Radon and CO2 tracer for radioxenon subsurface sampling in the On Site Inspection

The detection of anomalous concentration of Xenon radiosotopes in the subsurface gases during an On Site Inspection (OSI) is a strong indicator of a suspicious underground nuclear explosion. This implies that the sampling methodology ensure the collection of a reliable representative subsurface gaseous sample, avoiding the mixing with atmospheric gases. Radioxenon sampling in shallow layers can provide reliable results for desert areas, but different local geological features could result in more complex migration of subsurface gases to the very near superficial layers affecting the representativeness of the sample.

Radon is currently use as tracer to reveal the effective sampling of gases form the deep surface, so its measurement is coupled with the collection of radioxenon subsurface gases. The detection of radon anomalous concentration in subsurface gases could indicate different causes: high Radon content in subsurface indicate high radon concentration underground caused by the accumulation in an underground and confined cavity; on the other side, low radon detection in subsurface indicate low radon concentration underground that can be indicative of the absence of an underground cavity or the presence of rocks in the cavity absorbing radon. This lead to the consideration that radon is not a univocal tracer for Xe surface sampling in the OSI. A portable isotopic analyzer (that measures d13C and CO2) could be used to localize the faults and fracturing that could lead to a seeping of the subsurface gases. Therefore, this technique could be proposed as an auxiliary equipment for a preliminary activity during an OSI and a monitoring tool during subsurface gas sampling.