve a coefficient of variation of 10 percent at radon decay product concentrations of 0.02 WL or greater. An alternate measure of precision is a relative percent difference, defined as the difference between two duplicate measurements divided by their mean; note that these two measures of precision are not identical quantities. It is important that precision be monitored continuously over a range of radon concentrations and that a systematic and documented method for evaluating changes in precision be part of the operating procedures.
3.2.11 Quality Assurance
The quality assurance program for an RP system includes five parts: (1) calibration, (2) known exposure detectors, (3) duplicate (collocated) detectors, (4) control detectors, and (5) routine instrument checks. The purpose of a quality assurance program is to identify the accuracy and precision of the measurements and to ensure that the measurements are not influenced by exposure from sources outside the environment to be measured. The quality assurance program should include the maintenance of control charts (Goldin 1984); general information is also available (Taylor 1987, U.S. EPA 1984).
Users of electret-type RPs should follow the quality assurance guidance given for electret ion chamber devices in Section 2.3 of this document.
3.2.11.1 Calibration. Every RP should be calibrated in a radon calibration chamber before being put into service, and after any repairs or modifications. Subsequent recalibrations should be done once every 12 months, with cross-checks to a recently calibrated instrument at least semiannually. Calibration of RPs requires exposure in a controlled radon-exposure chamber where the radon decay product concentration is known during the exposure period. The detector must be exposed in the chamber using the normal operating flow rate for the RP sampling pumps. Calibration should include exposure of a minimum of four detectors exposed at different radon decay product concentrations representative of the range found in routine measurements. The relationship of TLD reader units or etched track reader units to working level (WL) for a given sample volume and the standard error associated with this measurement should be determined. Calibration of the RPs also includes testing to ensure accuracy of the flow rate measurement.
3.2.11.2 Known Exposure Devices. Anyone providing measurement services with RP devices should submit detectors with known decay product exposures (spiked samples) for analysis at a rate of three per 100 measurements, with a minimum of three per year and a maximum required of six per month. Known exposure detectors should be labeled in the same manner as the field detectors to assure blind processing. The results of the known exposure detector analysis should be monitored and recorded, and any significant deviation from the known concentration to which they were exposed should be investigated.
3.2.11.3 Duplicate (Collocated) Detectors. Anyone providing measurement services with RP devices should place duplicate detectors in enough houses to test the precision of the measurement. The number of duplicate detectors deployed should be approximately 10 percent of the number of detectors deployed each month or 50, whichever is smaller. The duplicate detectors should be shipped, stored, exposed, and analyzed under the same conditions. The samples selected for duplication should be distributed systematically throughout the entire population of samples. Groups selling measurement services to homeowners can do this by making two side-by-side measurements in a random selection of homes. Data from duplicate detectors should be evaluated using the procedures described by Goldin (section 5.3 in Goldin 1984), by Taylor (Taylor 1987), or by the EPA (U.S. EPA 1984). Whatever procedures are used must be documented prior to beginning measurements. Consistent failure in duplicate agreement may indicate a problem in the measurement process and should be investigated.
3.2.11.4 Control Detectors. TLD-type RPs use a TLD that is shielded from the gamma radiation emitted by the material on the filter. This TLD is incorporated in the detector assembly to measure the environmental gamma exposure of the sampling detector. The two TLDs are processed identically and the environmental gamma exposure is subtracted from the sample reading. Electret-type RPs also require an environmental gamma background correction.
3.2.11.4.1 Laboratory Control Detectors. The laboratory background level for each batch of assembled TLDs should be established by each supplier. Suppliers should measure the background of a statistically significant number of unexposed thermoluminescent assemblies that have been processed according to their standard operating procedures. To calculate the net readings used to calculate the reported sample radon concentrations, the analysis laboratory subtracts this laboratory blank value from the results obtained from the field detectors.
Similarly, the laboratory background level for each batch of ATD-type RPs should be established by each supplier of these detectors. Suppliers should measure the background of a statistically significant number of unexposed detector films that have been processed according to their standard operating procedures. The analysis laboratory will subtract this laboratory blank value from the results obtained from the field detectors before calculating the final result.
Users of electret-type RPs should follow similar control detector procedures discussed in section 2.3.11.1.
