MS-QA_06-2019 rev5

ANSI/AARST MS-QA 2019 30 Radon Measurement Systems Quality Assurance Laboratories—CAD / Gamma Spectroscopy 8.5.1 CAD / Gamma Spectroscopy—Continuing Calibration Verification Using Laboratory Control Devices Counter response can fluctuate over time, and a laboratory standard QC device ( matrix spike ) is to be analyzed at the beginning of each counting session to determine the stability of the counting system, ε , for that counter, that session and for the set of devices corresponding to the configuration of the laboratory standard QC device. Laboratory standard QC devices, also known as matrix spikes in this context, are devices that have been impregnated with a gamma emitter, preferably 226 Ra, and are sealed so that radon is in equilibrium with 226 Ra inside the device, which provides a benchmark indicator of individual counter response fluctuations. Use of radionuclides other than 226 Ra may require adjustment for decay over time and must be evaluated for applicability to the energy region of interest. The results of one or more continuing calibration verifications using one or more laboratory standard QC device results are to be incorporated into the calibration relationship described in Section 8.3 . In addition to being used in the algorithm to generate radon concentrations from the NCR, counting efficiencies are to be tracked and analyzed using statistically based control chart limits that have been generated from previous QC standard results for each configuration. Records of the continuing calibration verifications are necessary for documenting the counting system stability. 8.5.2 CAD / Gamma Spectroscopy—Characterization of the Background Count Rate (Lab Blanks) Characterization of the background count rate within the energy range (region of interest [ROI]) for each counting system requires measuring the count rate from unexposed, sealed devices that are dedicated solely for use as background devices, and which may be assigned a specific detector. The gamma emitters in the ROI that are inherent in the unexposed charcoal may vary from batch to batch , and it is the laboratory’s responsibility to measure and track the background associated with different sets of device serial numbers. The background count rate from one or more background counts on each counter is to be subtracted from the gross counts of exposed devices prior to applying the calibration relationship. 8.5.2.1 Initial calibration — background checks Note—At the initial calibration , background counts should be repeated as many times as necessary to demonstrate that the background counts do not vary more than would be expected due to Poisson counting statistics alone. 8.5.2.2 Routine operations — background checks During routine operations, background count rates are to be counted to measure counting-session- specific background, as well as differences between counters. At least one background count for a duration sufficient to acquire enough counts (i.e., 400) to reduce the error—due to assumed Poisson counting statistics alone—to 5% is to be conducted each counting session on each counter.