Environmental Engineering Reference
In-Depth Information
Figure 9.7
Radon sampling devices.
5.
Radon
Radon is sampled in indoor environments using both passive integrated
techniques and direct-read quasi-real-time instruments. The former are used
in screening measurements; the latter in evaluating sources and determining
the effectiveness of abatement measures. Techniques are available to deter-
mine radon concentrations in air, water, and soil.
Most commonly, radon is sampled using charcoal canisters (
Figure 9.7
,
right). The opened sampler is exposed for 2 to 7 days. Since radon diffuses
to charcoal at a known rate, the sample volume can be calculated. Concen-
trations are determined by measurement of gamma ray emissions from
radon decay products (RDPs) using gamma ray detectors. Concentrations
are calculated based on sample volume, radon half-life, and radon/RDP
equilibrium ratios. Radon concentrations collected on small charcoal sam-
plers can also be determined using scintillation counters.
Short-term (2 to 7 days) radon concentrations can also be measured with
passive devices which allow radon to diffuse into them where, on decay,
they produce positive ions, which are attracted to and deposit on a specially
charged plate (electret) (
Figure 9.7
,
left). The change (decrease) in voltage
can be directly related to the radon concentration and other sources of
gamma ray energy. Electret devices can also be used for longer-term sam-
pling (up to 3 months).
As indicated in Chapter 3, short-term measurements are not adequate
to determine long-term exposures which may pose a cancer risk. For such
determinations it is common to measure radon using a track-etch detector
(
Figure 9.7
,
center). The track-etch detector is a simple device that consists
of a plastic strip or film affixed to a small cylinder. The radon concentration
is determined by counting the microscopic tracks produced on the film by
alpha-particles. Track-etch detectors are typically exposed for 3 to 6 months.
A variety of quasi-real-time portable instruments are available for sam-
pling indoor radon concentrations. They use a variety of detection principles
including ionization. Measurements are typically reported as hourly averages.
