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

Area of Science:

• Environmental Science
• Radiochemistry
• Metrology

Background:

• Zinc sulfide coated scintillation cells are the standard method for radon-222 measurement at the U.S. Environmental Protection Agency (EPA).
• These cells are crucial for calibrating and testing devices used in exposure chambers.

Purpose of the Study:

• To detail the calibration procedure for zinc sulfide scintillation cells used in radon-222 measurements.
• To ensure the accuracy and reliability of radon concentration data obtained by the EPA.

Main Methods:

• Cells are calibrated using known radon-222 concentrations derived from National Institute of Standards and Technology (NIST) radium-226 solutions.
• Radon is transferred to evacuated scintillation cells, sealed, and allowed to reach secular equilibrium with its decay products.
• Calibration factors are calculated by comparing corrected count rates to radon activity, with multiple factors averaged per cell.

Main Results:

• Calibration factors are determined by decay-correcting radon to the time of collection and calculating the ratio of background-corrected count rate (cpm) to radon activity (Bq).
• Averages of at least four calibration factors are used for each cell.
• Recalibration occurs every six months, with factors outside the 95% confidence interval triggering cell inspection.

Conclusions:

• The described calibration method ensures accurate and reproducible radon-222 measurements.
• Regular recalibration and quality control checks maintain the integrity of the measurement system.
• This rigorous process supports the EPA's role in radiation monitoring and public health protection.