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Lasers Continue Across Nuclear Fission Threshold

Photonics Spectra
Apr 2000
Dr. James P. Smith

Scientists at the Rutherford Appleton Laboratory have demonstrated that today's powerful lasers can replace nuclear accelerators in some experimental studies of nuclear and plasma physics. Along with more general experiments, they used a terawatt laser to create plasma-induced gamma rays with sufficient energy to initiate nuclear fission.

Another group of researchers achieved nuclear fission with a petawatt laser at Lawrence Livermore National Laboratory in Livermore, Calif. Although there were some differences in the research, the experiments -- both detailed in the Jan. 31 issue of Physical Review Letters -- demonstrated the potential of laser-induced nuclear reactions.

The UK experiments used Rutherford's Vulcan laser, a powerful, Nd:glass laser system that can deliver up to 2.6 kJ in nanosecond pulses and more than 100 TW in picosecond pulses at 1054 nm. The laser uses chirped-pulse amplification to produce pulses of about 1 ps with an energy of about 50 to 80 J per pulse, said Ken Ledingham of the University of Glasgow's Laser Ionization Group. "We can normally get one pulse every 30 minutes, with about 25 to 30 pulses per week."

Ledingham, leader of the nuclear aspects of the UK program, said that the work done in his lab and in California were similar and entirely supportive, but that there were some differences. "The Livermore laser has a greater intensity, but Vulcan has a more accurately directed beam," he said. Livermore's laser struck a gold target mounted on copper-clad uranium; Rutherford's primary target was a tantalum slab with various materials behind it, including pure uranium to induce fission.

While Livermore's petawatt laser already has been dismantled, Ledingham said, the Vulcan will be upgraded to petawatt status within the next 18 months. "We want to understand in detail the plasma physics of these exotic plasmas, which underpin all the nuclear applications and without which none of this will be possible," he said.

Ledingham said the work has potential applications in nuclear medicine. "We are also keen to have a method for generating short-lived radioactive sources using a desktop laser rather than a reactor. This is especially important for generating positron emitting sources for [positron emission tomography] scanners."

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