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Radon is a
dangerous radioactive gas
that is odorless, tasteless,
and colorless. It is the
second leading cause of
lung cancer in America,
only behind cigarette smoking.
Excess levels of radon entry
into a property is not very
expensive to correct. It
is not prudent to waive
your rights to a radon test
as part of an agreement
of sale. EPA has reported
elevated radon levels in
every zip code across the
country.
Commonly Used
Radon Measurement Devices
Charcoal
Canisters - Charcoal
Canisters (CC) are passive
devices requiring no power
to function. The passive
nature of the activated
charcoal allows continual
adsorption and desorption
of radon. During the
measurement period, the
adsorbed radon undergoes
radioactive decay.
Therefore, the technique
does not uniformly integrate
radon concentrations during
the exposure period.
As with all devices that
store radon, the average
concentration calculated
using the midexposure time
is subject to error if the
ambient radon concentration
adsorbed during the first
half of the sampling period
is substantially higher
or lower than the average
over the period. For
a 2- to 7-day exposure period,
the lower level of detection
(LLD) should be 0.5 pCi/L
or less (U.S. EPA
Measurement Protocols 1989).
This level of sensitivity
can normally be achieved
with a counting time of
up to 30 minutes.
This LLD should be calculated
using the results of charcoal
background determinations.
The coefficient of variation
should not exceed 10 percent
(1 sigma) at radon concentrations
of 4 pCi/L or greater
(U.S. EPA Measurement Protocols
1989). This precision
should be monitored using
the results of duplicate
canister analyses.
CCs can achieve an average
coefficient of variation
of less than 5 percent at
concentrations of 4 pCi/L
or greater.
Alpha Track
Detectors - An alpha
track detector (ATD) consists
of a small piece of plastic
or film enclosed in a container
with a filter-covered opening.
Radon diffuses through the
filter into the container
and alpha particles emitted
by the radon and its decay
products strike the detector
and produce submicrocopic
damage tracks. At
the end of the measurement
period, the detectors are
returned to a laboratory.
Plastic detectors are placed
in a caustic solution that
accentuates the damage tracks
so they can be counted using
a microscope or an automated
counting system. The
number of tracks per unit
area is correlated to the
radon concentration in air,
using a conversion factor
derived from data generated
at a calibration facility.
The number of tracks produced
per unit of time is proportional
to the radon concentration,
so an ATD functions as a
true integrating detector
and measures the average
concentration over the measurement
period. The LLD (sensitivity)
and the precision of an
ATD system is dependent
upon the tracks counted,
and therefore the counting
achieves and LLD of 180
pCi/L-days. The coefficient
of variation (precision)
should be monitored using
the results of duplicate
detectors. The coefficient
of variation should not
exceed 20 percent (1 sigma)
at radon concentrations
of 4 pCi/L or greater (U.S.
EPA Measurement Protocols
1989).
Electret-Passive
Environmental Radon Monitors
-
Electret-passive
environmental radon monitors
(E-PERMs) required no power
and function as true integrating
detectors, measuring the
average concentration during
the measurement. E-PERMs
contain a permanently charged
electret (an electrostatically-charged
disk of Teflon), which collects
ions formed in the chamber
by radiation emitted from
radon decay products.
When the device is exposed,
radon diffused into the
chamber through filtered
openings. Ions which
are generated continuously
by the decay or radon and
radon decay products are
drawn to the surface of
the electret and reduce
its surface voltage.
The amount of voltage reduction
is directly related to the
average radon concentration
present during the exposure
period. There are
both short-term (2- to 7-day)
and long-term (1- to 12-month)
E-PERMs that are currently
marketed. The thickness
of the electret affects
the usable measurement period.
For a 7-day exposure period
using a short-term E-PERM,
as well as for a long-term
E-PERM, the LLD (sensitivity)
is about 0.3 pCi/L
(U.S. EPA Measurement Protocols
1989). The coefficient
of variation should not
exceed 10 percent (1 sigma)
at radon concentrations
of 4 pCi/L or greater (U.S.
EPA Measurement Protocols
1989). This precision
should be monitored by using
results of duplicate detector
analyses.
Continuous
Radon Monitors -
There are
three types of continuous
radon monitors (CRMs).
In the first type of CRM,
ambient air is sampled for
radon in a scintillation
cell after passing through
a filter that removes radon
decay products and dust.
Alpha particles (produced
by radon decays) strike
the zinc sulphide coating
of the cell, yielding scintillations
which are detected by a
photo-multiplier tube in
the detector. A second
type of CRM operates as
an ionization chamber.
Radon in the ambient air
diffuses into the chamber
through a filtered area
so that the radon concentration
in the chamber follows the
radon concentration in the
ambient air with some small
time lag. The third
type of CRM functions by
allowing ambient air to
diffuse through a filter
into a detection chamber.
As the radon decays, the
alpha particles are counted
using a solid state silicon
detector. Most CRMs
are capable of an LLD (sensitivity)
of 0.5 pCi/L or less
(U.S. EPA Measurement Protocols
1989). Special cells
are available for some CRMs
which have LLDs of 0.1 pCi/L.
The precision of most CRMs
can achieve a coefficient
of variation of less than
10 percent (1 sigma) at
4 pCi/L or greater
(U.S. EPA Measurement Protocols
1989). |
