Fixing a 50-year-old nuclear mistakeEngineering360 News Desk | July 22, 2019
Researchers at Idaho National Laboratory found a decades-old error that turns out to be important in the field of nuclear data.
The correction could help scientists who study experimental nuclear fuels for advanced reactors, possible fallout from terrorist attacks and other applications. The research appears online in Physical Review C, a journal covering nuclear physics.
The researchers described the gamma radiation spectrum for antimony-127. This particular radioisotope is associated with fallout from nuclear detonations. They said the baseline measurement data used to compare the radioisotope against others is inaccurate.
“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.”
Identifying 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. 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 off from the baseline data.
The researchers said that when uranium splits, the resulting radioisotopes emit gamma radiation until the atom becomes 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 spectrums were measured decades ago when nuclear research, equipment and methods were not as precise as they are today. For example, 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. “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 amounts that were too great for a minor instrumentation error. 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.
The research findings suggest that more work might be needed to validate data for other radioisotopes in the database, especially measurements taken with 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.