WOULD-BE plutonium smugglers could find life tougher thanks to a radiation detector based on optical fibres. The glass fibres emit light when bombarded with the neutrons that plutonium emits. The detector, which was developed at the Pacific Northwest National Laboratory in Richland, Washington, may also enable doctors to monitor the precise dose of radiation they are giving to patients.
"Knowing that neutrons are around is a big deal if you're looking for plutonium," says Mary Bliss, principal investigator on the project. "Other than a few applications in geology, there's no reason why someone would walk through an airport with a neutron source." However, traditional detectors-which consist of a metal tube filled with pressurised gas-are bulky, cannot safely be shipped by air, and can be damaged by vibrations.
The new fibre-optic detector is light and flexible. It is made from layers of plastic sandwiched between layers of the special fibres, which are impregnated with cerium (III) ions and the isotope lithium-6. When a neutron hits the sandwich, the plastic slows it down. It then collides with a lithium-6 atom, smashing it apart and releasing a shower of electrons.
The electrons excite nearby cerium(III) ions, which emit photons of visible light that travel to the ends of the fibres, where they can be detected. If four or more photons are detected within 200 nanoseconds, a neutron is almost certainly responsible.
"People have been trying since the early 1960s to use this stuff as radiation detectors," says Bliss. But making the necessary glass has proved to be the stumbling block.
The trouble is that cerium(III) is easily oxidised to cerium(IV). And if the fibres contain any cerium(IV) the detector won't work, because cerium(IV) will absorb any photons that are produced. To avoid contamination, the fibre must be made in a low-oxygen atmosphere, which requires precise control of the manufacturing process.
The laboratory has licensed the technology to Canberra Industries of Meriden, Connecticut. The company has produced a prototype plutonium detector, and the International Atomic Energy Agency has installed a unit at the border between Austria and Hungary.
So far, it has not uncovered any plutonium. It was, however, triggered by a woman on a bus who had recently received radiation therapy and was emitting gamma rays, to which the detector is also sensitive.
Glenn Knoll, a nuclear engineer at the University of Michigan is impressed by Bliss's technical accomplishment, but is less convinced of the practical value of the detector. "Although I can certainly recognise the novelty of what has been done," says Knoll, "I cannot, in all honesty, point to an important application where I think it has made a big difference."
But plutonium detection is not the only use for these fibres. They are also being tested at the University of Washington's Nuclear Radiation Center in Pullman as a way of monitoring the dose delivered to a brain tumour during radiation therapy. Bundles of the fibre are taped to the patient's skull and placed in the mouth and sinuses, allowing medical staff to deliver a precise dose of radiation.
Author: Henry Bortman
New Scientist issue 4 September 99
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