Calibration of Scintillation Cells for Radon-222 Measurements at the U.S. Environmental Protection Agency

Zinc sulfide coated scintillation cells are the primary method for measuring radon-222 at the U.S. Environmental Protection Agency (EPA), Office of Radiation Programs (ORP), Eastern Environmental Radiation Facility (EERF). These cells are used to measure concentrations of radon in exposure chambers that are used to calibrate or test other devices or instruments. Individual cells are calibrated by analyzing samples of air with known concentrations of radon produced by emanation of radon from standard radium-226 solutions obtained from the National Institute of Standards and Technology. The calibration procedure includes ingrowth of radon-222 into equilibrium with the radium in the standard solution, transfer from the solution into an evacuated container, and dilution with a measured volume of air. Samples of the radon in air mixture are transferred to evacuated scintillation cells and sealed for 4 h prior to counting, which allows secular equilibrium to be established between the radon and its decay products. Calibration factors for each individual cell are computed by decay correcting the radon to the time of collection and calculating the ratio of count rate (cpm), corrected for background, to radon activity (Bq) for the specific volume of the cell. Four or more calibration factors are determined for each cell and aver-aged to provide the calibration factor used for measurements. Calibrations are repeated at 6-mo intervals, and the results of each calibration are compared to the previous averages. When calibration factors fall outside the 95% confidence interval, they are rejected and the cell is checked for defects prior to recalibration.