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Hot
Spotter:
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| This “hot spotter” could be used to detect radiation from passengers’ baggage. Inset: Scintillating material for the next-generation detector. (Photo by Tommy Maxwell/Y-12; enhanced by Judy Neeley.) |
A sensitive radiation detector that could easily be held in your hand or attached to your belt has been designed and built by Oak Ridge researchers. The size of a videotape cassette, this detector could be used by first responders, customs officials, law enforcement personnel, and employees who scan passengers’ baggage at airports or train and bus stations. The device could indicate the presence of radioactive materials that might be intended for a “dirty bomb” that could be detonated using conventional explosives.
Detectors for both neutrons and gamma rays at low levels have been developed by Zane Bell of Oak Ridge’s Y-12 National Security Complex, Gil Brown of ORNL’s Chemical Sciences Division, and Michael Paulus and David Smith of ORNL’s Engineering Science and Technology Division. One instrument (the HotSpotter™) consists of a cadmium tungstate (CdWO4) scintillation crystal, microprocessor, high-voltage supply, analog pulse-processing circuitry, analog-digital converter, and photomultiplier tube. It also has firmware that acquires and analyzes gamma-ray spectra (the fingerprints for identifying radioactive elements) and compares them with known element spectra in a stored database.
The cadmium tungstate crystal, which emits a light when struck by a gamma ray, is clad with a 0.5-mm layer of boron-loaded epoxy—the novel part of the detector synthesized by Bell. When a neutron strikes the cladding, boron-10 (10B) disintegrates into an alpha particle and a lithium-7 (7Li) nucleus, which emits a high-energy gamma ray (478 kilovolts). Detection of this gamma ray (as opposed to the particles) results in a peak in the spectrum denoting the presence of neutrons.
“The probability of detecting gamma rays with our detector is larger than it is from conventional devices using cesium iodide or sodium iodide crystals,” Bell says, “because of the high density and high atomic number of cadmium tungstate.” Industrial interest has been expressed in commercializing the Oak Ridge device partly because it could be built for 25% of the cost of today’s radiation detectors.
For a second detector, Brown has synthesized a quasi-organic scintillator to replace CdWO4, which is an inorganic scintillator. By dissolving an organic scintillator, such as diphenyl oxazole, and a 10B-containing compound in silicone liquid, and then polymerizing the liquid while it is in a mold such as a jar, he makes a scintillating material with the mechanical characteristics of silicone rubber. Except for the phosphor and boron, the chemistry is essentially that of common bathroom caulk.
When neutrons strike the 10B atoms in this scintillator, the alpha particles and 7Li nuclei resulting from boron decay excite nearby benzene rings in the silicone. The rings become excited and transfer their energy to the scintillator molecules, which, in turn, emit visible light.
“This silicone scintillator is exquisitely sensitive to neutrons,” Bell says. “One radiation detector company in the Oak Ridge area is interested in our invention because it is five times more efficient in the detection of high-energy neutrons than is the conventionally used helium-3 tube detector. In addition, our silicone scintillator would be much hardier and less expensive than this commercial device.”
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