Pyramid-shaped nuclei

A group of European physicists believe they can solve the mystery behind dozens of nuclear experiments carried out more than a decade ago. The experiments—conducted with various detectors, at various energies, and involving various nuclear species—produced such puzzling results that many of the experimenters turned their attention to the study of other systems. Now, Jerzy Dudek of University Louis Pasteur (Strasbourg, France) and his colleagues at Warsaw University (Poland) and the Autonomous University of Madrid (Spain) think that the old results can be explained by nuclei having tetrahedral symmetry. Just as so-called magic nuclei have the right number of neutrons and protons to readily form stable spherical nuclei, so too can such magic numbers be expected for forming pyramidal nuclei. Stable, in this case, means that the state persists from 10¹² to 10¹⁴ times longer than the typical 10^–21 seconds for nuclear reactions. In tetrahedral nuclei, up to four nucleons of the same kind (neutrons or protons) could share a single energy level instead of the customary one or two nucleons permitted by classic nuclear theory. That rule-of-four would inhibit the normally observed decay modes by which nonspherical nuclei throw off energy, usually by emitting gamma rays. The inhibition would explain the puzzling results of earlier experiments. Dudek says that gadolinium-156 and ytterbium-160 are good candidate nuclei for stable pyramid configurations, and the physicists found tentative evidence for tetrahedral ¹⁵⁶Gd. The researchers plan to test their ideas in upcoming experiments. (J. Dudek et al., Phys. Rev. Lett. 97 , 072501, 2006 http://dx.doi.org/10.1103/PhysRevLett.97.072501 .)