What is the measurement principle behind activated charcoal devices?

The measurement principle behind activated charcoal devices is based on the adsorption of radon. In this method, activated charcoal, which has a porous structure, is used to capture radon gas from the air. When air containing radon passes over the surface of the activated charcoal, the radon atoms adhere to the charcoal due to their chemical properties and the physical characteristics of the charcoal's surface.

This adsorption process allows the device to trap radon for a specific period, enabling accurate measurement of radon concentration when the charcoal is later analyzed, usually through gamma spectroscopy or similar techniques. The ability of activated charcoal to effectively pull radon gas from the air makes it a popular choice for radon measurement in various environments.

Other principles mentioned in the choices do not accurately pertain to how activated charcoal devices work. Continuous ion counting involves monitoring ionization in a gas, ionization of air particles refers to a completely different measurement technique, and liquid scintillation is a method used for detecting radioactive isotopes in liquid form, not gas accumulation.