Unexpected detection of the 123Xe/123I pair and its impact on radioxenon measurements in the SPALAX-NG system.

This study reports the first documented detection of 123Xe in the context of the CTBTO's International Monitoring System (IMS), in samples collected by the SPALAX-NG system (CEA/DIF, France). 123Xe is a short-lived radioisotope (T1/2 = 2.05 h) produced during medical 123I synthesis; atmospheric releases from medical facilities can lead to 123Xe capture by nearby IMS stations, potentially causing false 131mXe detections due to spectral overlaps in regions of interest (ROIs). Using high-resolution gamma and coincidence spectroscopy, we confirmed the presence of 123Xe and its decay product, 123I, in collected air samples. To mitigate interference, we developed an analytical adaptation for SPALAX-NG that identifies 123Xe via its 149 keV gamma peak and adjusts ROIs 5 and 6, with minimal efficiency loss (≤2.8%). This solution ensures accurate discrimination between 123Xe and CTBT-relevant isotopes. Our findings demonstrate the need to account for non-CTBT radioxenon isotopes in IMS analyses, particularly near medical isotope production sites, and advocate for high-resolution detection systems to improve monitoring reliability under the CTBT.