3.2.11.4.2 Field Control Detectors (Blanks). Field control detectors (field blanks) should consist of a minimum of five percent of the detectors deployed each month or 25, whichever is smaller. Users should set these aside from each shipment, keep them sealed, label them in the same manner as the field detectors, and, where applicable, send them back to the analysis laboratory as blind controls with one shipment each month. These field blank detectors measure the background exposure that may accumulate during shipment or storage. The results should be monitored and recorded. If one or a few of the field blanks have concentrations significantly greater than the LLD established by the supplier, it may indicate defective material or procedures. If the average value from the background control detectors (field blanks) is significantly greater than the LLD established by the supplier, this average value should be subtracted from the individual values reported for the other detectors in the exposure group. The cause for the elevated field blank readings should then be investigated.
3.2.11.5 Routine Instrument Checks. Proper operation of all analysis equipment requires that their response to a reference source be constant to within established limits. Therefore, analysis equipment should be subject to routine checks to ensure proper operation. This is achieved by counting an instrument check source at least once per day during operation.
Pumps and flow meters should be checked routinely to ensure accuracy of volume measurements. This may be performed using a dry-gas meter or other flow measurement device of traceable accuracy.
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3.3 Protocol for Using Grab Sampling - Working Level (GW) to Measure Indoor Radon Decay Product Concentrations
3.3.1 Purpose
This protocol provides guidance for using the grab sampling-working level (GW) technique to provide accurate and reproducible measurements of indoor radon decay product concentrations. Adherence to this protocol will help ensure uniformity among measurement programs and allow valid intercomparison of results. Measurements made in accordance with this procedure will produce results representative of closed-building conditions. Measurements made under closed-building conditions have a smaller variability and are more reproducible than measurements made when the building conditions are not controlled.
The results of the GW method are influenced greatly by conditions that exist in the building during and for up to 12 hours prior to the measurement. It is therefore especially important when making grab measurements to conform to the closed-building conditions for 12 hours before the measurement. Grab sampling techniques are not recommended for measurements made to determine the need for remedial action. The investigator should also follow guidance provided by the EPA in Protocols for Radon and Radon Decay Product Measurements in Homes (U.S. EPA 1992c) or other appropriate EPA measurement guidance documents.
3.3.2 Scope
This procedure covers, in general terms, the equipment, procedures, and quality control objectives to be used in performing the measurements. It is not meant to replace an instrument manual but, rather, provides guidelines to be incorporated into standard operating procedures by anyone providing measurement services. Questions about these guidelines should be directed to the U.S. Environmental Protection Agency.
3.3.3 Method
Grab sampling measurements of radon decay product concentrations in air are performed by collecting the decay products from a known volume of air on a filter and by counting the activity on the filter during or following collection. Several methods for performing such measurements have been developed and have been described previously (George 1980). Comparable results may be obtained using all these methods. This procedure, however, will describe two methods that have been used most widely with good results. These are the Kusnetz procedure and the modified Tsivoglou procedure.
The Kusnetz procedure (ANSI 1973, Kusnetz 1956) may be used to obtain results in working levels (WL) when the concentration of individual decay products is unimportant. Decay products from up to 100 liters of air are collected on a filter in a five-minute sampling period. The total alpha activity on the filter is counted at any time between 40 and 90 minutes after the end of sampling. Counting can be done using a scintillation-type counter to obtain gross alpha counts for the selected period. Counts from the filter are converted to disintegrations using the appropriate counter efficiency. The disintegrations from the decay products collected from the known volume of air may be converted into WLs using the appropriate "Kusnetz factor" (see Section 3.3.11.3., Exhibit 3-1) for the counting time used.
The Tsivoglou procedure (Tsivoglou et al. 1953), as modified by Thomas (Thomas 1972), may be used to determine WL and the concentration of the individual radon decay products. Sampling is the same as that used for the Kusnetz procedure; however, the filter is counted three separate times following collection. The filter is counted between the interval of two to five minutes, six to 20 minutes, and 21 to 30 minutes, following completion of sampling. Count results are used in a series of equations to calculate concentrations of the three radon decay products and WL. These equations and an example calculation appear in Section 3.3.11.4.1.
