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Radon Calibration Facility

Radon-222 is a naturally occurring radioactive byproduct of radium-226. Radon is a known carcinogen, and the radon detection and mitigation industry depends on accurate environmental measurements. The National Institute of Standards and Technology (NIST) and its predecessor, the National Bureau of Standards (NBS), have provided the national standards for radon measurements and calibrations since the 1940s. 

A woman stands next to a system of connected tanks
Credit: NIST
Patricia Mullen working with NBS’s original radon measurement system, which she helped move to the new Gaithersburg campus in 1966. 

Today’s primary radon-222 calibration system, introduced in the early 1990s, is a modernized and expanded version based on the design of the original system from the 1940s. At the time this new system was being designed, consideration was given to possibly selecting an alternative measurement method for use as the primary standard. However, despite the passage of 50 years, researchers did not find any other available method or instrumentation that was more advantageous or superior to the original pulse ionization chamber. 

Schematic showing the gas-handling and gas-purification manifold and pulse ionization chambers.
Schematic of the NIST primary radon measurement system showing the complete gas-handling and gas-purification manifold used in conjunction with the pulse ionization chambers.

Cross section of pulse ionization chamber
Credit: NIST
Cross section of the 1990 NIST pulse ionization chamber

In this pulse ionization design, a gas-filled chamber with a cathode and anode measures the charge from the number of ion pairs created within the gas by the emission of alpha particles (two protons and two neutrons, identical to a helium nucleus) from radon and its radioactive daughters. In a pulse ionization chamber, a separate electrical pulse is generated by each alpha particle.

A rack hold four square boxes and four metal canisters, attached with tubing.
Credit: NIST
The complete radon-222 calibration facility showing the gas handling and the four pulse ionization chambers

Created July 29, 2019, Updated September 20, 2019