More clues come from levels of reactive agents, such as iron, added during processing and reprocessing.
For the Hansford sample, the team could tell that the plutonium was reprocessed using the bismuth-phosphate method, the only industrial-scale technique used in the U.S. in the 1940s.
Harold Smith, of the Goldman School of Public Policy at the University of California, Berkeley, noted that "nuclear forensics" efforts actually began in 1949.
That year, the U.S. used airplanes near the Chinese coast to monitor airborne debris and determined that the Soviet Union had detonated its first nuclear weapon.
Today, if plutonium is captured in transit or terrorists detonate a device, tracking techniques like those Schwantes used could help determine the material's origins.
But that requires comparing the stolen material, or the debris from an explosion, to data banks held by makers of nuclear material, Smith said. Such information is not universally available.
"You can have hundreds of fingerprints at a crime scene, but it won't do you much good unless you have a data bank of prints to compare it to. That data bank is going to be a pressing issue in the decade ahead."
Access to information on reactors and nuclear production is a sensitive national security issue. But Smith said that collaboration among nations is improving, and agencies such as the International Atomic Energy Agency have the beginnings of data banks.
Such a system could give pause to those who would acquire nuclear materials illegally.
"If Russia and the U.S. were willing to say they will be able to hold accountable anyone who supplies fissile material to terrorists, it could have a significant deterrent effect," Smith said.
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