|Charcoal canisters and similar charcoal-containing devices
are used for short-term tests lasting a minimum of 48 hours and up to 7
days. They contain a quantity of granular activated carbon that absorbs
the radon gas entering the canister from the surrounding air. At the end
of the test period, the canister is sealed and returned to the manufacturer
for analysis, who then reports the results to the buyer.
|Alpha-track detectors are long-term passive test devices
that operate on a very different principle. Inside there is a piece of
film that records the impacts (tracks) of alpha particles produced by the
decay of radon and its decay by-product, polonium. Depending on the design
of the detector, it may be set out for anywhere from 2 weeks to an entire
year. At the end of the test period, the detector is returned to the manufacturer
who counts the alpha tracks on the film, computes the radon concentration,
and reports the results to the buyer.
A device known as the Electret Passive Environmental Radon Monitor (E-PERM) is used by many professional radon testers to obtain fast results on radon concentration.
E-PERM is a passive integrating radon detection system consisting of a charged teflon disk (electret), an open-faced ionization chamber, a voltage reader, and a data logger. After placing the electret in the chamber, an electrostatic field is established. Radon gas diffuses passively into the chamber. The alpha particles emitted from the decay of radon ionize the air molecules. These ions are attracted to the charged surface of the electret, thus reducing the initial charge of the electret. The initial and the final voltages are measured using the voltage reader. The rate of change of the charge is proportional to the concentration of radon in the test area.
ACTIVE TEST DEVICES
Professional radon testers will frequently perform tests using an "active" device, such as an electronic Continuous Radon Monitor. For the test to be valid, the device must be used according to the US EPA protocol, which includes the requirement that it operate for a minimum of 48 hours, and record measurements hourly or more frequently. Radon mitigators may use devices that make radon measurements over shorter periods. However, these measurements are for diagnostic purposes only; for example, to assist the radon mitigators in determining whether a foundation opening is an entry point for radon. Such measurements are not valid tests for whole-house radon concentrations and should not be used to make mitigation decisions.
Occasionally the radon tester may also use an instrument
called a "Working Level Monitor." "Working level" is an alternative
measure of radon concentration that is often used in occupational settings,
such as in underground mines. Instead of measuring radon concentration
directly, the working level measures the amount of radon decay products,
which are an indirect measure of radon concentration. For a typical house,
a 0.02 working level (WL) measurement corresponds to about 4.0 pCi/l of
radon. This conversion between measurement units assumes that only half
of the radon decay products are available for breathing because the rest
have plated out on the walls, ceiling and furniture. The plate-out occurs
because the radon decay-product particles are positively charged and are
therefore attracted to surfaces of the opposite charge.