Somewhere in a busy American seaport, a gamma ray detector meant to spot smuggled nuclear weapons material will scan the contents of a cargo container, and suddenly — off goes the alarm. But this time, like the last time, the feared substance will turn out to be a load of… bananas.
Or kitty litter.
Or slabs of granite for kitchen countertops.
To my surprise, I found out that such false alarms are not uncommon with the extraordinarily sensitive detectors put in place to guard U.S. shipping centers. Although bananas don’t quite compare with weapons-grade U-235, the natural potassium isotopes found in the yellow fruit often produce enough particles in a large shipment to produce a false-positive signal. Kitty litter is made from clay containing faint traces of radionuclide, and granite slabs contain enough natural uranium and thorium to trip radiation monitors — requiring a time-consuming physical inspection.
Berkeley Lab's Stephen Derenzo
Well aware of these problems, Berkeley Lab researchers led by Stephen Derenzo, senior scientist in the Life Sciences Division and Edith Bourret, senior scientist in the Material Sciences Division, have been exploring materials for gamma ray detectors that can distinguish the difference between the signals of threatening materials and those that are harmless. “If a shipment is found to contain radioactivity, you want to know what it is. It’s really vital, even if there were no terrorism,” Derenzo told me during a recent visit to his lab.
The critical component of these improved detectors is the scintillator, a fist-sized crystal of material that gives off a tiny burst of light every time it catches and stops a gamma ray. That signal is distinctive for each isotope of the radioactive element that emitted it. Derenzo explained that almost all radioactive elements produce a unique pattern of gamma ray energies. “When gamma rays are stopped by the crystal, the pattern of light flashes identifies the radioactive element,” he said.
A scintillator sample
Most commercial scanners use plastic scintillators, which are low-cost but only scatter gamma rays rather than stopping them. They can pick up the presence of the rays, but they can’t measure their energies — so they can’t distinguish in a sealed container a load of bananas from a plutonium pit. Sodium iodide scintillators are low cost but cannot measure the energies accurately enough to do the job. Crystals made from lanthanum bromide are better but impractically expensive. Derenzo said Germanium has outstanding gamma ray energy accuracy but is also expensive and requires super-cooling to -190°C.
During a high-throughput screening process developed under a grant from the Department of Homeland Security, the scientists evaluated thousands of potential scintillator materials. The right combination of attributes? Dense enough to actually stop a gamma ray, inexpensive enough for handheld devices, and accurate enough to distinguish security threat isotopes from commonly shipped isotopes — like those in kitty litter and bananas. The process turned up dozens of new candidates that combined excellent gamma ray stopping power with excellent energy measurement accuracy. Small crystals of the best candidates were grown and evaluated as gamma ray detectors in Bourret’s lab with support from the Department of Energy NA22 office. A crystal made of cesium, barium, and iodine (CsBa2I5) produced the highest energy resolution ever reported.
Pleased with the results, the Department of Homeland Security arranged Small Business Innovation Research (SBIR) funding for companies to develop the Berkeley Lab scintillators into commercially viable products. This fall, grants totaling $500,000 were awarded to three small research firms in Massachusetts: Radiation Monitoring Devices, Inc., Agiltron, Inc., and Capesym, Inc. First, they have to demonstrate they can grow one or more of the most-promising crystals. They include europium-doped CsBa2I5, BaBrI, BaBrCl, and BaClI, all discovered at Berkeley Lab. In later phases, they may partner with large chemical firms to scale-up production.
If successful, a new generation of radiation detectors will become widely available, with improved cost, sensitivity, and accuracy. The new materials discovered at Berkeley Lab hold the promise of a less cumbersome and more reliable cargo screening process. “We knew that better scintillators were waiting to be discovered,’’ Derenzo told me. “And our search paid off.’’
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Written by Sabin Russell
