Forensic experts may soon be called on to learn yet another skill we hope they never have to use: determining the structure of a detonated nuclear bomb based on the debris it creates. In a paper released by PNAS on Monday, a group of researchers studied a piece of glass created by the Trinity nuclear test conducted by the US 65 years ago. They were able to find signatures of materials used in the bomb that allowed them to recreate its basic structure, a process that might be necessary to identify the origin of nuclear bombs after they've served their purpose.
On July 16, 1945, the United States Army detonated a nuclear bomb, known to its creators as "The Gadget," at a desert test site known as the Jornada del Muerto. The detonation of the plutonium-fueled bomb glassed the surrounding soil into "Trinitite," a material that the researchers thought might carry signatures of the explosion event and the design of the bomb.
Of course, the bomb's design is well-documented, so the authors were not looking to confirm or rewrite history. However, they realized that if they could analyze the Trinitite and find evidence of the bomb's structure and components, the process could be repeated for forensic use. If a bomb were ever detonated by an unknown group, analysis of type of fuel or other materials might lead back to the people that built it.
To conduct their investigation, the authors borrowed a piece of Trinitite from a mineral collector and applied various microscopic imaging tools to see what kinds of conclusions they could draw. First, they used light microscopy to characterize the minerals and glasses in the piece, and microfocusing X-ray fluorescence to map where they were.
In the Trinitite, only the external layers had been significantly affected by the bomb, leaving some preserved pieces inside for comparison. Some pieces of sand that made up the Trinitite had been elevated above the temperature needed to turn them to glass, but hadn't gotten hot enough to change shape, resulting in glass crystals. The sample also had background levels of a potassium isotope and swirls of calcium concentrations.
The authors made an autoradiograph of the sample to see its radioactivity, and used secondary ion mass spectrometry to see trace element concentrations and the abundance of isotopes that might have been created by the energy from the explosion. They found a small quantity of radioactive cesium-137, a fission product that doesn't indicate what the fuel was, but confirms it was a nuclear blast.
Some longer-lived elements were also present, such as isotopes of cobalt and barium, and a spike of americium. The highest source of radioactivity, however, was plutonium-239, mostly in the calcium swirls, along with an abundance of uranium and lead.
The abundance of various lead isotopes had a unique distribution, as lead is a decay product of uranium. This indicated that the uranium and lead originated from the bomb and not the environment. If lead were used in an improvised bomb as a tamper, or fission container, the authors note that its isotope composition could also form a sort of geographic signature that would allow them to determine where the lead came from.
They emphasize that analyzing the isotopic distribution, and not only the content, of the debris would provide this information. Of course, it wouldn't be hard for bomb makers to get their lead from somewhere else, so we would likely need a combination of tamper and fuel to have a better idea of origins.
Finding the origin of the fissile material, like plutonium or uranium, would involve more of an economic or political analysis, the authors say. Because nuclear fuel is currently hard to come by, it's likely that bomb builders would use whatever they could find on the black market or could be provided to them by sponsors. This would narrow the list of suspects considerably, as long as researchers know which of the fuels was used.
This kind of microanalysis has little application in the world we currently live in (and we hope that doesn't change), but it could prove vital for the hapless victims of cowardly antagonists. The analysis is also likely to be more complex if analyzing a bomb detonated somewhere less homogeneous than a desert, such as a city. Going forward, the researchers hope to subject debris from the Hiroshima and Nagasaki bombings to the same battery of tests to see how the setting changes the results.
PNAS, 2010. DOI: 10.1073/pnas.1010631107 (About DOIs).
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