Radionuclide data processing
Waveform data help identify the location of an event and qualify it as either natural or potentially man-made. However, they cannot reliably answer the question as to whether a man-made event, such as an explosion, was nuclear or not. The only reliable way to answer this question is to trace and analyse the radioactive remains of a potential nuclear explosion. Some of these remains provide the ultimate proof for the nuclear nature of an event and are therefore referred to as the ‘smoking gun’.Radionuclide monitoring data from the 80 radionuclide monitoring stations worldwide are expected to shed light on this key question. Data on radionuclide observations are sent to the International Data Centre (IDC), where they undergo an analysis process like waveform data. After the automatic analysis process, analysts refine the results during interactive review.
Only the analysis of radionuclide data can provide the ultimate proof, the ‘smoking gun’, to establish the nuclear nature of an explosion.
Each radionuclide particulate monitoring station sends one gamma ray spectrum per day. A gamma ray spectrum is a two-dimensional plot showing which radionuclides were observed in a single sample and in what quantity.
During radioactive decay, most radionuclides emit a very specific quantity of energy, which can take the form of alpha, beta or gamma radiation. As the energy of gamma radiation is known for each isotope’s radioactive decay process, the measuring of gamma radiation allows the conclusive identification of an isotope in the spectrum.
This is shown along the horizontal axis of the spectrum, where radionuclides present in the measured sample are identified by the energy of their gamma ray emission. The vertical value in the plot stands for the count rate of detected radiation and, thus, determines the quantity of each radioactive substance in the sample.
This entire process is automatic and its results are listed in the Automatic Radionuclide Report (ARR). Radionuclide analysts in the IDC review these reports and take corrective measures when needed. Their review results in the Reviewed Radionuclide Report (RRR).
Each radionuclide particulate monitoring station sends one gamma ray spectrum per day showing which radionuclides were observed in a single sample and in what quantity.
Automatic radionuclide event screening
It is known which radionuclides occur naturally or are man-made, and which man-made radionuclides are generated during a nuclear explosion. It is also known in what quantities and ratios these elements are produced during such an explosion. Based on this knowledge and with the results derived from the radiation spectra, State Parties can determine whether or not the sample suggests that a nuclear explosion has indeed taken place.
To filter out the results, samples are categorized in five groups according to the identified radionuclides. Radionuclides in the first two category groups are radioactive substances originating from natural sources.
Samples of radionuclides are examined and categorized in five groups according to the nature of the radionuclides contained in them.
Category 3 radionuclides are man-made radioactive isotopes that are known to be observed typically at certain stations. These include emissions from regular civilian application of nuclear technology, such as at nuclear power plants and in hospitals. For example, some monitoring stations in Europe still measure radionuclides that can be traced back to the accident at the Chernobyl nuclear power plant in 1986.
Categories 4 and 5, i.e. samples that contain radionuclides that are relevant for CTBT verification purposes, are considered more carefully in the analysis process. Of those two, level 5 detections are the ones that demand immediate attention as they may contain multiple specific radionuclides generated by a nuclear explosion.
A level 5 sample is therefore automatically sent to two of the 16 radionuclide laboratories for further analysis. While the initial analysis at station level takes only 24 hours, the specialized laboratories perform a very thorough review with highly specialized equipment which can take seven days.
Automatic radionuclide event screening cont.
The radionuclide screening process focuses on the concentrations of observed relevant radionuclides. Relevant radionuclides are radionuclides that are specific to nuclear explosions, comprising those from the nuclear fission itself, plus those resulting from, for example, interaction of neutrons emitted during the explosion with the surrounding material.
The relative quantities of different isotopes detected in a sample analysis can provide information about the time of an explosion and possibly about the environment in which the explosion took place, i.e. underwater, in the air or underground.
The findings of the screening process are presented in the Standard Screened Radionuclide Event Bulletin (SSREB). This report, along with raw data and the other bulletins, are made available to the Member States. It is their prerogative to make the final judgement on the findings.
Samples containing category 5 radionuclides demand immediate attention as they may contain multiple specific radionuclides generated by a nuclear explosion. They are sent to two of 16 laboratories for further analysis.
Radionuclide noble gases
Half of the 80 radionuclide particulate stations are expected to be equipped with noble gas monitoring technology. The systems currently under experimental testing can measure certain isotopes of the noble gas xenon. These isotopes are highly volatile and likely to be released even from underground explosions, whereby radionuclides particles are contained.
Some monitoring stations with noble gas technology employ the same detection method as that used by radionuclide particulate stations and send gamma ray spectra of daily measured samples. Other stations use a different method of measuring the radionuclide noble gases contained in a sample by looking at the combined beta and gamma radiation.
The 40 radionuclide particulate stations equipped with noble gas monitoring technology use several different detection methods to measure radionuclide noble gases.
Event categorization of noble gas samples is a highly demanding task, since some civilian sources may produce radioactive xenon in concentrations close to nuclear weapons test specifics. Work is continuing to advance the event categorization of noble gas findings.
Timelines of data processing and analysis