MS-QA_06-2019 rev5

ANSI/AARST MS-QA 2019 32 Radon Measurement Systems Quality Assurance Laboratories—CAD / Liquid Scintillation which reduces the number of photons that reach the photomultiplier tubes. The degree of quenching is dependent on the scintillator fluid used and is to be determined during each counting session for the samples being counted. Quenching can be determined for each sample using features built into many liquid scintillation counters. Each LS Counter is to be calibrated to the range of energies that it will detect, such as the commonly used calibration s based on laboratory exposures of the charcoal devices to 222 Rn. Where required by the manufacturer, LS counters used for this application must be calibrated using 226 Ra. Subsequent counter stability checks must be made with the laboratory standards that are recommended by the LS counter manufacturer and are usually made with Tritium ( 3 H) or 14 C. Note—The activity can be calculated as shown below in equation 9 : C = NCR / (CF * t e * ε *DF) (9) 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 ε = continuing calibration verification (cpm/pCi), which is often kept as unity (see Section 9.5 ) t e = exposure time (min) DF = decay factor correcting for decay between device sealing and analysis (unitless) • DF corrects for radon decay between the device sealing and its analysis; • CF is determined from the calibration functions (curves) that incorporate both moisture gain (if incorporated), as change of mass before and after exposure, as well as the duration of exposure. 9.4 CAD / LS—Establishment and Maintenance of In-Control Operations Conducted in accordance with the requirements in Section 7.7 , laboratories are to use results from at least 20 lab spikes , 20 lab blanks and 20 laboratory standard device counts to establish and monitor system stability. In addition to these categories of QC devices, the laboratory shall evaluate lab duplicates or recounts of the same device (with decay corrections) as part of investigations and periodic evaluation of system stability. Calibration function stability shall be based on the distribution (variability and mean at minimum) characterization of results from (a) counting session backgrounds, (b) laboratory standard QC devices and (c) the RPE for laboratory spikes . Note—Interferences include static electricity, which can be released as a burst of light from the cocktail. Static electricity counts can be reduced by using glass instead of plastic vials and wiping vials with antistatic sheets. 9.5 CAD / LS—Analysis Session Quality Assurance Continuing calibration verifications are required to determine the background and counter response by counting at least two laboratory standard QC devices and blanks . Blanks are also to be counted, recorded and assessed during each counting session. LS counter operations include automatic instrument stability checks that are to be part of every counting session to ensure consistent responses that are within the limits. Informative advisory —Laboratory standard QC devices should consist of a standard solution containing a known emission rate such as known for 226 Ra, 3 H and 14 C to verify counter stability and apply normalization factors that are specific to that counting session, as necessary.