MS-QA_06-2019 rev5

ANSI/AARST MS-QA 2019 31 Radon Measurement Systems Quality Assurance Laboratories—CAD / Liquid Scintillation 9 LAB QA FOR CHARCOAL ADSORPTION DEVICES—LIQUID SCINTILLATION METHOD This section applies to the method of analyzing charcoal adsorption devices using liquid scintillation (alpha/beta counting) analysis. For this method, all requirements in Sections 3 and 7 apply, including laboratory-specific QA/QC training, chain of custody, data validation , reporting, calibration , warning and control limits, responsibilities, QC of detector material and minimum intervals between system calibration s. Note—Liquid scintillation (LS) counts the alpha- and beta-particle emissions from radon and its decay products that have adsorbed onto activated carbon. In this method, a container of activated carbon is exposed during the measurement, sealed and sent to a lab, where a liquid medium capable of converting the kinetic energy of radioactive alpha and beta emissions into light energy is added to the container and mixed. The liquid medium includes several components (thus termed a “cocktail”), including a solvent and phosphor(s). When dissolved in the solvent, molecules of phosphor convert the absorbed energy from radiations emitted by radon decay products into light. After sufficient time is allowed for the elution process of desorbing the radon into the fluid, the container is analyzed in an automated, shielded, light-sealed counter using photomultiplier tubes, generally in a coincidence-counting pair to ensure that light counted is from radioactive emissions and not chemiluminescence. With every counting session, appropriate background and laboratory standards are also counted; the net count rate from the sample is proportional to the radon concentration to which the activated carbon was exposed. 9.1 CAD / LS—Standard Counting Configurations Note 1—LS counters can be automated machines capable of counting hundreds of vials in one counting session. Because the vials are placed in size-appropriate wells, configuration can be standardized. Each counter consists of a mechanism to move the vials into position and two photomultiplier tubes with associated coincidence counting software. Counting generally is conducted for as long as is necessary to obtain adequate counts. Note 2—Depending on the mass of charcoal used and other factors, some systems for analyzing activated carbon adsorption detectors via LS analysis may include a gravimetric scale, which is used to measure the mass of the device before and after exposure, and is used with exposure duration to determine the appropriate calibration factor (CF) for each device’s change in mass. 9.2 CAD / LS—QC of Detector Materials Because this method uses activated carbon, the requirements for assuring the homogeneity and stability of the carbon ( batch testing) are the same for this method as for carbon analyzed using gamma counting (Section 8.2). Informative advisory —Because the charcoal mass used in LS counting is generally small (e.g., 2 grams), the requirements for within- batch testing are critical. The liquid mixture (cocktail) is composed of several components and must also be standardized. This may be done by the cocktail manufacturer, in which certificates of uniformity that are supported by test results should be obtained so that the consistency of relevant cocktail characteristics is documented. In addition, cocktails degrade over time, and users are responsible to follow manufacturer’s recommendations regarding shelf life and use cocktails in laboratory standards and blanks . 9.3 CAD / LS—Calibration Relationship LS counting efficiency depends on several factors including the degree of quenching in the sample/cocktail mixture. Chemical quenching reduces the number of photons generated by the radioactive decay, and color quenching absorbs light in the wavelength range emitted by the scintillator,