Production and analysis of traditional and non-traditional radioxenon isotopes

Radioxenon releases can originate from fission during nuclear detonations (atmospheric, underground, and underwater), research and commercial reactors, and medical isotope production facilities. Their impacts on atmospheric sample analysis have to be well understood to distinguish between clandestine activities and commercial operations. The global community relies on atmospheric monitoring of radioxenon, among other technologies, to monitor emissions from underground nuclear tests. The Comprehensive Nuclear Test-Ban Treaty (CTBT) incorporates radioxenon monitoring within International Monitoring System (IMS) with a focus on the traditional radioxenon isotopes ¹³¹ [superscript m] Xe, ¹³³ [superscript m] Xe, ¹³³Xe, and ¹³⁵Xe. To strengthen environmental monitoring for radioxenon, a method to produce high purity radioxenon samples was developed. The University of Texas’ 1.1 MW TRIGA research reactor was used for radioactive sample production via neutron activation. The reactor facilities include a pneumatic system for precise timing when irradiating samples. In order to use the pneumatic facilities, gaseous samples have been encapsulated in quartz to fit into the polyethylene vials designed for the system; this method also minimizes leakage, and avoids contaminants from entering the sample. Enriched, stable, isotopically pure xenon gas was irradiated with neutrons in order to activate it to radioxenon isotopes, yielding a complete set of radioxenon isotopes including non-traditional – ¹²⁵Xe, ¹²⁷Xe, ¹²⁹ [superscript m] Xe, ¹³⁵ [superscript m] Xe and ¹³⁷Xe – and traditional radioxenon isotopes. The samples were analyzed with a β--γ coincidence detector; the measurement of the non-traditional isotopes in an ARSA-style β--γ coincidence detector were the first of their kind. Measurements of the ¹³¹ [superscript m] Xe, ¹³³ [superscript m] Xe, ¹³³Xe, and ¹³⁵Xe were used to determine the β--γ coincidence efficiency of the detector and the metastable versus ground state production ratio after irradiation of ¹³³Xe and ¹³⁵Xe. Regions of interest (ROI) were defined for ¹²⁵Xe, ¹²⁷Xe, ¹²⁹ [superscript m] Xe, and ¹³⁷Xe to estimate their interference with the traditional isotopes. The newly defined ROIs aid in distinguishing between radioxenon signatures originating from fission and those mainly originating from neutron activation, thus advancing atmospheric sample analysis in the context of CTBT verification.