The radionuclide technology is complementary to the three waveform verification technologies - seismic, infrasound and hydroacoustic - employed by the CTBTO verification regime. This technology is the only one that is able to confirm whether an explosion detected and located by the others is indicative of a nuclear test.
The radionuclide monitoring technology measures the abundance of radioactive particles and noble gases, i.e. radionuclides, in the air. A radionuclide is an isotope with an unstable nucleus that loses its excess energy by emitting radiation in the form of particles or electromagnetic waves. This process is called radioactive decay. Radionuclides may occur naturally, but they can also be artificially produced. Radionuclides are often called radioisotopes.
The radionuclide technology is the only one that can confirm whether an explosion is indicative of a nuclear test.
Primordial radionuclides originate mainly from the interiors of stars. Some of them, such as uranium and thorium, decay very slowly and are therefore still present in our universe today. There are also artificially produced radionuclides, which are generated by nuclear reactors, particle accelerators, radionuclide generators or nuclear explosions.
Noble gases are chemical elements that normally occur in their gaseous state. The name “noble gases” emphasizes the fact that these elements are inert and rarely react with other chemicals. In comparison to radionuclide particles, noble gas atoms are very small.
As other elements, noble gases also occur in nature in a number of isotopes, some of which are unstable and emit radiation. There are some radioactive noble gas isotopes, i.e. radionuclides, which do not occur naturally but can only be produced by nuclear reactions. Due to their nuclear properties, four isotopes of the noble gas xenon are particularly relevant to the detection of nuclear explosions.
Following an atmospheric nuclear explosion, solid fission products attach to dust particles that are propagated by prevailing winds over great distances.
Detecting nuclear explosions
Most of the energy of a nuclear explosion is transformed into the immediate blast, shockwaves and heat - explosive energy that is released within less than a minute. Initial radiation accounts for another small fraction of the energy released during a nuclear explosion. The remaining 10% of the energy is released as residual radiation, which is emitted over time, mainly through radioactive decay of the explosion’s fission products.
Fission products, in solid and gaseous form, are isotopes generated during the nuclear chain reaction. Some of these isotopes are stable. Most are not and undergo radioactive decay, i.e. they are radioactive. Following an atmospheric nuclear explosion, solid fission products attach to dust particles that are propagated by prevailing winds over great distances.
Underwater nuclear explosion also release radioactive particles into the atmosphere. Even shallow underground nuclear explosions can be detected by their release of radioactive debris.
Well-contained underground or deep underwater nuclear explosions, however, do not release any radioactive particles into the air. Another method is needed to detect them.
Radioactive noble gas isotopes – in particular xenon isotopes - are among the fission products generated in a nuclear explosion. Due to their property of being inert, these xenon isotopes will not attach to debris or dust to form larger particles. They remain in their gaseous state and some of them will seep through layers of rock and sediment until they escape into the air. Exposed to prevailing winds, they are dispersed in the atmosphere and may, after a certain period of time, be detected thousands of kilometres away from the explosion site.
Radioactive xenon from a well-contained underground nuclear explosion seeps through layers of rock, escapes into the atmosphere and can later be detected thousands of kilometres away.
The objective of the CTBTO’s radionuclide monitoring network is to detect this residual radiation in the form of radioactive particles or noble gas, even if only in miniscule amounts. By literally collecting and analysing the debris of a nuclear explosion, the radionuclide technology is the only one of the four technologies employed that can provide evidence that an explosion has been nuclear in nature.
Thus, this technology provides the means to identify the “smoking gun” needed to prove a possible violation of the Treaty. With its “forensic proof” of nuclear explosions, the radionuclide technology is of crucial importance to the entire verification effort.
The radionuclide technology provides the means to identify the “smoking gun” needed to prove a possible violation of the Treaty.
However, this does not mean that the CTBTO itself determines whether an explosion has been nuclear in nature or not. It is the prerogative of Member States
Building radionuclide stations
Stations must have a denser presence near the equator than in higher latitudes because global wind fields in the equatorial region are virtually vertical, while in the North and South they are more lateral. This means that, in the higher latitudes, radionuclides are transported horizontally most effectively. The more stations there are, the greater the probability of detection and the shorter the probable time period of doing so.
Basic components of a radionuclide station include housing for detection equipment (i.e. gamma ray detector, compressed filter, decay chamber) a high volume air sampler and a satellite antenna.
In terms of air sampling at the station site, it is advantageous to have a good mixing of surface air with upper layers of air. In principle, the site should be a windy, exposed place where the passing air really hits the sampler used to collect particulates transported by the wind. The larger the air volume, the greater is the efficiency for particulate sampling.
