Page 24 - Fall 2011

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3. UT - Unfiltered Track Detection
The unfiltered alpha track detector operates on the same principle as the alpha track detector,
except that there is no filter present to remove radon decay products and other alpha particle
emitters. Without a filter, the concentration of radon decay products decaying within the
"striking range" of the detector depends on the equilibrium ratio of radon decay products to
radon present in the area being tested, not simply the concentration of radon. Unfiltered
detectors that use cellulose nitrate film exhibit an energy dependency that causes radon decay
products that plate out on the detector not to be recorded.
This phenomenon lessens, but does not totally compensate for the dependency of the
calibration factor on equilibrium ratio. For this reason, EPA currently recommends that these
devices not be used when the equilibrium fraction is less than 0.35 or greater than 0.60 without
adjusting the calibration factor. EPA is currently evaluating this device further to determine
more precisely the effects of equilibrium fraction and other factors on performance. These
evaluations will lead to a determination as to whether to finalize the current protocol or remove
the method from the list of Program method categories.
4. LS - Charcoal Liquid Scintillation
This method employs a small vial containing activated charcoal for
sampling the radon. After an exposure period of 2 to 7 days (depending
on design) the vial is sealed and returned to a laboratory for analysis.
While the adsorption of radon onto the charcoal is the same as for the
AC method, analysis is accomplished by treating the charcoal with a
scintillation fluid, then analyzing the fluid using a scintillation counter.
The radon concentration of the sample site is determined by converting from counts per
5. CR - Continuous Radon Monitoring
This method category includes those devices that record real-time
continuous measurements of radon gas. Air is either pumped or
diffuses into a counting chamber. The counting chamber is typically a
scintillation cell or ionization chamber. Scintillation counts are
processed by electronics, and radon concentrations for predetermined
intervals are stored in the instrument's memory or transmitted directly to a printer.