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July 17, 2019
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A recent discovery by researchers at Idaho National Laboratory could correct a decades-old error important in the field of nuclear data. The correction could aid scientists studying fallout from nuclear detonations or experimental nuclear fuels for advanced reactors. The research appears online in a recent issue of Physical Review C, a journal covering nuclear physics.
In their publication, the INL researchers describe the gamma radiation spectrum for antimony-127, a radioisotope historically associated with the fallout from nuclear detonations. As discovered by the INL research team, the baseline measurement data used to compare this radioisotope against others is inaccurate. The baseline data was collected more than 50 years ago at the height of nuclear weapons testing in the United States and remains in use today. Correction of nuclear data is an important ongoing task for scientist studying short-lived isotopes formed from the fission process.
“Every nuclear device has its own fingerprint,” said Dr. Brian Bucher, a physicist with INL’s nuclear nonproliferation division, who discovered the error. “If there was a nuclear terrorism event and the origin was unknown, one of the first things analysts would do is collect fallout material and analyze the radioactivity.”
According to Bucher, which radioisotopes are present and the relative amounts of radioisotopes in relation to each other can tell analysts a lot about a suspect nuclear device. The discovery came while Bucher, his colleague Dr. Mathew Snow, and other researchers were performing experiments at the Idaho Accelerator Center, a research facility operated by Idaho State University.
Using a particle accelerator to split uranium into radioisotopes for training exercises by federal nuclear forensic teams, Bucher noticed the measurements for one of the fission products – antimony – was significantly off from the existing baseline data.
When uranium splits, the resulting radioisotopes emit gamma radiation until the atom is stable. Forensic teams can measure the energies and intensities of that gamma radiation—called a decay spectrum. They then compare that decay spectrum to a reference library of gamma radiation signatures to identify the radioisotopes in the fallout sample.
Some referenced spectrums recorded by past physicists were measured decades ago when nuclear research, equipment and methods for analyzing data were not as precise as they are today. The decay spectrum of antimony that Bucher was using was more than five decades old.
“I struggled at first trying to identify these peaks, because I calibrated the detector very carefully,” Bucher said, referring to a high purity germanium detector that he used to record a uranium fission spectrum. “There were three peaks, one was right on top of a peak that would normally indicate antimony, but the other two were pretty far off.”
Compared to the 50-year-old antimony decay spectrum, the two peaks were off by 1.1 keV and 0.8 keV, respectively, too much for a minor instrumentation error. To confirm his suspicions, Bucher asked colleagues to create a purified antimony sample and when measured again, the decay spectrum remained off from the baseline reference chart.
As it turns out, the authors of the previous study, performed in the 1960s, relied on inaccurate nuclear data to calibrate their own instruments.
“It was a very good measurement, especially for the time,” Bucher said. “But the data that they had at the time is not as accurate as it is now.”
With his findings now published, Bucher is eager to see if the reference charts, which are used worldwide, will be updated to reflect the more accurate measurement made at INL.
Bucher’s findings prove more work might be needed to validate data for other radioisotopes in the database, especially those measurements taken using older equipment. Isotopes with short half-lives are also more likely to have less accurate measurements. Antimony-127 has a half-life of just under four days.
“There are many fission isotopes to go back and check,” Bucher said. “But not all of them are important for nuclear forensics. There is a lot of potential for this process to be applied in other scientific fields where nuclear materials measurement is important.”
INL is one of the U.S. Department of Energy’s national laboratories. The laboratory performs work in each of DOE’s strategic goal areas: energy, national security, science and environment. INL is the nation’s leading center for nuclear energy research and development. Day-to-day management and operation of the laboratory is the responsibility of Battelle Energy Alliance.
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