Monday, November 11, 2013

CSI: Nuclear

Bruce Pierson has 90 seconds to escape the maze of concrete blocks. He starts at the back of the lab, next to the neutron generator that will start up once the area is clear. Then he weaves through the alcoves, hitting checkpoints that ensure that no one is hidden away working, though they should be alerted by the flashing lights and wailing siren. Anyone trying to come into the area is sensed and admonished by a recording. Once Pierson is out, he locks the door, and the experiment can begin.

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.
Actinides are some of the heaviest elements in the periodic table, including the nuclear weapon favorites uranium-235 and plutonium-239. If the authorities could measure confiscated bomb materials before they were detonated, the radioactive elements would be as recognizable as the voice of a DJ on the radio. Traditional nuclear weapon materials have been studied for decades, their radiation signatures are well known.

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.

Under interrogation

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. 
The trouble is, using radiation detectors to find the actinide gamma-ray signatures from fresh melt glass would be like trying to pick out the DJ’s voice from a recording of a crowd. The explosion would have turned many of the lighter nuclei in the glass into isotopes that are radioactive. As these light nuclei turn back into stable isotopes, many will also spit out gamma-rays, masking the gamma-rays emitted by the weapon’s active ingredient.

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.
He does this by thoroughly testing different isotopes that might be present in a nuclear weapon. For instance, a uranium weapon would be mostly the easy-to-fission uranium-235, with 92 protons and 143 neutrons, but much of the remainder would be naturally abundant uranium-238, with 146 neutrons. It would be useful to know how much uranium-235 is present compared to uranium-238, to probe the refining capabilities of the culprits. Through interrogation, both uranium isotopes reveal themselves.

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.

Speed demon

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.
An up-to-60-mph journey through metal tubing lands the pellet in the gamma-ray detector, made of a single germanium crystal. Pierson compares it to the pneumatic systems used for drive-through banking. Maybe if Elon Musk designed banks.

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.
To get the travel time down to less than 300 milliseconds, he has bypassed the routing points that could send the pellet to alternative detectors, saying with a hint of subversive pride that he has “defeated much of this multifaceted system.”

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.
But on rough days, when the neutron generator starts sending neutrons at the wrong rate, or the sample gets out of sync with the pneumatic timing, Pierson needs to fix the problem immediately. “I basically have to babysit the pneumatic system, detector and the neutron generator all day long during an experiment,” he said. So with the control system running on one laptop, he analyzes the data on his other computer continuously checking for variations in the system that would undermine his results.

Fingerprinting isotopes

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.

Photos by Joseph Xu, Michigan Engineering Communications & Marketing, unless otherwise noted. For more, see the Flickr set.

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