Answer to Question #12177 Submitted to "Ask the Experts"

For mapping radon radioactivity over a large area, such as a city, could one use an ion chamber to measure radon in different locations? What is the technique that is used to choose measurement sites to measure radon in a city that has a square shape?

In responding, I assume that you are referring to a direct-reading ion chamber detector system, not a laboratory measurement for radon gas concentration. Ion chamber survey meters can be used for measuring direct radiation levels but are not useful for measuring radon gas concentrations in air.

The ion chamber detector measures the amount of ionization produced in air (or other gas) in the enclosed volume of the chamber, so it is responsive to ionizing radiation such as the gamma photons emitted by radon decay products. Radon-222 (²²²Rn) is an alpha-particle-emitting decay product of radium-226 (²²⁶Ra). However, its short-half-life decay products, bismuth-214 (²¹⁴Bi) and lead-214 (²¹⁴Pb), emit beta and gamma radiation. The levels of direct gamma radiation could be measured using an ion chamber and possibly mapped for a city depending on the specific instrument set-up. Such measurements would reflect the concentration of ²²⁶Ra in soils, streets, building materials, etc., but not specifically radon. The concentration of the gamma-emitting decay products of radon in air at the surface under normal circumstances is not high enough to affect the gamma radiation exposure rate significantly.

A special form of ion chamber called an electret ion chamber can be used to measure radon concentrations at specific locations and can be used for residential and outdoor environmental radon concentration measurements. Mapping the individual radon results from electret ion chambers or other radon measurement systems such as alpha track detectors will give an indication of radon concentrations within a particular location such as a city.

With regard to choosing measurement sites to measure radon sites in a square-shaped city, as noted in your question, direct gamma radiation exposure measurements with an ion chamber instrument could identify elevated ²²⁶Ra levels, but that would not necessarily correlate to ²²²Rn concentrations to which members of the public are exposed. The number of residential radon measurements and the distribution of measurement sites needed to characterize average radon exposures and distribution of radon exposures would depend on the variability of the factors that influence radon concentrations, such as soil conditions, weather, and for indoor radon, type of building, construction, and lifestyle of the occupants. There is no easy answer to that question, but in this case, the number of measurements required would be independent of the shape of the city.

Janet A. Johnson, PhD, CHP, CIH