Physicists Challenge Reports of Accelerated Decay of Nuclear Excited State
LIVERMORE, Calif.—Physicists from the Lawrence Livermore National Laboratory, in collaboration with scientists at Los Alamos and Argonne national laboratories, have new results that strongly contradict recent reports claiming an accelerated emission of gamma rays from the nuclear isomer 31-yr. hafnium-178, and the opportunity for a controlled release of energy. The triggering source in the original experiment was a dental X-ray machine.
Using the Advanced Photon Source at Argonne, which has more than 100,000 times higher X-ray intensity than the dental X-ray machine used in the original experiment, and a sample of isomeric Hf-178 fabricated at Los Alamos, the team of physicists expected to see an enormous signal indicating a controlled release of energy stored in the long lived nuclear excited state. However, the scientists observed no such signal and established an upper limit consistent with nuclear science and orders of magnitude below previous reports.
The results measured by the collaboration including John A. Becker, Andreas Kraemer, Dennis McNabb and Tzu Fang Wang of LLNL; Joseph Banar, Geoffrey Miller, Laurence Pangault , Robert Rundberg and Jerry Wilhelmy of LANL; Irshad Ahmad, Frank Moore, Donald Gemmell, John Schiffer of the Argonne Physics Division; and Ali Mashayekhi and Sarvjit Shastri of the Advanced Photon Source at Argonne, appear in the August 13 edition of Physical Review Letters.
Nuclear isomers include excited states of nuclei that electromagnetically decay slowly enough for energy storage. However, the emitted gamma rays of the isomer decay come in a burst. Controlled triggering of the isomer decay allows stored energy to be released on demand, and nuclear isomers represent a potential stand-alone energy source. Barriers to developing a practical energy source are triggering and production. The tri-lab team decided to validate the earlier experiment and the conclusion of previous researchers: energy can be released in a controlled application that could be developed into a gamma ray laser.
The team set out to verify previous findings that stated a nuclear isomer, (hafnium) Hf-178, which has a half life of 31 years, is able to release a controlled amount of energy (decay quicker) when tickled with dental machine X-rays. However, when the team turned the APS X-ray beam onto the sample of 31-yr. Hf-178, no detectable increase of the isomer decay occurred. In other words, the X-ray irradiation did not decrease the time it takes for hafnium to decay; a result that Becker and the team claim is consistent with nuclear physics.
"We were trying for a verification of their (the previous scientists’) claims," Becker said. "Because the previous findings were so significant, our team felt the experiment deserved to be repeated and verified. Instead, we vigorously disagree with earlier results through direct experimental measurement. The earlier reports were also very unlikely, in any case, on very general physics arguments."
The new research shows that the practicality of this scheme of controlled release of energy and its application to gamma-ray lasers has fallen back into an interesting speculation. The experiment is a considerable contribution to the physics community. It has been selected as one of the best five works done at one of the Advanced Photon Source sectors during 2000.
The Department of Energy’s Nuclear Energy Research Initiative and the Office of Science/Nuclear Physics Division funded the research.