One disadvantage with this type of detection is that it is passive, relying on air currents to move the particles or gases to the radionuclide detection site. This is why so many stations are needed for the monitoring of radionuclides.
Ideal radionuclide monitoring sites are windy, exposed places where the passing air really hits the air sampler used to collect particulates transported by the wind.
First, a site survey is conducted to assess the suitability of the site to host a station and identify any specific conditions that would impact on station design. The Treaty lists the geographical coordinates for each station, but only a site survey determines the exact location of a radionuclide station and its elements.
For the manufacture and installation of the station, a single contractor is generally selected through an international tendering process. This contractor is responsible for the station design, manufacture, installation and testing.
The CTBTO reviews all aspects of each station to ensure that it meets all criteria to receive certification as a valid station within the IMS network. Once certified, operation and maintenance agreements are established between the CTBTO and a Station Operator. Long-term quality monitoring is then undertaken to ensure high standards of data quality, data availability; and station performance.
The radionuclide network
The 80-station radionuclide monitoring network enables a continuous worldwide observation of aerosol samples of radionuclides. The network is supported by 16 radionuclide laboratories hosting expertise in environmental monitoring and providing independent additional analysis of IMS samples.
The IMS’s 80-station radionuclide network is supported by 16 radionuclide laboratories hosting expertise in environmental monitoring and providing independent additional analysis of IMS samples.
A radionuclide particulate monitoring station contains an air sampler, detection equipment, computers and a communication set-up. At the air sampler, air is forced through a filter, which retains more than 85% of all particles that reach it. Filters are replaced daily. The used filter is first cooled for a period of 24 hours and then measured for another 24 hours in the detection device at the monitoring station. The result is a gamma ray spectrum that is sent to the International Data Centre for further analysis.
Data sent by the radionuclide stations to the IDC do not only include gamma radiation spectra, but also meteorological and state-of-health information. State-of-health data provide information on the station’s operational status and the quality of the raw monitoring data it transmits.
Support is provided by 16 radionuclide laboratories, which conduct sample analyses if and when necessary. The laboratories analyse samples suspected of containing radionuclide materials that may have been produced by a nuclear explosion. They also conduct routine analyses of regular samples to provide quality control of a station’s air sample measurements.
Radionuclide stations send monitoring data to the IDC, but also information on the station’s operational status and the quality of the transmitted monitoring data.
INGE (International Noble Gas Experiment)
On an experimental basis, half of all radionuclide stations (i.e. 40) are equipped with the noble gas monitoring technology. Complementary to radionuclide particulate monitoring, the International Noble Gas Experiment (INGE) was established in 1999 to test the measuring of radionuclide noble gases released by nuclear explosions.
The International Noble Gas Experiment (INGE) was established in 1999 to test the measuring of radionuclide noble gases released by nuclear explosions.
Four measurement systems were included in the experiment: Russia’s ARIX (Analyzer of Xenon Radioisotopes); the United States’ ARSA (Automated Radioxenon Sampler-Analyzer); Sweden’s SAUNA (Swedish Unattended Noble gas Analyzer); and France’s SPALAX (Systéme de Prélèvements et d’Analyse en Ligne. d’Air pour quantifier le Xénon), of which three are currently in use.
As a result of the experiment, the first radionuclide station with noble gas detection capabilities was formally integrated into the global verification regime on 19 August 2010. The noble gas measurement set-up is co-located with radionuclide station RN75 in Charlottesville, Virginia, United States.
All systems in the INGE project work in a similar way. Air is pumped into a charcoal-containing purification device where the noble gas xenon is isolated. Contaminants of different kinds, such as dust, water vapour and other chemical elements, are eliminated. The resulting air contains higher concentrations of xenon, both in its stable and unstable (i.e. radioactive) form. The radioactivity of the isolated and concentrated xenon is measured and the resulting spectrum is sent to the IDC in Vienna for further analysis.
Setting up the radionuclide labs
The 80 radionuclide stations are divided into four regions - the Americas, Europe and Eurasia, Asia and Oceania, and the Mediterranean and Africa - with each region supported by four radionuclide laboratories.
The IMS’s 80 radionuclide stations are divided into four regions - the Americas, Europe and Eurasia, Asia and Oceania, and the Mediterranean and Africa. Each region is supported by four radionuclide laboratories.
These 16 radionuclide labs, and their placements around the world, have been selected to support the station network. Their main function is the independent analysis of particulate samples to corroborate data from other types of stations and to provide quality control through routine sample analyses. Quality control for routine samples foresees one quarterly sample from each station so that the total number is about 320 per year.