Why Charcoal Devices Must Be Analyzed Soon after Measurement: Uncertainty and Minimum Detectable

The value of a radon measurement is meaningless unless one knows two additional pieces of information about the measurement; the total uncertainty and the Minimum Detectable Concentration (MDC). These statistics are particularly important when making measurements with devices that capture radon and then are analyzed later; such as grab scintillation cells and charcoal devices. During the delay between sampling and analysis, the quantity of radon in the device constantly decreases due to its decay with a half-life of about 3.8 days. Using 4-inch charcoal canisters as an example, and using typical values of analysis parameters, it is shown that the total uncertainty and the MDC for the measurement both increase with time after the sampling period and can become unacceptable, thus rendering the measurement useless. This illustrates the importance of analyzing these devices as quickly as possible after sampling.