|Pierson in the maze. The concrete structure on the left contains the neutron source.|
In an era of sophisticated terrorist organizations and rogue states attempting to develop nuclear weapons, efforts to prevent nuclear attacks have focused on controlling the materials that could be used for bombs. But if one of these groups did manage a nuclear attack, what information could authorities gain from a careful examination after the blast? Could the remnants left behind reveal anything about the source of the weapons-grade materials?
Pierson, a graduate student in the department of Nuclear Engineering and Radiological Sciences (NERS), is finding out. “In a situation where a nuclear bomb has gone off, you could go to ground zero get a piece of the blast debris from the melt glass underneath the crater. Trapped inside of that glass are the actinides that were used in the weapon,” he explained.
|Trinitite melt glass, from test of the Trinity plutonium bomb in 1945. Its composition bears evidence of the weapon's makeup. Photo by Wikipedia user Shaddack.|
Fissioning nuclei release energy in the form of gamma-rays, which are like souped-up x-rays, and atomic innards such as electrons and neutrons. Of these three forms of radiation, gamma-rays are the most abundant and easiest to measure after fission. By observing the energies of the gamma-rays released over time, nuclear engineers can narrow down what kind of nuclear fuel was used in the device, whether it’s uranium-235, plutonium-239 or some other mixture of materials.
The current methods for using melt glass to discover the composition of a bomb rely on chemically dissecting the glass into its different elements and isotopes, or combinations of protons and neutrons. “This approach requires days to weeks to finalize the analysis and determine the primary fuel, the percent of nuclear fuel that was converted to energy and weapon design sophistication,” said Pierson.
Rather than letting the trail go cold, authorities would rather have a system that could nondestructively characterize the materials within hours, perhaps allowing them to identify the culprits and prevent a second attack. At the same time, the sample would still be available for the more detailed analysis. “The idea is to gather more information faster without disrupting the time-tested and proven nuclear forensics methods,” said Pierson.
|The inquisitor: Pierson and fellow NERS grad student Anne Campbell check the coolant and high voltage connections on the neutron source before running an experiment back in January.|
But through a process known suggestively as interrogation, it’s possible to make the voices of the actinides speak up. Rather than firing questions at the material, researchers shoot high-energy neutrons into it. The active weapons materials are much more likely than lighter elements to fission or emit radiation in response to these neutrons. This means many more gamma-rays come from those bomb-friendly actinides. Pierson’s job is to figure out how to decode the gamma-ray response from such an interrogation.
|Pierson flips through a version of the periodic table showing elements and their various isotopes. The color-coding gives information about radioactivity.|
Other isotopes present in the weapon can give clues about where the initial fuel came from and what kind of reactor it was burned in. But in order to pick all that apart, nuclear engineers must first find out what each of these isotopes look like under individual interrogation.
Pierson loads a thorium-232 sample into the set-up. The pellet, about the size of a finger segment, spends 5 seconds in the neutron beam, where it is exposed to about 10 billion neutrons per second.
The end caps of the sample capsule get worn down over repeated journeys through the pipes.
But the speed is key for Pierson. Nuclear interrogation is all about probing the material quickly, and the radioactive nuclei that transform into other isotopes fast provide the most useful clues to the material’s composition.
“Before four seconds, the behavior is very different between different actinides,” said Pierson. “as time progresses the signal intensity diminishes, and you can’t tell the difference.”
|The system concedes defeat.|
The real killer on Pierson’s time is the five minutes of measurement that he has to do after the 10 seconds of interesting radiation are over. This records other, unimportant isotopes that build up as the sample is repeatedly sent back into the neutron beam. The sample may start out as thorium, but by the end of its run, Pierson says, “I’ve contaminated my sample with half the periodic table.”
The gamma rays measured during the wait are ignored when they appear in the next cycle’s data, like removing noise from an audio recording. On a good day, the cycle repeats for about nine hours.
|The gamma ray detector sits in the middle, receiving the sample through the central tube. Six neutron detectors complete the array.|
Actinides split into a range of “daughter” nuclei when they fission, but they have many of these daughters in common. For instance, tin-132 has a “magic” combination of 50 protons and 82 neutrons; for reasons yet mysterious, nature favors this configuration. It comes up a lot in actinide fissions. But if uranium and thorium both produce tin-132, their other daughters, made of the remaining protons and neutrons minus a few, will follow different trends.
Pierson explained that thorium is more likely to produce selenium-86, while uranium tends toward zirconium-99. But that isn’t the whole story - the fission daughters usually start out with an uncomfortably large number of neutrons. They then spit out particles and gamma-rays until they reach a stable ratio of protons and neutrons.
The streak shows how often isotopes with particular numbers of protons (Z) and mass numbers (A, or protons + neutrons) form when uranium fissions after being hit with a neutron of a particular energy.
Through the gamma-ray emission spectrum, Pierson watches how the isotopes walk from one element to another over about fifteen seconds of the measurement. This gives an additional handle on what thorium looks like under interrogation – he can recognize it by measuring the initial fission products and by the way that those products change over time.
At present, Pierson is still testing whether it is even possible to reliably pick different actinides apart in this way, but if the method is successful, he is beginning to build a database of gamma ray fingerprints for these elements. In addition to the thorium-232, he has studied uranium-238, and he’s looking into testing active weapons materials.