The stack monitor uses long-range alpha detection (LRAD) technology for the measurement of radon levels in the stack emissions. The basic principle behind LRAD is the collection of ions created in air through the energy loss mechanisms of decay alphas. This is accomplished by establishing an electric field in the region where alpha decays will occur, and directing the ions via the field onto a biased plate. Accumulation of charge on the plate results in a current in the biasing circuit which can be read with a sensitive electrometer. In electrostatic LRAD designs, the linearity of the measured current with gross alpha activity is well-established. In order to determine radon-222 levels in the presence of other radon isotopes, it is necessary to perform some type of isotopic analysis on the stack samples. In the present case, other radon isotopes of possible concern are radon-219, which occurs in the decay chain of uranium-235, and radon-220, found in the decay chain of thorium-232. Radon-219, with a half-life of four seconds, presents no difficulty for the situation in which emanations from the vitrification process undergo as little as one minute of delay before release into the stack. For example, an initial concentration of 200,000 pCi/l of radon-219 decays to 5 pCi/l in one minute. Radon-220, however, has a half-life of about 55 seconds. If initially present in a substantial ratio to radon-222, a radon gross-alpha measurement on stack emissions would have a significant error if used as a measure for radon-222, even with many minutes of processing delay before the sample was taken.