3.3.4 Equipment
Equipment required for radon decay product concentration determination by GW consists of the following items:
An air sampling pump capable of maintaining a flow rate of two to 25 liters per minute through the selected filter. The flow rate should not vary significantly during the sampling period;
A filter holder (with adapters for attachment) to accept a 25- or 47-mm diameter, 0.8-micron membrane or glass fiber filter;
A calibrated air flow measurement device to determine the air flow through the filter during sampling;
A stopwatch or timer for accurate timing of sampling and counting;
A scintillation counter and a zinc sulfide scintillation disc;
A National Institute of Standards and Technology (NIST)-traceable alpha calibration source to determine counter efficiency; and
A data collection log.
3.3.5 Predeployment Considerations
The plans of the occupant during the proposed measurement period should be considered before deployment. The GW measurement should not be made if the occupant will be moving during the measurement period. Deployment should be delayed until the new occupant is settled in the house.
The GW device should not be deployed if the user's schedule prohibits terminating the measurement at the appropriate time.
3.3.5.1 Premeasurement Testing
Prior to collection of the sample, proper operation of the equipment must be verified, and the counter efficiency and background must be determined. This is especially critical for the Tsivoglou procedure, in which the sample counting must begin two minutes following the end of sampling.
The air pump, filter assembly, and flow meter must be tested to ensure that there are no leaks in the system. The scintillation counter must be operated with the scintillation tray (where applicable) and scintillation disc in place to determine background for the counting system. Also, the counter must be operated with an NIST-traceable alpha calibration source in place of a filter in the counting location to determine system counting efficiency. Both the system background and system efficiency are used in the calculation of results from the actual sample.
3.3.6 Measurement Criteria
The reader should refer to Section 1.2.2 for the list of general conditions that must be met to ensure standardization of measurement conditions.
3.3.7 Deployment
3.3.7.1 Location in Room. The reader should refer to Section 1.2.3 for standard criteria that must be considered when choosing a measurement device location.
3.3.7.2 Sampling. A new filter should be placed in the filter holder prior to entering the building. Care should be taken to avoid puncturing the filter and to avoid leakage. The sampling is initiated by starting the pump and the clock simultaneously. The air flow rate should be noted and recorded in a log book. The time the sampling was begun should also be recorded. The sampling period should be five minutes, and the time from the beginning of sampling to the time of counting must be recorded precisely.
3.3.8 Documentation
The reader should refer to Section 1.2.4 for the list of standard information that must be documented so that data interpretation and comparison can be made.
3.3.9 Analysis Requirements
Analysis may be done using the Kusnetz procedure (ANSI 1973, Kusnetz 1956), the modified Tsivoglou procedure (Thomas 1972, Tsivoglou et al. 1953), or other procedures described elsewhere (George 1980). If the Tsivoglou procedure is used, the counting must be started two minutes following the end of sampling. Analysis using the Kusnetz procedure must be performed between 40 and 90 minutes following the end of sampling. A counting time of 10 minutes during this period is usually used. The reader should refer to Sections 3.3.3 and 3.3.11 for more information.
The filter from the holder must be removed using forceps, and placed carefully facing the scintillation phosphor. The side of the filter on which the decay products were collected must face the phosphor disc. The chamber containing the filter and disc should be closed and allowed to dark-adapt prior to starting counting. For the Tsivoglou method, this procedure of placing the filter in the counting position must be done quickly, since the first of the three counts must begin two minutes following the end of sampling. If the counter used has been shown to be slow to dark-adapt, the counting should be done in a darkened environment. Additional details on the procedure and calculations are available (Kusnetz 1956, Thomas 1972, Tsivoglou et al. 1953).
3.3.9.1 Sensitivity. For a five-minute sampling period (10 to 20 liters of air) on a 25-mm filter, the lower limit of detection (LLD [calculated using methods described by Altshuler and Pasternack 1963]) using the Kusnetz or modified Tsivoglou counting procedure can be approximately 0.0005 WL (George 1980).
3.3.9.2 Precision. Precision should be monitored using the results of duplicate measurements (refer to Section 3.4.10.2). Sources of error in the procedure may result from inaccuracies in measuring the volume of air sampled, characteristics of the filter used, and measurement of the amount of radioactivity on the filter. The method can produce duplicate measurements with a coefficient of variation of 10 percent or less at 0.02 WL or greater. An alternate measure of precision is a relative percent difference, defined as the difference between two duplicate measurements divided by their mean; note that these two measures of precision are not identical quantities. It is important that precision be monitored continuously over a range of radon concentrations and that a systematic and documented method for evaluating changes in precision be part of the operating procedures.
