posted at http://www.newscientist.com/ns/19990904/newsstory5.html
New Scientist, 4 September 1999
"Warning lights"
Henry Bortman
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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.
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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 [ie. BNCT]. 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.