How do you measure gas?

Answer 1

Radon gas is measured in a variety of different ways. These include charcoal canisters, alpha track detectors, and E-Perms (which all require analysis in a lab after the measurement period), grab sampling, and continuous radon monitors, each of which can use either scintillation cells or ion chambers. There are other methods but these are the most common. All these measurement methods depend upon the radioactivity of radon and its daughter products.

Charcoal Canisters

Charcoal is good at adsorbing radon gas. A charcoal canister contains a measured amount of charcoal and has an opening to permit radon gas to diffuse in and be absorbed. It takes about three days for the level of adsorbed radon to level off. The adsorbed radon gas will gradually decay and produce radon daughters, which are also radioactive. Some of these radon daughters produce gamma radiation which can be measured to estimate the amount of adsorbed radon.

Alpha Track Detectors

Alpha track detectors (ATDs) contain a piece of a special plastic which is sensitive to alpha radiation, which is emitted by radon gas and some of its daughters. Radon gas diffuses into the ATD and decays, followed by the decay of its daughters. The alpha particles produced during these decays produce damage tracks in the special piece of plastic. At the end of the measurement period the piece of plastic is removed from the ATD and etched in a caustic solution. The etching "develops" the damage tracks, producing pits in the surface of the plastic. These pits are counted and the number of pits per unit area is proportional to the radon exposure during the measurement period.

E-Perms

E-Perms also contain a special piece of plastic. In the case of E-Perms, however, the plastic is given an initial negative electrical charge which causes a surface voltage. The alpha particles from radon and its daughters, because they are electrically charged positive, gradually cancel the charge on the piece of plastic. The surface voltage on the plastic is measured before and after the exposure to radon and the change in voltage is proportional to the radon exposure.

Grab Sampling

In grab sampling, a sample of radon laden air is pulled into a measurement chamber or sample container. In the case of the sample container, the air is subsequently transferred from the sample container to a measurement chamber. The measurement chamber is either a scintillation cell or an ion chamber. Scintillation cells are lined with a phosphorescent material which produces a light flash when struck by an alpha particle. A photomultiplier tube and other electronics counts the light flashes to determine the amount of radon gas in the sample. An ion chamber depends on the fact that alpha particles (and other types of ionizing radiation) dislodge electrons from the molecules of gas in the chamber. These electrons (and the associated positive ions) are attracted respectively to the positive and negative electrodes in the chamber and the amount of electrical charge collected is measured, or the pulses of current produced are counted. The amount of electrical charge or current produced, or the number of pulses counted, is proportional to the amount of radon in the sample.

Continuous Radon Monitors (CRMs)

CRMs utilize measurement chambers as described for grab sampling, but instead of one time grab samples, gas continuously enters the measurement chamber either by pumping or through diffusion. The amount of radon in the chamber at any given time is determined just as for grab sampling, and in this manner a series of measurements is collected for an extended period of time.