The IMS radionuclide network of the CTBT

The Comprehensive Nuclear Test-Ban Treaty calls for the establishment of a world-wide network of monitoring stations (IMS, International Monitoring System). In total 321 monitoring stations will be installed: 50 primary seismic, 120 auxiliary seismic, 11 hydroacoustic, 60 infrasound and 80 radionuclide. The 80 radionuclide stations collect particulate from the air and measure its radioactivity. Among these radionuclide stations, 40 of them will also have the capability of measuring the concentration in the air of the noble gas Xenon. The particulate stations will have a cycle of 1 day collection, 1 day decay time and 1 day acquisition time whereas the noble gas stations will have 1 day of collection and 1 day of acquisition time. Equipment for particulate station consists of a high volume air sampler (flow rate ≥ 500 m-3 h-1 at standard pressure and temperature) and a HPGe detector system with a relative efficiency of at least 40 %. Stations can be manually or automatically operated depending on the choice of the host country. The technical specification of the equipment to be used, the distribution and the number of stations around the world were chosen in such a way to be able to detect nuclear explosions according to the following criterion: 90 % detection probability within approximately 14 days for a 1 kt nuclear explosion in the atmosphere or from venting by an underground or underwater detonation. All data collected are converted in a standard format and sent directly to Vienna where they are analysed. Most of the stations will send data through a very reliable satellite link. In fact it is required that particulate stations will send spectral data within 72 hours of collection start, within 48 hours for noble gas stations. In addition every station should reach 95 % of data availability; in particular it is required that the maximum consecutive down time is 7 days and a maximum of 15 days annually. In case of suspicious events detected by the stations, atmospheric transport models will be applied in order to localise the source as accurately as possible. After the treaty has entered into force, when a nuclear test is suspected, the state parties can take the decision to perform an on-site inspection at the location where this test is supposed having taken place. Currently about 11 stations are fully installed and it is planned to have additionally about 20 stations installed by the end of the year 2000. (author)