3.3.10 Quality Assurance
The quality assurance program for a GW system includes three parts: (1) calibration of the system, (2) duplicate measurements, and (3) routine instrument checks. The purpose of a quality assurance program is to identify the accuracy and precision of the measurements and to ensure that the measurements are not influenced by exposure from sources outside the environment to be measured. The quality assurance program should include the maintenance of control charts (Goldin 1984); general information is also available (Taylor 1987, U.S. EPA 1984).
3.3.10.1 Calibration. Pumps and flow meters used to sample air must be calibrated routinely to ensure accuracy of volume measurements. This may be performed using a dry-gas meter or other flow measurement device of traceable accuracy.
Every GW device should be calibrated in a radon (decay product) calibration chamber before being put into service, and after any repairs or modifications. Subsequent recalibrations should be done once every 12 months, with cross-checks to a recently calibrated instrument at least semiannually. Grab measurements should be made in a calibration chamber with known radon decay product concentrations to verify the calibration factor. These measurements should also be used to test the collection efficiency and self-absorption of the filter material being used for sampling. A change in the filter material being used requires that the new material be checked for collection efficiency in a calibration chamber.
3.3.10.2 Duplicate Measurements. Anyone providing measurement services with GW devices should place duplicate detectors in enough houses to test the precision of the measurement. The number of duplicate detectors deployed should be approximately 10 percent of the number of detectors deployed each month or 50, whichever is smaller. To the greatest extent possible, care should be taken to ensure that the samples are duplicates. The filter heads should be relatively close to each other and away from drafts. Care should also be taken to ensure that one filter is not in the discharge air stream of the other sampler. The measurements selected for duplication should be distributed systematically throughout the entire population of measurements. Data from duplicate samples should be evaluated using the procedures described by Goldin (section 5.3 of Goldin 1984), by Taylor (Taylor 1987), or by the EPA (U.S. EPA 1984). Whatever procedures are used must be documented prior to beginning measurements. Consistent failure in duplicate agreement may indicate a problem in the measurement process and should be investigated.
3.3.10.3 Routine Instrument Checks. Proper operation of all radiation counting instruments requires that their response to a reference source be constant to within established limits. Therefore, counting equipment should be subject to routine checks to ensure proper operation. This is achieved by counting an instrument check source at least once per day. The characteristics of the check source (i.e., geometry, type of radiation emitted, etc.) should, if possible, be similar to the samples to be analyzed. The count rate of the check source should be high enough to yield good counting statistics in a short time (for example, 1,000 to 10,000 counts per minute).
The radiological counters should have calibration checks run daily to determine counter efficiency. This is particularly important for portable counters taken into the field that may be subject to rugged use and temperature extremes. These checks are made using an NIST-traceable alpha calibration source such as Am-241. In addition, the system background count rate should be assessed regularly.
Pumps and flow meters should be checked routinely to ensure accuracy of volume measurements. This may be performed using a dry-gas meter or other flow measurement device of traceable accuracy.
3.3.11 Supplementary Information for the Grab Sampling-Working Level (GW) Method
3.3.11.1 Sample Collection. Two commonly used methods are described below. There are several other methods reported in the literature. Sampling using these methods requires collection of radon decay products on a filter, and measuring the alpha activity of the sample with a calibrated detector at time intervals that are specific for each method.
The filter is installed in the filter holder assembly and attached to the pump. The pump is then operated for exactly five minutes, pulling air through the filter. Starting time and air flow rate should be recorded. The pump is stopped at the end of the five-minute sampling time. At this time, the stopwatch should be started or reset.
3.3.11.2 Sample Counting. Sample counting for two different techniques is described below.
3.3.11.2.1 Modified Tsivoglou Technique (Thomas 1972, Tsivoglou et al. 1953). The filter is transferred carefully from the filter holder assembly to the detector. The collection side of the filter is oriented toward the face of the detector.
The counter is operated for the following time intervals (after sampling has stopped): two to five minutes, six to 20 minutes, and 21 to 30 minutes. The total counts for each time period are then recorded.
3.3.11.2.2 Kusnetz Technique (Kusnetz 1956). The filter is transferred carefully from the filter holder assembly to the detector. The collection side of the filter is oriented toward the face of the detector.
