  Deposition of $^{222}Rn$ daughters onto detector materials pose a risk to
ultra-low background experiments. To mitigate this risk, a common approach is
to enclose detector components in sealed plastic bags made of films known to be
effective barriers against radon. We describe a new method to evaluate radon
barriers which is unique in that (a) it gauges not only the intrinsic
resistance to radon penetration of a plastic film but also the integrity of
bags fabricated from the film and sealed following some protocol, and (b) it
employs gamma spectroscopy rather than alpha spectroscopy. We report the
results of applying this method to sealed bags fabricated from polypropylene,
Nylon, Mylar, metallized Mylar, FEP, and PFA. Evaluation of the fluoropolymers
FEP and PFA as radon barriers are the first such measurements.
