Ambient 133Xe levels in the Northeast US

Measurements of 133Xe (τ1/2 = 5.2 days) atmospheric concentrations were performed during the fall of 1993 and throughout 1995 on ‘noble gas concentrates’ from the northeastern US. These samples were obtained from a commercial air-reduction plant in Allentown, Pennsylvania. Following Chromatographic purification of the xenon gas, the 133Xe activity was determined using a high-purity germanium gamma-ray spectrometer. The average 133Xe concentrations were in the range 1–3 mBq m−3, which is consistent with nuclear power plant noble gas releases in the region surrounding the sampling point, but approximately 50–100 times lower than those reported in Albany, NY approximately 300 km to the northeast from 1975 through 1984. The lower atmospheric concentrations are also consistent with the 100-fold reduction in radioxenon release from 25 nuclear reactors in that region. Only an upper limit could be established for the 135Xe level (τ1/2 = 9.1 h), which was about 0.03 of the 133Xe level. These background levels are of concern in monitoring for atmospheric radioxenons to assure compliance with a Comprehensive nuclear Test Ban Treaty (CTBT).