MS-QA_06-2019 rev5

ANSI/AARST MS-QA 2019 29 Radon Measurement Systems Quality Assurance Laboratories—CAD / Gamma Spectroscopy 8.3 CAD / Gamma Spectroscopy—Calibration Relationship Note—The equation originally published by George (1984) can be used, as well as others, to calculate the radon concentration from the net count rate as shown below in equation 8: C = NCR / (CF * t e * ε * DF) (8) where C = Rn concentration (pCi/L) NCR = net count rate (cpm), including the subtraction of configuration- and counter-specific background count rate CF = calibration factor (L/min), specific to configuration t e = exposure time (min) ε = continued calibration verification check (cpm/pCi) which is often kept as unity and tracked separately (see 8.5.1 ) DF = decay factor (unitless) • DF corrects for radon decay between the device sealing and its analysis; • ε , the continuing calibration verification, sometimes known as the daily instrument check, is determined by counting a sealed, laboratory standard QC sample (standard can or device containing radon in equilibrium with a known activity of 226 Ra or containing other radionuclide such as 230 Th with gamma emission rates of between 1,000–10,000 counts per minute in the energy region of interest) of the same geometry as the devices to which ε is applied; and • CF determined from the calibration functions (curves) that incorporate both moisture gain, as change of mass before and after exposure, as well as the duration of exposure. 8.4 CAD / Gamma Spectroscopy—Establishment and Maintenance of In-Control Operations Determining calibration function stability shall be based on, and continually assessed, using the variability and mean of (a) analysis session background counts, (b) laboratory QC standards for session efficiency and (c) at a frequency of at least 3%, RPE for laboratory spikes . Results of counting session backgrounds and laboratory QC standards shall be recorded during every analysis session and compared to warning and control limits derived from previous responses. As presented in Section 7.7 , adequate statistics are obtained after at least 20 of each type of QC check that are used for monitoring system performance. 8.4.1 Batch Specific Calibration Relationships Activated carbon used in detectors is prepared in batches , with variation in efficiency and background between batches . Laboratories are to develop batch -specific calibration factors and background characterizations (through STAR exposures) and monitor within- batch variations in both. 8.5 CAD / Gamma Spectroscopy—Analysis Session Quality Assurance The session-to-session and between-counter variability of analysis system components, including gamma counting systems and associated electronics, shall be controlled and documented each day when analyses are performed. In-control operation of counting systems is to be verified through consistent use of QC checks specific to each device configuration and detector. The following checks are to be repeated daily or at least once during each counting session for each device configuration analyzed (Definitions— Significant Type Difference ), and each counting system used.