A comparison of electrostatic and filtered air collection of radon progeny

Experiments involving the collection of radon progeny from the atmosphere are described. Samples are collected for times ranging from 10 to 60 min using an electrostatic field and filtered air. A comparison is made of the total beta count obtained from samples collected via the two methods. The ratio (activity Bi 214 /activity Pb 214 ) at 15 min after the end of sampling is determined by a weighted least squares fit of the total beta count to theory. Clear differences are found in the composition of the collected samples using the two sampling methods. Two theoretical models are considered. They are (a) a model which assumes that the samples of radon progeny are in secular equilibrium, and (b) a model which assumes that only Po 218 is collected. Secular equilibrium occurs when the lifetime of the parent (Rn 222 ) is much larger than the lifetimes of the progeny in which case the activities ( N λ, where N is the number of a particular progeny and λ is the corresponding decay constant) of the progeny are equal. The results tend to indicate that the electrostatic method collects only Po 218 , whereas a mixture of radon progeny are collected via the filtered air method. However, in the filtered air case the mixture is not in secular equilibrium. Experiments of this kind are appropriate for inclusion in an experimental physics course at the junior or senior level. The experiments can be performed using inexpensive hardware and software and are particularly appropriate for modestly equipped physics departments.