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Lasers tackle radioactive waste

14 Aug 2003

A 360 J laser pulse transmutes an isotope with a half-life of 15.7 million years into a lighter isotope with a half-life of just 25 minutes.

One of the biggest challenges facing the nuclear industry today is the storage and disposal of waste that will remain radioactive for millions of years. One approach to this problem involves bombarding the waste with neutrons to speed up the decay of long-lived isotopes into nuclei with much shorter half-lives. However, physicists in the UK and Germany have now demonstrated a new laser-driven approach to "transmutation" by converting iodine-129, which has a half-life of 15.7 million years, into iodine-128. The half-life of this lighter isotope is just 25 minutes (J. Phys. D to be published).

Ken Ledingham and colleagues from Strathclyde University, Glasgow University, Imperial College, the Rutherford Appleton Laboratory and the Institute for Transuranium Elements in Karlsruhe, Germany, illuminated a small gold target with a 360 Joule laser pulse from the VULCAN glass laser at Rutherford. The pulse had a duration of 0.7 picoseconds and was focussed to give an intensity of 5x1020 Watts per square centimetre.

The laser ionized the gold to form a plasma and then accelerated the electrons in the plasma to relativistic energies. When the electrons struck the solid gold of the target they emitted gamma-rays as bremsstrahlung radiation. Ledingham and colleagues then placed a sample of nuclear waste containing radioactive iodine behind the gold target. Transmutation occurs when a gamma-ray ejects a neutron from a iodine-129 nucleus to leave behind short-lived iodine-128. Each laser shot produced about 3 million iodine-128 nuclei.

"We have shown for the first time that we can transmute isotopes with lasers," said Ledingham. "Now we need to scale up our methods so that we can deal with the sort of volumes likely to be produced by the nuclear industry in the future. Using lasers is a relatively cheap and very efficient way of disposing of nuclear waste." Laser-induced nuclear reactions could also have applications in the production of medical isotopes.

The field of nuclear physics with lasers took off in 1999 when Ledingham and co-workers, and an independent team using the Petawatt laser at the Lawrence Livermore National Laboratory in the US observed laser-induced nuclear fission in uranium-238 for the first time, along with a variety of other laser-induced nuclear reactions. Earlier this year a team at Friedrich Schiller University in Jena, Germany, managed to achieve photo-induced fission in U-238 and thorium-232 with a much smaller "table-top" laser. The Jena team has also observed the transmutation of iodine-129 with its system.

Peter Rodgers is the Editor of Physics World

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