A shape-isomerlike state has been discovered in the 66Ni nucleus. This is the lightest, ever, atomic nucleus exhibiting a photon decay hindered - solely - by a nuclear shape change. It is an extremely rare process involving a transition between totally different microscopic configurations, coexisting at similar excitation energy.
The experiment was conducted within an international collaboration lead by researchers of the Nuclear Structure group of the Milano Physics Department and of the GAMMA experiment of the INFN Nuclear Physics Committee III, in close connection with the Institute of Nuclear Physics PAN in Krakow, the Tandem Laboratory at the Horia Hulubei National Institute of Physics in Bucharest, the Free University of Brussels and the Tokyo University Theory group.
Nuclear isomers play a key role in understanding nuclear structure physics. Of special interest are shape isomers: they may arise when the nuclear potential energy surface, in the deformation space, has minima associated with different shapes and when these minima are separated by a high barrier.
So far, shape isomers, discovered in the 60’s, were known to occur only in the heavy actinide nuclei, although, since the 80’s, mean-field models predicted their existence also in lighter systems, pointing to 66Ni and 68Ni as the lightest candidates. Recently, fully microscopic, state-of-the-art shell-model calculations, based on ingenious (Monte Carlo) computational schemes and the use of very powerful supercomputing systems (the Japanese K-computer with 1 million parallel processors) elegantly confirmed shape coexistence in the Ni isotopes and indicated 66Ni as the most promising case, paving the way to a deeper understanding of shape isomerism in terms of quantum phase transition in the nuclear matter.
The experimental discovery of a shape-isomerlike structure in 66Ni, caught through high resolution gamma-ray spectroscopy and a very selective nuclear reaction mechanism, shows that shape isomerism is possible not only in very heavy nuclei: it remarkably appears in significantly lighter systems, as correctly predicted by theory, thus helping to solve the puzzle on the microscopic origin of nuclear deformation, that has been elusive over several decades.
The work was published in Physical Review Letters on April 20th 2017 and is has been selected as Editor’s Suggestion.
S. Leoni, B. Fornal, N. Marginean, M. Sferrazza, Y. Tsunoda et al., Phys. Rev. Lett. 118, 162502 (2017).
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