The counter is operated over any 10-minute time interval between 40 minutes and 90 minutes after sampling starts. The total counts for the sample and the time (in minutes after sampling) at the midpoint of the 10-minute time interval are then recorded.
3.3.11.3 Data Analysis. Data analysis for the two different techniques is described below.
3.3.11.3.1 Modified Tsivoglou Technique. The concentration, in picoCuries per liter (pCi/L), of each of the radon decay products (Po-218, Pb-214, and Po-214) can be determined by using the following calculations:
C2 = 1/FE (0.16921 G1- 0.08213 G2 + 0.07765 G3 - 0.5608 R)
C3 = 1/FE (0.001108 G1 - 0.02052 G2 + 0.04904 G3 - 0.1577 R)
C4 = 1/FE (-0.02236 G1 + 0.03310 G2 - 0.03765 G3 - 0.05720 R)
It is important to note that the constants in these equations are based on a 3.04-minute half-life of Po-218. The working level (WL) associated with these concentrations can then be calculated using the following relationship:
Where:
C2 = concentration of Po-218 (RaA) in pCi/L;
C3 = concentration of Pb-214 (RaB) in pCi/L;
C4 = concentration of Po-214 (RaC') in pCi/L;
F = sampling flow rate in liters per minute (Lpm);
E = counter efficiency in counts per minute/disintegrations per minute (cpm/dpm);
G1 = gross alpha counts for the time interval of two to five minutes;
G2 = gross alpha counts for the time interval of six to 20 minutes;
G3 = gross alpha counts for the time interval of 21 to 30 minutes; and
R = background counting rate in cpm.
Reference: (Thomas 1972).
3.3.11.3.2 Kusnetz Technique. WL is calculated as follows:
WL = C/Kt VE
Where:
C = sample cpm - background cpm;
Kt = factor determined from Exhibit 3-1 (PHS 1957) for time from end of collection to midpoint of counting;
V = total sample air volume in liters [calculated as flow rate (L/m) x sample time (m)]; and
E = counter efficiency in cpm/dpm.
Exhibit 3-1
Kusnetz Factors
(Public Health Service, 1957)
Time Kt Time Kt
40 150 66 98
42 146 68 94
44 142 70 90
46 138 72 87
48 134 74 84
50 130 76 82
52 126 78 78
54 122 80 75
56 118 82 73
58 114 84 69
60 110 86 66
62 106 88 63
64 102 90 60
3.3.11.4 Sample Problems
3.3.11.4.1 Sample Problem for the Modified Tsivoglou Technique
Given:
F = sampling flow rate = 3.5 Lpm
E = counting efficiency = 0.47 cpm/dpm
G1 = 880
G2 = 2660
G3 = 1460
R = 0.5
Calculate:
C2 = 1/3.5 x 0.47 (0.16921 x 880 - 0.08213 x 2660 + 0.07765 x 1460 - 0.05608 x 0.5)
C2 = 26.8 pCi/L
C3 = 1/3.5 x 0.47 (0.001108 x 880 - 0.02052 x 2660 + 0.04904 x 1460 - 0.1577 x 0.5)
C3 = 10.9 pCi/L
C4 = 1/3.5 x 0.47 (-0.02236 x 880 + 0.03310 x 2660 - 0.03766 x 1460 - 0.05720 x 0.5)
C4 = 8.1 pCi/L
WL = (1.028 x 10-3 x 26.8 + 5.07 x 10-3 x 10.9 + 3.728 x 10-3 x 8.1)
WL = 0.11
3.3.11.4.2 Sample Problem for the Kusnetz Technique
Background count = 3 counts in 5 minutes, or 0.6 cpm
Standard count = 5,985 counts in 5 minutes, or 1,197 cpm
Efficiency = 1197 cpm - 0.6 cpm/2430 dpm = 0.49 (known source of 2439 dpm)
Sample volume = 4.4 liter/minute x 5 minutes = 22 liters
Sample count at 45 minutes (time from end of sampling period to start of counting period) = 560 counts in 10 minutes, or 56 cpm
Kt at 50 minutes (from Exhibit 3-1) = 130
WL = 56 cpm - 0.6 cpm/130 x 22 L x 0.49
WL = 0.04
