Charcoal Canisters
In 1981 A. George described a simple method employing charcoal canisters to
measure indoor radon concentrations.
Charcoal canisters. Diffusion barrier type in
upper left. Canister on bottom shown with
top removed.
In brief, the method requires that a charcoal canister be deployed (opened and
exposed to the atmosphere) for the specific number of days (usually 2-6) and then
sealed. After deployment, the activity of the radon daughters in the canister is
determined by gamma spectrometry. The atmospheric radon concentration is
calculated from the daughter activity with the aid of a predetermined calibration
factor.
During deployment radon is passively adsorbed onto the surface of the charcoal
granules. At the same time, the adsorbed radon is decaying as well as desorbing
back to the atmosphere. If the exposure period is long enough, the adsorption and
desorption/decay rates will become equal. When this happens, extending the
duration of the deployment will not increase the total amount of adsorbed radon. In
other words, there is a law of diminishing returns with respect to increasing
exposure times. As such, charcoal canisters are rarely deployed for more than six
days. The EPA standard protocol employs as 2 day deployment.
Although the radon adsorption is relatively unaffected by temperature, it is
affected by humidity. For a given atmospheric radon concentration, the greater the
humidity the less radon will be adsorbed onto the charcoal. Therefore the
calibration factor depends on the humidity as well as on the length of deployment.
The humidity conditions are evaluated by weighing the canister before and after
exposure. Any increase in weight is due to adsorbed moisture.
At the end of deployment the canister is sealed to allow the radon daughters to
achieve secular equilibrium, a process that will take 3 - 4 hours. Following this the
canister will be counted on a NaI detector and the radon concentration calculated.
The analyses and calculations can be performed in a variety of ways. For example,
the analysis may determine the counts from the 242, 294, and 352 keV peaks of Pb214 and those from the 609 keV peak of Bi-214. On the other hand, the 242 keV
peak may ignored to eliminate possible interference from Pb-212 gammas that may
be seen due to the presence of thoron. The latter approach is used in the EPAs
standard protocol which determines the total counts in a region of interest from 270720 keV. A common variation on the standard protocol is to use two regions of
interest: one for the Pb-214 gammas and the other for the Bi-209 peak. To
calculate the net count rate, the count rate for the same region(s) of interest is
determined for a background canister and subtracted from the count rate for the
deployed canister.
The result is divided by the detector efficiency to determine the Rn-222 activity in
the canister and decay corrected back to the middle of the deployment/sampling
period.
Finally, to calculate the radon concentration in air, the collected activity on the
charcoal is divided by the volume of air (liters) "sampled" by the canister. Aside
from the length of sampling time, the volume of air sampled depends on the
construction of the canister and the humidity conditions.
The formula used to calculate the radon concentration is as follows:
Concentration (pCi/l) = net cpm/ (E x Ts x CF x DF)
where:
E
=
detector efficiency (cpm/pCi)
Ts
=
sampling time (min)
CF
=
calibration factor in 1/min analogous to a sampling rate
somewhat.
DF
=
decay correction factor to midpoint of sampling
t
(min)
=
e -0.000126 t
=
time from midpoint of sampling/deployment to count time
The calibration factor is obtained from a calibration curve similar to the
following:
Note that this calibration factor is a function of humidity (weight gain) and
deployment time as well as the canister construction. The EPA curve (above) is
for a two day deployment which they consider standard. Similar curves for other
deployment time could be generated but the EPA protocol uses adjustment factors
(see following figure).
The charcoal canister technique is simple, quick, inexpensive and makes it possible
for the homeowner to deploy the canister him or herself and mail it back to the
company for analysis. However, while it is sufficiently sensitive for measuring
concentrations at or above the EPA guideline level of 4 pCi/l, large errors are
associated with measurements below 1 pCi/l. This, and its sensitivity to humidity
and currents make the method unsuitable for environmental use. It should also be
kept in mind that all practical purposes, it is only measuring the radon
concentration for the last day or so of deployment.
Certain charcoal canister designs employ a "diffusion barrier". In typical design
the air enters through a 3/4" hole on the top of the can. Before it reaches the
charcoal the air must diffuse across a nylon/filter paper barrier. Such systems are
not as sensitive as the "open" canisters but they provide a better average
measurement since the effective collection time is two to four times as long as with
the open type. A desiccant is often added to the canister to reduce the influence of
humidity.